United States Department of Commerce

Technology Administration

National Institute of Standards and Technology '0

A111DM 123=131

NIST Special Publication 856

Electroacoustics for Characterization of

Particulates and Suspensions

Proceedings of Workshop held at the

National Institute of Standards and Technology

February 3-4, 1993

Gaithersburg, Maryland

Subhas G. Malghan, Editor

o QC

TOO

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//856

1993
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NIST Special Publication 856

Electroacoustics for Characterization of

Particulates and Suspensions

Proceedings of Workshop held at the

National Institute of Standards and Technology

February 3-4, 1993

Gaithersburg, Maryland

Subhas G. Malghan, Editor

Ceramics Division

Materials Science and Engineering Laboratory

National Institute of Standards and Technology

Gaithersburg, MD 20899

Issued September 1993

U.S. Department of Commerce

Ronald H. Brown, Secretary

Technology Administration

Mary L. Good, Under Secretary for Technology

National Institute of Standards and Technology

Arati Prabhakar, Director
National Institute of Standards U.S. Government Printing Office

and Technology Washington: 1993

Special Publication 856

Natl. Inst. Stand. Technol.

Spec. Publ. 856

308 pages (Sept. 1993)

CODEN: NSPUE2

For sale by the Superintendent

of Documents

U.S. Government Printing Office

Washington, DC 20402
PREFACE

Traditionally, interface chemistry of particulates and colloidal stability in liquids has been

characterized by techniques based on microelectrophoresis. Since the conception of the zeta

potential, colloid scientists have utilized microelectrophoresis and moving boundary

electrophoresis extensively for the measurement of electrophoretic mobilities. Significant

progress has been made in developing understanding and scientific data on many particulates

of commercial and technical interests. Some of the major drawbacks of these methods are

the need for use of low concentrations of particulates in the suspensions, the requirement of

optical opacity of particulates, and the exclusive restriction to colloidal size particulates.

The electroacoustic technique, based on physical effects described by Debye, is relatively

new and the laboratory instrumentation has become available only in the last decade. This

new instrumentation by Matec Applied Sciences and Pen Kern Inc. is based on the

application of either electrical fields or ultrasound as external fields to the particles and

measurement of the resulting electrical or ultrasonic response. These instruments, while

addressing the primary drawbacks of electrophoresis instruments, allow the determination of

signals that are related to the electrokinetic zeta potential for colloidal suspensions in polar

and nonpolar media. In recent years, the electroacoustic technique has been extensively

utilized for the characterization of a variety of particulates in dense suspensions of polar and

nonpolar liquids with great success. As a result of this surge in activity, a number of

technical issues have surfaced.

In order to discuss these issues and develop a common understanding of the fundamentals of

electroacou sties, a workshop was organized at the suggestion of the user community.

"Electroacoustics for Characterization of Particulates in Suspensions" is the first workshop on

this subject organized by the National Institute of Standards and Technology (NIST). This

workshop was held on February 3-4, 1993 at NIST, Gaithersburg, MD. The primary

purpose of this workshop was to bring together scientists and engineers working in this field to address relevant scientific and technical issues on the measurement of electroacoustic

parameters in the study of particulates. The participants included leading scientists,

manufacturers of electroacoustics, and researchers from minerals, ceramics, pharmaceutical,

inks, paints, cement and paper industries. The technical content of this workshop included

theoretical understanding of electroacoustics, latest developments in equipment, application of

electroacoustics to different industrial problems, methods and procedures in the measurement

of electroacoustic parameters, interpretation of these measurement parameters, and major

issues in the measurements.

The participants received a compilation of presentation viewgraphs after the meeting. This

proceedings of the workshop has been possible due to the efforts of a number of my

colleagues who not only presented their results but also provided their papers. Without their

dedication, this would not have been possible. This volume contains all but two papers

presented at the workshop. The papers were reviewed by the participants of the workshop.

iii
In addition, the session chairs prepared the attached notes based on the discussions held

under different groups.

This report is divided into six sections based on the sessions held at the workshop. The first session constituted papers on theoretical aspects and new data on electroacoustics

phenomenon, followed by application of this theory to the development of new

instrumentation. Prof. Hunter described the results of verification of electrophoretic mobility

and dynamic mobility for mono and polydisperse particles. In addition, data on measurements with the Matec Applied Science's new prototype version of Acoustosizer were

presented to illustrate the accuracy of determination of magnitude and phase of the dynamic

mobility over a frequency range of 0.3 to 11.1 MHz. Prof. Pendse presented the analysis of

high density suspensions by analyzing attenuation coefficient spectra over 1.0 to 100 MHz

frequency range to determine particle size distribution. In the area of new instrumentation,

both theory and experimental results of Matec Applied Sciences and Pen Kem Inc. were

presented by Messrs. D. W. Cannon and P. Goetz, respectively. The second generation of

these instruments have the capability to measure both particle size distribution and zeta

potential in dense suspensions.

In the second session, papers related to applications and procedural aspects of using

electroacoustics were presented. Dr. R. O. James discussed the calibration of Electrokinetic

Sonic Amplitude instrumentation using lattices and oxide colloids as a function of volume

fraction. Excellent precision of the ESA technique in comparison with the classical

electrophoresis techniques was demonstrated. The application of ESA technique to concentrated suspensions of carbon black in apolar liquids was used to study charging

mechanisms and influence of dispersants by Dr. R. E. Kornbrekke, I. D. Morrison and T.

Oja. Their results showed that particle charging was due to a combination of selective acidbase type interaction coupled with ion-pair dissociation. In a paper by Dr. Valdes, the

application of the ESA technique to the characterization of colloidal diamond particles as applied to the development of an electrophoretic coating process for selected area nucleation

and patterning of diamond thin films was presented. Background electrolyte correction in the

measurement of ESA was discussed by Dr. F. N. Desai. Under high ionic concentrations,

these effects become significant. If not corrected for, the results might indicate an apparent

shift in the iso-electric point.

In the third session, two papers were presented by Drs. J. Pollinger and V. A. Hackley on

the application of ESA measurements for silicon nitride powder processing. The powder

behavior in aqueous environment was studied by Dr. Pollinger to evaluate aging of powders

in slurries, characterization of dispersion of powders, development of multiple powder

dispersions, and correlation of ESA measurements with other suspension characterization

methods. The primary focus of Dr. Hackley's paper was on the role played by solution ionic

species in modifying surface charge behavior and the ionic contribution to the ESA signal of

the silicon nitride particles.

iv
The fourth session was devoted to the application of ESA measurements to minerals such as

clays, coal, and cements. Dr. N. D. Sanders presented the results of ESA measurements on

precipitated calcium carbonate for surface charge measurements. These results were

supported by calorimetric and Theological measurements in improving the understanding of

the coating behavior of this material. Prof. Chander discussed the role of ESA

measurements in studying the electrokinetic behavior of coal particulates. The ESA

measurements were used in the interpretation of surface charge and strength of adsorption of

species from an aqueous environment. In the study of portland cement, Dr. Darwin

presented a comparative data on electroacoustics, microelectrophoresis, and streaming

potential in the development of superplasticizers. Lack of strong agreement between the data

from these methods and difficulties in the determination of true zeta potential in such

complex systems was discussed. Prof. Moudgil presented the results of the application of

ESA measurements in mineral systems where solid-liquid or liquid-liquid interface chemistry

data are required.

The primary theme of the fifth session was coatings, though two related papers were

presented in other sessions. Dr. Krishnan's paper (not presented at the workshop) deals with

the application of the ESA technique in improving the performance of solvent based gravure

inks. A maximum value of ESA corresponded to a minimum in the particle size distribution,

thus providing a measure to control the coating process. The application of ESA

measurements was employed in the characterization of phosphors by Dr. A. Dutta. This

study involved the investigation of electrokinetic behavior of phosphors in the presence of

non-luminescent additives, surface coatings, and polyelectrolytes. An investigation on mechanisms of charging of liquid toner by ESA were presented by Dr. J. R. Larson, Besides

presenting a model for micelle ionization and particle charging in these systems, the ESA

technique was used in the study of liquid toner particles.

The sixth session was devoted to discussions and identification of issues. Based on the group

discussions, the session chairs were responsible for the development of the issues identified

in the last chapter of this report

We are pleased that Ceramics Division at the National Institute of Standards and Technology

sponsored this workshop. Our sincere thanks to Stephen W. Freiman, Division Chief, for

his support and encouragement. Special thanks to Margaret Robinson who was responsible

for workshop arrangements and preparation of the workshop proceedings. Also, thanks to Vincent A. Hackley for his assistance in organizing the workshop and reviewing papers.

Most of all, our thanks go to Lori Phillips and her colleagues at NIST for the excellent

arrangements of the workshop and help with registration.

Subhas G. Malghan

National Institute of Standards

and Technology

v
DISCLAIMER

Information on product names, manufacturers, or suppliers are included in this report for

clarification. This does not imply endorsement of the products or services by the National

Institute of Standards and Technology.

vi
TABLE OF CONTENTS

Preface iii

Session I. Fundamentals of Electroacoustics and Instrumentation

Chairman: R. O. James, Eastman Kodak Co.

Determining Charge and Size with the AcoustoSizer 1 Richard W. O'Brien, William N. Rowlands and Robert J. Hunter

University of Sydney

Theoretical Considerations in Acoustophoretic Analysis of Concentrated

Colloids 23

Hemant P. Pendse, Terry A. Strout and Arvind Shanna

University of Maine

New Developments in Electroacoustic Methods and Instrumentation 40

David W. Cannon, Matec Applied Sciences

Session II. Applications and Procedures

Chairman: H. P. Pendse, University of Maine

Calibration of Electroacoustic (ESA) Apparatus Using Lattices and Oxide

Colloidal 67

R. O. James, Eastman Kodak Company

Electrokinetic Measurements of Concentrated Suspensions in Apolar

Liquids 92

R. E. Kornbrekke, I. D. Morrison and T. Oja, Lubrizol Corp.

Acoustophoretic Characterization of Colloidal Diamond Particles Ill

J. L. Valdes, AT&T Bell Laboratories

Session III. Advanced Ceramics Applications

Chairman: R. Mountain, NIST

ESA Measurements for Silica and Alumina: Background Electrolyte

Corrections 129

F. N. Desai, H. R. Hammad and K. F. Hayes

Department of Civil and Environmental Engineering

Oxide and Non-Oxide Powder Processing Applications Using Electroacoustic

Characterization 143

J. P. Pollinger, Garrett Ceramic Company

vii
Investigation of Parameters and Secondary Components Affecting the

Electroacoustic Analysis of Silicon Nitride Powders

Vincent A. Hackley and Subhas G. Malghan, NIST

161

Session IV. Clays, Cements and Coa!

Chairman: B, M. Moudgil, University of Florida

The Impact of Surface Chemistry on Particle Electrostatic Charging and

Viscoelasticity of Precipitated Calcium Carbonate Slurries 180

Yan C. Huang, Nigel D. Sanders, Frederick M. Fowkes and Thomas B.

Lloyd

Lehigh University and Specialty Minerals Inc.

Characterization of Coal Slurries by Electrokinetic Sonic Amplitude

(ESA) Technique 200

H. Polat, M. Polat and S. Chander, Penn State University

Surface Charge Characterization of Minerals by Electroacoustic

Measurements 219

B.M. Moudgil and R. Damodaran, University of Florida

Surface Charge Characterization of Portland Cement in the Presence of

Superplasticizers 238

David C. Darwin, Roger Y. Leung and Trevor Taylor, W.R. Grace &

Company

Session V. Coatings

Chairman: N, Sanders, Minerals Inc.

Role of Particle Size and Electrokinetic Sonic Amplitude in Performance

of Solvent Based Gravure Inks 263

R. Krishnan, Sun Chemical Corp.

Electrokinetics of Phosphors 274

Arunava Dutta, Osram Sylvania Inc.

Advances in Liquid Toners Charging Mechanism: Electroacoustic

Measurement of Liquid Toner Charge 301

James R. Larson, Xerox Corporation

Session VI. Subgroups Discussions and Identification of Issues

Discussion Leaders Summary 315

VII. List of Participants 324

viii
SESSION I

FUNDAMENTALS OF

ELECTROACOUSTICS AND

INSTRUMENTATION

Session Chairman

R. O. James

Eastman Kodak Company

DETERMINING CHARGE AND SIZE WITH THE ACOUSTOSIZER

Richard W. O'Brien, William N. Rowlands and Robert J. Hunter

School of Chemistry

University of Sydney

NSW 2006 AUSTRALIA

ABSTRACT

The electrokinetic sonic amplitude (ESA) effect can be measured using the Matec ESA8000. The measured signal can be used to estimate the magnitude of the dynamic mobility at the measuring frequency (normally 1 MHz). Provided the particle size is known, this can be

related to the conventional zeta potential or electrophoretic mobility of the particles, using

the theory developed by O'Brien. We have verified the correlation between electrophoretic

(d.c.) mobility and dynamic mobility with suspensions of monodisperse smooth spherical

particles of inorganic colloids, and have shown that it can be extended as well to nonspherical and polydisperse systems by using the mass average particle size to characterize the

system. Even if the system is undergoing coagulation, it is possible to correlate the

measurements of d.c. and high frequency mobility, provided one can estimate the effective mass average particle size using, for example, a light scattering technique.

We have also made accurate determinations of both the magnitude and the phase of the

dynamic mobility, over the frequency range 0.3 to 11.1 MHz, using a prototype version of

the Matec AcoustoSizer. This makes it possible to determine the size independently as well

as the zeta potential or kinetic charge. The software provided with the instrument assumes

that the particles have a log-normal distribution of particle sizes and estimates the median

particle size and gives a measure of the spread. Good agreement is found between these

estimates at concentrations up to about 5% and those found using other methods of sizing.

Unstable systems lead to some deposition of particles on the electrode surfaces and this

contributes its own signal to the measured value. This may be sufficiently large to vitiate the

measurement at low particle concentrations, but should be less of a problem for the higher

particle concentrations for which the instrument is likely to find most use. In any case the

final version of the AcoustoSizer will incorporate arrangements which should eliminate the

worst effects of particle deposition on the electrodes. The main advantage of the

AcoustoSizer is that it is able to measure quite concentrated systems without the need for

dilution.

1
INITIAL EXPERIMENTS IN ELECTROACOUSTICS

O'Brien et al.

1

, Scales and Jones2 and James et al 3 have recently shown the value of

the electroacoustic method for the determination of electrokinetic properties. More recent

work in our laboratory has extended the application of the electro-acoustic method to the

study of adsorption of surfactants4 and hydrolysable metal ions5 onto clay mineral particles.

We have also examined in some detail, the conductance and electrokinetic behaviour of a

latex system which exhibits anomalous surface conductance6. All of these studies were

conducted using the standard electroacoustic apparatus (ESA-8Q00, Matec Applied Science,

Hopkinton, MA). Since much of that material has not yet appeared I shall describe some of

it here, along with the more recent results we have obtained with the Matec AcoustoSizer.

Our first step

1 was to establish that a monodisperse suspension of a colloid consisting of

smooth spherical particles gave an electroacoustic signal which was in quantitative agreement

with the predictions made by O'Brien's theory7

. The theory describes how electroacoustics

can be used to determine a quantity called the dynamic mobility, of the colloidal particles.

This quantity is analogous to the d.c. mobility jx E, as measured by electrophoresis, but is a

function of the frequency of the applied field. Since the motion of the particles lags behind

the applied field, nd is actually a complex quantity, with a magnitude which depends mainly

on the particle charge and a phase lag which depends on the size of the particles. It turns

out that nd is equal in magnitude to /x E and has no phase lag at the lowest frequencies, but

with increase in frequency (in the MHz region) the magnitude decreases and the phase lag

increases.

Establishing the validity of O'Brien's theory proved more difficult than had been

anticipated because of the difficulty of obtaining sufficient material to prepare a few hundred

mis of sample at the required particle concentration (at least 1%). We were limited here by

our suspicions about the behaviour of latex systems in electrokinetic experiments. Our

earlier studies8 had suggested that lattices were unusual in that they appeared to exhibit a phenomenon which we have called anomalous surface conduction; that is, the surface charge

which sits between the particle surface and the plane of shear appears to be able to conduct

2
an electric current. Such behaviour is not expected and is not included in the normal

treatment of the electrokinetic phenomena. We decided that in view of the uncertainties

connected with this phenomenon it would be unwise to predicate our examination of the

theory on such systems. Our first experiments were therefore done using a cobalt phosphate

sol and they gave very satisfactory agreement with the theory when the sol was highly

dispersed. The dynamic mobility, /xd could in that case be related to the normal

electrophoretic mobility, nE by the formula

ixd = nE.G(ua 2/v) [1]

where the function G measures the inertia of the particle, w is the measuring frequency, v is

the kinematic viscosity and a is the particle radius. [The theoretical expression for G is

described in Ref. 3]. In these experiments the particle radius, a, was known with high

precision so G could be accurately estimated.

When the suspension was treated with a reagent which reduced the effective charge to a

point where the sol became unstable 1

, the measured ESA signal decreased below the value

expected from the electrophoretic mobility. It appeared then, that coagulation reduced the

mobility of the particles in the high frequency (1MHz) field, and that was hardly surprising.

We next examined the behaviour of less idealized systems like alumina and silicon

nitride dispersions3 with a range of particle sizes and some variation in shape, It turned out

that, if one could estimate the mass average particle size from light scattering data, then this

value could be used to determine the effective value of the inertia! correction, G for a

polydisperse sol. Since G is a complex function, the proper procedure for taking it into

account is a little complicated; it is described in Ref 3. We were also able to show in that work that, even when the sol was undergoing

coagulation, it was still possible to measure the zeta potential, and get good agreement with

the value obtained from the electrophoretic method, if one could determine the effective

particle size during the coagulation. This we were able to do by withdrawing a sample of

the sol and diluting it rapidly with a solution with approximately the same composition as the

background electrolyte. The mass average particle size, estimated by light scattering, then

3
provided a suitable correction to bring the high frequency mobility into line with the static

value, as indicated by equation [1]. It would seem then that the electroacoustic effect and the

light scattering process both respond in the same way to a group of aggregating particles. [It

is important to note that the particle size which is measured in this way is not the size of the

aggregates but some augmented size for the primary particles. It is as though the particles

are somewhat restricted in their movement in the electric field and so they behave like larger

particles but the aggregate does not move as a single entity].

At this time we were already developing the AcoustoSizer, which could measure both

size and charge. It was the latter observation which encouraged us to believe that, in

addition to providing a means of measuring the particle size distribution in highly dispersed

samples, it might be possible to use the AcoustoSizer to determine some sort of effective

particle size, even in coagulating systems, and to use this to estimate the zeta potential and

charge. That may ultimately prove to be possible, but unfortunately, our preliminary attacks

on coagulating systems have run into some difficulties which will be discussed below.

We next set about using the ESA-8000 to look at some interesting systems in a more

qualitative way. We were able to follow the adsorption of cetyl pyridinium chloride (CPC)

onto the surface of kaolinite particles at different pH values4

. One could readily identify the

point at which the particle surface was covered with the surfactant, and hence estimate the

surface area of the solid from the known area per molecule of the adsorbate. The isoelectric

point of the clay also seemed to correlate well with the cation exchange capacity of the clay,

although one would need to repeat the experiment with a variety of clays to determine

whether the method was general. Kaolinite particles are flat discs and the edges of the disc

display a different sort of adsorption behaviour to the faces. It seemed to be possible to

identify the differences in adsorption and so to measure, rather crudely, the relative area of

the edges and faces, and hence to estimate the aspect ratio of the mineral crystals (i.e. the

ratio of width to thickness). That is an important determinant in the flow behaviour of such

systems and is not readily accessible by any other technique.

In another study on kaolinite5

, we could readily identify the adsorption of hydrolysable

metal ions onto the surface as a function of pH. Rather surprisingly, divalent ions like Co2+

, Cu2+ and Cd2+ show no special adsorption behaviour on the kaolinite surface under acid

4
conditions. As the pH is raised from acid values, a point is reached at which the metal ion

evidently becomes strongly adsorbed on the clay mineral surface, since the normally negative

surface becomes much less so. At sufficiently high concentrations of dissolved metal ion, the

underlying particle may become positive.

According to Healy and James9

, who studied such adsorption processes on the silica

surface, the divalent ion is not adsorbed because its hydration energy is too large. It becomes

adsorbable when it is converted at higher pH to a partially hydroxylated form: M(OH) +

, which has a lower hydration energy. On adsorption, that ion can interact with the surface

through hydrogen bonds to the surface oxygens. At still higher pH values, the zeta potential

goes through a maximum positive value and then may decrease to become negative at a pH

which correlates with the isoelectric point of the adsorbed metal oxide. In effect, the particle

behaves as though coated by a layer of the metal ion oxide or hydroxide, assuming the latter

is insoluble.

The ESA-8000 is very suitable for studying such adsorption processes for it can readily

evaluate the exact pH at which the electrokinetic potential changes from negative to positive

and back again. It can also measure, more or less quantitatively, the magnitudes of the zeta

potential, although there will be some error engendered by the fact that the particles undergo

some degree of coagulation as they pass through those iep values. This is again an area where the AcoustoSizer may prove capable of rendering more quantitative information, if it

can measure the particle size, even in coagulating suspensions. An important feature of the

apparatus is the ease with which measurements can be made as a function of pH over the

whole range of interest in a single titration experiment.

The detailed study of the polymer latex6 depended on measurements of both the complex

conductivity and the electroacoustic behaviour of a very well characterized system. In this

case we also had detailed potentiometric titration data, which gave us a measure of the

surface charge, and d.c. electrophoresis data which gave us the standard zeta potential value.

The story is too complicated to reproduce here but the upshot was that this latex system did

exhibit anomalous surface conduction. (That is to say, when placed in an electric field, there

is some ionic conduction which occurs in the region between the surface of the particle and

the shear plane. Normally that is considered to be a layer of solvent and ions which move

5

i
wholly with the particle). This is an area of active investigation at the moment. It may

prove to be of far-reaching importance, for it is possible that this anomalous surface

conduction is more prevalent than we had thought. Wherever it occurs it must be properly

taken into account if one is to obtain reliable zeta potential data from electrokinetics.

This is a new instrument which is about to be released onto the general market. Our

company, Colloidal Dynamics, is involved in the development of the instrument and so had

access to the first prototype and now has the first example of what is expected to be the

production model.

The instrument is based on

electric field causes the particles to

oscillate back and forwards due to their

charge and in so doing they generate a

sound wave. This is referred to as the Figure 1. The AcoustoSizer cell and electrode

system

Electrokinetic Sonic Amplitude (ESA)

effect. The sound wave travels down a glass delay line and is detected by a transducer

attached to the end of the rod. The magnitude of the sound signal can be related to the

charge on the particles (since this determines the amount of movement which is induced by a

given applied field). The particle motion in the AcoustoSizer is driven by the electric force

which is balanced by the hydrodynamic drag and the inertial force on the particle. At very

low frequency the inertial force is insignificant and the dynamic mobility is equal to the d.c.

(electrophoretic) mobility (i.e. G = 1 in eqn.fl]). As the frequency of the applied electric

field is increased, the inertial force on the particle becomes more significant, causing the

particle motion to lag behind the applied field. Since the inertial effect also increases with

THE ACOUSTOSIZER

O'Brien's (1992) patent

10

. A

sinusoidal voltage is applied to a pair of

gold plated disc electrodes immersed in

the colloidal suspension (Fig. 1). The

6
particle size, the frequency-dependent phase lag provides information about the size

distribution of the particles. As the frequency increases, the magnitude of the measured

signal also decreases more rapidly for particles of larger size, since their inertia effect is

larger. Both the magnitude and the phase of the measured signal can therefore be used to

obtain the best estimate of the particle size distribution and the particle charge. In the

AcoustoSizer the frequency range has been chosen to size particles ranging from 0.1 to 10

/*m.

Since the electroacoustic technique measures sound, it should be possible to determine

the electrokinetic potential and size of colloidal particles in opaque or photosensitive

suspensions at a much higher particle concentration (ranging from a few percent up to 40%

by volume) than is possible using typical light scattering methods.

Attempts to extend the measurements up to very high volume fraction encounter another

problem: the acoustic properties of the suspension, and hence the response, become

dependent on the particle concentration. In particular, the acoustic impedance will change

with concentration and this affects the magnitude of the resulting signals. [The acoustic

impedance, Z, is defined as follows. If the boundary of a system is caused to move with a

sinusoidal motion characterized by a velocity, Vexp(/oor), then the pressure, p, required to

produce this motion can be written p = Z V. For a homogeneous material, Z = pc where p

is the density and c is the velocity of sound in the material. For non-homogeneous systems,

like colloidal suspensions, Z can be complex (if there is absorption occurring) but we have

found it to be real, and proportional to particle concentration, over the frequency range we

have studied].

The second electrode and rod are used to provide a reference signal to which the signal

from the first electrode can be compared, and this corrects for any drift in the electronics, or

change in the acoustic impedance of the suspension, or small shifts in phase due to

temperature effects.

The frequency of the applied field varies from 0.3 to 11.1 MHz in 13 steps and the

magnitude and phase of the signal is determined at each of those frequencies. From the

spectrum of magnitudes and phases, both the charge and the size of the particles can be

7
determined, at least for the case where the particles are smooth and spherical. Non-spherical

particles, (disc shaped or cigar shaped spheroids) have been treated theoretically (subject to

certain restrictions discussed below) and are expected to behave like spheres with a modified

radius.

Instrument Output

In normal operation the computer associated with the instrument takes the spectrum of

the ESA magnitudes and phases and compares them with the reference signals to deduce the

dynamic mobility spectrum. The electrokinetic or zeta potential, the median size and

standard deviation, assuming the particles have a log-normal distribution, are then adjusted to

obtain the best fit to the mobility spectrum. Fig. 2 shows the fit obtained between theory

and experiment for a monodisperse silica sol of radius 1.5 /xm. Normally the user is unaware of the nature and treatment of the spectrum; only the charge and size are provided

as output.

There is a further complication to consider when the particle concentrations at which we

are working are towards the bottom end of the range for which the instrument is designed.

Under such conditions it is usually advisable to correct for the effect of the ESA signal from

the background electrolyte; this has to be subtracted (vectorially) from the measured signal.

Fortunately it turns out that even for the most dilute systems studied this effect usually has

only a very minor influence on the results and the correction must be applied only if one is

striving for maximum precision.

The instrument can also perform a pH titration, plotting pH as a function of added base

or acid and from this we can determine the magnitude of the titration charge, which is

usually regarded as the total surface charge. The charge derived from the electroacoustic

measurement is, on the other hand, a kinetic or effective charge, related to that part of the

charge which is in the diffuse part of the electrical double layer around the particle. It is

generally accepted that this latter charge gives a better indication of the likely coagulation

behaviour of the colloid.

8
DYNAMIC MOBILITY

Theory vs expt. for 1 .5 micron silica

e

1 .0 45

r*n n n n 1 1"1 1 rl p v - 40

0.8 - \ <1 *

* \ — phase 35

30

U.o 25

20

1 5 X

0.2 10

5

0.0 . i i i i 1 1 1

0

0.0 1.2 2.4 3.6 4.8 6.0 7.2 8.4 9.6 10.8 1 2.0 FREQUENCY MHz

Figure 2: The magnitude is measured in relative units and the phase angle is given in

degrees.

Although the theoretical analysis of the electroacoustic behaviour7 assumes the

suspension to consist of a dilute collection of smooth spherical particles, we know from our more recent theoretical work11 that for spheroidal particles the behaviour is qualitatively

similar. The result which is calculated for a system of anisometric particles is, therefore, an

equivalent electroacoustic radius, in much the same way as the sedimentation radius in such

cases is the equivalent settling radius.

THEORY OF THE ELECTROACOUSTIC EFFECT

O'Brien7 has shown that the ESA signal is related to the dynamic mobility, nd of the

particles by the following equation:

ESA = C(Q).<K^)-Hd-^%r [2]

P zg+ zs

where C(Q) is an instrument constant which does not depend on the properties of the

suspension, 4> is the volume fraction of the particles and Ap is the difference between the

density of the particles, pp and that of the medium, p. Z is the acoustic impedance and

subscripts g and s refer to the glass rods and the suspension. This expression is valid for

9
suspensions of arbitrary concentration. The dynamic mobility is a complex quantity which

depends on the measurement frequency.

The theoretical analysis of dilute suspensions of spheres shows that the dynamic

mobility can, in that case, be related to the radius, a, and zeta potential, of the particles

through the equations

2z\ n \ coa

2

-

3r]

where

(1 +/) , &

G(q) = j _ /fl[3+2(Ap/p)] "I

_1 [4] 9[l+(l-/)\/ft/2)]J

and a) is the measuring frequency and v is the kinematic viscosity. G(q) measures the inertia

effect and causes the magnitude of the mobility to decrease monotonically from its d.c. value

(fie) to about half that value at q = 10, provided /is constant. The value of q (= o)d2/p) in

water at 25 °C is 7.00a2 for a in /xm and a frequency of 1 MHz. [q = 6.26a2 at 20°C].

The function (l+f) is proportional to the tangential component of the electric field at the

particle surface and it, too, is complex:

l + /= 3(1

[5]

2(1 -/«') + (2X -iw'eje)

where X = IC/ICa is called the surface conductance parameter and ep is the particle

permittivity; w' = cos/K°° is the frequency divided by the relaxation frequency for ion

movement in the double layer (a few MHz in ordinary electrolytes). For many systems it

turns out that the second term in the denominator is much smaller than the first and so we

can take / = xh and equation [3] reduces to eqn [1]. This is the relation which is used in the

software associated with the Matec ESA-8000. The AcoustoSizer makes a more accurate

assessment by allowing X to vary as the estimated zeta potential varies. In this way the new

instrument is able to take account of 'surface conduction effects' provided that they are

'normal ' i.e. the system exhibits the amount of double layer conduction which would be

10
expected from the magnitude of zeta, assuming that the ions in the diffuse double layer have

their normal conductivity values and conduction occurs only outside the slipping plane where

zeta is measured.

Expected Behaviour of a Colloidal System

The predicted behaviour of a system of spherical particles exhibiting small surface

conduction can be seen from Fig. 2. Note that the magnitude of the dynamic mobility

decreases monotonically and the phase angle increases from zero to a limiting value as the

frequency increases. Bigger particles show bigger effects in both cases but the limiting value

for the phase angle is 45°.

The effect of size on phase angle is independent of the charge so the size and charge

can be uniquely determined. The output we obtain from the AcoustoSizer is the

dimensionless mobility, Ed , which is related to the dynamic mobility by:

so nd = 1.17xl0"8 Ed in SI units at 20°C.

The Sizing Algorithm

The mobility spectrum is determined by a series of steps: (1) the raw ESA signal is compared in magnitude with the reference signal taken from the second electrode; (2) the

background electrolyte signal is subtracted (in the case of dilute suspensions) and (3) the

resulting set of complex numbers is divided by the corresponding set for a system with the

same or similar conductivity and a known dynamic mobility. This last comparison takes

account of the effects of electrolyte conductance on the shape of the electric field inside the

cell. That calibration is the most crucial step in the process if one wants absolute values

rather than qualitative comparisons. We are still refining the values used by crossreferencing data obtained on different known systems.

11
Estimation of the size and charge of a system is based on obtaining a best fit to the

entire spectrum of mobility data both with respect to magnitude and phase. This is best

understood in terms of Figure 3 in which the anticipated values of the mobility are plotted

for different sized particles. The plot is done on an Argand diagram which shows the values

of the magnitude of the mobility ( | nd | ) and its phase, 6, where fid = X + iY and

| fx, I = (X2 + Y2)* ; tan 6 = (Y/X). [7]

Fig. 3(a) illustrates

equation (7). Fig. 3(b)

shows how the spectrum

of mobility values for a

particular system is

expected to look for

different sized particles.

The successive points

represent the values of the

complex quantity, fxd , at Figure 3: The mobility spectrum, (b) shows an enlarged picture

different frequencies of tne first quadrant of Fig (a).

(increasing in the

direction of the arrow). For a given zeta, a small particle (circles) will show a larger

magnitude than a larger particle (+) but the difference is slight at the lowest frequency and

increases as the frequency rises. The magnitudes always fall with increase in frequency

whilst the phase angles always rise and tend to the limiting value of 45°, if the particles are

large enough. Notice that for very small particles the magnitude does not change much, and

neither does the phase angle, so the points tend to be bunched together more than for the

larger particles. Changing the sign on the particles reflects the curve through the origin and

changes the phase by 180° (Figure 3(c)) but for systems with X » 0 the points are always

confined to the two wedge-shaped sectors from 6 = 0 to 45° and 6 = 180° to 225°.

12
The measured mobility < fx > for a dilute suspension is given by

<p> = fi(a).p(a)da [8]

where p(a)da is the (mass) fraction of particles with radius between a +da/2 and a-da/2. The

measured spectrum is fitted by a least squares procedure to obtain the best parameters to

describe the distribution function for the sizes. The particles are assumed to have a common

zeta potential and to have a log-normal distribution of radii. The fitting procedure then

determines the best fit values for zeta, the median radius, and a parameter which

characterizes the spread of the radii; the algorithm fits both the magnitude and the phase

angle spectrum over the entire frequency range. In practice the procedure is not simple but

the complexities are not apparent to the user.

RESULTS WITH THE ACOUSTOSIZER

The performance of the AcoustoSizer on a set of nominally monodisperse samples can

be judged from Fig. 4. The limits indicated by the solid horizontal lines cover the size from

the 15th percentile to the 85th percentile as measured by sedimentation, using the Horiba

Capa 700. The mark on the line indicates the position of the median size. The curves show

the log-normal distribution estimated from the AcoustoSizer data.

13
0.3

LA

SIZES OBTAINED ON SILICA SAMPLES

1 .0

1 .5

0.5

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 PARTICLE DIAMETER IN MICRONS

Figure 4: AcoustoSizer estimates of size are shown by the horizontal line. The mark

indicates the mean of the assumed log normal distribution.

Fig. 5 shows the result of the same procedure applied to a sample of material for which

the distribution is known with some confidence. These are crushed quartz samples prepared

and sized by the Bureau of Common Reference of the European Community. The sizing was

done by a settling procedure using direct measurements. As can be seen, the AcoustoSizer

suggests a slightly broader distribution than the measured one but the median size is

determined quite accurately and the result is certainly within the range of results obtained by

other methods.

14
MASS FRACTION UNDERSIZE

PARTICLE DIAMETER IN MICRONS

Figure 5: The full line represents the AcoustoSizer estimate. The points are the standard

estimates for these materials.

Fig. 6 shows the results obtained for a variety of other materials using the AcoustoSizer

and more conventional methods of sizing. It is clear that the new method gives results which

are as satisfactory and consistent as any of the other methods to which it is compared.

The above measurements of size have all been conducted on samples which were well

dispersed and at modest concentrations (up to about 5%). The sols were well removed from

their respective isoelectric points. The following table shows the results of a comparison of

the estimated zeta potentials from the measured dynamic mobilities, using equation [3],

compared with those calculated from the (d.c.) electrophoretic mobility.

Even when the zeta potential is brought close to the isoelectric point, (Ti02 at pH 8.2)

the agreement is excellent, provided one uses the correct apparent radius for the calculation.

Unfortunately, we have found that in some systems, when the particles are unstable they can form a deposit on the electrodes and this gives rise to a spurious ESA signal which interferes

with the measurement and leads to erroneous estimates of the phase angle. The size and

charge estimates then become impossible. We are currently working on methods to obviate

this difficulty.

15
Table 1 Comparison of Zeta potentials (in mV) measured by the AcoustoSizer

with measurements using the Rank Bros Mark II apparatus and the Malvern ZetaSizer

Colloid pH AcoustouUClRank Zeta

i IV-/

4 0 37 3R

R 9 u KJ

-—

0 ft -90 -99

013IN4 3 S 96 94

R 0 -3Q -43

rvaUlil 1 4 4 -98 -96

9.8 -37 -43

Alumina 5.0 47 58 53

Si3 N4 (Sample b)

10.0 -35 -42 -51

Ti02 (Sample b)4.0 40 50 57

4700C

ZetaSizer

Ti02

Honba

Acoust.

Y//////////////////////////////////A 4700C

Si3N4 Y//////////////////A ZetaSizer

V//////////////////////////////////////77777A Horiba

V///////////////J7ZZZ\ Acoust.

Alumina

4700C

ZetaSizer

Horiba

Acoust.

1 1 1 1 1 0.1 0.2 0.5 1.0 2.0

PARTICLE DIAMETER IN MICRONS

Figure 6: Comparison of various techniques for particle size analysis of different materials

16
MEASUREMENTS AT HIGHER PARTICLE CONCENTRATION

At particle concentrations up to about 10% it is possible to assume that the acoustic

impedance of the system is essentially the same as that of pure water. The zeta potential can

also be calculated from the dilute solution theory (eqn [3]) with reasonable confidence. When

the particle concentration is much higher it is necessary to take account of any changes in the

acoustic impedance of the system in order to obtain reliable estimates of the dynamic mobility.

There then remains the problem of determining the extent to which the theory can estimate

charge and size from these dynamic mobility measurements.

The variation in impedance with volume fraction for a variety of samples is shown in

Figure 7. Rather surprisingly, it turns out that the impedance varies linearly with volume

fraction for these systems, and it is a real quantity which is independent of frequency. If that

is found to be true for all systems, it will be a great deal easier to allow for it.

MEASURED ACOUSTIC IMPEDANCES

2.3 • Geltecl'O

2.2 o Geltecl «5

+ GeUecO'5

2.1 o Alumina Ti02

„ 20

u1

19

o.e i.8

1.7 / * / a jy^ •

1.6 +

1.5 •+ I.I.I 0.00 0.05 0.10 0 15 0.20 0.25 0.30 Volume Fraction

Figure 7: Measured acoustic impedance of different systems. The two lines are drawn using a simple relationship which assumes that the impedance is a function only of the volume

fraction of the particles and the particle density.

17
Even with this simplification, we still need a theoretical model to describe how the particle

concentration is expected to influence the dynamic mobility. The most likely sort of model to

use is a form of cell in which each particle is surrounded by an appropriate volume of liquid and

the mathematical boundary conditions on the surface of the cell are arranged to produce a

reasonable approximation to the conditions operating in the total volume of the suspension. We

have set up such a cell model and from it have calculated the anticipated dependence of the

magnitude and phase of the dynamic mobility as a function of frequency.

The result is shown in Fig. 8 and it is obvious that this particular cell model is not

representing the behaviour as well as we would have hoped. That is rather surprising since the

form of the boundary conditions used here has been used successfully in other applications of

a very similar sort. The cell model does not suggest nearly enough influence of the

concentration at the highest frequencies and it is difficult to see how to modify it. We have, as

yet, no inkling of what could be causing the large effect at the uppermost frequency.

Fortunately it is possible to adopt an alternative, semi-empirical procedure. We can take

a known system (like the Geltech silica) and determine the values of f, am and o in dilute (less

than 5%) suspension. In practice we assume that the particles have a log-normal size

distribution and determine the median size, am and the spread, a of that distribution where

± a covers the range from 15 to 85% of the sizes. We can then determine how and a

appear to depend on the volume fraction, <f> for the Geltech. (It turns out that f and both

decrease and a increases with volume fraction). We can then correct the estimated value for the

unknown f at a given 4> value by multiplying by the ratio of the true f to the apparent f for the

standard Geltech system. The same procedure can be applied to the other variables.

18
MOBILITY MAGNITUDES PHASE LAG

Frequency (MHz) Frequency (MHz)

Figure 8. The magnitude and phase lag measured for a typical system and compared to the

theoretical expectation for the current cell model.

The result of this procedure when applied to suspensions of Ti02 and alumina is shown in

Figures 9 and 10. The estimated median size of the alumina is very nearly constant up to about

20% whilst the values for Ti02 show a smooth variation of around 50% over the range. The

zeta values show a steady drop of around 10% for the alumina and perhaps 20% variation for

the titania. The result is, however, extremely promising and is certainly greatly superior to any

alternative procedure for examining these quantities in concentrated systems.

In many industrial situations the particle concentration is not varying greatly and one would

expect that even if the measured values have no exact quantitative significance, the estimated

values should be able to tell something about what is happening in such systems. We presume

that a particular system will show a reproducible response in terms of the magnitude and phase

of the dynamic mobility as a function of frequency, even if the behaviour is not well described

by the theory for smooth spherical particles described above. Indeed, we have done a

preliminary investigation on one such system - a suspension of the clay mineral kaolinite.

19
CALCULATED SIZES FOR ALUMINA AND Ti02 ZETA MAGNITUDES

• T,Ox

0.02 0.04 0.06 0.0 0.1 0 0.12 0.14 0.16 0.18 0.20 0.22 0.24 0.26 Volume fraction

100

90

80

70

60

30

40

30

20

10

0

0

o Alumina

• Ti02

02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 0.22 0.24 0.26 Volume fraction

Figure 9. Calculated sizes for alumina and titania as a function of volume fraction using the

quasi-empirical procedure described in the text. The bars in this case represent the

range from the 15th to the 85th percentile.

Figure 10. Estimated zeta potentials for alumina and titania using the same procedure as for

Figure 9.

Kaolinite particles show a number of anomalous electroacoustic properties. To begin with,

both the magnitude and the phase of the dynamic mobility, as a function of frequency, are

smaller than would be expected from other measurements. The fact that these particles behave

as though they were smaller than their nominal size is not surprising. They are disc-shaped and

our theoretical calculations 10 suggest that such particles will give a phase angle signal

corresponding to a particle with a radius reduced in the ratio Vk where k is the axial ratio of the

particles (ratio of width of the disc to its thickness). More surprising at first sight is the fact that

the magnitude of the signal actually increases withfrequency. Once again this is understandable,

at least qualitatively, by the fact that the parameter X in equation [3] is very large. Instead of

the value of 0.02-0.03 expected from the estimated zeta potential, its measured value is close

to unity. This large value for X is referred to as anomalous surface conductance and the fact

that kaolinite exhibits it to such a high degree means that it will have to be treated using a

special version of the sizing algorithm if we are to obtain the necessary accuracy for both size

and charge.

20
CONCLUSIONS

Electroacoustics provides a very convenient procedure for following the zeta potential or

charge of a colloidal particle in suspension with the addition of some reagent. The most readily

exploited is the effect of acid and base on the surface charge of oxide and similar materials. We

have extended that to an investigation of the effect of added surface active ions and have shown

that the electroacoustic method is rapid and convenient.

With the advent of the AcoustoSizer the range of application of the method is much wider.

Particle size can be measured directly and this makes determination of the zeta potential more

reliable as well. The new device can make adequate estimates of both size and charge in

suspensions with particle concentrations up to almost 30% by volume (about 60% by weight).

The mathematical analysis takes some 2 minutes using our 80286 PC but with the 80486 cpu

which will be used in the commercial instrument the analysis will be done in real time if the

titration is conducted at the usual rate to ensure that the solution remains in equilibrium with the

particle surface.

ACKNOWLEDGMENTS

We thank Matec Applied Sciences for the loan of the prototype version of the AcoustoSizer

and the Australian Department of Industry Technology and Commerce and the Australian

Research Council for grants to assist us in the development of the instrument and the basic

science of electroacoustics respectively.

21
REFERENCES

1. O'Brien, R. W., Midmore, B. R., Lamb, Alice, and Hunter, R. J. (1990) Faraday Disc.

Chem. Soc, 90, 301-12.

2. Scales, P. and Jones, E. (1992) Langmuir 8, 385.

3. James, M., Hunter, R. J. and O'Brien, R. W. (1992) Langmuir 8, 420-3.

4. Rowlands, W. N. and Hunter, R. J. (1992) Clays and Clay Minerals 40, 287-91.

5. Hunter, R. J. and James, M. (1993) Clays and Clay Minerals (accepted).

6. Shubin, V., Hunter, R. J. and O'Brien, R. W. (1993) 7. Colloid Interface Sci.,

(accepted).

7. O'Brien, R. W. (1988)7. Fluid. Mech. 190, 71-86.

8. Midmore, B. R., Hunter, R. J. and O'Brien, R. W. (1987) 7. Colloid Interface Sci. 120,

210.

Midmore, B. R. and Hunter, R. J. (1988) 7. Colloid Interface Sci, 122, 521.

9. Healy, T. W. and James, R. O. (1972) 7. Colloid Interface Sci. 40, 42, 53 & 65.

10. O'Brien, R. W. (1992) US Patent No 5,059,909, Oct 22, 1991.

11. Loewenberg, M. and O'Brien, R. W. (1992)7. Colloid Interface Sci. 150, 158-68.

22
THEORETICAL CONSIDERATIONS IN ACOUSTOPHQRETIC ANALYSIS OF

CONCENTRATED COLLOIDS

Hemant P. Pendse, Terry A. Strout and Arvind Shanna

Industrial Process Control Sensor Systems Program

UNIVERSITY OF MAINE

ORONO, ME 04469

ABSTRACT

In recent years several advances have been made in ultrasound based instruments and

sensors for monitoring zeta potential and particle size distributions of colloidal suspensions.

This paper deals with the ultrasonic response of colloidal slurries, over a wide frequency

range, with particular attention to the effects of particle size distribution (PSD) and

concentration. Measurements of ultrasonic attenuation spectra are used in a newly developed

AcoustoPhor System to get PSD of colloidal dispersions.

INTRODUCTION

As sound travels through a suspension, the particles undergo oscillations relative to the

suspending medium as consequence of difference in the densities of particle and medium

phases. This relative motion is termed acoustophoresis.

Acoustophoresis induces a macroscopic colloid vibration potential (CVP) arising from

the relaxation of ionic atmospheres of suspended particles. Measurements of CVP amplitude

at a fixed operating frequency have been used to monitor zeta potential of concentrated

aqueous pigment slurries in the Acoustophoretic Titrator (Pen Kem System 7000). In the

Acoustophoretic Titrator the CVP phase is used to determine the zeta potential sign (Pendse,

1987; Marlow, Fairhurst and Pendse, 1988). In general, CVP amplitude and phase are

found to depend on operating frequency, as well as particle size and concentration.

Monitoring changes in the CVP phase during repeated acoustophoretic titrations has been

suggested as a tool for detecting degree of flocculation (Pendse and Strout, 1990a).

However, ability to use measurements of amplitude and phase of the CVP to determine the

zeta potential and the PSD simultaneously, is inherently dependent on availability of

23
appropriate predictive models that account for electric double layer relaxation associated with

the acoustophoresis at moderate and high particle volume fractions. Preliminary analysis

based on available dilute-limit models indicates that the frequency-dependent response of

vibration potential amplitude and phase is complicated by (i) interacting roles of relative

magnitudes of electric double layer thickness and oscillatory boundary layer thickness

compared with particle size, and (ii) issues related to superposition of contributions of

different sized particles in a polydisperse suspension, in addition to the contributions of

dissolved ions and polyelectrolytes.

Acoustophoresis also causes attenuation of sound due to viscous dissipation associated

with the particle motion relative to the suspending medium. Pen Kern has developed a new

commercial laboratory instrument, called AcoustoPhor System for monitoring particle size

distributions of concentrated colloidal slurries. The instrument is based on estimation of PSD

functions using measurements of ultrasound attenuation coefficients at several frequencies

over 1 to 100 MHz by solving appropriate integral equations. The mathematical algorithm

employed in the deconvolution or inversion of measured spectra utilizes the results of a

recent theoretical model developed, by the University of Maine researchers, to predict the

behavior of ultrasound as it passes through a slurry of polydisperse particles at a known

concentration, as high as 50 volume percent. In a cooperative research project, Pen Kem

developed and tested prototype sensor electronics and computer system with the measurement

software capable of programmed ultrasonic spectroscopy. The slurry characterization

chambers and the sample delivery system along with the sensor operating software were

developed by P&G Systems with design and fabrication assistance from Techna Machine.

One on-line and two laboratory sensor prototypes have been developed and tested extensively

in laboratory and production settings using several industrial mineral pigment slurries. The

prototype sensor instrument system was tested successfully in the University of Maine and

Du Pont research laboratories. Evaluation to date shows that the sensor is capable of

providing particle size distributions, without any assumptions regarding their functional form,

over diameters ranging from 0. 1 to 100 micrometers in slurries with particle concentrations

of 5 to 50 volume percents.

24
An on-line prototype was evaluated at Du Pont's semiworks facility for pigment

production during several months of field testing. Field evaluation has shown that the new

ultrasound based PSD sensor system is capable of handling several types of mineral pigment

slurries commonly encountered in industry under processing concentrations. During the field

tests, the system has operated reliably as an on-line monitoring tool during protracted runs of

continuous operation extending over several days, providing data sensitive to changes in the

processing conditions. The main objectives in these tests were to study sensitivity of the

analyzer to changes in the processing conditions, and to study robustness of the prototype

design for extended operations in manufacturing plant environments. During several twentyfour hours per day and five days per week operations, the analyzer was found to be operating

in a robust and reliable manner. The repeatability of data was also found to be excellent

(Pendse, 1992).

In the following sections we present a brief theoretical background on this technique

along with some illustrative results.

BACKGROUND

Acoustophoresis and Associated Sound Attenuation

The extent of attenuation depends on the length of the acoustic path, the concentration

and sizes of the particles, the frequency of sound, and the physical properties of the

particulate and fluid phases. In particular, the density contrast, a defined as the difference

between the densities of the particulate and fluid phases divided by the fluid density is an

important characteristic parameter in connection with the acoustophoresis and hence the

associated viscous attenuation. The acoustophoretic velocity is zero when the density

contrast is zero. The expression for the acoustophoretic velocity, vK normalized by the

undisturbed fluid velocity, u takes the form:

^=VgCv

(s,4>) [1] u 9

25
with

s = D

2 G)p [2]

where

D = particle diameter,

co = sound frequency in radians/second, (2*/),

f = sound frequency in cycles/seconds,

po = density of the suspending medium,

770 = viscosity of the suspending medium,

vibrational Reynold's number, corresponds to

(particle radius/oscillatory boundary layer thickness)2

,

,2

density contrast, Po

0 = volume fraction of the suspended particles, and

Cv (^,0) = acoustophoretic velocity correction factor

Based on the considerations of the rate of viscous energy dissipation in a suspension of

spherical solid particles subjected to a sound wave, one obtains the following expression for

the attenuation coefficient contribution due to monodisperse particles, amoao : ^TM^=ls2 o

2CAM) [3]

CO<J) 9

where

c0 = sound speed in the suspending medium, and

CA(s,<j>) = attenuation coefficient correction factor

For the case of polydisperse particles, the attenuation coefficient contribution as a function of

operating sound frequency/, a(f) becomes:

26
a(/)=a medium

[4]

where

g(D) PSD by mass fraction with g(D) dD equal to the mass fraction

of particles having diameters in range of D to D + dD, and

<Xmoao(f>D) = attenuation coefficient contribution at a frequency /due to a

monodisperse population with all particles having a diameter

equal to D

The particular expression for amooo mentioned above takes into consideration the viscous

dissipation associated with the acoustophoresis properly accounted for finite values of both

the vibrational Reynold's number and the particle volume fraction. The non-linearity

associated with the concentration dependence of attenuation coefficients is accounted for in

the factor CA (s,4>). For very dilute suspensions (4> -* 0), and for small diameters and low

frequencies (s -» 0), Cw (s,4>) and CA(s,4>) approach 1. For suspensions of submicron particles

with considerable density contrast the viscous contribution to the attenuation often dominates

over the thermal and scattering contributions in the frequency range of practical interest

corresponding to the long-wavelength limit (X> >tcD and o> > 1). These conditions

correspond to ultrafine inorganic pigment slurries commonly encountered in industry.

The correction factor for the attenuation coefficient, CA(s,<f>) is obtained using the coupledphase formulation for sound propagation through suspensions with the viscous dissipation term

based on the oscillating sphere-in-cell model (Strout, 1991; Sharma, 1993). Use of the

oscillatory sphere-in-cell model extends the coupled-phase approach to a broad range of

concentrations while retaining appropriate size and frequency dependence in predictions of

viscous attenuation coefficients. The frequency response of the attenuation coefficient is of

interest because the frequency of the applied sound can be readily varied in practice to obtain a

broad-band spectrum which uniquely characterizes a suspension. For frequencies and particle

sizes corresponding to vibrational Reynold's numbers larger than unity, the drag force corrections

become significantly different from those estimated on the basis of steady-state cell models.

Ability of our present model to account for attenuation coefficient dependence of vibrational

27
Reynold's number over a wide range at moderate and high particle volume fractions allows us to

properly interpret measured attenuation spectra in terms of particle size distributions.

General Formulation of the Integral Problem for Estimating PSD Using Ultrasound Attenuation

Spectra

Using measurements of the attenuation coefficient as a function of operating frequency, a(j)

covering an appropriate frequency range and a known expression for afmooo(/*,D), one can obtain

the PSD function, g(D) by inverting the above mentioned integral equation. The problem

encountered in determining g(D) from known a(J) is known by mathematicians as a Fredholm

integral problem of the first kind. It is important to reformulate the governing integral equation

in terms of transformed variables as follows:

R(f)={K(fPOp(X)dX

where

and

[5]

ptf)-

*®'****TM®

[6]

r(f)

X=q(D) H

p(X)dX=g(D)d(D) fg]

K(fJO=*°

u>"o(fMX))

[9]

rif)

The transformations defined by Equations (6) and (7) are selected to obtain appropriately

normalized and non-dimensionalized variables R and X. Proper selection of functions r(f)

28
and q(D) can yield a well-behaved Fredholm integral problem. Note that r(f) is independent

of diameter D, and q(D) is independent of frequency/. In this context the function R(f) is

referred to as the response function, and the function K(f,X) as the kernel function.

In practice the measured response is not known accurately. Hence the above equation is

only satisfied within measurement errors. This integral problem does not possess a solution

if the kernel function K(f,X) is separable into factors as K(f)-K"(X). Hence, for a separable

kernel function one can not determine p(X) from known R(f). Another peculiar nature of this

integral problem can be appreciated by noting that for any K(f,X) as long as it is reasonably

smooth and not a sharply peaked kernel.

Therefore, a true solution p(X) and a test function obtained by an addition of a highly

oscillatory error function (p(X) + Qsin(\X)} both will satisfy the governing integral

equation. Thus a Fredholm integral problem of the first kind becomes a non-trivial problem.

The success in inverting the Fredholm integral equation of the first kind depends on the

measurement errors and the shape of the kernel function.

Representation of the Integral Problem in Matrix Equation Form

The integral equation can be transformed to a matrix equation by first considering

discrete values of the operation variable (frequency f), and then replacing the integral by an

appropriate quadrature formula using a weighted sum of the base-point functional values of

[10]

as x

-* 00 for an arbitrary constant Q

29
the integrands as shown below:

RrfKi (X)p(X)dX for i=\X .-M [11]

K^Kif.J) for [12]

Ri-'E KpjWjPj with summation over j=l2,...>M

for i = IX .-M [13]

where

Defining

Wj

K„

R,

Quadrature weights for the numerical integration

formulation,

K(fitXj)

R(j), and

P(Xj)

sr w. Pj [14]

we get

where

R = KS

R. = K. S. with summation over j = 1,2,.. .N;

i /—j ij j

30
for i=\X .-M [15]

Here, K is an M x N matrix, and R and S are column vectors of dimension M and N,

respectively. One needs to pay attention to problems associated with near-singular

coefficient matrix, K involved in the above equation. There are several algorithms reported

in literature to solve Fredholm integral problems of the first kind. However, no single

algorithm is found to be suitable for handling several widely different kernel functions when

the response function is known with only modest accuracy as commonly encountered in

practice. One needs to properly account for measurement as well as modeling errors. The

solution depends on the choice of the kernel function and particular implementation of

inversion algorithm.

We have extensively studied several deconvolution algorithms using simulated response

functions appropriate for ultrasonic spectroscopy of industrial colloidal slurries (Pendse and

Strout, 1990b). We have been quite successful in working with unimodal, as well as

bimodal distributions. In terms of its ability to differentiate two distributions with relatively

small differences in means and standard deviations our present deconvolution software is

remarkably successful especially in the light of the fact that no assumptions regarding the

form of the size distribution function are involved.

31
ATTENUATION SPECTRA AND PSD ESTIMATION

A theoretical expression for a^^D), appropriate for finite particle concentrations (1 to

50 volume %), forms the basis of our present deconvolution scheme. Focusing our attention

on aqueous slurries of inorganic pigments with high density-contrast, we set out to test

adequacy of the our model results using well-dispersed aqueous suspensions of titanium

dioxide pigment. We obtained attenuation coefficient spectra over a wide frequency range on

slurries of different particle concentrations. These slurries were prepared by careful dilutions

of a pre-dispersed base slurry formulated at 75 wt% solids. Independent PSD analyses of

these slurries using SediGraph 5100 confirmed that all of these slurries are characterized by a

single particle distribution curve independent of their particle concentrations. The SediGraph

PSD corresponds to median diameter of 0.35 /xm, modal diameter of 0.31 /*m, and geometric

standard deviation (gsd) of 1.6. Comparison of experimental measurements and

corresponding predictions of attenuation coefficient spectra is shown in Figure 1.

Experimental data is shown for attenuation coefficients covering ranges of 3.5 to 42.3 vol%

(i.e., 13 to 75 wt%) for particle concentration and 3 to 100 MHz for sound frequencies. The

seven concentration response curves correspond to seven operating frequencies of 3.00, 5.57,

10.34, 19.20, 35.65, 66.20 and 100.00 MHz. The curves shift up with increasing

frequency. The coefficient of variation, i.e., ratio of standard deviation to mean for the

measured attenuation coefficients is found to be less than 2%. The attenuation coefficients

range from 2 to 2000 dB/cm increasing monotonically with increasing frequency.

The model predictions reported here were based on an assumed log-normal PSD

characterized by geometric mean (gm) of 0.33 fxm and gsd of 1.65. The overall agreement

32
Model Predictions

Eu

10000

1000

100

10 * _ fl 1

0 10 20 30 40 50

Particle Concentration (vol )

Experimental Data

10000

1000

100

0 10 20 30 40 50

ParticJe C oncentxatton (vol )

Figure 1. Variation of attenuation coefficient with particle concentration and

frequency for well-dispersed titanium dioxide slurries

33
between predictions and experimental measurements is remarkably well, with the overall

root-mean-square relative deviation being 19% over such wide ranges of volume fraction (4

to 40 vol%) and vibrational Reynold's number (0.1 to 10). From Figure 1 it is clear that the

attenuation coefficient versus volume fraction relationship is non-monotonic, with the peak

location shifting to higher volume fractions with increasing frequency. This comparison of

measurements done on a set of well characterized, well-dispersed slurries and corresponding

predictions based on a single realistic PSD shows that the theory predicts appropriate

dependence of attenuation coefficient on particle volume fraction and operating sound

frequency over very wide ranges. The quantitative agreement of course will depend on

adequacy of the assumed size distribution.

Figure 2 shows frequency response of the attenuation coefficients for two test slurries with 3.5

and 42.3 vol% solids concentration. These correspond to 13 and 75 wt% solids concentrations,

respectively. The attenuation coefficients are normalized by dividing them by corresponding volume

fractions and frequencies, for convenience. The peak in the normalized spectrum for 13 wt% slurry

is apparent. The peak frequency for the spectrum for 75 wt% slurry is above 100 MHz. The two

spectra corresponding to 13 and 75 wt% solids concentrations are quite different in magnitude and

shape, even when normalized by the corresponding particle volume fractions. The need to properly

account for dependence of ultrasonic attenuation spectra on particle volume fraction is obvious.

Also shown in Figure 2 are model predictions based on three different assumed log-normal particle

size distributions: (i) gm = 0.33 y.m and gsd = 1.65, (ii) gm = 0.27 /xm and gsd = 1.65, and (iii)

gm — 0.33 fim and gsd = 1.35. None of the three test log-normal particle distributions appear to

fit experimental spectra over the entire frequency range. The calculated attenuation coefficient

34
120

1 10 100

f, MHz —9— Experimental data

b— Model predictions with gm=0.27 jj,m and gsd=1.65

-e— Model predictions with gm=0.33 /xm and gsd=1.35 — Model predictions with gm=0.33 fim and gsd=1.65

Figure 2. Attenuation spectra for two test slurries

35
spectra are seen to be sensitive to shifts in the gm, as well as shifts in the gsd. Upward shift in the

peak frequency with the downward shift in the gm diameter is especially clear from the model

predictions for the 13 wt% slurry. Broadening of the PSD is reflected in the broadening of the

attenuation spectra.

Using measured attenuation spectra one can estimate particle size distributions for the test

slurries. Using our present deconvolution methodology we obtain particle size distributions without

imposing any functional form. We impose the obvious nonnegativity constraint. Estimated particle

size distributions obtained using the measured spectra are shown in Figure 3 for the two extreme

cases of solids concentrations, 75 and 13 wt%, respectively. It is interesting to compare the PSD

estimates obtained by deconvolution of measured attenuation spectra for slurries of widely different

solids concentrations with the results obtained from the SediGraph on samples further diluted prior

to analysis. Ultrasonic PSD estimates appear to be a little broader and more skewed that the

SediGraph PSD. The median diameters based on the ultrasonic PSD estimates are less than that

reported by the SediGraph analysis, and are in fact closer to the modal diameter reported by the

SediGraph analysis. Our experience with several pigment slurries shows this trend consistently.

Similarities in the these PSD results are notable.

CONCLUSIONS

Ultrasonic spectroscopy of colloidal dispersions appears to be a powerful tool for their

characterization. Recent instrument developments allow one to obtain attenuation coefficient spectra

over 1 to 100 MHz covering the range of 1 to 2000 dB/cm. For the case of high density contrast

36
—a— SediGraph 5100 analysis —©— Estimated PSD based on deconvolution for 42.3 vol% (75 wt%) —®~- Estimated PSD based on deconvolution for 3.5 vol% (13 wt%) '

Figure 3. Particle size distributions for two test slurries

37
suspensions, viscous loss associated with acoustophoresis is the dominant contribution to sound

attenuation. Availability of a predictive, theoretical model that accounts for hydrodynamic

interactions underlying acoustophoresis in multiparticle systems with moderate and high particle

volume fractions, over a wide range of vibrational Reynolds numbers allows one to interpret

measured attenuation spectra in terms of particle size distributions. This model is validated in the

light of experimental data on attenuation spectra obtained using a set of well-dispersed suspensions

of different solids concentrations. Attempts to use CVP phase measurements for monitoring particle

size distributions can be expected to involve difficulties associated with proper accounting of

interacting effects of variations in particle concentration, as well as dissolved ion and polyelectrolyte

concentrations. The strategy (Pendse and Strout, 1990b) of using the two acoustophoresis related

effects together in a complimentary fashion, electric double layer relaxation for zeta potential

analysis combined with viscous attenuation effect for PSD characterization, may prove to be quite

successful. Ultrasound spectroscopy may find interesting applications in characterization and quality

control of a wide variety of industrial colloidal dispersions including suspensions and emulsions.

ACKNOWLEDGMENT

The authors are grateful for the support of Department of Energy, Office of Industrial

Technologies under grant number DE-FC05-88CE40864, Du Pont Chemicals, and Pen Kern. This

paper is based in-part on a presentation at the NIST Workshop entitled "Electroacoustics for

Characterization of Particulates in Suspension," held in Gaithersburg, MD in February 1993.

38
REFERENCES

1. Marlow B. J., D. Fairhurst, and H. P. Pendse, "Colloid Vibration Potential and the

Electrokinetic Characterization of Concentrated Colloids," Langinuir, 4, 611-626 (1988).

2. Pendse, H. P., "Electrokinetic Vibration Potential of Colloid Suspensions," NSF Project

Report, Grant Number CBT-8410523 (1987).

3. Pendse, H. P., "Particle Size Distribution Sensor: Current Success and Future Work,"

DOE/Industry Advanced Sensors Technical Conference, Warwick, RI, (1992).

4. Pendse, H. P., and Strout, T. A., "On Effects of Particle Size in Acoustophoretic

Analysis of Colloidal Zeta Potential," AlChE Annual Meeting, Chicago, IL (1990a).

5. Pendse, H. P., and Strout, T. A., "Ultrasound-Based Sensors for Characterization of

Colloidal Slurries," Proceedings Sensors Expo West, 205B (1990b).

6. Sharma, A., "Experimental Aspects of Ultrasonic Spectroscopy," Ph.D. Dissertation,

University of Maine (1993).

7. Strout, T. A., "Attenuation of Sound in High-Concentration Suspensions: Development

and Application of an Oscillatory Cell Model," Ph.D. Dissertation, University of Maine

(1991).

39
NEW DEVELOPMENTS IN ELECTROACOUSTIC METHODS AND

INSTRUMENTATION

David W. Cannon

Matec Applied Sciences

Hopkinton, MA

ABSTRACT

Electroacoustic phenomena arise due to the interaction of electric fields and sound waves

in a suspension of charged particles. The Electrokinetic Sonic Amplitude (ESA) is the sound

wave generated by the particles when an alternating electric field is applied to the

suspension. The Colloid Vibration Potential (CVP) is the alternating potential developed

between two points when a sound wave is propagated through the suspension. Commercial

instruments for measuring the ESA and CVP have been available since the middle 1980's and

have been used to determine the particle charge or zeta potential. Recent developments in

both theory and instrumentation have resulted in the Matec AcoustoSizer, a new instrument

for measuring both the particle size and zeta potential in concentrated suspensions. The

historical development of the technique and the basic measurement principles for

determining particle size and charge in the AcoustoSizer are presented.

INTRODUCTION

Significant progress in the science of electroacoustic phenomena in colloids has been

made over the past several years. The availability of commercial instruments coupled with

important advances in theory have enabled the technology to move from a research curiosity

to one of the most significant particle characterization techniques to emerge since light

scattering. Electroacoustic instruments are being used for electrokinetic characterization in

major industrial and academic research laboratories around the world for applications ranging

from the characterization of ceramic powders to apolar liquid toners used in imaging

technology. The popularity of electroacoustic methods has grown rapidly over the past

several years because of the significant advantages offered over traditional methods for

determining electrokinetic properties.

40
These advantages include:

1 . Measurements can be performed on concentrated suspensions that are inaccessible to

optical techniques such as microelectrophoresis and electrophoretic light scattering.

2. The measurements can be made on stirred or flowing samples allowing rapidly

sedimenting or viscous materials to be characterized. There is also promise for the

technique to be applied to in-line monitoring of process streams.

3. Data acquisition is rapid and can be combined with automatic titrators to provide

electrokinetic data over a wide range of solution conditions.

The promise of electroacoustics has been the ability to determine the electrokinetic

charge or zeta potential in concentrated systems without the need for dilution. While

progress has been made, the existing commercial instruments on the market have not been

able to fully achieve this goal. Important issues like the dependence on particle size,

instrument calibration, and the interpretation of measurements at high particle concentrations

have been raised by several authors15

. Through the theory developed by 0'Brien6,7 we now

have a much better understanding of electroacoustic effects in concentrated systems. In

particular, the role of particle size has been studied in great detail and has enabled the

development of new instrumental methods that allow zeta potential and particle size

to be determined simultaneously. For the past 5 years, Matec Applied Sciences has been

working with an Australian company, Colloidal Dynamics, and the Colloids Group at the

University of Sydney to develop new electroacoustic measurement techniques and theory to measure particle size and zeta potential in concentrated suspensions. This effort has resulted

in the Matec AcoustoSizer. The objective of this paper is to describe the design criteria for

the instrument and the instrumental methods developed to achieve them.

41
ELECTROACOUSTIC EFFECTS AND THE DYNAMIC MOBILITY

Ultrasonic Vibration Potentials

All of the early developments in electroacoustic instrumentation were concerned with

the measurement of Ultrasonic Vibration Potentials (UVP). The ESA effect was not

discovered until 1982. 8 The UVP effect was first predicted by Debye9 in 1933 where he

suggested it as a method for measuring the effective mass of ions in solution. Later,

Rutgers10 and Hermans11 independently proposed UVP for measuring the particle charge in

colloidal suspensions. Ultrasonic Vibration Potentials have been studied extensively by

Yeager and coworkers who were the first to detect the "Debye effect" in electrolytes. A

thorough review of research in the area prior to 1982 is given in reference (12). Marlow

et al. n describe a commercial instrument marketed by the Pen Kem company for applying

the UVP technique to studying the electrokinetic properties of colloids.

Ultrasonic Vibration Potentials are created when sound waves are propagated through a

suspension of charged particles if the material wave creates a periodic displacement

(polarization current) between oppositely charged particles. In the case of electrolyte

solutions the effect occurs if the cation and anion have different effective masses and

frictional coefficients. The viscous and inertial forces acting on the ions due to the passage

of the sound wave cause oppositely charged ions to take on different velocities creating

periodically changing electric charge densities in the solution. In a colloidal suspension, the

UVP effect arises from a periodic displacement of the particles relative to their surrounding

electrical double layers. If there is density difference between the particles and

the liquid, the particles will take on a different velocity in the sound wave than the

surrounding liquid. Since the ions in the double layer will move with the liquid, there will

be a relative motion between the double layer ions and the particle leading to a periodic

polarization of the double layer. The dipole fields created by this polarization sum to

produce a macroscopic alternating electric field at the frequency of the sound wave.

According to the terminology established by Yeager14

, the UVP of an electrolyte is known as

the Ion Vibration Potential (IVP) and the UVP of a colloid is termed the Colloidal Vibration

Potential (CVP). The IVP and CVP are reported as the amplitude of the measured potential

42
normalized by the velocity amplitude of the sound wave measured at the electrodes.

The UVP is measured by placing electrodes in the sound field with an electrode spacing

other than an integer multiple of the sound wavelength and measuring the open circuit

voltage. O'Brien6 has pointed out that the electrical response at the electrodes can also be

measured as the short circuit current which has the advantage that the current is directly

proportional to the charge on the particles while the UVP is proportional to the charge

divided by the electrical conductivity of the suspension. Keeping with the foregoing

terminology it seems appropriate to name the short circuit current as the Colloidal Vibration

Current (CVC). The CVC is easily measured but has not been implemented in any of the

available commercial instruments.

Electrokinetic Sonic Amplitude

A second type of electroacoustic effect occurs when an alternating electric field is applied

to a suspension of charged particles. The particles move back and forth in the liquid due to

the force of the electric field creating tiny pressure disturbances around the particles in the

liquid. If there is a density difference between the particles and the liquid a macroscopic

acoustic wave is developed at the boundaries of the suspension (the electrodes). This effect

has been termed the Electrokinetic Sonic Amplitude (ESA) and was discovered at Matec8 in

1982. Matec Applied Sciences markets the ESA-8000 system which can be configured to

measure either the ESA or CVP to characterize the electrokinetic properties of colloidal

suspensions.

The Dynamic Mobility

O'Brien6 was the first to develop a theory for the ESA effect and showed that it is linked

to the CVP through a reciprocal relation. More significantly he showed that the ESA and

CVP effects are related to a fundamental property of the particles known as the dynamic

mobility. The dynamic mobility, fi d , is defined as the particle velocity, V divided by the

electric field, E for a charged particle moving under the force of a sinusoidal electric field

with angular frequency co.

43
Electric Field

Particle Velocity

E = Eq sin 03

V = V0 sin co

n V0

(t + At)

Time

Figure 1. The definition of the dynamic mobility

The dynamic mobility is a complex quantity meaning that it has both a magnitude and

argument (or phase angle). The magnitude of /*d is Vq/Eq, the amplitude of the particle

velocity divided by the amplitude of the electric field. The argument of fid relative to the

electric field is co(At). The argument of /* d gives the time delay or phase lag between the

particle motion and the applied field. The dynamic mobility of a colloidal particle is a

function of the particle zeta potential, particle radius, double layer thickness, and the

frequency of the applied field. As the frequency of the electric field approaches zero the

dynamic mobility reduces to the familiar electrophoretic mobility defined for a DC electric field. When the frequency of the electric field is increased, the particle velocity

will begin to lag the field and the velocity amplitude will drop because of inertial forces

which are proportional to the particle size. The AcoustoSizer is a new instrument designed

to make electroacoustic measurements over a range of frequencies to yield the dynamic

mobility spectrum of a concentrated suspension. The particle size and zeta potential of the

suspension can be calculated from the dynamic mobility spectrum. The first step in

determining particle charge and size requires highly accurate and reproducible measurements

of an electroacoustic effect.

44
MATEC ESA-8000 MEASUREMENT PRINCIPLES

The existing commercial instruments for making electroacoustic measurements lack certain

capabilities required in a device for determining particle size and charge. The AcoustoSizer

does, however, draw several important design features from the Matec ESA-8000 system and

an overview of that instrument serves as a good starting point for explaining the motivation

behind the design principles of the AcoustoSizer. Figure 2 gives a schematic block diagram

of the ESA-8000 system with the model PPL-80 flow through sensor.

1 MHz

Frequency

Synthesizer

Phase

Splitter

cos cot

sin oo t

Quadrature Phase

Sensitive Detector

Amplifier

Modulation

Impedance

Matching

&

Voltage Control

High Voltage

RF Pulse

Electrodes

Variable Gain

PPL - 80 Sensor

Colloid to

Low Level

ESA

Sample

and

Hold

A/D

Converter

Computer

Preamp

Piezoelectric Transducer

Delay Line

Figure 2. ESA-8000 System Block Diagram

45
The instrument can be configured to measure either the ESA or CVP of a suspension

depending upon how connections are made to the sensor. Figure 2 gives the arrangement for

the ESA measurement mode. The PPL-80 measurement cell consists of two parallel gold

electrodes laminated to solid rods sealed in a teflon housing inside the PPL-80. One of the

rods has a piezoelectric transducer mounted on the end opposite the electrode. The colloid

under test is pumped through the housing between the two gold electrodes. An ESA

measurement begins by applying a 1 MHz pulsed sinusoidal voltage to the electrodes in the

sensor. When the electric field is applied to the electrodes, the charged particles in the

suspension are accelerated back and forth generating tiny pressure waves in the liquid. The

pressure fields from neighboring particles tend to cancel except at the boundaries

of the suspension. The net effect is to generate a macroscopic acoustic wave at each

electrode surface that behaves as if the electrodes were vibrating instead of the particles. A

portion of the acoustic energy is transmitted into the solid rods supporting the electrodes and

the rest travels through the suspension toward the opposite electrode. In the PPL-80 design

the electrode spacing is chosen such that the sound wave being generated at one electrode

adds constructively to the sound wave arriving from the opposite electrode. This leads to

acoustic resonance in the cavity between the electrodes. The required spacing must be

approximately an odd multiple of one half the acoustic wavelength, X, in the colloid. The

PPL-80 design uses a spacing of 1.5 X which is roughly 2.2 mm in water at 1 MHz. The

acoustic resonance amplifies the ESA effect. Depending on the construction details of the

sensor, the ESA signal is amplified by a factor of 3 to 5 times. The pressure wave travels

down the solid rod and generates a voltage at the piezoelectric transducer.

In a CVP experiment, virtually the same arrangement is used except the voltage pulse

from the gated amplifier is applied to the piezoelectric transducer. The voltage applied

across the transducer causes it to expand and contract at the frequency of the applied voltage

causing a sound pulse to travel down the rod. When the sound pulse reaches the

electrode-colloid interface, some of the sound energy is transmitted into the cavity formed by

the electrodes and the rest is reflected back towards the transducer. Acoustic resonance

occurs if the electrode spacing is an integer multiple of one half the acoustic wavelength. If

the electrodes are spaced an odd multiple of one half the acoustic wavelength, the CVP effect

46
is maximized and can be measured by connecting the electrodes to a high input impedance

amplifier. O'Brien6 derived a reciprocal relation linking the ESA and CVP effects

and it can be written as: vV = zv (l)

ESA CVP

is the voltage generated at the transducer divided by the voltage applied to the

electrodes in an ESA experiment and VCVP is the the CVP voltage generated by the sound

wave in the colloid divided by the voltage applied across the transducer to generate the sound

wave. Z is the electrical impedance of the transducer divided by the cell impedance. From

this relationship it is easy to see that the ESA effect will give a higher signal to noise ratio

when the value of Z is greater than one. In the ESA-8000 system, this occurs in aqueous

media at electrolyte concentrations of about 10"3 M and greater. At lower suspension

conductivities the CVP will give the best signal to noise but the conductivity of the

suspension at the frequency of the electroacoustic effect must be measured to calculate the

dynamic mobility from the CVP. The existing commercial instruments measure the

conductivity at a lower frequency which gives only the real part of the conductivity at the

frequency used in the electroacoustic measurements. This in turn leads to a measurement

error when converting the CVP to a dynamic mobility. The ESA measurement has the

advantage that it is independent of the suspension conductivity and can be used over a wide

conductivity range. The ESA method is being successfully applied to electrokinetic

measurements in non-aqueous low dielectric constant media15,16

.

Pulse Modulation vs Continuous Wave Methods

Sinusoidal voltage pulses are used in the ESA system instead of continuous wave signals

for a very important reason, At the frequencies used in the ESA system, it is very difficult

to prevent the large voltages developed by the transmitter from interfering with the small

signals generated by the ESA or CVP effects. The ESA and CVP signals are often on the

order of microvolts while the transmitter voltages are typically hundreds of volts. By using

voltage pulses and the solid rods in the sensor, the signal from the electroacoustic effect can

47
be separated in time from the transmitter signal. This is illustrated in Figure 3 which shows

a typical oscilloscope display of signals from various points in the ESA signal processing

schematic (Figure 2). The top waveform is the sine wave output of the frequency

synthesizer which operates at a frequency near 1 MHz. The low level synthesizer

output is amplified into a high voltage pulse by a gated amplifier. The second waveform in

Figure 3 is the modulation gate which turns the gated amplifier on and off. The gated

amplifier output is typically 100 volts peak to peak for aqueous suspensions but voltages as

high as 1000 volts peak to peak are used in low dielectric constant non-aqueous media. The

bottom trace in Figure 3 is the amplified signal from the piezoelectric transducer in the

sensor. The first waveform from the left hand side is the "cross talk" or electromagnetically

induced interference from the transmitted voltage pulse. The second waveform in the bottom

trace is the ESA signal which is delayed in time by the time it takes for sound waves to

travel the length of the solid rod (acoustic delay line). The time duration of the voltage pulse

and the length of the rod are selected so that the voltage pulse is long enough for the

resonance in the cavity to reach steady state but short enough that the electrode

voltage is turned off by the time the ESA signal arrives at the transducer. The pulse

modulation technique allows very small signals to be measured with high accuracy and was

employed by Yeager17 in the first successful quantitative measurements of the IVP.

48
Frequency Synthesizer Sine Wave Output

Modulation Pulse

J

Gated Amplifer Pulse Output r- 100 Volts J

Delayed

Transducer Signal Electroacoustic

Signal Crosstalk -mamma,—

Acoustic

Reflections

Acoustic Delay

Figure 3. Pulse modulated acoustic measurement concepts.

ESA-8000 Signal Processing and the Calculation of Zeta Potential

The signal processing of the ESA and CVP is the same except for the connections to the

sensor (Figure 2). The ESA or CVP signals from the measurement cell are amplified first by a high impedance narrow band preamplifier located near the measurement cell. The

output of the preamplifier is further amplified by a variable gain amplifier before being

mixed with the reference synthesized sine wave source in a quadrature phase sensitive

detector. Quadrature detection allows both the amplitude and phase angle of the signal to be

measured. The two outputs of the phase sensitive detector, V0 and are proportional to

the in phase and quadrature components of the measured signal. That is:

VQ = Acos$ and V9Q = AsinQ (2)

49
where A is the amplitude of the signal being measured and $ is the phase angle of the signal

relative to the sine wave reference. The voltages V0 and V90 are measured by a sample and

hold circuit and read by the computer through an A/D converter. The amplitude and phase

angle of the signal at the input to the phase sensitive detector are given by:

A = (V0 2 + V90

2

) 1/2 and* = ran'Mv^/Vo) < 3 >

In practice, there are error terms which must be included in these equations that result from

the phase splitter signals not being perfectly in quadrature. Reference (18) describes the

procedure for eliminating the error terms.

In order to convert the measured electrical signal to the dynamic mobility and eventually

to a zeta potential, a formula is required linking the electroacoustic effects to the dynamic

mobility. This formula is obtained by solving the Electroacoustic Equations derived by

O'Brien7 using boundary conditions appropriate for the PPL-80 measurement cell. Using a

simple one dimensional parallel plate model of the sensor the following formula is obtained

for the ESA pressure wave, P, in the delay rods per unit electric field, E, between the

electrodes19

.

1 + (o)M - iZr ) cot kL (4)

where $ is the particle volume fraction, Ap is the difference between the particle density and

the liquid density, p. Zr is the acoustic impedance of the solid delay rod. The acoustic

impedance is defined as the ratio of pressure to particle velocity in an acoustic wave and is

analogous to electrical impedance defined as the ratio of voltage to current. The term inside

the brackets in equation [4] describes the acoustic resonance of the electrode cavity. Here a>

is the angular frequency (2irf), M is the mass per unit area of the electrodes and k is the

propagation constant for the acoustic wave in the suspension given by:

50
k

2 = «ix

(1 + ico(j)Apa)

(5)

where x is the mass averaged bulk compressibility of the suspension and a is a term defined

by O'Brien as a transport property of the suspension linking the particle velocity to the

macroscopic pressure gradient in the suspension7

.

Equations [4] and [5] are valid for suspensions of arbitrary concentration. Unfortunately,

they contain terms that depend on the properties of the suspension which vary with the

particle volume fraction. Both a and x are functions of particle concentration and must be

determined experimentally in a concentrated suspension. In a dilute suspension, the term (1

+ ico#Apa) in equation [5] approaches 1 and k reduces to the frequency, w, divided by the

speed of sound, c. Under this approximation, and with a fixed frequency and separation

distance between the electrodes, the expression for the ESA pressure wave reduces to

equation [6] where the term in the square brackets can be taken to be a constant under the

restriction of dilute suspensions.

Z„ is the acoustic impedance of the suspension which is the same as the pure solvent for a

dilute suspension. For a fixed electrode spacing and frequency, the terms to right of /id in

equation [6] can be taken as a constant and the ESA voltage measured by the ESA-8000

electronics given by:

The C(Q) term in equation [7] lumps several instrument constants together with the constant

terms in equation [6]. Instrument constants include the voltage - pressure calibration for the

transducer, fixed acoustic propagation losses through the delay rod and fixed electronic gains

in the instrument. The voltage measured by the instrument electronics can now be related to

(6)

VESA = C(Q)<I>Apnd

(7)

51
the 1 MHz dynamic mobility of the suspension with an expression involving only instrument

constants, the volume fraction of the particles and the density difference between the particles

and the liquid. The ESA-8000 software computes the particle zeta potential using the

following formulae 8 and 1 1 for the dynamic mobility of a dilute suspension of spheres6

:

(a) For aqueous media assuming thin double layers

a

r\

0)3' (8)

G(a) = 1 ia(3+2Ap/p)

9[l+ (l-i)/a72]

(9)

where: a = , a is the particle radius and v is the kinematic viscosity of the liquid.

(b) For non-aqueous media assuming thick double layers

f 2\ (x>a 3n V v J

(10)

<?'(«) = 1 + -i

\ 2

+ 9

2PD p m

(ii)

Equations [8] and [10] are identical to the well known Smoluchowski and Huckel equations20

for the static DC electrophoretic mobility except for the G(a) and G'(<*) terms. The G terms

describe the effect of particle inertia on the dynamic mobility which causes it to decrease

with both increasing frequency and increasing particle size. This dependency on particle size

and frequency has been exploited in the AcoustoSizer for measuring both particle size and

52
zeta potential. Because the ESA-8000 system operates at only one frequency, the particle

size must be known to calculate zeta potential from the dynamic mobility.

Standard Colloid Calibration

In order to obtain a quantitative measurement of the dynamic mobility a calibration must

be performed to determine the instrument constant in equation [7]. This is accomplished by

placing a colloid with known particle size and zeta potential in the cell. The first step in

calibrating involves sweeping the frequency of the applied field over a narrow frequency

range to find the resonant frequency of the cell where the acoustic signal goes through a maximum. The resonant frequency can vary due to changes in the sound velocity of the

medium which is a function of temperature, the particle volume fraction and electrolyte

concentration. The magnitude and phase of the reference colloid signal is then measured at

the resonant frequency of the cell. The instrument constant is determined by calculating the

dynamic mobility of the standard using equation [8] or [10] and solving for C(Q) using

equation [7], Reference (2) describes the calibration procedure using several different

standards as a cross check.

ESA Phase Angle Calibration

The phase angle of the signal measured by the ESA-8000 electronics is not the phase

angle of the ESA pressure wave relative to the electric field. Rather, it is the phase angle of

the ESA signal relative to the sine wave reference and is arbitrary. The phase angle

measurement includes phase shifts through the measurement electronics and also depends

strongly on the time it takes the sound pulse to travel the length of the acoustic delay rod.

From Figure 3 it should be apparent that a small change in the time delay of the ESA signal

results in a large change in the phase relationship between the signal and the reference sine

wave. The ESA-8000 system uses the phase of the ESA signal to determine the polarity of

the particle charge. When the particle charge changes sign, the phase of the acoustic signal

changes by 180 degrees. By following the change in the ESA phase angle from some known

reference point the polarity of the particle charge can be determined. This process

requires a reference measurement on a particle sample with a known charge polarity. The

53
phase angle measured for the reference colloid is stored and used to calculate the relative

phase angle of the sample to be analyzed. The phase angle reported by the ESA system is

then the phase angle measured on the sample under test minus the phase angle of the

reference colloid. If temperature is held constant, any phase difference between the sample

and the reference is associated with the particle properties since the phase shifts through the

electronics and acoustic delay line will cancel out, There are other factors which affect the

measured phase angle in the ESA system besides the polarity of the particle charge. These

include phase shifts due to changes in particle size and also changes in the acoustic resonance

in the sample cell that arise from differences between the acoustic properties of the phase

reference sample and the colloid under test. These effects are rarely large enough to cause

problems with determining the charge polarity. A strong background electrolyte signal will

also affect the ESA signal phase and magnitude and these signals will not, in general, be

in phase with one another. The measured ESA signal is the vectorial sum of the ESA signal

from the colloid and the electrolyte. Strong background electrolyte signals can result in an

apparent shift in the colloid isoelectric point.

Once calibrated, the ESA-8000 system provides a robust and rapid way to determine the 1 MHz dynamic mobility and zeta potential of moderately concentrated colloidal suspensions.

The measurement technique is accurate to within +/- 2 % for suspension concentrations up

to 5 volume percent for systems where the particle size is not too large ( < 1 micron) and

well known. For higher particle concentrations the assumptions used to derive equation [7]

are no longer valid and an error will be introduced that is proportional to <f>Ap/p. In practice

the acoustic properties of a concentrated suspension cannot be calculated and must be

measured directly. In spite of this limitation, the relative dynamic mobility or zeta potential

measured in highly concentrated systems provides a powerful and rapid method for

monitoring the change in electrokinetic charge as a function of pH or chemical additive. For

samples with particle sizes significantly greater than one micron, the 1 MHz dynamic

mobility becomes a strong function of the particle size distribution. Under these conditions,

the size distribution must be measured and the correct inertial term for the dynamic mobility

must be calculated to accurately calculate the particle zeta potential. James et al? describe

the procedure for calculating the correct G terms under these conditions.

54
ACOUSTOSIZER DESIGN PRINCIPLES

The AcoustoSizer takes advantage of the dynamic mobility's sensitivity to particle size and

the frequency of the applied field by measuring the frequency dependence of the dynamic

mobility. As the frequency of the applied field is increased, the inertial forces acting on the

particles increase causing both a decrease in particle mobility magnitude and an increase in

phase lag. The magnitude spectrum combined with the phase lag spectrum provide enough

information to calculate zeta potential and size. Figure 4 illustrates how the magnitude and

phase lag of the dynamic mobility vary with frequency for two different particle sizes with

the same zeta potential.

2.50-

2.00-

1.50-

Mag

1.00-

0.50-

0.00-

0.3 Micron Dia. —A

5 Micron Dia.

4 6

Frequency (MHz)

r

10

50.0-

40.0-

Phase Lag

Degrees

30.0-

20.0-

10.0-

A,-A"A -A-A"A~A-A AA

03 Micron Dia.

5 Micron Dia.

12

Frequency (MHz)

Figure 4. Dynamic mobility magnitude and phase lag vs frequency for 0.3 and 5.0 micron

diameter spheres. Ap / p = 1.0 ; C = 30 mV

The mobility spectrum data is acquired by measuring the ESA of the suspension over a

range of frequencies. The frequency range spanned by the instrument determines the range

of particle sizes that can be measured. Particles at the lower sizing limit must have a

measurable phase lag at the highest frequency in order to be sized. Particles at the upper

sizing limit must move at a sufficient velocity at the lowest frequency to produce a

measurable electroacoustic signal. The frequency must also be low enough so that the phase

lag of the largest particle size is smaller than the asymptotic limit of 45 degrees for particles

55
with thin double layers. These conditions are met when the quantity a = is near

unity. The AcoustoSizer measures ESA over a frequency range of 300 kHz to 12 MHz to

determine particle size in the range of 0.1 to 10 microns.

Several new instrumental methods had to be developed for the AcoustoSizer because the

design of the ESA-8000 system was not suitable for frequency domain measurements.

Factors limiting the ESA-8000 design for sizing purposes include:

1. Narrow band measurement cell design

The measurement cell is designed to operate over a narrow frequency range centered about 1 MHz. The design has been tuned and optimized to provide high sensitivity at the resonant

frequency of the cell. Matching the frequency response of the narrow band piezoelectric

transducer to the frequency response of the electrode cavity is very difficult over the wide

frequency range required for particle size measurements.

2. Phase angle measurement accuracy

The ESA-8000 system was not designed to measure the phase angle of the ESA signal with

high accuracy since only changes of 180 degrees need to be detected to determine charge

polarity. The phase of the ESA signal measured by the ESA-8000 system is very sensitive to

changes in temperature, the acoustic properties of the suspension and phase shifts in the

electronics. Electroacoustic particle sizing requires very accurate and reproducible phase

angle measurements over the entire frequency range of the instrument.

3. Acoustic Properties of the suspension

The acoustic properties of the suspension must be measured in order to convert the measured

ESA signal to the dynamic mobility in concentrated suspensions. There is no provision for

this measurement in the ESA-8000 design.

56
Essential Features of the AcoustoSizer Design

The AcoustoSizer has been designed to overcome the problems described above and can

accurately measure the dynamic mobility spectrum of a suspension over a very wide particle

concentration range. New designs have been developed for both the measurement cell and

the instrument electronics. Figure 5 gives an overall schematic diagram of the measurement

system.

Windows

User Interface V

Dyn Mob,

Size & Zeta

,

V

DSP Module

ESA Fourier Transform

Operator Computer

Terminal

80486 CPU

5 2

Instrument Control Analog Signal Processor Processing

pH, Temp, Cond,

RPM, Torque

i

6. Digital Titrator Module

1

15

'/v/ss/.yr.-v.

..-...-..-.-^..->. H r

1 . Measurement Cell

Figure 5. AcoustoSizer System Block Diagram.

57
The instrument system is highly integrated combining measurement cell, signal processing

electronics, computer and automatic titrator into a single instrument enclosure. The

electroacoustic measurement sub-system consists of the measurement cell (1), analog signal

processing electronics (2), and the DSP module (3). Each of the major instrument

subsections is described below.

1. The AcoustoSizer Measurement Cell

A simplified schematic diagram of the AcoustoSizer measurement cell and front end signal

processing is shown in Figure 6. The suspension under test is contained in an inert epoxy

housing and kept under constant agitation by an overhead propeller type mixer. Probes for

measuring pH, conductivity and temperature are inserted into the suspension from above and

are not shown in Figure 6. A heat exchanger for controlling the sample temperature is also

inserted into the cell housing from above.

Acoustic Impedance

Sensor

Mixer

Reference

and Acoustic

Impedance Signal

Shielded Housing Piezoelectric 'Transducer

Electrode

RFpulse

Matching

Networks

X 1

Preamp

ESA Signal

RF Multiplexer

Voltage

Sample

Current and

Voltage

Sample

Gated Amplifier To Analog

Signal

Processing

Broadband

RF Amplifier

Figure 6. AcoustoSizer measurement cell and front end signal processing schematic.

58
Two solid acoustic delay lines are mounted on opposite walls of the housing for carrying out

ESA measurements on the suspension. Inert metal electrodes are mounted on the suspension

side of each delay line and are sealed into the wall of the housing so that the electrodes are

in contact with the suspension.

There are several important features of the AcoustoSizer cell design that distinguish it from the ESA-8000 sensors. The first is the use of two acoustic delay lines. Like the ESA

system, the AcoustoSizer employs pulse modulation techniques to avoid problems with

cross-talk between the transmitter and receiver electronics. The right hand delay line in

Figure 6 is used to measure the ESA spectrum of the suspension. For each test frequency, a

radio frequency (rf) pulse generated by a gated amplifier is applied to the electrodes in the

measurement cell. The high frequency electric field applied across the suspension generates

ESA pressure waves at each electrode which travel out into the suspension and down each

delay line. The ESA pressure wave in the right hand delay line is detected by a piezoelectric

transducer which produces an electrical signal that is amplified and directed to the signal

processing electronics through a switch. The spacing between the electrodes in the

AcoustoSizer measurement cell is large compared with the PPL-80 design. A wide spacing

is used so that the ESA signals from each electrode do not interfere with one another. The

ESA pulse width and electrode spacing is chosen so that the time duration of the ESA signal

is shorter than the time required for the ESA signal from the opposite electrode to travel the

distance between the electrodes. This eliminates the acoustic resonance between the

electrodes that is employed in the PPL-80 design. Eliminating the resonance makes it much

easier to carry out ESA measurements over a wide frequency range.

Solving the Electroacoustic Equations for the geometry used in the AcoustoSizer cell the

following formula is obtained for the ESA pressure wave in the delay line.

V P ) ESA = C(Q) (J) \id—^- (12)

z, +zr

where C(fi) is an instrument constant that does not depend on the properties of the particles,

and the subsripts s and r refer to the suspension and delay rod respectively. Equation [12] is

valid for both dilute and concentrated suspensions. In order to use this equation to compute

59
the dynamic mobility, the acoustic impedance (Zs) of the suspension must be measured. This

is the purpose of the second delay line in the measurement cell. Accurate measurement of

the acoustic impedance is critical for converting the measured ESA signal in the delay line to

the dynamic mobility in concentrated suspensions. The dependence of the ESA signal on

important factors such as the presence of air bubbles and acoustic dissipation by the particles

is governed by the Zs term in equation [12]. Measurement of the acoustic impedance is

carried out by switching the output of the gated amplifier to a proprietary sensor mounted on

the left hand delay line. The sensor generates an acoustic signal that varies with the acoustic

impedance of the suspension and is detected by a piezoelectric transducer on the opposite end

of the delay line. A very important feature of this arrangement is that the ESA

and acoustic impedance signals travel through identical signal processing paths. This

provides a means for ratioing out phase shifts in the signals that result from the physical

measurement system. Examples include phase shifts due to thermal expansion of the delay

lines and the effect of variable propagation delays in the electronics. The AcoustoSizer

measurement cell also contains circuitry for measuring the current and voltage waveforms

applied to the electrodes and the voltage applied to the acoustic impedance sensor. The

unique design of the AcoustoSizer measurement cell and signal processing achieves absolute

phase angle measurement accuracies better than one degree and magnitude measurement

accuracies better than one percent over the entire operating frequency range.

2. Analog Signal Processing Electronics

The low-level signals from the measurement cell are switched to the input of the

AcoustoSizer analog signal processing electronics where they undergo further treatment to

produce the Fourier Transform of the signals. The Fourier Transform of the signals is

required because the theory developed for relating the ESA spectrum to the dynamic mobility

spectrum assumes that the applied fields are continuous sinusoids of a pure frequency, w. A

pulsed sinusoid has its own characteristic frequency spectrum which varies with the width

and shape of the pulse. The analog signal processing electronics measure the Fourier

component of the ESA pulse at the frequency of the synthesized sine wave source supplied to

the gated amplifier. The ESA spectrum is constructed by varying the frequency of the rf

60
pulse applied to the electrodes in a series of steps over the frequency range of 300 kHz to 12

MHz. The process is repeated with the acoustic impedance sensor to produce the acoustic

impedance spectrum of the suspension. Using equation [12] the dynamic mobility spectrum

is calculated from the measured ESA and acoustic impedance spectra.

3. DSP Processor

The DSP module is a high speed "Digital Signal Processor" based microcomputer

optimized for computationally intensive routines such as matrix operations. The DSP module

controls the ESA data acquisition process and calculates the dynamic mobility, particle size

distribution and zeta potential from the measured ESA and acoustic impedance spectra. The

use of a high speed DSP co-processor in conjunction with the central operator computer

terminal allows particle size and zeta potential data to be collected very rapidly achieving

close to real time operation. This is an important feature for carrying out automated

titrations.

For a polydisperse suspension, the dynamic mobility is given by the following integral

equation3

.

where (/O is the mass averaged dynamic mobility of the suspension, p(r) dr is the mass

fraction of particles having radii between r-dr/2 and r+dr/2, and nJj) is the dynamic

mobility for a particle of radius r. The calculation of (fxd) is straight forward using equation

[12] once the ESA and acoustic impedance spectra have been measured. The inversion of

equation [13] to solve for the zeta potential and particle size distribution is not a trivial

exercise however. Special numerical techniques are required to deconvolve the mobility

spectrum. The software developed for the AcoustoSizer to date assumes the size distribution

function p(r) dr is a known function such as the log-normal distribution. The mass median

size, 15 to 85 mass percentile size range and zeta potential of the suspension is adjusted

using a least squares fitting routine to obtain the best fit to the measured dynamic mobility

spectrum. The equation used for /x d (r) in [13] depends on the nature of the sample

(13)

61
under test. For thin double layer systems with concentrations below 10 volume percent,

equation [8] can be used. At higher solids levels analytical formulae are not available for the

dynamic mobility and a modified cell model approach is used. The details of the procedure

along with AcoustoSizer results for a series of dilute and concentrated model colloidal

systems are presented in a companion paper

21

.

4. Operator Computer Terminal

The AcoustoSizer Operator Computer Terminal consists of a fully imbedded 80486

computer including hard disk and floppy disk mass storage capabilities. The instrument

control software utilizes the Microsoft Windows 3.1 user interface for intuitive and simple

instrument operation. The central 80486 computer communicates with the DSP and

Instrument Control Processor to acquire and process data. The distributed computing

approach used in the instrument ensures that the data acquisition and analysis process

proceeds smoothly and quickly while allowing the user to archive, analyze or print data

simultaneously. All of the mobility, size, and zeta potential data are displayed

on the high resolution color display. Temperature, pH, conductivity, mixer speed,

suspension relative viscosity data and temperature control functions are also provided on a

front panel display located next to the measurement cell.

5. Instrument Control Processor

The AcoustoSizer instrument control processor is a microcontroller responsible for low

level non-electroacoustic control and data acquisition functions in the AcoustoSizer. This

module measures the pH, conductivity, and temperature of the suspension under test.

Additional functions include mixer speed control, torque and viscosity measurements, and

temperature control utilizing a solid state Peltier temperature control system.

6. Digital Titrator Module

A dual syringe pump automatic titrator module is also incorporated in the AcoustoSizer

instrument system. This module and accompanying control software provides for carrying

out automated additions of reagents such as dispersants, flocculants, acids and bases for

62
rapidly assessing their affect on the suspension zeta potential and particle size distribution.

Calibration of the AcoustoSizer

Similar to the ESA-8000 system, instrument constants must be determined for the

AcoustoSizer in order to relate the voltage spectra measured at the sensing transducers to the

actual ESA pressure wave in the delay line. The instrument constant is given as C(D) in

equation [12] and is a function of frequency and instrument gain but not the properties of the

particles. A standard colloid with a known zeta potential and particle size distribution is

required for determining C(Q). In practice it is very difficult to obtain suspensions with a

well known size distribution and zeta potential. We have found that a series of spherical

mono-disperse silica powders manufactured by GELTECH22 serve as excellent calibration

standards for the AcoustoSizer. GELTECH's nominal 1.0 micron diameter material was

chosen for calibrating the instrument. Typically the material is dispersed at high pH and

at a sufficient background electrolyte concentration so that the assumption of a thin double

layer is valid. The particle volume fraction is usually in the range of 2 - 3 volume percent to

provide good signal to noise while remaining in a concentration range where particle-particle

interactions can be neglected. The zeta potential of the calibrant is determined by

microelectrophoresis. The GELTECH 1.0 micron standard was found to be reasonably

mono-disperse. The particle size of the calibration standard was verified by SEM and a

centrifugal sedimentation based particle size analyzer, the Horiba Capa-700.

SUMMARY AND CONCLUSIONS

New developments in theory and electroacoustic measurement technology have resulted in

the AcoustoSizer, an instrument for determining the particle size distribution and zeta

potential of concentrated suspensions. It is the first commercial instrument capable of

making accurate electroacoustic measurements in highly concentrated systems. The

frequency dependent dynamic mobility can be measured in suspensions ranging from 1 to

over 40 volume percent and particle size can be determined in the range of 0. 1 to 10 microns

diameter. The emergence of this technology should provide a rich set of new data on the

63
microstructure of concentrated systems. The technology is well suited to a broad set of

slurry processing applications in the minerals, ceramics, pigments and coatings industries.

ACKNOWLEDGEMENTS

I would particularly like to recognize my principal collaborator in the development of the

AcoustoSizer, Richard O'Brien, who has played a pivotal role in the development of both the

theory instrumentation for the technology. In preparing this overview I have drawn heavily

from his work including many private communications. The contribution of Gary Petersen

and Bruce Chick of Ritec, Inc. in the development of key aspects of the signal processing

electronics is also gratefully acknowledged.

64
REFERENCES

(1) O'Brien, R. W.; Midmore, B. R.; Lamb, Alice; Hunter, R. J., Faraday Disc. Chem.

Soc. 1990, 90, 301.

(2) Klingbiel, R. T.; Coll, H.; James, R. O.; Texter, J., Colloids and Surfaces, 1992, 68,

103.

(3) James, M.; Hunter, R. J.; O'Brien, R. W., Langmuir, 1992, 8, 420.

(4) Scales, P. J.; Jones, E., Langmuir, 1992, 8, 385.

(5) Isaacs, E. E.; Huang, H.; Babchin, A. J.; Chow, R. S., Colloids and Surfaces, 1990,

46, 177.

(6) O'Brien, R. W., J. Fluid Mech., 1988, 190, 71.

(7) O'Brien, R. W., J. Fluid Mech., 1990, 272, 81.

(8) Oja, T.; Petersen, G. L.; Cannon, D. W., US Patent #4,497,208 (1985).

(9) Debye, P. J., Chem. Phys., 1933, 1, 13.

(10) Rutgers, A. Physica, 1938, 5, 674.

(11) Hermans, J., Phil. Mag., 1938, 25, 426.

(12) Zana, R.; Yeager, E. B., Mod. Aspects Electrochem. , 1982, 14, 1.

(13) Marlow, B. J.; Fairhurst, D.; Pendse, H. P., Langmuir, 1988, 4, 611.

(14) Yeager, E.; Hovorka, F., J. Acoust. Soc. Am., 1953, 25, 443.

(15) Pearlstine, K.; Page, L.; El-Saued, L., Jour. Imaging Science, 1991, 35, 55.

(16) Pearlstine, K.; Swanson, J. R., J. Colloid Interface ScL, 1992, 151, 343.

(17) Yeager, E.; Dietrick, H.; Hovorka, F., J. Acoust. Soc. Am., 1953, 25, 456.

(18) Petersen, G. P., Matec Instruments' Application Note; "The Measurement of Acoustic

Velocity and Attenuation Using Phase Detection and Matec Instruments' MBS-8000

Computer Controlled System" (1986), Matec Instruments, 75 South Street Hopkinton,

MA 01748.

(19) O'Brien, R. W., Private communication, (1990).

(20) Hunter, R. J., In Zeta Potential in Colloid Science; Academic Press: New York, 1981.

65
(21) O'Brien, R. W.; Rowlands, W. N.; Hunter, R. J., Proceedings of The NIST Workshop

on Electroacoustics for Characterization of Particulates in Suspensions, 1993, National

Institute of Standards and Technology, Gaithersburg, MD 20899.

(22) GELTECH Inc., 1 Progress Blvd., Box 8, Alachua, FL 32615.

66
SESSION II

APPLICATIONS AND PROCEDURES

Session Chairman

H. P. Pendse

University of Maine

CALIBRATION OF ELECTROACOUSTIC (ESA) APPARATUS

USING LATTICES AND OXIDE COLLOIDS

R. O. James

Imaging Research Laboratories

Eastman Kodak Company

Rochester, NY 14650-2112

ABSTRACT

The electrokinetic sonic amplitude (ESA) of aqueous suspensions of polystyrene and

poly(methyl methacrylate) lattices and of oxide colloids is reported as a function of volume

fraction. The use of these readily available materials as convenient standards for the

calibration of ESA apparatus is described. We review recent results obtained in our

laboratories. The precision of these techniques appears competitive with other classical

electrokinetic methods.

INTRODUCTION

In many industrial activities including mineral processing, water treatment and in

particular, the specialist coatings industries, it is important to assess and control the colloid

stability of suspended particles. A wide variety of methods have been used to monitor

colloid stability or as sometimes termed "the dispersion quality" or "the emulsion quality",

depending on the particular colloid system under scrutiny.

These methods have included electrokinetic techniques such as microelectrophoresis for

measurement of the electrophoretic mobility and calculation of the "zeta potential" of the

particles in dilute suspension. New methods to determine the mobility, or zeta potential,

in moderately concentrated suspensions have been developed in the last decade. The

methods are based upon electroacoustic effects that are associated with charged colloids

(1,2). These effects involve either the generation of sound waves by colloidal particles

driven by ac electric fields, or the generation of ac electric fields, or currents by the

67
vibration of particles driven by a sound wave (3,4). A specific example of a commercially

available instrument is the electrokinetic sonic amplitude (ESA) apparatus produced by Matec

Applied Sciences (3). In this paper, we will discuss methods and materials to calibrate the

ESA apparatus to provide data that can be compared under appropriate conditions to data

collected with classical methods, e.g., microelectrophoresis. Before proceeding with that

discussion, a few comments about the need for new techniques will be offered.

In the area of coatings industries, quality assessment can be made on semi-finished or

coated materials by observation of physical properties like surface roughness, gloss,

reflectivity, transmission, haze, scattering, and absorption of light and by various

microscopic techniques, e.g., optical microscopy, electron microscopy, and force microscopy

methods. All these methods can be sensitive to defects and nonuniformities such as coating

lines, variable roughness, and high or low optical density areas that may be caused or

associated with over-sized or aggregated solid particles, e.g., dispersed dyes, pigments or

latexes.

While some of these techniques can also be used directly on the coating fluid, few of

them have a direct relation to the fundamental factors involved in the colloid stability of the

fluid. In most coating applications, the stability of dispersions and the uniformity of the

particle size distribution is controlled by at least two principal factors: (a) the mutual

repulsion of charged particles and their electrical double layers, (b) The presence of

adsorbed polymeric or condensed layers on the particle that reduce the attractive van der

Waals or dispersion forces between the coated particles in the fluid.

Of course, in most coating applications, polymeric film-forming materials are present to

ensure adhesion to the substrate and to modify other physico-chemical properties of the

coating. Some of these polymers or binders function to reduce the van der Waals attraction

between particles. For example, in the photographic industry, gelatin is famous for its role

in the stabilization of silver halide particles in photographic emulsions. In addition, gelatin

may show zwitterionic character due to ionizable anionic and cationic functional groups.

Thus adsorbed polymers like gelatin can also dominate the interfacial charge and potential of

the particles and may cause some gel-coated particles to become pH sensitive.

68
Conventional microelectrophoresis has a drawback to direct application in many coating

formulations owing to its optical detection requirement that light scattered by a particle then

passes to the viewer without interaction with another particle. This often limits the

dispersion concentration to about 0.01 % w/w depending somewhat on the size and refractive

index of the particles. In many practical systems the percent solids (of particles) in a coating

fluid may be from about 1 % to about 60% w/w. Hence, the effect of pH adjustments with

acids or bases or of added dispersants and polymeric stabilizers on the particle charge is

often not directly observable at the coating solids level by microelectrophoresis. In addition,

because of solution handling and dilutions it is not a very rapid characterization technique.

Thus the arrival in the 1980s of an electroacoustic technique (3), and a theoretical basis

(4-6) for the evaluation of dynamic electrophoretic mobilities of more concentrated

dispersions has been welcomed by a number of research and development groups in the

formulation and coating community. It offers the possibility of making assessment of the

parameters that relate more directly to ionic adsorption and to the repulsive interaction part

of the total particle interaction energy. These parameters can be vital to colloid stability and

to uniform particle distribution in a wide variety of dispersion and coating applications.

ESA CALIBRATION STRATEGY AND PROCEDURES

The ESA apparatus measures the ultrasonic (acoustic) response of a colloidal suspension

of non-bouyant particles to a burst of applied radio-frequency (RF) field. The ESA is the

ratio of the pressure wave amplitude to the RF field amplitude. The pressure wave arises

from the combined vibrations of charged particles as they follow the driving electric field

(7,8).

Because the ESA apparatus involves various gain factors and instrumental constants there

is a need for a calibration procedure so that ESA signal data can be transformed to dynamic

electrophoretic mobility data for use in fundamental studies of colloid stability and for

applied stability and rheological studies. In these areas the sign and value of the mobility

and of the derived zeta potential may be crucial to interpretation of stability data.

69
The calibration procedure described here rests upon a theoretical basis for the

relationship between the dynamic electroacoustic mobility (at say 1 MHz) and the static or dc

electrophoretic mobility of particles as observed by microelectrophoresis in conventional

apparatus. The theoretical results described by O'Brien (4-8) and by others (9,10) that relate

the frequency dependent ESA and dynamic mobility, /x(co), to the static mobility, ^i(O), have

allowed the development of methods of calibration by commercially available colloidal

materials.

The empirical relationship of ESA to experimental signals (S) and the theoretically

derived dependence of ESA to the properties of the colloidal material in the limit of low

volume fraction are expressed as (2):

Sy = ESA = 0.01 c <f> Ap /x(co) Gf (1)

where S is the observed signal (mPa m/V),

7 is an instrumental constant (at low volume fraction),

ESA is the electrokinetic sonic amplitude (mPa m/V),

c is the speed of sound of the continuous phase (m sec"

1 ),

Ap is the difference in density (g cm"3) between the particle and the continuous phase,

/x(co) is the dynamic mobility (^cm cm sec

1 V"1 ) at field frequency co (rad sec1 ) and 8,45

is the particle volume fraction.

70
Gf is a geometrical and acoustic gain factor that describes the coupling between the

colloid suspension and the piezo-electric transducer and depends on the elastic properties on

the buffer rod.

In practice it is convenient to lump Gf and 7 together and determine an instrumental

constant 7/Gf such that:

S(T/Gf) = ESA/Gf = 0.01 c Ap 0 (2)

where the quantities on the RHS of Eq. 2 are only properties of the colloid dispersion. At

higher volume fractions, outside the linear S-<j> range, 7/Gf is dependent on electrode/probe

geometry and on the volume fraction. This discussion of calibration procedures is limited to

only the linear region which, in our experience, usually applies for volume fractions less than

0. 05.

The strategy for calibration of ESA apparatus involves use of appropriate materials to

obtain data that may be evaluated using the theoretical models that are relevant to the particle

or electrolyte data set. In the selection of materials for the calibration experiments, a number of the factors that should be considered are listed below.

1. Suitable colloid particles must be essentially insoluble in the solvent (water) with simple

pH dependent surface charge reactions, preferably not redox reactions, e.g., oxides and

lattices. Also, it is preferable that there is a pH region in which the electrophoretic

mobility is not strongly dependent on pH.

2. The particles must be non-bouyant. For aqueous systems near room temperature the

particles should have a density greater than 1.05 g cm"3 or less than 0.95 g cm"3 and

preferably have a density greater than about 2 g cm"3 in order that the Ap term in Eq. 1

ensures a high signal-to-noise ratio.

3. The particles are preferably spherical or spheroidal and of narrow size distribution. In

addition, they should be of such size and refractive index that they are easily observable

71
in the microelectrophoresis cell. For example, the particle radius should be about or more than 100 nm, but not so large as to sediment too quickly. Also the particle should

be small enough that the acoustophoretic mobility is approximately the same as the

electrophoretic mobility, n(0). When these quantities are of very similar magnitude,

errors and differences in sizing methods and in the details of model calculations are

potentially less critical. For many particles, this requires that the radius be in the

approximate range from 100 to 300 nm depending on the particle density (1,2,11).

Also, in order to use the theory developed by O'Brien (4), which is based on thin double

layer approximations, i.e., «a > > 1, it is desirable to adjust the ionic strength of the

electrolyte to about 0.01 mol dm 3 so that /ca is greater than 30 and preferably greater

than 50, where \Ik is the Debye length or diffuse electrical double layer thickness.

For the relationship between the electroacoustic mobility ju(co) and the electrophoretic

mobility, /x(0), we will use the theory developed by O'Brien (4). The important relations

from that paper are given below.

where f —0.5, a = coa

2/y, a is the particle radius, v = i)lp is the kinematic viscosity, 17 and

p are the viscosity and density of the continuous phase, and

Mco) = (2ef/3i? )G(a)(l + f) = M0)G(a) (3)

G(a) = {1 - i a(3 + 2 Ap/p)/9(l + (1 - i)(a/2)05)}" 1 (4)

This leads to the modulus of the dynamic to dc mobility ratio

|^M/M(0)| = |{1 - ia(3 + 2 Ap/p)/9(l + (1 - iXa/2)05)}- 11 (5)

72
The modulus |/*(u)//i(0) | is plotted in Fig. 1 as a function of radius for several values of

particle density in aqueous solutions and for ac field frequency at w/2x = 1 MHz (1,11). It

can be seen that for radius a <0.2 /xm, | /x(co)/ju(0) | >0.9.

Thus particles that satisfy most of the above criteria should be quite useful as calibration

materials for ESA using microelectrophoresis as the reference method.

Having calculated ^c(co) for each particle, at say, fixed pH, ionic strength, particle size

and electrophoretic mobility, /a(0), the ESA signal amplitude, S, is measured as a function of

volume fraction, 4>, of particles in the suspension under the same solution conditions, e.g., pH and ionic strength. From the linear S - <f> regime at low volume fraction, the limiting

slope, dS/d0, is determined and used to obtain the instrument calibration factor from:

7/Gf = 0.01 c Ap /i(o)/(dS/d0) (6)

Once y/Gf has been obtained and is self-consistent for several different colloidal

materials it may be adopted as the calibration factor for the instrument.

At this point, one may then choose to establish other materials for evaluation as

secondary standards that may offer materials advantages like availability or shelf-life and

chemical stability. For example, there are a range of commercially available colloidal silica

sols that could serve as secondary standards. An example is Du Pont Ludox TM, which is a

stable silica sol and has a low concentration of Na2 0 so that sols of fairly high volume

fraction can be prepared by dilution without greatly increasing the ionic strength above 0.01

mol dm"3

. Ludox TM is a useful silica sol because it has particle density of 2.2 g cm"3 and a

radius about 11 nm, it is stable for at least a year and gives a good signal-to-noise ratio over

a range of volume fractions. These silica sols are too small for conventional

microelectrophoresis but have been investigated by moving boundary electrophoresis (12,13)

and also a large body of mobility or zeta potential data exists for the silica/water interface

(14-24). Using ESA apparatus, one can determine the ESA signal, S, as a function of

volume fraction of silica and use y/G{, Ap and c to obtain the dynamic mobility /x(w) from

Eq. 2. Then it is possible to compute the static mobility from O'Brien's equations. In this

case the particle size is small enough that these mobilities are the same, /x(0) = ju(co).

73
In addition, one referee reported that Ludox TM has been investigated using laser light

scattering using a Coulter DELSA 440 apparatus (26). The result gave the electrophoretic

mobility at pH 9.9 of -3.59 (/xm/sec) (cm/V) at 34.2° scattering angle.

EXPERIMENTAL

Details of the experimental conditions and apparatus have been reported elsewhere (1,2)

so only a brief summary is given here. Electrophoretic mobilities were determined with a Rank Brothers Model Mk II microelectrophoresis apparatus at 25 °C. The microscope

graticule was calibrated with a ruled scale on a microscope slide. The flat cell constant was

61.7+0.6 cm"1

. Particles were timed in both directions by alternate reversal of the field of

7.28 V cm"1 and observed at the stationary fluid layer inside the front and back walls. The

ESA signals were measured at 25 °C with different Matec Applied Science MBS-8000

electrokinetic sonic amplitude systems. A low volume parallel-plate flow cell (Matec PPL80) was used with electrode spacing of 3/2 wavelengths. All measurements were done on

samples adjusted to 0.01 mol dm"3 KN03. The ESA apparatus field factor was first adjusted

to give a signal S equal to -3.67 mPa m/V for a 10.0% v/v suspension of Ludox TM silica

sol following the manufacturer's recommendation.

Poly(methyl methacrylate) latex (PMMA) (Soken MP-1201, Nachem, Braintree, MA)

was obtained as a dry powder and dispersed by ultrasonic agitation in water at 10% v/v.

The particle density was 1.191 g cm"3

.

Colloidal silica, Du Pont Ludox TM was obtained as a 51.2% w/w sol. The particle

density was taken as 2.2 g cm"3

.

Fine alumina (a-A£ 2 03) abrasive powder, AKP-30, was obtained from Sumitomo

Chemical Company, Ltd, Tokyo, was dispersed with a high speed Cowles mixer at 67.1%

w/w (33.8% v/v) for 24 hr at pH 4.0. The particle density was taken as 3.98 g cm"3 and the

size distribution was determined by centrifugal sedimentation.

Poly(styrene) latex (PS) with surface carboxyl groups was made by Dr. R. Sutton

(Clinical Diagnostics Research Laboratories, Eastman Kodak Company). The particle

74
density was 1.0541 g cm"3

. Particle size for PS, PMMA and silica sols were determined

from histograms obtained by electron-microscopy. For PS the average particle radius was

302+23 nm.

RESULTS AND DISCUSSION

Results of the size analysis for the PMMA latex and silica sol are shown as normalized

probability density distributions in Figs. 2 and 3. For PMMA the number average radius is 162+10 nm, while for the Ludox TM the average radius is 11.5+2.3 nm. This leads to *ca

values of 52.3 and 3.78, respectively, for sols adjusted to 0.01 mol dm"3 KN03

. The size

distribution for the AKP alumina sol is shown in Fig. 4 as a cumulative weight percent

oversize plot for repeat determinations. The line is log normal distribution fit where the

geometric mean radius by mass is 195 nm with a geometric standard deviation of 1.366.

The arithmetic mean and volume mean radii and geometric mean radius by count were 153,

169, and 146 nm, respectively (1). Using the volume mean radius (169 nm), the value of *a

in the electrolyte solution is 55; this is the value used to calculate G(a) for AKP.

ESA and microelectrophoresis measurements were made in 0.01 N KN03 and at various

constant pH values (6.8 for PS, 10 for PMMA, 4 for AKP, and 9 for Ludox TM). The

static mobilities, //(0), for PS, PMMA, and AKP are shown in Table I together with the zeta

potential, computed from the theoretical work of O'Brien and White (25).

The modulus of the ratio of dynamic mobility to static mobility |/x(oo)//x(0) | calculated

using Eq. 5 is shown in Fig. 1 at field frequency w/2x = 1 MHz as a function of particle

radius and for various values of particle density, e.g., 1.1, 2.5, 4, and 6 g cm"3

. It can be

seen that for radius a less than 0.2 /xm, | ^t(co)//x(0) | is greater than 0.9 for most kinds of

particles. For larger particles, it becomes more important to know the density and particle

size distribution accurately to get a good estimate of | ^(co)/^i(0) | for subsequent evaluation of

the instrument factor 7/G1-.

Values of the mobility ratio were calculated for all of the sols (PS, PMMA, AKP and

TM) at w/27T = 1 MHz and these appear in Table II. In all cases, owing to the combination

75
of particle size, density and field frequency the mobility ratio is greater than 0.9.

To determine the instrument calibration factors it remains to evaluate the slope, dS/d<£,

from study of the volume fraction dependence of the ESA signal. Data for PS and AKP are shown in Fig. 5 where each sol exhibits a linear regime at low volume fraction (</> < 0.05)

with slopes of -2.76 and 139.5 mPa m/V, respectively. The low density of PS results in the

low magnitude of the ESA signal and the slope and the negative sign reflects the charge of

the dissociating carboxylate surface groups whose intrinsic dissociation constant is of the

order 10"5

. Likewise, the high density alumina AKP (which has an isoelectric point at pH =

9) yields a high positive slope due to the excess adsorption of protons at pH 4. Values for

dS/d<£ and ylGi derived from Eq. 6 are also listed in Table II. For the other sols, PMMA

and TM, data for each sol was obtained on two different instruments and the ESA signal is

plotted as a function of particle volume fraction in Figs. 6 and 7, respectively. Again, these

sols demonstrate linear behavior at low volume fraction, giving slopes of -6.89±0.24 mPa

m/V and -41.4+0.5 mPa m/V, respectively.

Using the slopes for PS, PMMA, and AKP and Eq. 6 the calibration factor was

computed for each sol as 1.48 (1), 1.54 (2), and 1.23 (1), respectively, and listed in Table

II. In an earlier report (1) we obtained a calibration factor of 1.4+0.15 from an average of

values for PS, PMMA, and AKP on a single instrument. However, since data presented

here for PMMA was available from two different instruments and it had a narrower size

distribution than the AKP or PS we will use the value 1.54±0.18 from the PMMA data for

7/Gf

.

This factor and the other sol properties now allow the estimation of the dynamic mobility

of Ludox TM silica in 0.01 mol/dm3 KN03 . Using Ap = 1.20, dS/d0 = -41.4 and -ylGf =

1.54 gives a value -3.59 + 0.41 jxm cm s"

! V"1 for Since the silica particle radius is

only 11.5 nm, the ^(w)/^t(0) ratio is unity and the implied static mobility is also -3.59 /xm cm

s"

1 V"1 at pH 9. The result is consistent with a number of earlier studies using the moving

boundary method on various Ludox silica samples (12,13). The static mobility data may also

be used to estimate the zeta potential of the sol particles using the theory of O'Brien and

White (25,26). We used their computer program to evaluate the relationship between zeta

potential and mobility for *a values of 52.3 and 3.8, which apply to PMMA and Ludox TM

76
in 0.01 mol dm"3 univalent electrolyte. The results of calculations are shown in Fig. 8. From

these plots, Tpmma is about -52 mV while and £TM is about -78 mV with a range from -64 to

-98 mV for the 90% confidence level. Because the ka value for our Ludox TM experiments

is near where a maximum exists in the O'Brien and White theory (25), larger uncertainty

exists in the upper value of this range. This result, fXM = -78 mV, is consistent with other

electrokinetic data for silica/electrolyte systems from a wide range of methods.

In conclusion, we have described a strategy and suitable materials that allow calibration

of ESA apparatus by comparison with results obtained using the selected colloids with data

obtained using standard microelectrophoresis methods. After calibration, dynamic mobilities

can be converted to the steady state mobility and zeta potential, which may have useful

application in the stability of dispersions and coating fluids.

ACKNOWLEDGMENTS

The collaboration and co-authorship of Hans Coll, Richard Klingbiel, Peter Scales, and

John Texter is gratefully acknowledged. Discussions and correspondence with David

Cannon, Robert Hunter, and Richard O'Brien over an extended time were very stimulating.

I would also like to acknowledge private communications and pre-publication data or comments from Alex Babchin. In addition, one referee kindly supplied data on the mobility

of Ludox TM in 0.01 M KC1 at pH 9.9, which was obtained by Karin Evans formerly of

Langley-Ford Instruments (Division of Coulter, Inc.).

77
REFERENCES

(1) James, R. O.; Texter, J.; Scales, P. J. Langmuir 1991, 7, 1993.

(2) Klingbiel, R. T.; Coll, H.; James, R. O.; Texter, J. Colloids and Surfaces 1992,

68, 103.

(3) Oja, T.; Peterson, G. L.; Cannon, D. W. US Patent #4,497,208 (1985).

(4) O'Brien, R. W. J. Fluid Mech. 1988, 190, 71.

(5) O'Brien, R. W. J. Fluid Mech. 1990, 2T2, 81.

(6) Loewenberg, M.; O'Brien, R. W. J. Colloid Interface Sci. 1992, 150, 158.

(7) O'Brien, R. W. Chemistry in Australia 1992, 59, 158.

(8) O'Brien, R. W. Australia Patent #597,442 1988; US Patent 5,059,909 (1991).

(9) Babchin, A. J.; Chow, R. S.; Sawatzky, R. P. Adv. Colloid Interface Sci. 1989,

30, 111.

(10) Sawatzky, R. P.; Babchin A. J. J. Fluid Mech. 1993, 246, 321.

(11) Scales, P. J.; Jones, E. Langmuir 1992, 8, 385.

(12) Long, R. P.; Ross, S. J. Colloid Sci. 1965, 20, 438.

(13) Allen, L. H.; Matejevic, E. J. Colloid Interface Sci. 1969, 3_L 237; 1971, 35, 560.

(14) Depasse, J.; Watillon, A. J. Colloid Interface Sci. 1970, 33, 430.

(15) Goodwin, J. W.; Harbron, R. S.; Reynolds, P. A. Colloid Polym. Sci. 1990, 268,

766.

(16) Li, H. C; de Bruyn, P. L. Surf. Sci. 1966, 5, 203.

(17) Wiese, G. R.; James, R. O.; Healy, T. W. Discuss. Faraday Soc. 1971, 52, 302.

(18) Smit, W.; Holten, C. L. M.; Stein, H. N.; de Groeij J. J. N.; Theelen, H. M. J.

J. Colloid Interface Sci. 1978, 63, 120.

(19) Prasad, N.; Pooley, F. D. J. Appl. Chem. Biotechnol. 1973, 23, 675.

(20) Bousse, L.; Mostarshed, S.; van der Shoot, b.; de Rooij, N. F.; Gimmel, P;

Gopel, W. J. Colloid Interface Sci. 1991, 147, 22.

(21) Scales, P. J.; Grieser, F.; Healy, T. W.; White, L. R.; Chan, D. Y. C. Langmuir

1992, 8, 965.

78
(22) Churaev, N. V.; Nikologorskaja, E. A. Colloids Surf. 1991, 59, 71.

(23) Kosmulski, M.; Matijevic, E. Langmuir 1992, 8, 1060.

(24) Kosmulski. M.; Matijevic, E. Colloid Polym. Sci. 1992, 270, 1046.

(25) O'Brien R. W.; White, L. R. J. Chem. Soc. Faraday Trans. 2 1978, 74, 1607.

(26) Unpublished data: Karin Evans, Langley-Ford Instruments, Amherst, MA.

79
TABLE

Properties of Test Sols

Sol P

g cm"3

Radius

(a, nm) K2l

dS/d0

mPa m/V

m

0*m/S) (cm/V) f mV

PS 1.054 302 99 -2.76 -5.13 -70

PMMA 1.191 162 53 -6.89°

-7.27"

-3.80 -52

AKP 3.98 169' 55 139.5b +4.06 +56

TM 2.20 11.5 3.8 -41.

a

-43.

b

"Reference 2. "^Reference 1.

c Volume mean radius.

80
TABLE II Comparison of Calibration Constants for Test Sols

m(0)

Sol \u{uj)im\ K/Gf //m m/s V f mV

PS 0.95O 1.49" -5.13 -70

PMMA 0.987' 1.54°

(1.48b)

-3.80 -52

AKP 0.957* 1.23b +4.06 +56

TM 1.00^ 1.54"

(1.40b)

(-3.59) (-78)

"Reference 2. "Reference 1.

cComputed from Eq. 5.

81
FIGURE CAPTIONS

Figure 1 . Dependence of the modulus of the dynamic to static mobility ratio on

particle size (a) and particle density p at frequency oj/Itt = 1 MHz

from Eq. 5 for p = 1.1 (), 2.5 (•), 4 (a), and 6 () g cm"3

. The viscosity and density of the continuous phase were taken as 0.01

poise and 1 g cm"3

, respectively.

Figure 2. Number density (p(a)) normalized particle size distribution as a

function of particle radius a for the PMMA latex, MP1201.

Figure 3. Number density (p(a)) normalized particle size distribution as a

function of radius a for the colloidal silica sol Ludox TM.

Figure 4. Cumulative percent oversize distribution for alumina AKP by

centrifugal sedimentation analysis. The line is for a log normal

distribution where the median radius by mass is 195 nm and the

geometric standard deviation is 1.37.

Figure 5. ESA signal, S, as a function of volume fraction ((f)) for PS latex at

pH 6.8 (ordinate values magnified 10X) and for alumina AKP at pH

4. The alumina particles have a positive charge while the PS latex is

negative. The solution was 0.01 mol/dm3 KN03 at 25°C.

82
Figure 6. ESA signal, S, as a function of volume fraction (f) for PMMA latex

MP- 1201. The data points were obtained on two different MBS8000 instruments. The continuous line is a least-squares fit for 4> <

0.03 and has a slope of -6.89+0.24 mPa m/V.

Figure 7. ESA signal, S, as a function of volume fraction (</>) for the colloidal

silica Ludox TM. The data points were obtained using two different

MBS-8000 systems. The continuous line is a least-squares fit to data

for 0 < 0.07 and has a slope of -41.4+0.5 mPa m/V.

Figure 8. Relationship between zeta potential and electrophoretic mobility for

PMMA latex, *a = 53, and for Ludox TM, /ca = 3.8 calculated

using the O'Brien and White method (25) and computer program.

83
84
^—,_.

r„1 _ ^ - , —, -t

—| i—i—i—i—|—i—i r—i f
1 00 ¥

90

80

70

60

50

40

30

20

10

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;

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PARTICLE DIAMETER tyim)

Figure 3

86
r 1 i i ""i i

I t i —r —i—

j—I—I—i—i—

0.20

0.15

Q_ 0.10

0.05

0.00

j i i i i Ai i i j t » « » < t 0 100 200

a (nm)

300

Figure 4

87
88
Figure 6

89
90
91
ELHCTROKINETiC MEASUREMENTS OF CONCENTRATED SUSPENSIONS IN

APOLAR LIQUIDS

R. E. Kornbrekke

The Lubrizol Corporation

Wickliffe, OH

I. D. Morrison

Xerox Corporation

Webster, NY

and

T. Oja

Matec Applied Sciences

Hopkinton, MA

ABSTRACT

We have used electroacoustic (Electrokinetic Sonic Amplitude, ESA) measurements to study the charging of carbon black suspended in alkanes containing dispersants. Systems

with various types of dispersants and 0.5 to 15 volume % carbon black were used. The state of adsorption was determined from FTIR measurements, and the conductivity of the mixtures

was also measured. These measurements have given insight into some of the mechanisms of

particle charging in low dielectric liquids. The particles charge only after a sufficient amount

of dispersant has adsorbed onto the solid. Charging is due to a combination of selective

interaction (an acid-base type) coupled with ion pair dissociation which is probably facilitated

by associative interactions. The degree of charging is affected by the particle concentration;

the charging per particle decreases as the concentration increases. This is probably related to double layer overlap. We have found that particle charging in these systems correlates, to a

large extent, with the suspension stability.

(Since the complete manuscript could not be released for publication, only the presentation

slides are included for this paper).

92
Llo

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LU

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53

Q.

CO

a co

CO o

93
Non-Aqueous Aqueous

D « 2- 10 D = 80

Ao < 1 pS/cm

to

o

* 1000 pS/cm

Ao ~ i-i fio/cm

to

Ao « 10,000 jaS/cm

Particles not

Initially

Charged

Particles

Charge

Spontaneously

94
95
L.o

L_ +*

€0

L.mmo0 A

iiI

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+-> o

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CO CO .Q

Q.

96
EXPERIMENTAL PROCEDURE

SYSTEM:

Sterling NS Carbon Black

Polyisobutylene Succinimide (PIBSI)

(Mn=1695, TBN=46)

Dodecane or Isopar G, L

Mix, equilibrate,

Measure:

* Conductivity - 18Hz.

* ESA - Zeta Potential

* Liquid (supernatant) Concentration

FTIR Carbonyl (1705/cm.)

* Dispersion Stability

97
>

98
CO

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101
102
[IMOllOQ] (AW) e;9Z

103
o

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104
SUSPENSION STABILITY

System Sediment Zeta(mV)

5 V% C

%PIBSI

0.01 not resuspend -1.1

0.35 resuspends -17.9

0.69 resuspends easily - 19.2

9 V% C

%PIBSI

0.11 not resuspend -0.3

0.60 resuspends easily -11.4

1.31 resuspends easily -12.0

12.5% C

%PIBSI

0.14 resuspends easily -0.1

0.95 resuspends easily -7.9

1.89 resuspends easily -8.9

105
ELECTROSTATIC STABILIZATION

I) Huckel regime:

radius(um) zeta(mV)

0.05 -100

0.1 -71

0.3 -41

1.0 -22

5.0 -10

^Partial Electrostatic Stabilization

106
Electrophoretic Mobility = u = — £ - i@j

Non-Aqueous Aqueous

2 eC

Huckel Regime

u = —

Smoluchowski Regime

Ka « 1

1»

a

K

Ka » 1

i«i

a

K

1_ = (

V

e kT

O e2 V n,

2

i2

1

K

= Thickness of the Diffuse Layer

Electrical Potential Decays to 2% of

the Original Value at

K

107
At 0.5 V% for Carbon Black

a = 100 nm

Ri =

1.81x100 nm

i

= 1060 nm

But 1

K

(0.005 V3

291 nm

>-4 1060 nm

- )

100 nm

' u/ 100—nm

K

3- = 900 nm

K

Electrical Double Layer Overlap occurs even

at Relatively Low Particle Volume Fractions.

108
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CODozoo ©©

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110
ACQUSTOPHORETIC CHARACTERIZATION OF COLLOIDAL DIAMOND

PARTICLES

J. L. Valdes

AT&T Bell Laboratories

Murray Hill, New Jersey 07974

ABSTRACT

The interfacial chemistry of sub-micron diamond particles is of great practical importance

in determining the stability and electrophoretic behavior of these colloidal dispersions. The

electrokinetic sonic amplitude (ESA) technique is used to probe concentrated dispersions of

natural and synthetic diamond particles in both aqueous and non-aqueous solutions.

Acoustophoretic characterization of colloidal diamond dispersions have been studied over a

broad pH range (2 < pH < 12). In aqueous dispersions, the zeta potential of both synthetic

and natural diamond particles are dependent on the pH of the solution. Natural diamond was

found to have an appreciably more negative zeta potential than synthetic dispersions of

diamond under the same conditions. These results are consistent with the presence of very

reactive, unsaturated covalent bonds that lead to the formation of oxygen-containing ionizable

groups on the surface of natural diamond. The zeta potential of colloidal diamond

dispersions was found to be considerably diminished in non-aqueous solvents (e.g. £=2-5

mV in methanol). Acoustophoretic characterization of colloidal diamond dispersions is essential to the development of an electrophoretic process for the selected-area nucleation and

patterning of diamond thin films.

INTRODUCTION

High thermal conductivity, excellent dielectric and optical properties, and superior

mechanical strength make diamond thin films promising for a variety of important

technological applications. In a recent paper, we demonstrated a new electrophoretic process

for selected-area nucleation and patterning of silicon substrates for chemical vapor deposition

(CVD) growth of polycrystalline diamond films (1-3). In this process, seeding of the

substrate to be nucleated for diamond film growth is accomplished by electrophoretic

deposition from a dispersion of colloidal diamond particles. Once deposited on a suitable

substrate, the diamond crystallites serve as homoepitaxial centers for CVD growth of a

contiguous film.

Ill
The design of an optimized electrophoretic process for the deposition of colloidal

diamond particles requires a consideration of numerous process parameters. One of the most

important variables in this process is the electrophoretic mobility of diamond dispersions.

The mobility controls not only the electromigrational transport of particles in the bulk and the

degree of stability of the dispersion itself, but also has been found to impact on the density

and selectivity of particle nucleation on the substrate.

Despite all of the unique physical and chemical properties ordinarily associated with

diamond as a material, colloidal particles of diamond have a remarkably rich surface

chemistry. The surface chemistry on diamond is believed to derive from the presence of very

reactive, unsaturated covalent bonds. In the presence of water or oxygen, these covalent

"dangling" bonds react to form a multitude of carbon-oxygen surface groups (4). Infrared

spectroscopy of diamond powders has revealed the presence of various oxygen-containing

functional groups on the surface, including: carbonyl, carboxylic, lactone, tertiary alcohol,

cross-linked oxygen, and epoxide. When diamond particles are immersed in a polar liquid,

the surface acquires a charge due to the ionization of these groups, and also from selective

ion adsorption, dissolution or ion exchange. The electrophoretic mobility is determined

largely by the degree of particle charging and will in general depend on the type of particles,

the dispersion medium, ions present in the solution, and on the pretreatment of the diamond

powders.

Electrokinetic measurements provide one of the most powerful tools for probing the

interfacial chemistry and stability of colloidal diamond dispersions. Electrokinetic properties

such as the particle mobility, zeta potential, surface charge density, and the iso-electric point

are essential in developing an optimized electrophoretic nucleation process. In this work, we

conducted an investigation using the electrokinetic sonic amplitude (ESA) method to

characterize and explore the interfacial chemistry of colloidal diamond particles.

In the ESA method, an alternating electric field (E) is applied to a dispersion containing a known concentration (volume fraction, $) of colloidal diamond particles (5). In response to

their surface electrical charge, the particles will move in an oscillatory manner causing the

transfer of momentum to the fluid phase with concomitant generation of an acoustical

pressure sound wave. The magnitude of the electrokinetic sonic amplitude defined as the

112
magnitude of the sound wave (P) per unit electric field is shown to be directly proportional

to the dynamic electrophoretic mobility n'd by the following relationship (6,7),

ESA(v) =— =cAp$u* (a) (1)

E d

where (go is the angular frequency, c is the velocity of sound in the suspension, and Ap is the

density difference between the particles and the medium. For most systems of practical

interest, the double layer is usually thin compared to the characteristic size of the colloidal

particle. For these systems, we will adopt the zero limit formula for the DC electrophoretic

mobility as given by the Smoluchowski equation and represent the dynamic mobility as,

H* = £C G ((D^) (2)

d n v

where f is the zeta potential, y\ is the Newtonian shear viscosity, and v is the kinematic

viscosity of the bulk suspension. For sub-micrometer colloidal particles of diamond having

an intrinsic density of 3.51 g/cc in an aqueous dispersion and at operating frequencies of ~

1 MHz, the inertial correction term is relatively unimportant, G ~ unity.

EXPERIMENTAL

Electroacoustic experiments were conducted with the MATEC-ESA MBS-8000 electronics

system. Electrokinetic measurements were performed in a continuously stirred Teflon®

vessel containing approximately 200 cm3 suspension. Concurrent with the electroacoustic

response the pH, temperature, and electrical conductivity of the solution were measured.

Characterization of colloidal diamond dispersions was studied by electroacoustic

measurements of the zeta potential under controlled temperature (25 °C) and over a broad pH

range (2 < pH < 12).

113
Colloidal dispersions were made using either synthetic or natural diamond particles

(Industrial Diamond Corp. (IDC), PA or Diamond Abrasives Corp. (DAC), NY) in high

purity, organic-free, 18 Mft-cm resistivity water. The volume fraction of particles in all of

the experiments reported here was maintained constant at $ = 0.5%. Colloidal dispersions were prepared just prior to electroacoustic measurements by combining the requisite

quantities of diamond powders in high purity water and ultrasonicating for approximately

fifteen minutes. The natural pH of the resultant solution was approximately 5.6. The pH

was adjusted from this initial value to around 12.2 by the addition of small aliquots from a 1M solution of electronic grade potassium hydroxide. Zeta potential-pH titrations were

carried out using a 1M solution of hydrochloric acid. No other additional salts or indifferent

electrolyte was used in these experiments. Electroacoustic experiments were also conducted

with colloidal dispersions of diamond in low sodium MOS-grade methanol.

RESULTS AND DISCUSSION

In Figure 1 are shown experimental data derived from the electroacoustic

characterization of colloidal diamond particles. The contribution of the acoustophoretic

signal from the background electrolyte in these experiments is shown in Figure 2 in terms of

the zeta potential. Over the range of pH values studied there is essentially no contribution

from the electrolyte medium. In aqueous media, the zeta potential of both synthetic and

natural diamond particles were found to be dependent on the pH of the solution. Natural

diamond particles were found to exhibit considerably more negative zeta potentials than their

synthetic counterparts. For both natural and synthetic diamond, the zeta potential is

relatively insensitive to pH in the range 4.0 to 12.0. For pH values below pH=4, the zeta

potential of natural diamond decreases rapidly with pH as it approaches the iso-electric point.

The iso-electric point for natural diamond (Type IDC#3, i.e., Batch #3 from IDC) was found

to be at a pH — 2 whereas an iso-electric point at pH ~ 3.4 was measured for the synthetic

diamond.

114
The hydrophilic nature of colloidal diamond particles is a property attributed in large

part to the population density of oxygen-containing functional groups on the surface. In

these systems, the principal particle charging mechanism results from the ionization of

acidic-hydrogen associated with certain functional groups on the surface of diamond. The

zeta potential measurements shown in Figure 1 suggest that synthetic diamond is considerably

more hydrophobic in nature than natural diamond. Produced under high pressure and

temperature conditions, synthetic diamond is likely to contain a dearth of oxygen surface

groups which are responsible for the apparent higher degree of particle charging observed for

natural diamond particles. Moreover, a reduction in the number of "dangling" bonds and the

possible formation of a graphite-type layer on the surface may also explain the

hydrophobicity of synthetic diamond.

The development of a practical electrophoretic process for the nucleation and patterning

of silicon substrates requires a dispersion with consistent electrokinetic properties. We

examined several samples of natural colloidal diamond to ascertain the degree of variability

in these dispersions. In Figure 3 are shown experimental data for the acoustophoretic

characterization of various natural diamond dispersions. The curves labeled IDC#1, IDC#2,

and IDC#3 correspond to three different batches of 0 - 1/2 fim natural diamond particles

obtained from IDC. Diamond powder obtained from DAC was specified to be of similar

grade (with respect to particle size and its distribution) as the natural diamond obtained from

IDC. Despite the apparent similarity in grade specifications, IDC and DAC powders exhibit

a clearly visible difference in hue. Moreover, the zeta potential data shown in Figure 3 also

suggests that a significant variation in the degree of particle charging exists among the

various batches of the same grade natural diamond. The degree of particle charging in batch

IDC#2 was found to be considerably less (A£* —20 mV at pH = 8) than that measured for

either IDC#1 and IDC#3. However, all of the diamond powders analyzed from IDC showed

similar characteristics in the shape of the overall zeta potential-pH curves. In contrast,

dispersions containing DAC particles exhibit a pronounced kink in the zeta potential curve

around a pH of 6-7 which is not observed with any of the IDC material. This distinct

feature suggests that the charging of DAC colloidal particles may derive from a wholly

115
different surface functional group; one that has an appreciably less acidic hydrogen associated

with it. The type of surface group on diamond may have an important role in determining

the level of adhesion of colloidal particles on the nucleated substrate.

The electrophoretic nucleation process relies on the development of sufficient charge on

the particle to allow for effective bulk transport of the colloids. To this end, we investigated

the feasibility of improving the electrophoretic mobility of natural diamond particles by

increasing the number of surface active sites. Strong oxidizing agents such as perchloric acid

or nitric acid are effective in promoting the oxidation of surfaces for many materials. In

Figure 4 are plotted experimental data on the zeta potential showing the effect of oxidizing

the surface of natural diamond (IDC#2) using concentrated nitric acid. We found that while

the shape of the overall zeta potential-pH curve remains approximately the same, a

substantial increase (A£ ~ -25 mV) in the zeta potential is afforded by treating the surface of

natural diamond with a strong oxidizing agent. Only a minor shift of the iso-electric point

(A i.e. p. — 0.4), to lower pH values, was measured for the treated surface. These results

suggest that oxidizing the surface of natural diamond increases the density of surface sites

rather than alter the specific chemical moieties on the surface.

Chemical modification of the surface of natural diamond was examined using a hydrogen

microwave plasma discharge. Natural diamond powder (IDC#2) was treated in a hydrogen

microwave plasma reactor at the following conditions: temperature, 800°C; pressure, 10

torr; microwave power, 600 watts; for 5 hours. The zeta potential-pH curve of the treated

sample is compared to the untreated IDC#2 in Figure 5. Plasma treatment of the diamond

material significantly alters both the magnitude (Af ~ 22 mV) and shape of the overall zeta

potential-pH curve. The iso-electric point of the treated diamond (pH=7.0) is also shifted by

about 5 decades from the pH value corresponding to the isoelectric point of the untreated

diamond powder (pH=2.0). These results strongly suggest that hydrogen plasma activation

of the diamond surface leads to considerable modification in the surface chemistry of

diamond. The electroacoustic characterization shows that plasma-treated diamond affords a

material with functional groups that have less acidic type of protons. Further investigation of

the surface by spectroscopic techniques is necessary to fully investigate the various functional

derivatizations effected by the plasma treatment.

116
In non-aqueous systems, the zeta potential of colloidal particles is generally not as

pronounced as in protic solvents unless chemical agents and/or surfactants are added

deliberatively in order to promote charging of particles. In Figure 6 are shown

electroacoustic data on the zeta potential of various natural diamond particles in methanol.

As expected, the zeta potential of natural diamond in methanol was found to be considerably

diminished (e.g. ~ -2 to -5 mV).

Electrophoretic deposition was conducted from dispersions containing approximately

0.2 g/1 of IDC natural diamond in high purity water and at the natural pH of the solution

(pH=5.6). Typically, the deposition substrate consisted of a 4-inch p-type silicon wafer (10

ohm-cm conductivity) biased to positive potentials. A gold-plated metal disk of equivalent

diameter served as the negative counter-electrode. Electric fields of 5-40 V/cm and

deposition times of 1-10 minutes were employed to provide effective nucleation of the

substrate. A scanning electron micrograph (SEM) of diamond particles electrophoretically

deposited onto a silicon substrate is shown in Figure 7. Diamond particles were found to be

well-adherent to the silicon surface. The nucleation density afforded by this electrophoretic

process (10

8 - 109 nuclei/cm2) is many orders of magnitude higher than that can be achieved

by any other substrate nucleation method.

Using conventional photolithographic techniques, the electrophoretic process can be

applied to selected-area nucleation and patterning for the growth of CVD diamond films. As

shown in Figure 8, a silicon wafer is first patterned with oxide (Si02). When immersed in

an aqueous solution at the appropriate pH, regions masked by the oxide develop an inherent

negative surface charge that effectively precludes the deposition of diamond particles with a

negative zeta potential. Deposition of diamond on silicon biased at a positive potential

remains largely uninterrupted. Colloidal diamond particles with high zeta potentials are

desirable for maximizing the coulombic repulsion forces in order to effect the selective

deposition of diamond.

117
CONCLUSIONS

The electrokinetic properties of sub-micron diamond particles are of great practical

importance in determining the stability and chemical behavior of colloidal dispersions. We

have used the electrokinetic sonic amplitude (ESA) technique to investigate the interfacial

chemistry and particle charging mechanisms of colloidal diamond dispersions in aqueous and

non-aqueous solutions. Such information is of paramount importance to the development of

an optimal process for the nucleation and patterning of diamond thin films.

118
REFERENCES

[1] Valdes, J. L., Mitchell, J. W., Mucha, J. A., Seibles, L., and Huggins, H., J.

Electrochem. Soc, 138, 635 (1991).

[2] Valdes, J. L., Mitchell, J. W., Mucha, J. A., Seibles, L., and Huggins, H.,

Applications of Diamond Films and Related Materials, 73, 423 (1991).

[3] Valdes, J. L., Mitchell, I. W., US Patent # 5,128,006, "Deposition of Diamond

Films on Semiconductor Substrates."

[4] Shergold, H. L., and Hartley, C. J., Int. J. Mineral. Processing, 9, 219 (1982).

[5] O'Brien, R. W., Midmore, B. R., Lambs, A., Hunter, R. J., Faraday Discuss. Chem.

Soc, 90, 301-312 (1990).

[6] O'Brien, R. W., J. Fluid Meek, 190, 71 (1988).

[7] O'Brien, R. W., J. Fluid Mech., 212, 81 (1990).

119
FIGURE CAPTIONS

Figure 1 . Acoustophoretic characterization of natural and synthetic colloidal diamond.

Experiments were conducted with 0 - l/2ium-grade diamond powder at a volume fraction of $ = 0.5% and at room temperature. Isoelectric point for

natural diamond was found to be at pH ~ 2 and for synthetic diamond at pH ~ 3.4.

Figure 2. Acoustophoretic signal from the electrolyte medium.

Figure 3. Dependence of zeta potential on pH for several samples of sub-micron natural

diamond powders.

Figure 4. The effect of surface oxidation by treatment with concentrated nitric acid on the

zeta potential of natural diamond.

Figure 5. Chemical modification of the surface of natural diamond powder using a

hydrogen microwave plasma.

Figure 6. The zeta potential of various colloidal dispersions of natural diamond in

methanol.

Figure 7. Scanning electron micrograph (SEM) of natural diamond particles deposited

electrophoretically onto a silicon < 100 > wafer. Deposition was effected from

a dispersion containing 0.2 g/L of natural diamond (IDC#2) at the natural pH

of the solution (pH=5.6).

Figure 8. Electrophoretic nucleation process for selected-area nucleation and patterning of

diamond thin films.

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128
SESSION III

ADVANCED CERAMICS APPLICATION

Session Chairman

R. Mountain

National Institute of Standards and

Technology

ESA MEASUREMENTS FOR SILICA AND ALUMINA: BACKGROUND

ELECTROLYTE CORRECTIONS

F. N. Desai2

, H. R. Hammad1

, and K. F. Hayes*' 1

'Department of Civil and Environmental Engineering

The University of Michigan

Ann Arbor, Michigan 48109-2125

2The Procter & Gamble Company

Winton Hill Technical Center

6300 Center Hill Road

Cincinnati, OH 45224

*To whom all correspondence should be addressed

ABSTRACT

The ESA technique has been used to determine the electrophoretic mobility and zeta

potential of colloidal systems. For a solid suspension in an electrolyte solution, the measured

ESA signal is a vector sum of two signals, one for the solid and the other for the background

electrolyte. Under certain conditions, for example when the ionic strength is high, the

background contribution can become a significant fraction of the measured ESA signal and

background subtraction has to be done. In this paper, the background correction technique

has been applied to silica and alumina over a range of pH and ionic strengths.

INTRODUCTION

The Electrokinetic Sonic Amplitude (ESA) technique is an electroacoustic method that

has recently been used to determine the electrophoretic mobility and zeta potential of

colloidal systems1 "7

. The ESA technique has a wider range of applicability than conventional

microelectrophoresis; for example, ESA measurements can be made on transparent or opaque

particles that could be as large as 150 /xm with solid concentration as high as 50% by

volume. However, for converting ESA to zeta potential, the upper limit of particle

concentration is 5% by volume3 ' 4

.

129
One limitation of the ESA technique is that a significant error is introduced in the

measured ESA value when the ionic strength is high. In the presence of a background

electrolyte, the measured ESA signal is a vector sum of two signals: one for the solid and

the other for the background electrolyte. The background electrolyte signal can become

important when i) the ionic strength is high, ii) the pH is close to the isoelectric point (iep),

iii) the density difference between the solid and the liquid is small, iv) the solid concentration

is relatively low, or v) the solid has a low charge density. For example, background

corrections will be important for polymer solids for which the density difference between the

solid and the liquid is small and hence the measured ESA is relatively low. Under these

circumstances, the background electrolyte signal has to be subtracted from the measured ESA

signal to give the true ESA.

Some researchers 3 ' 4 have avoided the problem of background correction by using a high

enough solid concentration such that the background contribution was less than 5% of the

measured ESA signal. This is not possible in all cases since the solid concentration is

limited to 5% by volume if the ESA data has to be converted to zeta potential. Furthermore,

if the solid is expensive or difficult to purify, it is advantageous to use as little solid as

possible. Based on the background correction procedure for the colloid vibration potential

(CVP) outlined by Mariow et al.

2

, Desai et al.

8 have demonstrated how the background

correction technique can be applied to ESA measurements on quartz. The objective of this

paper is to apply this technique to silica and alumina suspensions over a wide range of pH

and ionic strengths.

BACKGROUND ELECTROLYTE CORRECTIONS

The magnitude and the phase angle of the ESA signal can be measured with the Matec

ESA-8000 (Matec Applied Sciences, Hopkinton, MA). The phase angle is measured with

respect to a phase reference. The phase reference material is generally taken as the solid in

water in the absence of any background electrolyte, assuming that the polarity of this solid is known a priori.

130
Figure 1 gives the magnitude and the phase angles for the various ESA signals. The

measured ESA(ESA m^) is a vector sum of the true ESA for the solid (ESA^) and the ESA

for the background electrolyte (£&4 bkgd).

ESA meM = ESA^ + ESA bk„ d (1)

Equation (1) can be resolved into its two components:

ESA^ cos0 = ESA^ cos/3 + ESAbkgdcos</> (2)

ESAmeas sin# = ESAtrue sin/3 + ESAbkgd sin<£ (3)

Here $, <t>, and (3 are the phase angles of the measured, background, and true ESA

signals respectively. It should be noted that the absolute ESA values are used in equations

(2) and (3) above. The actual sign of ESA^ can be determined from the phase angle, jff. If

jS<90°, the sign of ESA^,. is the same as that of the phase reference; whereas, if /3>90°,

the sign of ESA^ is opposite to that of the phase reference. If ESA,ueas , ESAbkgd , 6, and <f>

are measured experimentally, equations (2) and (3) can be solved simultaneously to give

ESA^ and (3.

EXPERIMENTAL

Materials

The silica used in this study was purchased from Geltech Corp. (Alachua, FL). The

particles were spherical and monodispersed with particle density of 2 g/cm3

. The median

particle diameter was about 1.6 /^m as measured with the Capa-500 Centrifugal Particle Size

Distribution Analyzer (Horiba Instruments, Irvine, CA). This value is close to the diameter

of 1.5 fim specified by Geltech Corp. The Geltech silica had a purity of 99.9% and was

used without any purification.

131
The alpha alumina was purchased from Buehler (Lake Bluff, IL). The median particle

diameter, measured with the Capa-500, was about 4 [xm, which is larger than the 1 /xm size

specified by the manufacturer. This was the case even when the sample was sonicated

anywhere between 2 and 6 hours. The particle density was 3.97 g/cm3

. This alumina was

used without any purification.

Method

Potentiometric titrations were performed on the ESA-8000 for silica and alumina at

three ionic strengths. The concentration of solids was 5% by volume. The solids were

hydrated for 24 hours, followed by sonication for about 2 hours. The ESA probe was

calibrated with 10% by volume Ludox TM (Du Pont, Wilmington, DE). The ESA value for

the Ludox should be -5.32 mPa m/V at 25 °C or the zeta potential should be -38 mV as

specified by Matec Applied Sciences. In all experiments, the temperature was kept constant

at 25.0±0.2°C and a nitrogen blanket was maintained above the solution in the titration cell

to prevent carbon dioxide contamination.

The following experimental procedure was followed for each solid at a particular ionic

strength. After calibrating the ESA probe with Ludox, the solid in pure water was poured

into the titration cell and used as the phase reference material. Next, the ionic strength was

adjusted with the appropriate amount of NaCl. Potentiometric titrations were performed by

measuring the magnitude and phase angle of the ESA signal, temperature, electrical

conductivity, and amount of acid or base added as a function of pH. For silica, the pH was

raised to 8 and the potentiometric titration conducted from pH 8 to 2. For alumina, the pH

was raised to 10 and the titration was carried out from pH 10 to 6.

Blank titrations were also performed in the absence of the solid to give the background

electrolyte signal as a function of pH and ionic strength. These titrations were performed

using the Geltech and alpha alumina phase references. Keeping the phase reference the

same, in effect, keeps the frequency of the ac signal constant. The precision of the ESA

measurements was +0.01 mPa m/V and that of the phase angle measurements was ±5°.

132
RESULTS AND DISCUSSION

Geltech Silica

Even though the ESA signal for the background electrolyte was measured as a function

of pH and ionic strength, only the mean values of ESAbkgd and 4> were used for each ionic

strength since the ESA signal for the background electrolyte did not depend significantly on pH in the pH range 2 to 10. Table 1 gives the background signal (ESA5kgd and 0) as a

function of ionic strength with Geltech silica as the phase reference.

Figure 2 shows the measured ESA and the phase angle for the Geltech silica as a

function of pH and ionic strength. For 0.001 and 0.01 M ionic strengths, the measured

phase angle, 6, is close to zero indicating that the ESA does not change sign in the pH range

of 8 to 2. However, for 0.1M ionic strength, the uncorrected ESA becomes positive at a pH

of about 4.7. This change in sign of the ESA signal is also reflected in the measured phase

angle, which goes from about 0 to 180° at a pH of about 4.7.

The data for 0.1M ionic strength suggests that the iep is at a pH of 4.7. However, the

background correction shows that for 0.1M ionic strength, the background corrected ESA

value is negative at least down to pH 2 (Figure 3) as expected9 ' 10

. The background

correction has little effect at low ionic strength (at least less than 0.01M), but a substantial

effect at 0.1M ionic strength. As the ionic strength increases, two factors tend to increase

the relative importance of the background signal: ESAbkgJ increases and ESA^ decreases.

Alpha alumina

Table 2 gives the background electrolyte signal as a function of ionic strength with

alpha alumina as the phase reference. Comparison of Tables 1 and 2 shows that ESA^ is

approximately independent of the phase reference material within experimental error,

whereas 0 is a strong function of the phase reference. This underscores the need for

measuring the background electrolyte signal whenever a new phase reference material is

used.

Figure 4 shows the measured ESA and phase angle as a function of pH and ionic

strength. The common intersection point (cip) occurs at an ESA value greater than zero with

an iep of about 8.7 suggesting that there may be contamination in the sample. However, the

133
electrolyte background correction shifts the cip to ESA value of zero (Figure 5) indicating

that there is no impurity, with the iep at pH 9, the same as the value that has been previously

reported for alpha alumina11

. As for silica, the background electrolyte has a significant effect

at 0.1M ionic strength and a negligible effect at the lower ionic strengths.

Several factors, including type and concentration of solid, electrolyte concentration, and

pH, determine the ionic strength at which background corrections become important. As

shown by Hackley and Malghan 12 in a companion paper, the background electrolyte signal

also depends on the ions that make up the electrolyte; for example, the ESA signal for

Ba(N03)2 is about twice that for NaCl. The relative importance of each of the factors must

be considered to insure that interpretation of the ESA signal is made correctly.

CONCLUSIONS

The background electrolyte correction method extends the range of experimental

conditions (like pH, ionic strength, and solid concentration) over which ESA data can be

converted to electrophoretic mobility and zeta potential. The background electrolyte signal is

only a weak function of pH in the pH range 2 to 10, but a strong function of ionic strength.

The magnitude of the ESA signal for the background electrolyte, ESAbkgd , is independent of

the phase reference material, whereas the phase angle, </>, is a strong function of the phase

reference.

For both silica and alumina, the background electrolyte has a negligible effect below

0.01M ionic strength, but a significant effect at 0. 1M ionic strength. For silica, at 0.1M

ionic strength, the measured ESA changes sign around a pH of 4.7 suggesting that silica is

positively charged below that pH. However, background correction rightly shows that the

true ESA does not change sign at least down to a pH of 2. For alumina, ESA measurements

without background correction suggest possible contamination in the sample. However, after

the background correction is made, the iep and cip are coincident indicating no

contamination.

134
ACKNOWLEDGMENT

We gratefully acknowledge the support of Dr. Frank J. Wobber, Program Manager,

Subsurface Science Program, Office of Health and Environmental Research, U.S.

Department of Energy (Grant DOE-FG02-89-ER60820) for funding this research.

NOMENCLATURE

CVP colloid vibration potential

ESAbkgd magnitude of the measured ESA signal for the background electrolyte

ESAmeas magnitude of the measured ESA signal for the solid

ESA^ magnitude of the background-corrected ESA signal for the solid

Greek symbols

£ phase angle of the background-corrected (true) ESA signal for the solid

4> phase angle of the measured ESA signal for the background electrolyte

0 phase angle of the measured ESA signal for the solid

BIBLIOGRAPHY

(1) O'Brien, R. W., J. Fluid Mech., 1988, 190, 71.

(2) Marlow, B. J.; Fairhurst, D.; Pendse, H. P., Langmuir 1988, 4, 611.

(3) James, R. O.; Texter, J.; and Scales, P. J., Langmuir 1991, 7, 1993.

(4) Scales, P. J.; Jones, E., Langmuir 1992, 8, 385.

(5) James, M.; Hunter, R. J.; O'Brien, R. W., Langmuir 1992, 8, 420.

(6) Babchin, A. J.; Chow, R. S.; Sawatzky, R. P., Adv. Colloid Interface Sci. 1989,

30, 111.

(7) Sawatzky, R. P.; Babchin, A. J., J. Fluid Mech., 1992.

(8) Desai, F. N.; Hammad, H. R.; Hayes, K. F., submitted to Langmuir April 1993.

135
(9) Allen, L. H.; Matijevic, E., J. Colloid Interface Sci. 1969, 31(3), 287.

(10) Li, H. C; de Bruijn, P. L., SuffSci. 1966, 5, 203.

(11) Hunter, R. J., Zeta Potential in Colloidal Science. Principles and Applications,

Academic Press, London, 1981, 229.

(12) Hackley, V.; Malghan, S. Workshop on Electroacoustics for Characterization of

Particulates, 1993, NIST, Gaithersburg, MD.

136
Table 1: Background Electrolyte Signals with Geltech Silica as Phase Reference

[NaCl] (M) ESAbkgd (mPa M/V) Phase angle, 4>, (deg.)

0.001 0.026 165

0.01 0.050 150

0.1 0.125 175

Table 2: Background Electrolyte Signals with Alpha Alumina as Phase Reference

[NaCl] (M) ESAbkgd (mPa M/V) Phase angle, <f>, (deg.)

0.001 0.020 -8

0.01 0.045 -20

0.1 0.130 0

137
Figure 1: Magnitude and phase angle for the ESA signals

138
Figure 2: Measured ESA and phase angle for Geltech silica

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142
OXIDE AND NON-OXIDE POWDER PROCESSING APPLICATIONS USING

ELECTROACOUSTIC CHARACTERIZATION

J. P. Pollinger

AlliedSignal Ceramic Components

Torrance, California

ABSTRACT

Aqueous suspensions of ceramic powders are used in a variety of powder compact

forming techniques for structural ceramics. The development and implementation of

practical electroacoustic techniques for characterization and development of aqueous powder

suspensions is presented. Topics to be presented include evaluation of powder aging in

suspension, characterization of dispersion of powders by pH and dispersant additions,

development of compatible multiple powder suspensions, intrepretation of high solids content

suspension information, and correlation of acoustophoresis measurements with other

suspension characterization techniques. The example powder system discussed is silicon

nitride and yttrium oxide, typically used in fabricating structural silicon nitride material.

INTRODUCTION

Most structural ceramic components are fabricated by densification (sintering) of powder

compacts composed of a mixture of powders. Generation of ceramic components with high

strengths, reliability (Weibull modulus), and dimensional control places many requirements

on the as-formed powder compact, such as a homogeneous distribution of the powder

constituents with no agglomerates and uniform high powder packing density. If colloidal

filtration is the powder compact forming route, then the suspension properties will control

the resulting powder compact properties. If it is assumed that the powder composition and

particle sizes/distributions are constrained by the final desired microstructure and properties,

then the colloidal suspension properties that can be manipulated are the degree of dispersion

of the powders and the suspension solids content.

143
The dispersion of colloidal particles in an aqueous medium by the generation of

electrostatic or steric repulsion has been extensively discussed in the literature [1-5]. In the

case of electrostatic forces, the generation of high zeta potentials is generally observed to

result in stable dispersions through the generation of large mutually repulsive forces between

particles. The suspension will pack into high density compacts during filtration due to the

mutual repulsion between particles. Low zeta potentials result in the domination of short

distance attractive forces such as Van der Waals forces, which result in the generation of

poorly packed floe structures (agglomerates) which rapidly settle out and generate increased

suspension viscosities and instability. The poor packing, results in inhomogeneous, low

packing density compacts.

For multicomponent powder suspensions, powders with same-sign and large magnitude

of zeta potentials exhibit well dispersed stable suspensions, while powders with opposite sign

of zeta potentials exhibit hetero-coagulation, instability, and rapid settling [6,7]. Heterocoagulation can be a benefit and has been utilized in suspensions where the powder

constituents have greatly different densities and differential segregation can occur in a highly

dispersed suspension [8-10]. Electrostatic forces are typically manipulated by changing pH

of the suspension through the addition of acids, bases, and polyelectrolytes.

Steric repulsion can be generated simultaneously with electrostatic repulsion by the

addition of long chain polyion dispersants to the suspension. One end of the polymer

attaches to the particle surface, while the other end extends out and repels like-polymer chain

ends attached to other particles. If the polymer is added to particles with low surface charge,

the charged polymers also generate a large charge and create an appreciable electrostatic

repulsion[l 1]. In practice, pH manipulation and dispersant additions are used in combination

to stabilize suspensions.

Solids content of suspension also influences a number of suspension and cast part

properties. Besides exhibiting higher viscosities, higher solids content suspensions may

exhibit greater tendencies towards shear thinning and instability (manifested as gelation)

because of decreased particle repulsive forces due to double layer overlap and compression

when the particle separation distances in the suspension approach the particle double layer

thicknesses [12,13]. As a result, pourability and conformability of suspensions in molds will

144
be reduced, as well as the time over which the suspensions can be used. Higher solids

content do result in shorter casting times and higher powder compact packing densities due to

the smaller amount of liquid to be removed during filtration and the smaller amount of

rearrangement needed for particles transitioning from the suspension to cast compact[14].

Thus, the characterization of colloidal suspensions is critical to allowing insight into

suspension properties and how they can be manipulated into generating the desired

suspension characteristics and resulting powder compact properties. Many techniques are

used to characterize colloidal suspensions. Techniques that measure macroscopic suspension

properties include rheological measurements, settling tests, and measurement of packing

densities of cast powder compacts. But insight into the fundamental nature of particle

interactions allows the highest degree of understanding of colloidal suspension stability, and

subsequently its manipulation. The most widely used technique is electrophoresis which

allows determination of electophoretic mobility as a function of suspension properties, such

as pH and electrolyte concentration. A major limitation of electrophoresis is that it is

restricted to very dilute suspensions.

Recently acoustophoresis equipment has been developed that utilizes high frequency

electric fields to generate relative motion in charged colloidal particles with surface charge in

suspension, called the electrokinetic sonic amplitude (ESA) effect. The movement of the

particles generates a high frequency sound wave which is related to the charge on the particle

surfaces and from which the electrophoretic mobility and zeta potential of the particles can be

calculated. The mobility of particles in suspension can be monitored as the pH of the

suspension is varied, and as dispersant is added. The acoustophoresis equipment is capable

of measuring highly loaded colloidal suspensions, up to 50 vol %.

At AlliedSignal Ceramic Components, the acoustophoresis technique has been evaluated,

developed, and implemented for the characterization of oxide and non-oxide powder based

aqueous suspensions, to be used in the fabrication of structural ceramics by colloidal

filtration, or by compaction of spray dried granules generated from suspensions. The

following sections will discuss the development and implementation of acoustophoresis

characterization, comparison and correlation with other colloidal suspension characterization

techniques, examination of pH and surfactant titration techniques, effect of suspension solids

145
content, and applications correlating colloidal properties with other particle characteristics.

The example ceramic system discussed is silicon nitride and yttrium oxide in aqueous

suspensions (a typical silicon nitride structural ceramic composition would have 4 to 8 wt%

yttrium oxide added as a sintering aid).

ACOUSTOPHORESIS TECHNIQUE DEVELOPMENT

Acoustophoresis techniques have been developed and refined at AlliedSignal Ceramic

Components over the last five years, for application with silicon nitride based aqueous

suspensions. The first step in the characterization process is to prepare the colloidal

suspension. For characterization of a single powder suspension, a 1 vol % solids suspension

is prepared in a teflon container (nominal suspension volume is 250 ml). The powder is

added to deionized water and the suspension is mixed for 15 minutes using a teflon coated

magnetic stir bar. The stir bar speed is adjusted to ensure that no powder settles to the

bottom of the container. The suspension is then ultrasonicated using an ultrasonic horn for 1

minute at 100 watts power, to break up any agglomerates and assure that maximum colloid

surface area is available for evaluation. The suspension is then placed into the

acoustophoresis measurement equipment" and constantly stirred. The suspension properties

are then monitored until they have equilibrated. Properties monitored are zeta potential,

suspension pH, and suspension conductivity. Through experience and knowledge of the

planned suspension applications, the suspension is considered to be equilibrated when none of

the measured properties has varied more than 5% in 15 minutes. It has been demonstrated

that silicon nitride powders react in water, due to the relative instability of surface amine

groups and their replacement on the surface over time by silanol groups[7]. But it has been

observed in this example powder system that the yttium oxide powder is much more reactive

in water (as demonstrated by the aging results in Figure 1) than representative silicon nitride

powders. Yttrium oxide powder hydrolyzes readily in water and is generally observed to become unstable in aqueous silicon nitride-yttrium oxide suspensions long before the silicon

Matec Model 8000

146
nitride powders. After the powder suspension is determined to be equilibrated, the pH of the

suspension is noted, as it is a characteristic of the powder as a result of its desorbed and

dissolved surface species.

The properties of the suspension are then characterized. Acoustophoresis measurements

are made using the ESA method. The Ultrasonic Vibration Potential method is not used, as

it requires knowing the suspension high-frequency conductivity, which is not measured. The

zeta potential is typically determined by titrating the pH, from the equilibrated suspension

pH, to either the desired low or high pH endpoints. Thus two experiments are required to

generate a pH vs. zeta potential curve. Titrations are only performed in one direction for

each experiment, since this approximates suspension preparation for actual components, i.e.

the suspension is only adjusted to its desired final pH and there is no buildup of undesirable

counterions. The titrants used are 2. ON HN03 and 2. ON NH4OH, since this acid and base

are used in production suspension pH manipulation, as they contain no detrimental cations or

anions. The relative strength of the titrants limits the achievable pH range to 2.0 -10.5. A

25 second hold is utilized between titrant additions to allow the suspension time to

equilibrate. Figure 2 shows a typical zeta potential vs. pH curve for a silicon nitride

powder. It is important to note that no electrolyte is added to these suspensions.

Electrolytes such as KC1 and NaCl are typically added to colloidal suspensions for

electrophoresis experiments to generate a constant ionic environment for suspension property

measurements. But alkali and alkaline earth cations and halide anions are detrimental to

silicon nitride (and many other structural ceramics) high temperature properties and are not

added to colloidal suspensions used in component fabrication. Experiments have been

performed to evaluate the influence of electrolyte additions on measured zeta potentials in pH

titrations of silicon nitride powders (see Figure 3). The only influence of the electrolyte was

to decrease the relative zeta potential, with the greatest effect at low and high pH. Thus, no

detriment is observed by eliminating electrolyte additions for acoustophoresis measurements.

The effect of dispersant additions on zeta potential is determined by titrating the

dispersants into suspensions. In some cases, the pH is manually adjusted to be maintained at

a desired level during the titration, so that the effect of the dispersant can be determined at a

constant pH.

147
0 12 3

AGING TIME (HOURS)

Figure 1 . Aging Comparison of Si3 N4 and Y2 03 Si3 N4 and Y2 03 Powders in Water.

10 12

Figure 2. Typical Zeta Potential vs. pH Curve for Si3 N4 Powder.

The time between dispersant additions is typically 25 seconds, to allow for suspension

equilibration after the previous dispersant addition. A typical dispersant titration curve for

silicon nitride is presented in Figure 4.

20

10

0

I <: -10

i

s:

LlJ

i—o -20

D.<

tn -30

-40

-50

** - ^\

%M0--1 KN03

XJ0--2 KN03

NO KN03 \

2 4 6 8 10 12

pH

Figure 3. Effect of Electrolyte Additions on

Zeta Potential of Si3 N4 Powder inWater.

0 5 10 15 20 25 30

DISPERSANT ADDITION (mg/g Powder)

Figure 4. Example of Surfactant

Titration Curve For Si3 N4 Powder.

148
APPLICATION TO SILICON NITRIDE - YTTRIA SUSPENSIONS

Single Component Suspension Characterization

The first step taken in the development of silicon nitride based aqueous suspensions is to

characterize the individual powder zeta potentials as a function of pH. In order to correlate

zeta potential with other suspension characterization techniques that require higher solids

content suspensions for measurement sensitivity, 5 vol% solids suspensions were utilized.

Figure 5 shows the zeta potential vs. pH curves of silicon nitride (Denka SN-9S) and yttria

(Shin-Etsu SU) powders. The Y2 03 was only evaluated above pH 7, since it dissolves under

acidic conditions. The isoelectric point (iep) of the Si3 N4 is 4.0 and the iep of the Y203 is

9.25. The powders exhibit opposite sign zeta potentials at pH 7. As the pH is increased, the

Y2 03 moves through its iep and becomes negatively charged, the same sign charge as the

Si3 N4 powder. As pH 10.5 is reached, zeta potential values of the two powders increase to

large values. The zeta potential values measured were correlated with three other suspension

characterization methods - suspension viscosity, suspension sediment height, and suspension

supernatent turbidity.

The suspensions used for these methods were prepared at specific pH values, not titrated.

The viscosity of the suspensions, measured using a high sensitivity couvette viscometerb is shown in Figure 6. Viscosity measurements correlate well with zeta potential values, i.e.

viscosities are lowest where zeta potential values are highest, and the Y2 03 viscosity is

highest at its iep, where there are no repulsive forces and floccing is allowed to occur.

bHaake Model CV-100

149
80 r

60

40

20

1zUJ

1

OCu 0

<

-20

-40

-60

\ Y203

\

\

\

X \

1

-

i_ 0 2 4 6 8 10 12

pH

Figure 5. Zeta Potential vs. pH Curves for

Si3 N4 and Y2 03 Powders.

6.5 7.5 8.5 9.5 10.5

pH

Figure 6. Viscosity of Si3N4 and

Y2 03 Powders as a Function

of pH.

Sediment height tests were conducted with equal amounts of suspension placed into test tubes

for 48 hours. The relative sediment heights were then examined. The intrepretation of

sediment height tests is that if a suspension is allowed to stand for an extended time, the

particles will settle due to gravity. Powders that are well dispersed will pack into high

density structures, forming a short sediment height. Poorly dispersed (low zeta potential)

powders will pack poorly into low density structures, resulting in high sediment heights.

Figure 7 shows relative sediment height results as a function of pH. The results correlate

with zeta potential measurements in that highest sediment heights are achieved at the lowest

zeta potential values. Supernatant turbidity tests were conducted by examining the settling

test suspensions placed in test tubes after 48 hours. The suspension supernatant turbidity was

rated by visual inspection from 1 to 5, with 1 being clear and 5 being opaque. Interpretation

of turbidity tests is that well dispersed particles will mutually repel one another and that

some will stay suspended for long times, clouding the supernatant. Poorly dispersed

particles will floe and settle rapidly, leaving a clear supernatant. The results again correlate

with zeta potential measurements in that opaque or nearly opaque supernatants are seen for

high zeta potential suspensions, while a clear supernatant is observed for Y2 03 at its iep (see

Figure 8).

150
w. o

U. / o

U-l U.D

UJ 0.5 —

LlJ oo 0.4

LlJ 0.3

LkJ

0.1

Y203 ^

0

$

i i 1

6.5 7.5 8.5

pH

9.5 10.5

Figure 7. Sediment Height of Si3 N4 and Y2 03 Suspensions as a Function of pH.

6.5 7.5 8.5 9.5 10.5

pH

Figure 8. Supernatent Turbidity of Si3N4 Y2 03 Suspensions as a Function

of pH.

The correlation is excellent for all three techniques, when correlated with the zeta

potential measurement results, but the advantages of acoustophoretic measurement of the zeta

potential is clear. It is both more rapid (two pH titrations vs. preparation of a number of

different pH suspensions, and waiting for sedimentation to take place in the case of sediment

height and supernatent turbidity tests), and it provides information and insight on the

fundamental forces controlling suspension properties.

Multicomponent Suspension Characterization

The second step taken in the development of Si3 N4

- Y203 suspensions was to examine

the effect of combining the two powders in suspension. Examination of the individual

powder zeta potential vs. pH curves in Figure 5 indicates that powders will have opposite

sign zeta potentials at pH 7 and hetero-coagulation will occur. As the pH is increased, the

Y2 03 will pass through its iep and become the same sign charge. Mutual repulsion will then

occur with dispersion being maximized at pH 10.5, where both powders exhibit large

magnitude zeta potentials. To determine if the individual zeta potential values of the two

powders could be used to predict multicomponent suspension properties, two suspension

compositions were prepared - 50 wt% Si3 N4/50 wt% Y2 03 , and 95 wt% Si3 N4/5 wt% Y2 03.

151
Earlier experiments indicated that titrating the suspensions through regions of opposite sign

zeta potentials induces hetero-coagulation that could not be reversed as the titration is

continued. Thus, individual suspensions were prepared at specific pH values of 7, 8, 9, 10,

and 10.5. Since the acoustophoresis equipment measures the sum signal of the suspension,

the zeta potential measured for multicomponent powder suspensions is a relative value. It is

useful to calculate the relative zeta potential value for qualitative use in evaluating

multicomponent suspensions, so the particle density and particle size inputs to the zeta

potential calculation software are weighted using weight percent. The results are presented

in Figure 9. For the 95 wt% Si3 N4 suspension, the zeta potential measured is essentially that

of the Si3 N4 powder by itself in suspension. This can be expected due to the fact that the

acoustophoresis equipment is measuring the resulting zeta potential of the entire suspension

and that 5% Y2 03 will have a minimal effect on the suspension properties, even if it has an

opposite sign zeta potential below pH 9, compared to the Si3 N4. On the other hand, the 50

wt% Si3 N4 suspension shows a dramatic effect as a function of pH.

152
Its measured zeta potential is approximately halfway between that of the Si3 N4 and Y203 powders between pH values 8.8 and 10.5. This can be expected since the powders are both

negatively charged and the suspension zeta potential should be approximately an average

between the two powder zeta potentials. Below pH 8, the zeta potential tracks with the Y203

zeta potential, indicating that either the Y2 03 particles have heterocoagulated onto the larger

Si3 N4 particles, thereby shielding the Si3 N4 surface charge, or possibly that Yttrium cations

have dissolved from the Y2 03 particle surfaces and adsorbed onto the Si3 N4 surfaces, a

process observed in other oxide materials by Wiese and Healy[15-16].

The viscosity of the two multicomponent suspensions was also measured and the results

are presented in Figure 10. The 95 wt% Si3 N4 suspension tracks with the viscosity of the

Si3 N4 suspension, while the 50 wt% Si3 N4 suspension viscosity tracks with its measured zeta

potential. At pH 7, although strong hetero-coagulation is predicted from the individual

powder suspension zeta potentials, the result is a low viscosity suspension with high zeta

45 r 40

35

^30 •

<_> i"

25

oo> 15

10 •

5

0

- 6.5 7 7.5 8 8.5 9 9.5 10 10.5 11

pH

Figure 10. Viscosity of Multicomponent Suspensions as a Function of pH, Compared with

the Separate Components.

potential. In addition, a sediment height evaluation was performed at pH 7 on the 50 wt%

Si3 N4 suspension and it showed a higher sediment height than either of the individual powder

suspensions at pH 7. This indicates that there is some type of hetero-coagulation occurring,

but that it is not strongly influencing rheology.

153
In summary, evaluation of multicomponent suspensions by acoustophoresis (even though

a qualitative measurement) is valuable in determining how the individual constituents will

react when placed together, and how they can be manipulated, even though the

acoustophoresis technique only measures the overall suspension zeta potential. But in order

to effectively interpret results, the zeta potential of the individual powder must be known as a

function of pH before trying to interpret the multicomponent suspension results, and

suspension characterization techniques such as viscosity measurements, can provide

additional useful information and insight.

Manipulation of Zeta Potential Sign by Dispersants

Multicomponent suspensions have a desired pH level at which they must be prepared, in

order to maximize the zeta potential and degree of dispersion by the major powder

component. Many times the minor powder additions have the opposite sign zeta potential of

the major powder constituent at that pH and if added directly to the suspension will result in

hetero-coagulation. The zeta potential sign can potentially be manipulated by addition of a

dispersant and the result can be measured using acoustophoresis. A specific example is the

following Si3 N4-based suspension. The Si 3 N4 powder shown in Figure 2 can be most

effectively dispersed at a pH range from 9 to 10.5. But a selected Y2 03 powder0 has the

opposite sign zeta potential (positive) at that pH range. In order to prevent

heterocoagulation, the zeta potential of the Y2 03 was changed to negative by the addition of a sodium polyacrylate dispersant. d The dispersant titration curve is shown in Figure 11.

Obviously, the Y2 03 must be predispersed with the dispersant before addition to the Si3 N4 , and the Si 3 N4 powder must also be compatible with the dispersant, because some of the

dispersant adsorbed on the Y203 particles may desorb and interact with the Si3 N4

.

c Molycorp #5600

dW.R. Grace Daxad 30

154
Effect of Solids Content

Acoustophoresis has the ability to measure zeta potential in high solids content

suspensions. But the equations relating the electroacoustic effects to mobility and zeta

potential are based on dilute suspensions of particles. As the solids content is increased to

where electrical double-layer interactions and hydrodynamic interactions occur, the

relationships become nonlinear. At even higher solids contents, particle-particle interactions

occur and reduce dynamic mobility. For most aqueous suspensions, the non-linear behavior

occurs above approximately 10 vol %. Si3 N4 aqueous suspensions were prepared at a range

of solids contents to evaluate acoustophoretic zeta potential measurements. The results are

presented in Figure 12. The calculated zeta potential is in fact the same for 10 vol % solids

content and less. The calculated zeta potential does decrease above 10 vol % solids.

Viscosity was also measured for each suspension prepared and the results are shown in

Figure 12. The viscosity does not start to increase appreciably until 20 vol% solids.

In order to determine the sensitivity of acoustophoresis at high solids loadings, Si3 N4

suspensions and 95% Si3 N4/5% Y2 03 suspensions were prepared at 37.7 vol% solids at pH 9

and 10.5.

0 10 15 20 25

DISPERSANT ADDITION (mg/g Powder)

30

1 1.5

1 1

10.5

10

9.5

35

Figure 1 1 . Reversal of Zeta Potential Sign of Y203 Powder in Aqueous Suspension by

Dispersant Additions.

155
i

zLxJ OQ_

<r.

•15

-25

-35

-45

-55

10 20 30

SOLIDS CONTENT (VOL %)

o

on

Figure 12. Effect of Si3 N4 Suspension Solids Content on Zeta Potential Measured by

Electrokinetic Sonic Analysis.

The suspensions were characterized using acoustophoresis and viscosity measurements. The

results are presented in Figure 13. The comparison of 5 wt% Y2 03 substitution indicates that

the calculated zeta potential and viscosity measurements correlate - zeta potential decrease

and viscosity increase with Y2 03 substitution, although the viscosity measurement is much

more sensitive to small changes at the high solids content. Thus, to evaluate high solids

content suspensions, viscosity measurements are a more desirable choice due to their greater

sensitivity.

i <

si

1 o

<I

0

10

-20

-30

-40

®

SI3N4

95% SI3N4

———

— ©

~~—

<

i i J

8.5 9.5 10

pH

10.5

120

100

80

60

40

20

0

COO

co

1 1

Figure 13. Comparison of Viscosity and Measured Zeta Potential of 37.7 vol% Solids

Suspensions of Si3 N4 and 95%Si3 N4/5% Y2 03

.

156
Correlation of Zeta Potential with ShK, Oxygen Content

Zeta potential measurements can also be used to evaluate other properties that affect zeta

potential of powders. An example is the correlation that can be made between Si3 N4

isoelectric point (iep) and surface oxygen content. In an experiment to evaluate this

correlation, Si3 N4 aqueous suspensions were milled in deionized water for a range of times,

up to 168 hours. For each milling time, the suspensions were titrated using acoustophoresis

to determine the iep, then the suspension was quickly dried and the oxygen content of the

powders determined using a fusion technique/ The results are presented in Figure 14. The

Si3 N4 powder iep decreases in pH with increasing oxygen content. This is a result of the

substitution of surface amine groups by silanol groups, which result in the surface becoming

more Si02-like. The Si3 N4 iep varies nearly linearly with oxygen content up to

approximately 2.5 wt% oxygen, then the oxygen content increases rapidly with little change

in iep. The transition at 2.5 wt% oxygen potentially indicates the complete coverage of the

Si3 N4 particle surface by silanol groups. Further increases in oxygen content would not be

expected to affect the Si3 N4 iep, since the surface will continue to be fully covered with

silanol groups.

12

2 2.5 3 3.5 4

4.5

ISOELECTRIC POINT (pH)

Figure 14. Correlation of the Isoelectric Point of Si3 N4 in Aqueous Suspension (Milled at Various Times) with Oxygen Content.

eLECO Model TC-346

157
SUMMARY

In conclusion, the development and implementation of acoustophoresis techniques for the

determination of zeta potential in aqueous ceramic powder suspensions has been described.

The acoustophoretic measurements correlate well with other suspension characterization

methods for low solids content suspensions and are more rapid than conventional methods.

At high solids contents, viscosity is a more sensitive indicator of suspension properties.

Acoustophoresis techniques can also be used to characterize multicomponent powder

suspensions, if single component suspension properties of the constituent powders are known.

Powder properties which affect zeta potential, such as Si3 N4 powder surface oxygen content,

can also be correlated with zeta potential measured by acoustophoresis.

158
REFERENCES

1. J. T. G. Overbeek, "Recent Developments in the Understanding of Colloid Stability,"

J. Colloid Interface Sci., 58 [2] 408- 22 (1977).

2. J. T. G. Overbeek, "How Colloid Stability Affects the Behavior of Suspensions," in

Emergent Process Methods for High Technology Ceramics, edited by R. F. Davis, H.

Palmour III, and R. L. Porter, Plenum Press, NY, 25-44 (1982).

3. J. Lyklema, "Interfacial Electrochemistry of Disperse Systems," ibid 1-24.

4. M. D. Sacks et al., "Dispersion and Rheology in Ceramic Processing," in Ceramic

Powder Science, edited by G. L. Messing, K. S. Mazdiyasni, J. W. McCauley, and R.

A. Haber, Am. Ceram. Soc, Ohio, 495-516 (1987).

5. R. O. James, "Characterization of Colloids in Aqueous Systems," in Ceramic Powder

Science, edited by G. L. Messing, K. S. Mazdiyasni, J. W. McCauley, and R. A.

Haber, Am. Ceram. Soc, Ohio, 350-410 (1987).

6. T. W. Healy, G. R. Wiese, D. E. Yates, and B. V. Kavanaugh, "Hetero-coagulation in

Mixed Oxide Colloidal Dispersions," J. Colloid Interface Sci., 42 [3] 647-9 (1973).

7. J. P. Pollinger, D. D. Newson, and J. J. Nick, "The Implementation of

Acoustophoresis for the Characterization of Silicon Nitride Based Aqueous

Suspensions," to be Published in Characterization Techniques for Solid Solution

Interfaces in Ceramic Systems, edited by J. H. Adair and J. A. Casey, Am. Ceram.

Soc.

8. E. Carlstrom and F. F. Lange, "Mixing of Flocced Suspensions," J. Am. Ceram. Soc,

67 [8] CI 69-70 (1984).

9. F. F. Lange, B. I. Davis, and E. Wright, "Processing-Related Fracture Origins: IV,

Elimination of Voids Produced by Organic Inclusions," J. Am. Ceram. Soc, 69 [1] 66-

69 (1986).

10. T. M. Shaw and B. A. Pethica, "Preparation and Sintering of Homogeneous Silicon

Nitride Green Compacts," J. Am. Ceram. Soc, 69 [2] 88-93 (1986).

159
11. R. G. Horn, "Surface Forces and Their Action in Ceramic Materials," J. Am. Ceram.

Soc, 73 [5] 1117-35 (1990).

12. W. Albers and J. T. G. Overbeek, "Stability of Emulsions of Water in Oil: II. Charge

as a Factor of Stabilization Against Flocculation," J. Colloid Sci., 14 [5] 510-18

(1959).

13. E. M. Deliso, A. S. Rao, and W. R. Cannon, "Electrokinetic Behavior of A12 03 and

Zr02 Powders in Dilute and Concentrated Aqueous Dispersions," in Ceramic Powder

Science, edited by G. L. Messing, K. S. Mazdiyasni, J. W. McCauley, and R. A.

Haber, Am. Ceram. Soc, Ohio, 525-536 (1987).

14. N. Ise, et. al., "Ordering of Charged Particles in Solution," Naturwissenschaften, 69,

S.544 (1982).

15. G. R. Wiese and T. W. Healy, "Coagulation and Electrokinetic Behavior of Ti02 and

A12 03 Colloidal Dispersions," J. Colloid Interface Sci., 51 [3] 427-33 (1975).

16. G. R. Wiese and T. W. Healy, "Adsorption of Al(III) at the Ti02-H2 0 Interface," J.

Colloid Interface Sci., 51 [3] 434-42 (1975).

160
INVESTIGATION OF PARAMETERS AND SECONDARY COMPONENTS AFFECTING

THE ELECTROACOUSTIC ANALYSIS OF

SILICON NITRIDE POWDERS

Vincent A. Hackley and Subhas G. Malghan

National Institute of Standards & Technology

Ceramics Division

Gaithersburg, Maryland 20899

ABSTRACT

The electrokinetic sonic amplitude (esa) measurement may be used to characterize silicon

nitride powders in aqueous suspension at solids concentrations approaching those encountered

in real processing environments. The effects of powder and measurement parameters on esa

analysis has been investigated. The role played by solution ionic species in modifying

surface charge behavior and the ionic contribution to the esa signal of the solid phase was

examined in detail.

INTRODUCTION

Colloidal filtration (slip casting) and spray drying methods for processing of silicon

nitride powders into structural ceramics require the application of aqueous colloidal

dispersion techniques. The preparation and property control of dense powder slurries

containing multiple components (e.g., sintering aids, dispersants and binders) which interact

in complex ways is a difficult problem [1]. In addition, the silicon nitride surface may,

depending on powder history, contain varying amounts of oxide or inorganic contaminants.

These components can alter the interfacial chemistry during processing. Wide variations in

physical and surface chemical properties of powders often occur between manufacturers and

even between batches from the same source. It is desirable to understand how these

variations will affect suspension behavior, then apply this information to compensate for

property fluctuations during or prior to processing.

The colloid stability and rheology of aqueous suspensions derive primarily from the

interfacial electrical properties of the dispersed particles. For Si3 N4 , the surface is

161
zwitterionic consisting primarily of silanol (Si-OH) and amine (=N-H) functional groups,

which can dissociate to produce a negative or positive charge that is pH dependent [2]. The

isoelectric point (iep) defines the pH at which positive and negative species are balanced,

yielding a net potential of zero on the particles. This condition is usually determined by

electrokinetic measurements of the shear-plane or zeta potential [3]. The iep for Si3 N4 can

vary from about pH 2 to 9, and is greatly affected by the adsorption of ionized solution

species and the degree of surface oxidation [4-7]. Electroacoustic techniques, based on the

response of charged particles to an alternating electric field or sound wave, have recently

been developed for the analysis of interfacial electrochemistry in dense suspensions. Detailed

descriptions of measurement systems based on the electroacoustic effect are to be found

elsewhere within this volume.

We have measured the electrokinetic sonic amplitude (esa) which is related to the colloid

dynamic mobility, and zeta potential through the following relationship [8]:

p

esa = — = c Ap 4>/ u.(o) (1)

E

*

where PIE is the pressure amplitude over the applied ac electric field, c is the speed of

sound, Ap is the difference in density between the particle and suspending medium, 0 is the

solid volume fraction and f

g is a geometric factor. The dynamic and static (electrophoretic)

mobility differ by an inertial factor which is close to 1 for the conditions of most of the

experiments reported in this paper. The polarity of the particle charge is calibrated against a known material, after which any change in polarity is characterized by a rapid shift (through

180°) in the measured phase angle.

162
EXPERIMENTAL

The Si3 N4 powder used in these studies has a mean particle size of 0.4^m (determined by

light diffraction) and a BET surface area of 10.1 m

2/g. This material, type SNE-10+

, is

produced by Ube Industries (Tokyo, Japan). In general, sample suspensions were prepared

in the following manner. A weighed amount of powder was mixed into the appropriate

amount of solution containing the desired salt and deionized water (pre-acidified with nitric

acid in most cases) and allowed to equilibrate for 10 min. The suspension was then

dispersed using ultrasonication. If multivalent ions were used in an experiment, they were

added in solution form following sonication and then equilibrated for 10 min. The esa

measurements and acid-base titrations were performed using an ESA-8000 system from

Matec Applied Sciences (Hopkinton, MA). Head space in the sample cell was blanketed

with nitrogen gas and maintained at positive pressure to exclude atmospheric C02 during an

experiment. Samples were titrated at 60 s intervals unless otherwise noted. Nitric acid and

sodium hydroxide titrants were used. All chemicals were analytical grade or higher.

All particle concentrations are reported as % volume fraction of solids. Ion

concentrations are expressed in either molar or equivalent weight units (eq/L). We define an

equivalent weight as that fraction of the molecular weight corresponding to the reciprocal

valance of an ion as given in the text or graphs. For example, Mg2+ and HP04

2" have an

equivalent weight equal to lA of the molecular weight, and therefore a 1 mol/L solution of

these ions contains 2 eq/L. For all 1:1 electrolytes, the molecular and equivalent weights are

identical.

t Certain trade names and company products are mentioned in the text or identified in illustrations in order to adequately specify the experimental procedure and equipment used. In no case does such

identification imply recommendation or endorsement by National Institute of Standards and Technology,

nor does it imply that the products are necessarily the best available for the purpose.

163
RESULTS AND DISCUSSION

Measurement and Powder Parameters

Relative to more established methods, such as microelectrophoresis, the esa technique has

not been tested over a sufficiently broad range of experimental conditions and sample

diversity to warrant a priori acceptance of results. Our first goal is to examine the

dependence of esa measurements on various physicochemical properties of aqueous ceramic

powder suspensions, and establish procedures and conditions for obtaining reproducible

results that can be interpreted in a manner consistent with known properties. Our second

goal is to perform fundamental studies of non-oxide surface chemistry which may relate

directly to processing concerns.

Figure 1 illustrates the high degree of reproducibility obtained from base titrations

performed on typical 1 % suspensions over several days and under controlled experimental

conditions. In particular the iep, found to be 6.4 in this case, is determined with an error

not exceeding ±0.05 pH units. At higher solids concentrations, errors associated with the

measurement of pH using a glass-membrane electrode may play a significant role in

determining the overall error in the iep value. In Figure 2 we examine the effect of titration

speed on esa as a function of pH. The adjustable parameter is the delay time between each

incremental titrant addition and the next measurement. Although significant variations in esa

are observed with titration speed in the alkaline pH range, the iep (6.15 in this case), exhibits

only a small spread. There is, however, indication of a trend toward increasing esa with

longer delay times. The different iep in Figs. 1 and 2 is partly a consequence of using

titrants which differed in dissolved C02

. This effect will be discussed at greater length in the

section on Effects of Surface Contamination.

Three sample-related parameters will now be addressed, namely sample stability, volume

fraction and extent of surface oxidation. The notion of sample stability is a complex problem

in which several factors may play a role simultaneously. Evidence has indicated that for

many non-oxide powders, the surface is unstable in aqueous solution [4], possibly undergoing

hydrolysis, dissolution, or other yet undefined reactions. Other factors that may affect

164
stability are changes in the state of dispersion and slow surface reactions in which

equilibrium is not reached during the duration of an experiment. In most experimental

scenarios, stability becomes important over a period of one to several hours, whereas in a

processing application this period may be extended. The results of three contiguous cyclic

acid-base titrations of Si3 N4 in 0.1 eq/L NaN03 solution are plotted in Figure 3. Each cycle

took about 1.6 h to complete, and no significant hysteresis or iep shift was observed. These

results indicate that this suspension is electrochemically stable over at least a 5 h period.

In Equation (1) the esa signal is linearly dependent on the particle volume fraction.

Typically, this relationship is observed up to roughly 10% solids [5,9]. However, significant

deviations from dilute behavior often occur above 5% solids, and changes in the effective

particle size due to aggregation may lead to further deviations from linearity. Figure 4

shows the esa signal as a function of solids concentration and suspension pH for a series of

base titrations (pH 4 to 8). The iep is independent of volume fraction up to 6% solids, at

which point a small shift is observed. Examination of the iso-pH curves in the lower part of

Figure 4 indicates some deviation from linearity at 6%, particularly for negative polarity

values. Furthermore, the curves are not symmetric about the iep. These effects may be

partially explained by irreversible changes in particle aggregate mass that occur upon passing

through the iep. In any event, this data clearly illustrates a demarcation between 4% and

6%, which may be interpreted as a transition from dilute to moderately concentrated

regimes.

Depending on the synthesis route and treatment history, commercial Si3 N4 powders may

contain varying amounts of oxygen, both in the bulk phase and as a surface oxide phase [6].

The presence of oxygen greatly effects the distribution and dissociation behavior of surface

charge groups, which in turn modifies the iep and dispersion properties. The surface oxide

exists essentially as a silicate layer encapsulating the bulk phase. Assuming a silica

structure, the surface oxide thickness can be estimated from electron spectroscopy methods.

In previous work we showed that the iep, determined from esa measurements on several

commercial powders, decreased linearly with increasing oxide thickness [4], Presumably,

the iep decrease is a consequence of acidic silanol groups derived from surface hydrolysis.

165
Linear regression analysis predicts an iep of about 9.7 for pure Si3N4 and estimates a surface

oxide thickness of 1.5 nm is necessary to obtain a silica-like surface. Additionally,

experiments indicate that surface oxide can be removed and the iep increased using mild

surface treatment such as aqueous soxhlet extraction.

The Role of Ions in Solution

There are generally four mechanisms by which ions dissolved in solution can affect the

electroacoustic measurement of particulates in suspension: the ion vibration potential,

electrostatic screening, specific adsorption and heterogeneous precipitation. In the first mechanism, or more accurately its reciprocal effect in an applied ac potential, the ions make

an independent contribution to the esa signal and phase angle. This occurs as a result of,

and the amplitude is proportional to, the difference in relative effective mass between cations

and anions during field induced charge separation. The ion vibration potential was predicted

by Debye in 1933 [10], and has been used to obtain information about ion size in solution.

For colloid systems this effect represents a background signal that may require a correction.

The ion signal is relatively small in most cases, but can become significant when the particle

concentration is low, the ionic strength is high or the measurement is made close to the iep

of the particles. Additionally, this signal can cause the apparent iep to shift, and lead to an

incorrect interpretation of results [11]. Figure 5 shows the measured ion contribution to the

esa signal for a series of ion pairs at 0.1 eq/L concentration. This data may be used to

predict potential background problems with an electrolyte. A blank measurement of the ion

signal from an equivalent solution or supernatant, followed by a simple subtraction procedure

will generally suffice as background correction. Since the ion signal is nearly pHindependent from pH 5 to 9, a single background measurement is usually sufficient in this

range. Of the four mechanisms listed above, only the ion vibration potential is unrelated to

the solid phase.

The second mechanism, screening, is generally indifferent toward the nature of the

particle surface, and depends only on ionic strength /, through it's affect on the electrical

double-layer parameter k [3]:

166
k = 3.288 y/l (nrn

-1 ) (2)

in water at 25 °C. As / increases, the effective thickness of the counter-ion envelope \Ik

around each particle is compressed (potential falls off more rapidly), which reduces the zeta

potential and mobility accordingly. This effect is illustrated in Fig. 6 for Si3 N4 in NaN03

solutions of increasing /. The data are presented in the form of dynamic mobility calculated

directly from esa. The well-defined common intersection point (cip) in Fig. 6 is indicative

of an "indifferent" electrolyte. The cip defines the "natural" iep which characterizes a powder surface, while the absence of a cip implies that specific interactions are involved.

The latter situation occurs when an ion interacts chemically with the particle surface,

penetrating the inner or compact part of the double-layer. This type of interaction may

consist of varying degrees of covalent or coordinate (complexation) bonding, and constitutes

the third mechanism by which ions affect the esa measurement.

Generally, specific adsorption of anions and cations will cause the iep to shift toward the

acidic and alkaline directions, respectively (see Fig. 7). This affect is easily explained. In

the case of specific anion adsorption, more protons are necessary to compensate for the

additional negative charge brought to the surface by the anion. The pH at which this new

charge equilibrium is balanced (the iep) must therefore decrease. Specific adsorption of

cations is explained in an analogous manner.

In Fig. 8, mobility curves calculated from esa measurements of Si3 N4 suspensions are compared for the (potassium) halide series relative to the curve for NaN03

. It is seen that

only fluoride, which is known to interact strongly with silicon, gives any indication of

specificity for the Si3 N4 surface, and then only a small interaction is apparent. The other

electrolytes are indifferent. Figure 9 shows the mobility curves for Si3 N4 in the presence of

several (potassium and sodium) oxyanions. The interaction with this class of ions is

observably stronger. This is particularly true for sodium hexametaphosphate (NaP03)6 , which does not have a measurable iep. Figure 10 shows mobility curves for divalent cations

including the alkaline-earth (nitrate) series and the transition metal ion copper. The alkaline167
earths behave similarly, exhibiting weak specificity with no clear dependence upon ion size.

In contrast, the interaction of Cu2+ with the Si3 N4-solution interface is clearly more complex,

and belongs to a separate class. This type of mobility curve is typical for hydrolyzable metal

cations on hydrous oxide surfaces [12], and is an example of the fourth mechanism,

heterogeneous precipitation.

The interaction of hydrolyzable cations with Si3 N4 is illustrated in Fig. 11, which shows a

series of dynamic mobility curves for suspensions containing increasing yttrium nitrate

concentrations in 0.01 eq/L NaN03 as supporting electrolyte. At acidic pH values (below 6)

the hydrated Y

3+ ion adsorbs weakly to the positively charged Si3 N4 surface. With

increasing concentration of Y(N03 ) 3

, a maximum in the mobility curve develops between pH

6 and 8 and the iep is shifted toward increasingly alkaline pH values. The sharp increase in

mobility which occurs near pH 8 is due to strong adsorption of hydrolyzed Y(OH)x

(3 X)+ and

cationic polynuclear species which predominate in solution in this pH and concentration

range [13]. As the pH increases, further hydrolysis leads to precipitation of a new metalhydroxide surface phase. This new phase controls the electrokinetic behavior of the

dispersed particles, conferring on them the iep of the freshly precipitated hydroxide. This

process has been investigated as a method for adding sintering aids based on oxides of

yttrium, aluminum and the rare-earths [1].

Effects of Surface Contamination

Commercial Si3 N4 powders often contain minor and trace constituents that may affect

surface properties. Oxygen and carbon are ubiquitous, being found to some degree on all

non-oxide powder surfaces exposed to air. The effect of oxygen on the electrochemical

behavior was discussed previously. Some other common elemental contaminants are Fe, Al,

Ca, Mg, Co, Ni and F. The effect these contaminants may have on the surface properties

are contingent upon their form and their concentrations near the surface. Still, based on data

presented above we can predict that metallic impurities such as Fe, if they exist in oxidized

forms, will generally produce a more basic Si3 N4 surface (higher iep). Synthetic routes

based on nitridation of Si sometimes employ hydrofluoric acid leaching to remove residual

Si, resulting in fluorine contamination [4]. This impurity will most likely exist as F or

168
bound to silicon near the surface. Our results indicate that F interacts weakly with Si3N4 ,

causing a small increase in surface acidity (lower iep). Contamination may also occur during

processing if the powders come in contact with metallic surfaces, for instance in the milling

operation

Another potential source of contamination is atmospheric C02

, a component that is not

normally controlled in the processing environment, but may affect interpretation of titration

experiments. It has previously been demonstrated for hydrous iron oxides [14] that C02

dissolved in solution can react chemically with surface hydroxyls to form an adsorbed

carbonate anion. Evidence for this type of interaction has recently been observed in our

laboratory for Si3 N4 using esa measurements. In this case, contamination is thought to have

originated in the base titrant, where C02 has a substantial solubility. During a series of

titration experiments, a sudden and consistent shift in the measured iep was observed. All

samples were purged with nitrogen, had come from the same powder batch and were

analyzed over a relatively short period of time under identical conditions. The difference had

occurred when the base titrant being used was replaced with a new low-carbonate standard

solution. Figure 12 illustrates this effect for 2% volume fraction Si3 N4 in 0.01 eq/L NaN03

. These titrations were performed using a low-carbonate 0.1N NaOH standard and a prepared

NaOH solution in which C02 contamination was not controlled. Further investigation of the

C02 effect is on-going, but caution should be exercised when preparing or storing alkaline

titrant solutions if C02 is a suspect reactant.

SUMMARY

We have employed electroacoustic methods to characterize and study the electrokinetic

behavior of Si3 N4 precursor powders in moderately concentrated aqueous suspension. The

electrokinetic properties of a suspension are closely connected to its dispersion properties,

and often reflect changes which occur as a result of reactions at the particle-solution

interface. Various parameters that affect the measurement of Si3 N4 suspensions have been

analyzed. The effects of dissolved inorganic ionic species on the electrokinetic sonic

169
amplitude (esa) has been examined in terms of four general mechanisms. These

mechanisisms are the result of ion electroacoustic effects or ion interactions (screening,

adsorption and precipitation) at the solid-solution interface. The implication of these

investigations regarding the effects of contamination and the use of supporting electrolytes is

discussed in detail.

REFERENCES

[I] . P. Greil, "Processing of Silicon Nitride Ceramics," Mat. Sci. Engineer., A109, 27-35

(1989).

[2]. G. Busca, V. Lorenzelli, G. Porcile, M. Baraton, P. Quintard and R. Marchand, "FT

-IR Study of the Surface Properties of Silicon Nitride," Mat. Chem. Phys., 14,

123-140 (1986).

[3], R. J. Hunter, "Zcta Potential in Colloid Science," Academic Press, London (1988).

[4]. V. A. Hackley, P. S. Wang and S. G. Malghan, "Effects of Soxhlet Extraction on the

Surface Oxide Layer of Silicon Nitride Powders," to be published in Mat. Chem.

Phys. (1993).

[5]. J. P. Pollinger, D. D. Newson and J. J. Nick, "The Implementation of

Acoustophoresis for the Characterization of Silicon Nitride Based Aqueous

Suspensions, Multicomponent Silicon Nitride Based Aqueous Suspensions," to be

published in Ceramic Transactions (1993).

[6]. P. Greil, "Evaluation of Oxygen Content on Silicon Nitride Powder Surface from the

Measurement of the Isoelectric Point," J. Europ. Ceram. Soc, 7, 353-59 (1991).

[7]. L. Bergstrom, "Surface Chemistry of Silicon Nitride Powders: Electrokinetic

Behavior and ESCA Studies," Colloids Surf., 49, 183-197 (1990).

[8]. R. W. O'Brien, "The Electroacoustic Equations for a Colloidal Suspension," J. Fluid

Mech., 212, 81-93 (1990).

[9]. V. A. Hackley, NIST, unpublished work.

[10]. P. Debye, "A Method for the Determination of the Mass of Electrolytic Ions,"

J. Chem Phys., 1, 13-16 (1933).

[II] . V. A. Hackley, R. S. Premachandran and S. G. Malghan, "Electrokinetic Sonic

Analysis of Silicon Nitride Suspensions," to be published in Ceramic Transactions

(1993).

170
[12]. R. O. James and T. W. Healy, "Adsorption of Hydrolyzable Metal Ions at the Oxide

Water Interface," J. Colloid Interface Set, 40, 53-64 (1972).

[13]. C. F. Baes, Jr. and R. E. Mesmer, "The Hydrolysis of Cations," John Wiley & Sons,

New York (1976).

[14]. W. A. Zeltner and M. A. Anderson, "Surface Charge Development at the

Goethite/Aqueous Solution Interface: Effects of C02 Adsorption," Langmuir, 4,

469-74 (1988); V. A. Hackley, University of Wisconsin, unpublished data.

171

Fig. 1 Reproducibility of electroacoustic measurements and isoelectric point

determination during NH4OH titrations of 1 % volume fraction Si3N4 aqueous suspensions.

-3 -

Fig. 2 The effect of the delay time between incremental titrant additions on the

electroacoustic measurement of dynamic mobility for a 2% volume fraction Si3N4 suspension.

173
.5

0.0

-0.5 -

-1.0

o base, run 1 • acid, run 1

v base, run 2

acid, run 2

base, run 3

acid, run 3

Fig. 3 The stability of Si3N4 during three contiguous acid-base titration cycles

0.1 eq/L NaNQ3.

174
1 2 3 4 5 6

% solids

Fig. 4 The dependence of the electrokinetic sonic amplitude and isoelectric point

on the volume fraction of Si3 N4 and suspension pH.

175
>

g

a.

a,

m

m

0/1 4

0.1 2

0.10

0.08

0.06

0.04

0.02

0.00

-0.02

-0.04

i—i—i—i—i—i—i—i—j—m—i—i—i—ir—i

Fig. 5

l—

-

-

! : H

• i

1 2 3 4 5 6 7 8 9 1011121314151617

ion pair number

The background esa signal for a series of ion pairs in solution at 0.01 eq/L.

Ion pair number corresponds to following species: (1) NaN03 , (2) KN03 , (3) CsN03 , (4)

NH4N03 , (5) LiCl, (6) NaCl, (7) KF, (8) KC1, (9) KBr, (10) KI, (11) Mg(N03)2 , (12)

Ca(N03)2 , (13) Ba(N03)2 , (14) K2Cr04 , (15) Na2Si03 , (16) K2 HP04 , (17) (NaP03) 6

.

176
Fig. 7 Schematic illustration of the effect of specific adsorption on the mobility and

isoelectric point of a colloid suspension.

o CI 0.01 eq/L

^ Br

Fig. 8 Dynamic mobility as a function of pH for 2% volume fraction Si3N4 in a

series of (potassium) halide solutions at 0.01 eq/L, relative to the curve for NaN03

. Fluoride shows a small degree of specificity for the surface.

177
Fig. 9 Dynamic mobility as a function of pH for 2% volume fraction Si3 N4 in a

series of oxyanion solutions at 0.01 eq/L, relative to the curve for NaN03

.

Fig. 10 Dynamic mobility as a function of pH for 2% volume fraction Si3N4 in a

series of divalent cation (nitrate salt) solutions at 0.01 eq/L, relative to the curve for

NaN03

. Note the distinct difference in behavior between the hydrolyzable Cu2+ ion and

the alkaline-earth ions.

178
03 >

CV2

-4->

43

O

s

o 0.005 mol/L Y(N0„)3'

3

2

1

0

-1

-2

-3

Fig. 11 Dynamic mobility as a function of pH and Y(N03)3 concentration for 2%

volume fraction Si3 N4 in 0.01 eq/L NaN03

. The formation of a maximum in the mobility

curve is due to the heterogeneous precipitation of Y(OH)3 at the Si3N4 surface.

1

-

S3 PU

oa

-1

Fig. 12 The effect of C02 contamination of a base titrant on the isoelectric point of

a 2% volume fraction Si3N4 suspension in 0.01 eq/L NaN03

.

179

SESSION IV

CLAYS, CEMENTS, AND COAL

Session Chairman

B. M. Moudgil

University of Florida
THE IMPACT OF SURFACE CHEMISTRY ON PARTICLE ELECTROSTATIC

CHARGING AND VISCOELASTICITY OF PRECIPITATED CALCIUM CARBONATE

SLURRIES1

Yan C. Huang2

Research Associate

Department of Chemistry

Lehigh University

Bethlehem, PA

USA

Frederick M. Fowkes*

Professor of Chemistry

Department of Chemistry

Lehigh University

Bethlehem, PA

USA

* Deceased.

Nigel D. Sanders

Project Manager

Surface & Colloid Science

Specialty Minerals Inc. Bethlehem, PA

USA

Thomas B. Lloyd

Research Scientist

Department of Chemistry

Lehigh University

Bethlehem, PA

USA

ABSTRACT

The rheology of coating colors is now well recognized to be a function of both particle

packing and surface chemical effects. In this paper we explore the complex rheology of

precipitated calcium carbonate (PCC) slurries where surface chemistry has been carefully

controlled. Surface properties of PCC were characterized with a combination of surface

charge measurements (via a Matec electrokinetic sonic amplitude analyzer) and adsorption

calorimetry. Viscoelasticity was assessed with a Bohlin rheometer. We have found that the

unique surface chemical features of PCC (namely, the presence of strongly adsorbed calcium

ions as interactive sites) determine the nature of the binding configuration of conventional

anionic polyelectrolytes to the PCC surface and hence the rheology of the slurry. We further

suggest ways in which these unique features may be used to improve the rheology of a PCCbased coating colo( where particle size distribution (packing) has been chosen to optimize

coating structure.

'This paper is an updated version from the original which was presented at the 1991 TAPPI

Coating Conference, Montreal, Quebec, May 1991, Copyright TAPPI 1991.

2 Current Address: International Paper, Corporate Research Center, Long Meadow Road,

Tuxedo Park, New York, USA

180
INTRODUCTION

Precipitated calcium carbonate (PCC) is a bright, white pigment which is being used

extensively for high and low gloss paper coatings. Generally a sodium polyacrylate

dispersant is added to stabilize calcium carbonate slurries at high solids content. The calcium

carbonate particles are generally regarded to be negatively charged, i.e. the dispersion has an

anionic character. [1] The precise relationship between the charging effect of the

polyelectrolyte and pigment dispersion stability in terms of Theological properties of slurries

is not well known though many theories have been presented.

A recent study by the authors of this paper shows that the surface of undispersed

calcium carbonate has a finite number of reactive sites (1.58 x 10"6 moles/m2

) which

spontaneously and uniformly interact with calcium ions in solutions (aH = -6.8 kcal/mole

and aG° = -6.9 kcal/mole at 23° C). [2] Slurries of undispersed calcium carbonate in

solutions of ca. 4ppm or above Ca2+ have a cationic character because the Ca2+ adsorption

favors a positive ^-potential on the CaC03 particles.

It has been found that determining electrostatic charging via ^-potential measurements

is a useful analytical tool to monitor interactions between dispersion components and hence to

access dispersion stability. [3] The purpose of the present study is to relate various

rheological properties of calcium carbonate dispersions to the mechanism of electrostatic

charging on the calcium carbonate particles, which is in turn related to the surface chemistry

at the solid/liquid interfaces.

EXPERIMENTAL

Materials

The PCC used in the studies is the prismatic form of calcite. This material consists of

0.7 fxm particles with a specific surface area of 8.63 m

2/g and a true density of 2.442 g/ml.

Doppler electrophoretic light-scattering measurement at low solids (0.1%; Coulter DELSA

440) yield an average ^-potential of +18.3 ± 0.8 mV at pH = 8 and pCa = 3.1 in 0.01 M

181
KC1, which is in good agreement with the results of Foxall, et al. (+21 mV). [4] Ion

scattering spectroscopy (ISS) and X-Ray Photoelectron Spectroscopy (ESCA) surface

elemental composition data confirm that the material has a clean CaC03 surface.

The sodium polyacrylate dispersant (NaPA, 43.7 wt% in water) is a commercial

product and is a 90% neutralized polyacrylic acid (Mw = 3400). Calcium chloride (CaCl2 2H20) is of reagent grade.

Methods

Potentiometric titrations were performed at room temperature with a Matec

electrokinetic sonic amplitude analyzer (ESA; Model 8050), interfaced with an AT personal

computer. The ESA, which can be related to particle mobility and therefore ^-potential, is

the acoustic response of a pigment slurry under an external, alternating electric field (ca. 1 MHz). An aliquot quantity of a titrant solution was injected stepwise by a Hamilton

motorized microsyringe (Microlab 900) into the reaction vessel where ESA, conductivity and

pH (all pre-calibrated) were simultaneously determined for each step of the addition of the

titrant solution. A complete titration curve was obtained by dividing the whole titration

process into several segments to ensure that the frequency of the electrical field was always

optimized.

Calcium ion concentrations were determined at room temperature with a Ca2+ ion

selective electrode (ISE; Orion, Model 93-20) connected to an pH/ISE analyzer (Orion,

Model 940). The ion analyzer was precalibrated to the Ca2+ concentration range of interest.

Dynamic light scattering was performed at 23° C with a Coulter N4 MD multi-angle,

photon correlation spectrometer (PCS). An instrumental capability (via a package of

hardwired computer firmware) to determine the pre-scale factor and the delay time increment

of the autocorrelation function was always used in the PCS experiments.

Electrophoretic light scattering was performed at 25° C with a Coulter Doppler-shift

electrophoretic light-scattering apparatus (DELSA, Model 440) which is a PC computer

software-driven instrument. The electrical field across the cell was programmed at a

constant current of 30 mA and a frequency shift of 500 Hz.

182
Flow and viscosity curves were determined at 25° C with a Bohlin VOR rheometer. A

coaxial cylinder (C25) with a moving cup of radius 27.5 mm and a fixed bob of radius 25.0

mm and a torsion bar of 3.76 gem were used.

Solution titration microcalorimetry to determine heats of interaction of the sodium

polyacrylate with PCC and calcium ions, respectively, was performed with a Tronac 1250

batch-type microcalorimeter running in an isoperibol mode.

RESULTS AND DISCUSSION

Electrostatic Charging of CaCO, Particles in Sodium Polyacrylate Solutions

PCC slurries, 11.54 % and 20.69 % by weight, were titrated with a sodium

polyacrylate dispersant solution, ^-potentials were determined during the titration process by

the Matec ESA-8050 system, and 20 cm3 of the NaPA titrant solution (0.3 g/cm3 ) were

added to each of the slurries by the end of the titration. The PCC slurries were prepared by

dispersing 30 and 60 g CaC03 , respectively, into 230 cm3 DDI water. The pH of these

dispersions was always about 9.5 ± 0.2 and rose only slightly upon addition of sodium

polyacrylate in accord with a slight decrease in calcium ion concentration and the CaC03

solubility equilibria. [2] Figures 1 and 2 depict the ^-potential and conductivity versus the

NaPA addition, respectively. In Figure 2, the conductivity of the CaC03 slurries was plotted

against the NaPA concentration shown as the cumulative grams of the NaPA added per liter

of the continuous phase. The conductivity curve of the 11.54 wt% slurry is drawn with a

dotted line and a dashed line is drawn for the 20.69 wt% slurry. Since the two slurries were

prepared with the same volume of DDI water, there was no significant difference in the

conductivity of the two slurries. The conductivity of the NaPA solution in particle-free,

CaC03 -saturated DDI water was found to bear a linear relationship with the NaPA

concentration (the solid line). The particle-free, CaC03 -saturated DDI water was prepared

by storing DDI water over CaC03 and then filtering the CaC03 suspension through 0.2 ^tm

pore-size syringe filters (Acrodisc Chemical Resistant, Gelman Sciences).

Comparing the conductivity in the solutions with and without the CaC03 particles, both

of the two slurries had slightly higher conductivity than the solution without any CaC03

183
particles at low levels of the NaPA addition, which suggests that the negatively charged

CaC03 particles were slightly conductive. As the NaPA concentration became higher, the

conductivity of the two slurries approached that of the particle-free NaPA solution. The

conductivity of the 11.54 wt% slurry became equal to that of the particle-free NaPA solution

at a lower level of the NaPA addition than the 20.69 wt% slurry. This effect was very likely

an indication that the negatively charged CaC03 particles were experiencing a viscous drag,

which lowered their drift velocity and therefore the conductivity. When the NaPA addition

was increased, polyacrylate concentrations both on the CaC03 surface and in the bulk phase,

also increased and the viscous drag was enhanced, thus decreasing the conductivity of the

particles. For the less concentrated slurry (11.54 wt%), the build-up of the surface

polyacrylate-concentration occurred sooner than the 20.69 wt % slurry; hence, the viscous

drag was seen at an earlier stage of the titration.

In Figure 1, the ^-potentials were plotted as a function of the NaPA addition, expressed

in the NaPA-to-CaCo3 w/w percentage (i.e. the dispersant level). Before the addition of the

NaPA dispersant, the CaC03 slurries were positively charged (about 3 mV); with the

addition, the polarity of the ^-potentials altered immediately. The positive ^-potentials were

provided by adsorbed Ca2+ ions while the negative f-potentials were due to the anionic

polyacrylate chains. Microcalorimetric determination shows the adsorption of NaPA to be

endothermic (i.e. heat was absorbed). The magnitude of the heat is high (+4.5 kcal/mole of

monomer), demonstrating a strong interaction. [10]

As can be seen, the negative ^-potentials increased with the dispersant level until a

peak in the electrostatic charging was reached. The peak electrostatic charging corresponds

to an optimum dispersant level at 1-2 w/w % sodium polyacrylate. The optimum dispersant

level was independent of the CaC03 concentration, but the peak electrostatic charging was

lower for the 20.69 wt% slurry. The humps on the f-potentiometric titration diagrams,

occurring not far beyond the optimum NaPA level, are probably an indication of multilayer

adsorption and/or changing orientation of the adsorbed polyacrylate chains. [1] The

multilayer/orientation change effect was also seen in the microcalorimetric titration curves

which show a heat "tail" after the initial heat pulse due to NaPA adsorption.

184
When the dispersant concentration went beyond the optimum level, the ^-potentials

started to decrease from their peak values. The decrease in ^-potential with increasing

electrolyte concentration is due to an effect known as electrical double layer compression.

Charged particles have an electrical double layer around them. The electrical double layer

can be regarded as consisting of two regions: an inner region which included adsorbed f

-

potential-determining ions, and a diffuse region in which ions are distributed according to the

influence of electrical forces and random thermal motions. The extent of the diffuse double

layer around a particle is, in general, on the order of some tens of nanometers, but the exact

magnitude depends on the electrolyte concentration. Increasing the electrolyte concentration

causes the diffuse double layer to shrink closer to the particle, so that the electrostatic -

potential falls more quickly with distance [5]. Since ^-potential is the electrostatic f

-

potential measured at the shear plane, which is located at some fixed distance from the

particle surface, it decreases as the bulk electrolyte concentration increases.

Also shown in Figure 1, the negative ^-potential was higher in the less concentrated

slurry at any dispersant level around and beyond the peak electrostatic charging. The same

trend of the ^-potential dependence was reported in cationic CaC03 suspensions where Ca2+

was the ^-potential determining ion [2]. The effect of particle concentration on ^-potential,

especially in concentrated dispersions, has not been studied extensively so far; however, this

effect is genuine and has been observed also in oil-in-water emulsions, [6] clay-particle

dispersions, [7] and silica and PMMA latex suspensions [8]. As detailed in the following

section, studies of the bulk electrolyte-concentration effect and the particle-concentration

effect on ^-potential were conducted in our laboratory using the adsorption of Ca2+ ions from

CaCl2 solutions on CaC03 particles [2] as a model system.

Electrical Double Layer Compression due to Bulk Electrolyte Concentration and Charged

Particle Concentration

^-potential and conductivity plots, similar to those in Figures 1 and 2, for six CaCQ3

slurries are shown in Figures 3 and 4. These slurries, ranging from 8 to 21 wt%, were

prepared by dispersing proper amounts of CaC03 into 230 ml DDI water, respectively, each

followed by a ^-potentiometric titration with 20 ml of a 1.02 molar CaCl2 solution. As can

185
be seen, the adsorbed Ca2+ ions were the ^-potential-determining ions, giving the slurries a

cationic nature. The peak electrostatic charging was about +18.5 mV at the addition of 0.1

millimoles of Ca2+ ions per gram CaC03

, which is in excellent agreement with our previous

studies on the adsorption of Ca2+ ions from CaCl2 solutions [2]. In this case, the addition of

Ca2+ causes a drop in pH from the initial 9.5 ± 0.2 to 8.1 + 0.1 as a direct consequence of

CaC03 solubility equilibria [2]. The ^-potentials in the six slurries rose uniformly with the

addition of the Ca2+ ions before reaching the peak electrostatic charging; no CaC03 concentration-dependence was observed at this stage of the titration. As soon as the addition

of Ca2+ ions exceeded this level, the f-potential began to decrease. The more concentrated

the slurry, the more rapidly the ^-potentials decreased.

In Figure 4 the conductivity of the slurries, for the data points beyond the peak

electrostatic charging, was plotted against the bulk Ca2+ equilibrium concentration, which

was evaluated by subtracting the monolayer adsorption coverage (13.6 /xmoles/gm CaC03 [2]

from the total amount of the Ca2+ ion addition. In the particle free, CaC03 -saturated DDI

water, the conductivity was found to increase linearly with the Ca2+ equilibrium

concentration (the solid line). At any bulk Ca2+ concentrations, there was little difference in

the conductivity of the six slurries of the various particle concentrations, indicating saturation

of the Ca2+ adsorption. Again, the positively charged CaC03 particles were seen to be

slightly conductive; in other words, the CaC03 particles with Ca2+ adsorbed are actually

conducting species somewhat like ions. Being huge in size and therefore very slow in drift

velocity as compared to the Ca2+ (C03)

2

", CI", H

+

, and OH" ions in the solutions, the

positively charged CaC03 particles contributed to the total conductivity far less than the ions.

The viscous drag effect observed in the CaC03 /NaPA system was virtually insignificant in

the CaC03 / CaCl2 system, largely because Ca2+

, (C03 )

2

, CI", H

+

, and OH" ions are by

orders of magnitude smaller than that of the anionic polyacrylate chains.

Further studies on the dependance of ^-potential upon bulk Ca2+ concentration and

CaC03 particle concentration were performed in the CaC03 slurries of 11.54, 13.53 and

16.67 wt%, respectively, by determining the bulk Ca2+ equilibrium concentrations at various

stages of the titration using a Ca2+ ion selective electrode. In Figure 5 the ^-potentials and

conductivity (the left and right ordinates, respectively) of the slurries were both plotted

186
against the bulk Ca2+ ion equilibrium millimolarity. Data points for the conductivity

measurements are all overprinted with a small square-shaped dot, and the average

conductivity of the three slurries is drawn with a dashed line. The ^-potentials were found to

decrease with the increasing bulk Ca2+ concentration and with the increasing CaC03 particle

concentration. The ^"-potential-dependence upon the CaC03 particle concentration is not a

conductivity effect. As can be seen, the conductivity of the three slurries all increased

linearly with the bulk Ca2+ equilibrium concentration; at any bulk Ca2+ concentration, the

conductivity values of the three slurries are nearly identical, but this is obviously not the case

for the ^-potentials.

For the three CaC03 particle concentrations, the ^-potential versus Ca2+ equilibrium

concentration plots appear to be three straight lines almost parallel to one another with the

least concentrated slurry (11.54 wt%) on the top and the most concentrated (16.67 wt%) at

the bottom. The fact that the rate of ^-potential decrease with respect to the bulk Ca2+

concentration (-0.0962 mV/mM) is independent of particle concentration suggests that in the

CaC03 /CaCl2 system the CaC03 particle concentration is an additional factor to be

considered in the analysis. One possibility is that the electrical double layer compression due

to increasing bulk electrolyte concentration and due to increasing charged particle

concentration work on the same principle, because the positively charged CaC03 particles are

of ion-like behavior to some extent. When the particle concentration is increased, the diffuse

double layer is compressed towards the particle surface as if the electrolyte concentration

were increased, and the electrostatic ^-potential decays more quickly with distance.

In Figure 6 the ^-potential were plotted against interparticle separation distance for the

bulk Ca2+ concentrations at 25, 40, 60 and 80 mM, respectively. Based on the geometry of

a cube with a CaC03 particle at the center of the cube, the calculated interparticle separation

distances are 616, 718 and 801 nm for the 16.67; 13.53 and 11.54 wt% slurries,

respectively. The higher the solids concentration, the shorter the interparticle separation

distance. As can be seen, at the four bulk Ca2+ concentrations, the f-potential increased with

the interparticle separation distance at a same rate (0.00663 mV/nm).

By combining the two factors, bulk electrolyte concentration and charged particle

concentration, into one equation, the change in ^-potential after the peak electrostatic

187
charging can be written as d£ = -0.0962dCaq + Q.00663dH where the f stands for fpotential in mV, the Caq is the bulk Ca2+ equilibrium concentration in millimolarity, and the

H is the interparticle separation distance in nm. In the foregoing analysis we have, however,

assumed a constant dispersed particle radius. If flocculation affects the effective radius for

ESA conversion to ^-potential slightly, this could be an additional CaC03 solids-dependent

factor.

Mesophase Structural Elements of Polyacrylate Aggregates in Solutions

To ascertain the self-association between polyacrylate chains, two NaPA solutions were

prepared: one of 4.3 wt% NaPA in DDI water at pH 7 and the other of 5.0 wt% NaPA in

particle-free, CaC03 -saturated DDI Water at pH 9.5. This concentration range of 4.3 - 5.0

wt% NaPA in solutions corresponds to an NaPA level of 1.8 - 2.1 w/w % in a 70 wt%

CaC03 slurry. The solutions were repeatedly filtered through the Acrodisc 0.2 fim pore-size

syringe filters and then stored in glass vials for two days before dynamic light scattering

experiments via the Coulter N4 MD photon correlation spectrometer.

As shown in Figures 7 and 8, mesophase structural elements with well defined

dynamic correlation lengths were observed in both the solutions. The smoothly decaying and

well-converged autocorrelation functions at the lower right-hand corner of Figures 7 and 8

ascertains the detection of the structural elements in solutions. The fact that the solutions

have been repeatedly filtered excluded possibilities of the presence of adventitious dust, so

the structural elements must be aggregates of polyacrylate chains. The light scattering angle,

15.5°, was chosen for illustration because it is more sensitive to weak scatterers than a larger

angle. In the solution that contains about 7.04 x 10"5 M Ca2+ ions (Figure 8) the scattered

light intensity (1.95 X 104 photon counts per second) was about 2.3 times higher than the one

without Ca2+ (6.94 x 103 counts/sec. in Figure 7), indicating a higher degree of aggregation.

As can be seen through a comparison between Figures 7 and 8, the dynamic correlation

lengths of the mesophase structural elements were also increased due to the higher degree of

aggregation. Results of mean aggregate sizes from the unimodal analysis increased from

1830 to 3150 nm (almost double). The size distribution processor (SDP) gave both the

systems bimodal diagnosis and suggested that a small aggregate size fraction at 326 nm was

188
present in the solution in DDI water only but not in the Ca2+ -containing solution which

instead had a large size fraction at 5600 nm. Both solutions, however, contain aggregates of

1800 to 2310 nm in size.

The self-association (or aggregation) between the polyacrylate chains is, most likely,

brought about by the bridging of the carboxylate groups which are surrounded with the Na+

counter-ions. It is conceivable that the divalent Ca2+ causes an even greater degree of

bridging (specific interactions) than is seen for Na+

. Thus, the mesophase structural

elements with Ca2+ are larger in dimension.

Specific Interactions Between Calcium Ions and Polyacrylate Chains

The electrophoretic mobilities of the same two NaPA solutions (CaC03 particle-free) were measured in the Coulter DELSA and ^-potentials calculated. Here the velocity of the

charged mesophase structural elements in an electrical field is measured by determining the

Doppler shift of light scattered at four angles, 8.6°, 17.1°, 25.6° and 34.2°. Figures 9 and 10

show that both the systems have negative ^-potentials with the Ca2+ -containing system being

less negative, approximately -30 mV versus -40 mV. Also the latter (Figure 10) has a much

narrower distribution of the ^-potential than the DDI water-only system (Figure 9). It

appears that under the electrical field the mesophase structural elements are of net negative

charge, most likely due to carboxlyate end groups with Na+ and H

+ as counter-ions. The

weakly held aggregates may be in a flux of changing size and charge. When Ca2+was

added, the attractive forces were greater and the aggregates more stable as to size and

charge. Some Ca2+ ions must have been trapped inside the polyacrylate mesophase structural

elements, thus neutralizing part of the negative electrostatic charging. This is the evidence

that among the three kinds of counter-ions in the system, namely, Ca2+

, Na+ and H

+

, the

Ca2+ ions have the strongest specific interactions with the polyacrylate chains.

The specific interactions observed in the solution phase between Ca2+ and polyacrylate

chains may well be the analogy of the interactions occurring at a CaC03 /polyacrylate

interface; thus, further investigation was made by studying flow and viscosity curves of

CaC03 slurries (Figure 11). Two 50 wt% CaC03 slurries were prepared each with the

addition of 2.9 w/w % NaPA based on the weight of the CaC03

. Then 1.6 w/w % CaCl3

189
was added to one of the slurries, also based on the weight of the CaC03

. After mixing

thoroughly in a blender, the two slurries were tumbled on a laboratory rolling mill for 10

days to let the dispersion contents equilibrate. Twenty minutes before the viscometry

experiments, the two slurries were removed from the rolling mill and sonicated vigorously

for ten minutes each using an ultrasonic probe.

In Figure 1 1 the viscosity and shear stress (the left and right ordinates, respectively) were plotted against the shear rate, which was programmed to sweep downwards from 1500

to 1.2 s

1 in 32 steps (each step at a fixed shear rate). At each shear rate, a ten-second shear

was applied prior to the measurements: then, the viscosity and shear stress were measured

over a twenty-second period.

The two slurries showed shear-thinning behavior. For the shear rates lower than 10

s"

1

, the NaPA-only system had a more rapidly decreasing viscosity profile which started to

level off as the shear rate further increased, but the viscosity of the Ca2+ -containing system

continued to decrease and became lower than the NaPA-only system when the shear rate

exceeded 300 s"

1

. The flow curves (the correlation between shear stress and shear rate)

indicate that the NaPA-only system had some dilatancy, while the Ca2+ "containing system had

a higher yield-point value (i.e., more elastic), which suggests that due to the addition of Ca2+

the polyacrylate chains are bonded more strongly to the CaC03 surface as compared to the

NaPA-only system. [9]

In the adsorption of spin-labelled polyacrylic acid onto calcium phosphate surface, it was demonstrated by electron spin resonance (ESR) spectroscopy that the specific interactions

between Ca2+ and polyacrylate chains are more intense than the interactions between Na+

and polyacrylate chains [1]. The polymer was found to be adsorbed in a flat conformation

with more than 90% of its segment adsorbed (immobilized) on the surface. The adsorption

was favored by the Ca2+ because considerably more polymer segments were found in the

conformation of loops when a significant part of the surface Ca2+was replaced by Na+

.

190
CONCLUSIONS

In this paper we have demonstrated that the detailed chemical nature of the surfaces of

a coating pigment can strongly impact the way in which the pigment particles interact with

commercial dispersants. The conformation of a polyacrylate dispersant on the surface of

PCC, for example, is highly dependent upon the concentration of Ca2+ surface sites. By

regulating the surface Ca2+

site concentration to achieve an optimum conformation, the

rheology of PCC-based coating colors may be improved.

ACKNOWLEDGEMENTS

We would like to thank John H. Schaefer of Specialty Minerals for performing the

particle size and surface area measurements and Ali R. Bashey of Specialty Minerals for the

ISS, ESCA and microcalorimetric data.

191
REFERENCES

1. F. Th. Hesselink, "Adsorption of Polyelectrolytes from Dilute Solution" in Adsorption

from Solution at the Solid/Liquid Interface . G. D. Parfitt and C. H. Rochester, Eds.,

Academic Press, Orlando, 1983. p. 377.

2. Y. C. Huang, F. M. Fowkes, T. B. Lloyd, and N. D. Sanders "The Adsorption of

Calcium Ions from Calcium Chloride Solutions onto Calcium Carbonate Particles"

Langmuir, 7(8): 1742 - 1748 (1991).

3. N. D. Sanders and J. H. Schaefer, TAPPI Proceedings, "Zeta Potential Distributions

in Pulp/PCC Mixtures" 1989 Papermakers Conference: 69 (1989).

4. T. Foxall, G. C. Peterson, H. M. Rendall, and A. L. Smith, J. Chem. Soc., Faraday

Trans. I, "Charge Determination at Calcium Salt/Aqueous Solution Interface"

75: 1034 (1979).

5. R. J. Hunter, Foundations of Colloid Science. Oxford University Press, New York,

1987, p. 91.

6. R. J. Goetz, dissertation study, "Formation and Stabilization of Oil in Water

Miniemulsions", in progress, Lehigh University.

7. A. Tuncan, dissertation study, "The Influence of Ocean Disposal of Wastes on

Depositional and Post Depositional Mechanics of Marine Clays", in progress, Lehigh

University.

8. R. T. Klingbiel, H. Coll, R. O. James, and J. Texter, "Colloidal Standards for the

Measurement of Electrokinetic Sonic Amplitude", presented at the 21st Annual

Meeting of the Fine Particle Society, San Diego, California, August, 1990.

192
9. S. Ross and I. D. Morrison, Colloidal Systems and Interfaces . Wiley, New York,

1988, p. 17.

10. N. D. Sanders and A. R. Bashey, Proceedings 1991 International Paper Physics

Conference, TAPPI Press, Atlanta, pp. 473-482.

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199
CHARACTERIZATION OF COAL SLURRIES BY

ELECTROKINETIC SONIC AMPLITUDE (ESA) TECHNIQUE

H. Polat, M. Polat and S. Chander

Mineral Processing Section

Penn State University

University Park, PA 16802

ABSTRACT

The feasibility of electrokinetic sonic amplitude (ESA) technique to characterize coal

slurries was investigated in this study. The ESA of coal samples of different rank was

measured as a function of particle size and electrolyte concentrations. For comparison,

measurements were also made with alumina suspensions. Substantial differences were

observed in the electro-kinetic behavior between coal and alumina samples. The reasons for

the unusual behavior of coal were discussed.

INTRODUCTION

Electrokinetic characterization of concentrated slurries is important in process control.

Concentrated slurries are processed in the ceramics, paint, pharmaceutical and coal industries

daily. Traditional methods of electrokinetic characterization require samples of low solids

concentration which necessitates dilution of the original slurry. In addition, many

characterization methods are constrained to very narrow particle sizes, usually very fine,

which may not be the actual state of the sample to be examined. Although the phenomenon

of acoustophoresis, which is a type of electrokinetic phenomena, has been known for several

decades (Debye, 1933), only recently was it exploited in the study of concentrated slurries

(O'Brien, 1986). Depending on the signal being measured, acoustophoretic measurements

can be divided into two distinct categories: ultrasonic vibration potential (UVP) and

electrokinetic sonic amplitude (ESA). In the former case, an alternating pressure wave, or

sound wave, is applied to a slurry which causes the displacement of the particles suspended

in liquid. A measurable alternating electric field will be developed due to this displacement

200
whose magnitude is a function of inertia of the particles. In the latter case, an alternating

electric field is applied to a suspension of particles. The resulting motion is a vibration of

particles between the electrodes which creates an alternating pressure wave that can be

measured quantitatively and is related to the zeta potential (O'Brien, 1986; James et al.,

1992).

The electrokinetic behavior of coal particles in concentrated slurries may provide

information on coal surface properties and on the interactions among coal particles suspended

in liquid. Such information may be extremely important in explaining coal-water slurry

properties. Coal is predominantly composed of organic material with imbedded mineral

matter and shows a different structure depending on its rank, degree of oxidation and mineral

matter content. Hence, its electrokinetic behavior is difficult to predict. There are numerous

studies in the literature on the electrokinetic behavior of coals (Sun, 1954; Fuerstaneau,

1983; Guiterrez-Rodriguez and Apian, 1984). Nevertheless, they all involve measurements

of the electrophoretic mobility in dilute suspensions. Introduction of the ESA and UVP

techniques provides a good opportunity toward carrying out an investigation with more

realistic slurries. In this study, the feasibility of the ESA technique to characterize coal

slurries was investigated.

EXPERIMENTAL PROCEDURE

The instrument used in the experimental work was a Model 8000 Electrokinetic Sonic

Amplitude Measurement system manufactured by Matec Applied Sciences. It consists of a

main unit which controls four separate probes to measure ESA, pH, conductivity, and

temperature. The system also includes a frequency synthesizer to regulate the alternating

electrical field, an automatic titrator for continuous pH adjustment and a computer to acquire

and process data. All the measurement probes are inserted from the top in a teflon cell of a volume of about 300 ml.

The relationship between the electrokinetic sonic amplitude, ESA, and the zeta potential

was developed by O'Brien (1986) and may be written as:

201
C G(«) = (1)

where

ia(3 + ^)

6(0)

9[l+(l-i) °

] 2J

and

a = ttaap "« (3)

ESA is the electrokinetic sonic amplitude which is the pressure sensed by the probe due to

oscillating particles per unit length of electrical field in units of mPa/V.m, £ is the zeta

potential of the particle in V, 77 is the viscosity of the liquid in kg/m.sec, e is the dielectric

permittivity of the suspension, $ is the volume fraction of the particles, Ap is the density

difference between the particles and the liquid in kg/m3

, pliq is the density of the liquid in

kg/m3

, *> liq is the velocity of sound in the suspension in m/sec, G(a)"1 is the inertial term, a is

the particle radius in m and co is the angular frequency in sec"

1

. The complex quantity, G(a), takes into account the inertial effects of particles. This

term is equal to unity and drops out for particles less than about 0.3 /xm for alumina and

about 1.0 /xm for coal. For particles larger than these sizes, the zeta potential has to be

corrected for the inertia effect using G(a). Following appropriate mathematical

manipulations, G(a) can be expresses as a real quantity as:

202
G(a)

1

= i (1 +

9 (A 2 + A + 0.5)

(3^ 3 + 2A 3 _£L_ )2 + ( il+A) (3A 2 + 2A 2

9 (A 2 + A + 0.5)

(4)

where

A = (5)

N 2

It has to be noted that the calculations given above are valid for relatively dilute slurries, less

than 10% by volume, and for /ca values greater than 50 where k is the reciprocal thickness of

the double layer. Calculations using the ionic strength of the double distilled water used in

our studies resulted in a /ca value of 48 which is well within the limitations of the formulae

above.

Ions in solution generate electroacoustic signals as was predicted by Debye in 1933 and

lead to an appreciable background signal at high ionic strengths. This phenomenon might

become especially important in cases where a weak ESA signal should be expected, such as

at low solids concentration and where there are small density differences between the solution

and particles. In addition, small ESA readings near the point of zero charge is another factor

for amplifying the significance of the background signal. Hence, a background correction

which would eliminate the contributions from the ion in solution may be necessary under

certain circumstances.

A 7-alumina and two coal samples, namely anthracite and a sub bituminous A, were used

in this study. The 7-alumina was used to establish the measurement procedure since its

electrokinetic behavior was well known. Alumina samples of three nominal sizes, namely

0.05, 0.3 and 1.0 ^m, were obtained from the Buehler Company, Illinois.

MATERIALS

203
The sub bituminous A coal and anthracite samples were obtained from Penn State Coal

Data Bank. The samples were from Colorado Q Seam in Moffat County-Craig Township,

Colorado and the Primrose Seam in Schuylkill County-Coaldale Township, Pennsylvania,

respectively. Approximate analysis of the coal samples are given in Table 1. Before

the electrokinetic analysis the coal samples were ground below nominal -37 fim and observed

to have similar size distributions. The mean size of the samples were about 12 jum and well

within the range of the practical size requirements of the ESA device. According to the

manufacturer the device should be able to measure electrostatic signal for particles ranging in

size from 150 //m down to the molecular sizes (Matec manual, 1989).

A suspension of the sample was prepared using double distilled water. It was placed in

an ultrasonic bath for 30 minutes to disperse the particles and the slurry temperature was

adjusted to 23.0° in a temperature bath. The sample was placed in the cell of the ESA

device. A moderate stirring was applied in the cell to prevent settling of the particles.

Table 1. Characteristics of coal samples used in this investigation.

Sub-bituminous A coal Anthracite

Component, % As Received Dry As Received Dry

Moisture 13.03 3.15

Ash 5.65 6.50 11.00 11.36

Carbon 61.14 70.30 81.09 83.73

Hvdrogen 4.10 4.71 1.50 1.55

Nitrogen 1.64 1.88 0.67 0.69

Total Sulfur 0.32 0.37 0.46 0.48

Oxygen 14.12 16.24 2.12 2.19

204
RESULTS AND DISCUSSIONS

Effect of Solids Concentration

To determine the change in ESA with varying amounts of solids and to choose an

appropriate solids concentration for the subsequent tests, experiments were carried out with

7-alumina, at different slurry concentrations. Three slurry concentration of 0.1%, 1.0%,

and 5.0% by weight were investigated. The results are given in Figure 1. As expected, the

point of zero charge did not change when the amount of alumina in the slurry was

increased. However, the strength of the ESA signal decreased drastically with decreasing

alumina concentration as predicted by Equation 1 . It was observed that a solids

concentration of at least 5.0% was desirable to obtain a strong signal, especially in the case

of coal samples whose densities were small compared to the alumina sample.

Effect of Particle Size

A set of experiments were conducted with alumina using three different samples with

mean particle sizes of 0.05^m, 0.3 ^tm and 1.0 ^im. The results are given in Figure 2.

Particle size was seen to have an effect on the magnitude of the ESA signal. The point of

zero charge also displayed a slight shift to the acidic side with increase in particle size.

Increase in the ESA signal strength with increasing particle size is contrary to Equations 1

through 4 which predict the signal strength should decrease with increasing particle size

above 0.3 /xm and no change in the point of zero charge should take place. To explain this

behavior, zeta potential was measured using conventional electrophoresis technique with the

0.05 [im and 1.0 alumina samples. The sample displayed the same amount of shift in the

point of zero charge as in the ESA experiments, suggesting that this behavior was due to some chemical differences in the sample characteristics. Further experimentation with this

sample was discontinued.

To determine the effect of particle size on ESA measurements with coal, sub bituminous

A coal sample was used. Samples with different size distributions were obtained after

grinding for 1, 2, 4, 8 and 16 minutes in a laboratory Bleuler mill. Size distributions of the

samples were determined with a light scattering device, Malvern 2600c and are given in

205
Figure 3. The measured ESA are given for various grinding times in Figure 4. In order to

correct for the inertial effects, an effective inertial term, G(a)eff, was computed with the

assumption that a given sample consisted of n discrete size intervals. The following

relationship was used:

where f; is the volume fraction of the zth size interval and G(a\ is the interval term for the

zth size interval. The term, G(or)eff was inserted in Equation 1 to calculate the corrected zeta

potentials for each grinding time and the results are given in Figure 5. The figure shows a good agreement for the zeta potentials between the samples towards high pH values. The

shift in the point of zero charge with increase in the grinding time at lower pH values might

be due to creation of new unoxidized surface by grinding. It is well known that the point of

zero charge moves to acidic side as the oxidation of the coal sample increases (Wen and Sun,

1977). The higher than expected zeta potential for the 1 minute ground sample was ascribed

to the partial settling of this sample in the ESA cell which would have reduced the average

size of the particles in the suspension. Since the G(a)eff term in Figure 5 was estimated using

the actual size distribution, it resulted in overestimation of the zeta potentials (closed circles

in Figure 5). If the G(a)eff term computed for the sample ground for 2 minutes was used to

correct for the inertial effects, the 1 minute-curve approached the rest of the curves (open

circles in Figure 5).

Effect of Electrolyte Concentration

Background Correction

Presence of ions in solution generate an electroacoustic signal. Addition of ions during

the titration or adjustment of the ionic strength of the solution by electrolyte addition may

increase the magnitude of this secondary signal to such an extent that it may be necessary to

correct for it. In such a situation, the background signal should be subtracted from the actual

ESA readings. The subtraction process has to be done vectorially since the ESA is a vector

206
quantity. In a recent study to determine the effect of electrolyte concentration on the

background signal on the ESA, Desai et al. (1993) measured ESA of different electrolyte

solutions as a function of pH, which were subtracted from the ESA of the slurries. The

procedure required the selection of a common reference for both electrolyte solutions and

the slurries. A dispersion of sample solids in distilled water was chosen as the reference

material against which both the actual and the background signal were measured as a function

of pH for different electrolyte concentrations. The "true" ESA signal was obtained using the

equations:

ESA^Cotf = ESAmeasuredCos0 - ESAbackgroundCos$ (7)

and

ESA^SinjS = ESAmeasuredSin0 - ESAbackgroundSin$ (8)

The various angles in Equations 7 and 8 are defined in Figure 6. The authors found that the

electrolyte concentrations above 0.01M required background correction for the samples they

utilized. They also stated that the background signal due to addition of acid or base during

the titration cycle was negligible in the pH range of 4 to 10.

A second procedure was followed in our experiments in which the reference selection was different. Instead of choosing a dispersion of solids in pure water as the reference, the

dispersion of solids containing the appropriate amount of electrolyte was employed.

Measurements on both the actual suspension and the blank electrolyte solution were carried

out as a function of pH with respect to this reference. Equations, 7 and 8 were used to

calculate the true magnitude of the ESA signal after background electrolyte correction. A

comparison of the background correction using both the procedure of Desai et al., (1993) and

that used in this study is given in Figure 7 for an the alumina sample in 0. 1M KN03

solution. The figure shows that the true ESA signal after correction is significantly different

than the measured ESA signal at this high electrolyte concentration, including a notable shift

in the point of zero charge. The true ESA signal estimated by both the methods was quite

similar. It was also observed in our study that the background electrolyte correction was not

necessary for a pH range of 2 to 12 and when the electrolyte concentration was less than or

207
equal to 0.01 M. For a concentration of 0.1 M the background electrolyte correction was

necessary. Therefore, the background correction in the absence of electrolytes was ignored

in the other sections of this investigation. The ability to measure zeta potentials at higher

electrolyte concentrations is quite useful since difficulties are encountered with conventional

electrophoresis methods.

ESA Corrected for Background Electrolyte for Alumina and Coal Slurries

A set of experiments with alumina were carried out at different ionic strengths of 0.1 M,

0.01 M, and 0.001 M obtained by adding an indifferent electrolyte, KN03

. The ESA results

corrected for background electrolyte are given in Figure 8. It was found that the background

electrolyte correction was necessary only for the electrolyte concentration of 0.1 M. There

was no significant changes in the ESA at lower electrolyte concentrations. However, the

ESA signal decreased appreciably at 0.1 M electrolyte concentration, most probably due to

the depression of the double layer and, therefore, small zeta potential values at the slipping

plane. This data is in agreement with the data reported by Malghan and Lum (1990) for

Si3 N4 powders in the presence of electrolytes. It may be noted that a shift in the point of

zero charge to the basic side at high ionic strength prevailed even after the background

correction, suggesting a different phenomenon to be responsible for such a behavior.

Another set of experiments at the same electrolyte concentrations were carried with the

anthracite and sub-bituminous A coal samples and the results are given in Figures 9 and 10,

respectively. A decrease in the ESA signal was observed with increasing electrolyte

concentrations as expected with no noticeable change in the point of zero charge at low

concentrations of KN03

. At an electrolyte concentration of 0.1 M, the ESA signal remained

in the negative side for both coal samples and did not change its polarity in the pH range

studied. Yoon and Sabey (1988) observed a decrease in the electrophoretic mobility of coal

and a shift in the point of zero charge to the acidic side for all the electrolyte concentrations.

The observed behavior suggests that the ESA measurements may be affected by some

secondary factors at very high ionic strengths such as strong adsorption of the anions to the

coal surface. Nevertheless, further studies are needed to explain the reasons behind such a

behavior. Therefore, meaningful interpretation of the electrophoretic data should be done

208
with caution under these circumstances.

Reversibility of Titration

To determine the reversibility of titration the suspension was subjected to a titration with

an alkali after completing the acid titration. For the alumina sample, it was observed that

neither the magnitude of the measured signal nor the point of zero charge changed

significantly. However, both coal samples studied showed a different behavior when the

same suspension was subjected to a second titration with an alkali. The results are given in

Figures 11 and 12 for anthracite and sub-bituminous A coal, respectively. For both coals,

the ESA signal remained in the positive side throughout the pH range covered during the

second titration. A change in the sign of the ESA was not observed. To understand this

behavior, an additional experiment was conducted in which second titration was carried out

after filtering the solids and adding fresh water following the first titration. In the case of

anthracite, the signal obtained was identical to the one obtained in the first titration process

and crossed the point of zero charge value at the expected pH ("Second titration after

filtering" curve in Figure 11). For the sub-bituminous coal, on the other hand, filtration

alone was not enough to restore the original surface characteristics since the ESA signal still

remained positive as can be seen from the data marked as "Second titration after filtering" in

Figure 12. Filtration followed by a thorough washing of the solids was necessary to restore

the electrokinetic characteristics of the sub-bituminous coal. For the filtered and washed

sample the point of zero charge was observed at the expected pH as can be seen from the

data marked as "Second titration after filtering and washing" in Figure 12. These results

show that the surface characteristics of the coal particles were restored after filtration and

thorough washing. The change in the ESA is probably due to some surface active species

which weakly adsorb on the surface.

209
SUMMARY AND CONCLUSIONS

The feasibility of electrokinetic sonic amplitude (ESA) technique to characterize coal

slurries was determined in this investigation and the following conclusions were made:

1 . Zeta potential could be estimated using correction factors for inertial effects. To

determine the correction factor a good estimate of particle size distribution was

necessary. In addition, some secondary effects such as settling in the cell or changes in

the surface with change in particle size might also have to be taken into account.

2. Two procedures were used to make corrections for background electrolyte. Both the

procedures gave comparable results. The background ESA correction was negligible for

electrolyte concentrations less than or equal to 0.01M for KN03

. Hence, for most

purposes, this correction might not be necessary.

3. Reversibility of titration was observed for alumina but not for the coal samples. For

both the coal samples, the values of ESA in the second titration with alkali were

markedly different than those obtained in the first titration with acid. In the reverse

titration, the ESA did not change sign at the point of zero charge. For anthracite,

filtration of the suspension after the first titration and addition of fresh water restored the

original behavior, suggesting some form of weak specific adsorption onto the anthracite

surface. For the sub-bituminous A coal, the adsorption was reversed only after filtration

and thorough washing of the coal surface. These results imply that the strength of

adsorption was a function of coal type. Further studies are needed to establish the

mechanisms.

Acknowledgements

The authors acknowledge the financial assistance from the U.S. Department of Energy

through its cooperative agreement No. DE-FC22-92PC92162 in support of this work.

210
REFERENCES

1. Debye, P. W., J. Chem. Phys., Vol. 1, pp. 13, 1933.

2. Desai, F. N., Hammand, H. R., and Hayes, K. F., Background Electrolyte

Correction for Electrokinetic Sonic Amplitude (ESA) Measurements, This publication,

1993.

3. Fuerstenau, D. W., Rosenbaum, J. M., and Laskowski, J., Effect of Surface

Functional Groups in the Floatability of Coal, Colloids and Surfaces, Vol. 8, pp. 153-

174, 1983.

4. Gutierrez-Rodriguez, J. A. and Apian, F. F., The Effect of Oxygen on the

Hydrophobicity and Floatability of Coal, Colloids and Surfaces, Vol. 12, pp. 27-51,

1984.

5. James, M., Hunter, R. J., and O'Brien, R. W., Langmuir, Vol. 8, pp. 420, 1992.

6. Malghan, S. G. and Lum, L., Factors Effecting Interface Properties of Silicon Nitride

Powders in Aqueous Environment, Am. Cer. Soc., Ed. G. L. Messing et al., Vol. 12,

pp. 403-411, 1990.

7. MATEC ESA-8000 System Manual, MATEC Applied Sciences, 75 South Street,

Hopkinton, Ma, 01748, 1989.

8. O'Brien, R. W., Electro-Acoustic Effects in a Dilute Suspension of Spherical Particles,

J. of Fluid Mechanics, Vol. 190, pp. 71-86, 1986.

9. Sun, S. C, Hypothesis for Different Floatabilities of Coals, Carbons and Hydrocarbon

Minerals, Transactions of AIME, Vol. 199, pp. 67-75, 1954.

10. Wen, W. W. and Sun, S. C, Transactions of AIME, Vol. 262, pp. 174-180, 1977

11. Yoon, R. H. and Sabey, J. B., Coal Flotation in Organic Salt Solutions, Interfacial

Phenomena in Coal Technology, Edited by Gregory D. Botsaris and Yuli M. Glazman,

Vol. 32, pp. 87, 1988.

211
FIGURE NAMES

Figure 1 . Effect of solids concentration on ESA for 7-alumina.

Figure 2. Effect of particle size on ESA for 7-alumina.

Figure 3. Size distributions of the sub-bituminous A coal samples for various grinding

times.

Figure 4. Measured ESA values for the sub-bituminous A coal samples for various

grinding times.

Figure 5. Zeta potential of sub-bituminous A coal samples calculated from ESA values.

Figure 6. Background correction for electrolyte signal.

Figure 7. ESA values measured and corrected for background electrolyte for 7-alumina

sample in 0.1M KN03 solution.

Figure 8. Effect of electrolyte concentration on ESA for 7- alumina.

Figure 9. Effect of electrolyte concentration on ESA for anthracite.

Figure 10. Effect of electrolyte concentration on ESA for sub-bituminous A coal.

Figure 11. Effect of acid-base titrations on ESA for anthracite.

Figure 12. Effect of acid-base titrations on ESA for sub-bituminous A coal.

212
E

>

"re o_E

<

LU

0.8

0.4

0.0

-0.4

-0.8

1 ' 1

1 1 r ALUMINA

V 5.0%SOUDS

1.0% SOLIDS n 0.1% SOLIDS

V

Do °onnnnnn»»^ ffl y° ° U U B V

V

V

V

V

6 8

PH

-J i_

10 12

Figure 1. Effect of solids concentration on the ESA for 7-alumina.

E

>

CL

E

<

COUJ

0.8

0.4

0.0

-0.4

-0.8

Jg&iv ALUMINA/5%' solid's MEAN PARTICLE SIZE O 1.0 iim "Ttln V 0.3 pm

""••a Cg_ • 0.05pm

5.

1 i . 1

V

\

"

V

1 8 10 12

PH

Figure 2. Effect of particle size on the ESA for 7-alumina.

213
Figure 3. Size distributions of the sub-bituminous A coal samples ground for various times.

214
10

>

E

-J<

Z

UJ

I-O

UJN

•10 -

-20

-30

—1— 1 1 1

SUB BITUMINOUS A

^\\\V urUIMUINu 1 IML S>^ta|. #01 min. V/Nx^ v 2 min. XSJ. * 4 min. ^SL. o 8 min. >v ^^L, 16 min.

1 I I I

PH

Figure 5. Zeta potential of sub-bituminous A coal samples calculated from ESA values.

ESA.

ESA

REFERENCE . , (Solids in electrolyte solution)

Figure 6. Background correction for electrolyte (Desai et al., 1993)

215
0.8

>

E

<

m

0.4

0.0

-0.4 ALUMINA IN 0.1M KNQ,

O ESA

-0.8

ESA^ (reference: pure water)

ESA^ (reference: electrolyte coin.)

2 10 12

PH

Figure 7. Measured and corrected ESA's for 7-alumina sample at 0.1M KN03

.

2 4 6 8 10 12

PH

Figure 8. Effect of electrolyte concentration on the ESA for 7- alumina.
o IDO o

o

in

o

G o in

o

in©

o

I— in

6 d d d d d d d dg d d

uj-A/BdUJ 'VS3 uj"A/BdLU 'VS3

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00 uj-A/BdUJ 'VS3 WA/Bdw 'VS3

217
i

a.

0.15

0 10

0.05

0.00

S -0.05

UJ

-0.10 -

-0.15

ANTHRACSTE

AAAAaaaAAAAA

SECOND TITRATION

1

DQ FIRST TITRATION

°oBb

SECOND TITRATION

AFTER FILTERINGHON * BHBiBBi

8

PH

Figure 1 1 . Effect of acid-base titrations on the ESA for anthracite.

(CP!

0.10

0.05 -

0.00

-0.05 -

-0.10 -

-0.15

r SECOND TITRATION AFTER FILTERING _ ,»V' V SECOND

TITRATION

SECOND TITRATION AFTER

FILTERING & WASHING

FIRST TITRATION

SUB BITUMINOUS A

1 2 3 4 5 6

pH

Figure 12. Effect of acid-base titrations on the ESA for sub-bituminous A coal.

218
SURFACE CHARGE CHARACTERIZATION OF MINERALS BY ELECTROACOUSTIC

MEASUREMENTS

B. M. Moudgil and R. Damodaran

Department of Materials Science and Engineering

University of Florida

Gainesville, FL 32611

ABSTRACT

Surface charge characterization of minerals is important in developing suitable solid-solid

and solid-liquid separation techniques. Adsorption of surfactants or polymers on solids,

which is dependent on the surface charge characteristics, govern the efficiency of physicochemical processes such as flotation and flocculation. Generally, relative surface charge

measurements are adequate for process development purposes. Electrokinetics measurements were employed until a few years ago to evaluate the surface charge behavior of dilute as well

as concentrated suspensions of minerals. Presently, electroacoustic techniques are being

evaluated for surface charge characterization. In this paper, the opportunities and limitations

of electroacoustic measurements, as applied to mineral processing operations are discussed.

INTRODUCTION

The study of colloidal phenomena is important in a vast array of materials, processes and

physical properties. Colloids are typically liquids containing very small particles ranging in

size from 1 to 1000 nanometers, common examples of which are milk, blood, pigments,

pharmaceuticals, inks, dyes, ceramics and clays. In mineral processing, the theories of

colloidal phenomena have been used for surface charge characterization of suspensions in

which the particle sizes are in the range 0.2 to 10 micrometers. Many properties of colloidal

suspensions, such as dispersion/aggregation, polymer and surfactant adsorption on solid

particles, and aging phenomena are determined, directly or indirectly, by the presence of

electrostatic charge on the surface of the particles in suspensions. The charge on the

particles can develop from a variety of mechanisms, such as ionization of surface chemical

groups, adsorption of ions, or unequal dissolution of ions from an ionic crystal lattice. The

surface charge of the particle is balanced by an equal and opposite charge of "counter-ions"

219
distributed as a diffuse cloud around the particle. The combined region of surface and

diffuse charge is known as the "electrical double layer". The electrical potential at the plane

of shear is termed the zeta potential which determines the electrostatic interaction forces

between the particles. Important colloid properties, such as suspension rheology and

suspension stability towards aggregation, are strongly influenced by the zeta potential.

Surface charge characterization of particles in suspensions find wide applications in

important mineral processing operations such as flotation, filtration, sedimentation and

selective flocculation. The desired state of dispersion or aggregation of the particles in

suspension, required in such operations are achieved by control of the surface properties

including its charge characteristics. Modification of the surface properties may be brought

about by adsorption of surfactants or polymers. Polymer and surfactant interactions with the

mineral particles are governed by hydrogen bonding, covalent bonding, electrostatic or

hydrophobic interactions. Electrostatic interactions of polymer and surfactants are

determined by the charge of the polymer or the surfactant as well as that of the solid

substrate. Surface reactions in particles such as aging phenomena, which play an important

role in determining surface properties as a function of time, can be monitored by surface

charge characterization of the particles. Both dilute and concentrated slurries are

encountered in mineral processing, which emphasizes the use of techniques to characterize

surface charge both under dilute and concentrated conditions.

ELECTROKINETIC MEASUREMENTS

Until recently, the measurement and control of zeta potential in industrial processes has

been done by electrokinetic measurement techniques which involve one of the four classical

electrokinetic phenomena; electrophoresis, electro-osmosis, streaming potential and

sedimentation potential. Of these four electrokinetic effects, electrophoresis and the

streaming potential are more frequently employed in studies related to minerals. In

microelectrophoresis, the velocity of individual particles under the influence of a known dc

or low frequency electric field, is measured in extremely dilute suspensions (less than 100

ppm). Such low solids concentrations do not truly represent the practical systems, hence, the

220
data obtained from such measurements has to be extrapolated to the conditions of the real

system, which may not always be correct. Nevertheless, microelectrophoresis has been used

extensively to characterize surface charge behavior of solid-liquid and liquid-liquid interfaces.

It should be noted that more often it is the value of the iso-electric point and the trends in the

behavior of the disperse systems that have significance, compared to the specific calculated

values of the zeta potentials.

Applications in Mineral Processing

Electrokinetic measurements of zeta potential have been used to correlate bulk properties

such as viscosity of suspensions with the surface chemical behavior of particles. A typical

correlation between viscosity of concentrated alumina slurries (40 vol%) with the zeta

potential of alumina measured using dilute suspensions (100 ppm) is shown in Figure l. m A

sharp increase in viscosity (at pH 4.5-5.0 and again at pH 10.8-11.2) is observed with a

slight increase in the zeta potential. These observations suggest that, zeta potentials

measured using microelectrophoresis may not be directly correlated with the viscosity of a

concentrated suspension. Electrokinetic measurements have also been used to study polymer

adsorption onto mineral particles and polymer-surfactant interactions. Zeta potential

measurements were conducted to investigate the nature of interactions between

pol yaery1 amide type polymers, and anionic and cationic surfactants, in a study on the effects

of such interactions on adsorption of various species on hematite and silica (see Figure 2).

[2]

The effect of higher pulp densities encountered in mineral processing operations, however,

can only be speculated because under such conditions the suspensions could aggregate,

making it difficult to keep the particles suspended during the electrophoretic measurements.

The electrokinetic behavior of the individual minerals and the changes taking place at the

solid/liquid interface, when other minerals are present, are important considerations in

flotation. Changes in the zeta potential of apatite and dolomite as a function of pH were

used to study dissolution behavior, which affected the flotation results during separation of

phosphate rock from dolomite.'31 A correlation between mobility, settling, and surfactant

adsorption density, shown in Figure 3,

141 emphasizes the use of electrokinetic measurements

to monitor bulk properties. Again, the effect of solids loading can only be speculated. An

221
500

400

o 300

>—o-

& 200

>

100

• viscosity

o zeta potential

0

40

30

20

10

>

0 ui

Shear Rate: 20 s \ .

-10

-20

-30

-40

o

CL

UJ

6 8

pH UNITS

10 12

Figure 1 . Correlation between zeta potential and viscosity (40 vol% alumina)

222

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£

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cr

o

CO

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<

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CO

-3

-2

0

+ 1

+ 2

J + 3

+4

J + 5

io

3

0.

EQUILIBRIUM CONCENTRATION, mole/ liter

Figure 3. Correlation between mobility, settling and surfactant adsorption density'41

224
useful review of zeta potential studies in mineral processing has been published by

Mackenzie. [5] The following technique for surface charge characterization of relatively

concentrated suspensions became available in the last decade.

ELECTROACOUSTIC MEASUREMENTS

Acoustophoretic technique can be applied to characterize surface charge of colloidal

particles (less than 1.0 micrometers). A wide range of sample concentrations, ranging from

100 ppm to 50 vol% can be measured using this technique. Absence of optical imaging

permits characterization of opaque systems. The most important advantage is the potential

use of acoustophoresis in on-line analysis of real systems as in flocculation processes, quality

control, dispersion and emulsion formulation, raw materials screening and process

monitoring.

The electroacoustic instrument measures the response of charged particles to alternating

electric and pressure fields instead of using static electric or pressure fields such as in

electrophoresis or streaming potential methods. An applied alternating field induces a

tangential motion between the particles and the liquid, resulting in a periodic distortion of the

particle's electrical double later. If there is a density difference between the suspended

particles and the liquid, the following electroacoustic phenomena occur;

1. When an alternating electric field is applied, the relative motion between the particles

and liquid generates a sound wave at the frequency of the applied field. The particles

vibrating in the electric field radiates sound which sums up to a coherent sound wave.

This effect has been termed the Electrokinetic Sonic Amplitude or ESA of the colloid

and is defined as the pressure amplitude per unit electric field generated by the colloid,

and has units of pascals per volt per meter.

2. In the case of an applied alternating pressure field (sound wave), the inverse of the ESA

effect occurs. The relative motion between the particles and liquid, due to their density

difference, creates a periodic polarization of the electrical double layers and an

alternating dipole moment at the frequency of the applied field, which sum to a potential

225
that can be detected by placing a pair of electrodes in the sound field. This effect is

called the Ultrasonic Vibration Potential or UVP and was first predicted for electrolyte

solutions by Debye in 1933. [6] UVP has units of volts per meter per second. The UVP

in colloidal systems was first studied theoretically by Enderbym in 1951 and more

recently by O'Brien 181 and White.'91

The electroacoustic measurement system as illustrated in Figure 4, generally consists of a

signal generator with a frequency synthesizer, a titrator assembly and a computer system

controller. The titrator assembly consists of a sample cell and the sensors for detecting pH,

temperature, conductivity, electroacoustic signals, and a high torque stirrer. The probe

assembly can be placed into a cell of any geometry and hence is ideal for on-line analysis.

Figure 4. Schematic diagram of an ESA sample cell

Relation Between ESA. UVP and Zeta Potential

Rigorous theoretical treatments have been developed by O'Brien, 181 Marlow et al.,

[10] and

Babchin et al.

[U1

, to arrive at relationships between UVP, ESA, the zeta potential and the

electrophoretic mobility. A relationship between ESA, UVP and a high frequency or

dynamic mobility (jxj of the form

226
UVP(K*) = ESA = cAp0/xu7

has been predicted, where K* is the complex conductivity of the dispersion, c is the speed of

sound in the fluid, Ap is the density difference between the particle and the fluid, 4> is the

volume fraction of the particles in the fluid, and 7 is a geometrical and acoustic coupling

factor. O'Brien gives an approximate relationship between p. w and f for the case where /ca is

less than 10:

^ = (2er/3u)(l+/)G(a)

where e is the dielectric permittivity of the fluid, rj is the fluid viscosity, /is a complex function

of the frequency of the applied field and the diffusivity of the electrolyte ions in solution; G(cv),

a damping term, is a complex function of the frequency of the applied field (co), the average

radius of the particles in solution (a), and the kinematic viscosity (p), such that

a = coa

2 /V

and

G(a) = [1 -

1 w, 2A,ck —ia{3 + ——

9 P

1 + (1-0

a

2

where p is the fluid density. At high /ca, the ratio of na to p. e may be approximated to

pjpt = 2/3(1 +/)G(o)

In general, the magnitude of the ESA depends on the solids concentration in the suspension, the

zeta potential of the particles and the density difference between the particles and the suspension

medium.

227
Experimental Verifications

A number of researchers have demonstrated excellent agreement between the calculated

zeta potentials from electroacoustics and microelectrophoresis for a range of solids using the

above mentioned theoretical treatments. However, it has been found that frequently the

magnitude of the zeta potential calculated from dynamic mobilities are different from those

calculated from other electrokinetic techniques. It was demonstrated by Scales and Jones[12]

that this discrepancy was due to an error in the input of the particle size. These investigators

have arrived at correlations between the dynamic mobility and the electrophoretic mobility

for different particle sizes and frequencies (see Figure 5). The required volume fraction to

ensure an error of less than 5% in mobility, as a function of the density difference, plotted in

Figure 6, for two different values of ionic strengths, arrived at, highlights the importance of

electrolyte signal in such measurements. (12] The density difference between most minerals

and water is normally high enough to use a suspension concentration of 1 to 2 vol%. The

use of electroacoustic techniques for the accurate measurement of zeta potential is

straightforward for monosized particles and for polydisperse dispersions where the particle

size is distributed in a normal fashion about the mean. Marlow et al. [10] have reported good agreement between measured values of

electrophoretic and electroacoustic mobilities for a 700 ppm sample of Ti02 in 0.001M KC1

as a function of pH (see Figure 7). The electroacoustic mobilities agreed to within 10% of

the electrophoretic mobilities, thus independently verifying the UVP theory for dilute

suspensions. For higher suspension concentrations, a knowledge of the continuous phase

conductivity is required to compare experimental and theoretical values. Electroacoustic

mobilities for colloidal silica (3.5 nm particles) at 20% w/w as a function of pH, measured

by Marlow et al. [101 highlights the advantage over electrokinetic techniques in characterizing

colloids which are optically opaque.

For polydisperse samples, O'Brien et al.

[13] have shown that the effective mass average

particle size has to be taken into consideration for accurate measurements of zeta potential

using the electroacoustic techniques. A comparison of zeta potential, obtained from

microelectrophoresis and electroacoustics, as a function of pH is provided in Figure 8.

228
2.0 <j cm

3.0 " ~

4.0 ** ~~

5.0 -

-

-5 \ \

v- \\\\\\

o

0.01

' 4 * A 0.10 1.00

PARTICLE RAOIUS (/zm)

10.00

Figure 5. Calculated ratio of dynamic to electrophoretic mobilities (for KCI at 0.1 mol dm*

particle relative permittivity of 5 and a zeta potential value of 30mV)I12]

229
0.0 1.0 2.0 3.0 4.0 5.0

Ap

Figure 6. Volume fraction versus density difference for a 5% error cut

(for two KCI concentrations) 1121

230
Ac oustophor etic

-4

2 ~T" ~T~

~~5~"

10 12

pH

jure 7. Comparison of electrophoretic and acoustophoretic mobilities

(data is for a 700 ppm Ti02 sample in 0.001 M KCI) 1101

231
Figure 8. Effect of effective mass average radius correction on zeta potentials

(alumina in 10^M KCI solution) 1131

232
The values of zeta potential, obtained using initial estimates of the mass average particle size on a 0.5 vol % suspension, are low compared to the data obtained using

microelectrophoresis. However, electroacoustic results of a well dispersed sample show

excellent agreement with the microelectrophoresis results. There is an excellent agreement

of the corrected electroacoustic results, using light scattering estimates for the effective mass

average radius at each pH, with the microelectrophoresis values.

Miller and Berg[141 found good agreement of microelectrophoresis mobility data for

titania, to inertially-corrected mobility data calculated from ESA measurements upto the isoelectric point of titania, beyond which particle aggregation was reported to result in deviation

between the two curves. Figure 9 compares the two techniques for titania/PVA adlayer

system. Electroacoustic techniques have been demonstrated to be useful in measuring the

effect of polymer molecular weight on zeta potential. It has been suggested that

electroacoustic studies on titania dispersions with an adsorbed layer of PVA can be used to

calculate an effective adsorbed layer thickness on the particles.

All the above mentioned studies have been conducted in suspensions of not more than 5

vol% solids loading, which again, is not representative of the slurries encountered in mineral

processing applications. Such operations involve solids loading of 20 vol% or more.

Additionally, typical ceramic powders processed are at solids loading around 40 vol% or

more. At such high concentrations, where particle-particle interactions cannot be ignored,

existing theories of particle behavior cannot predict the bulk properties satisfactorily. Direct

measurements of mobility at such a high solids loading have been reported by Springgate[15]

using electroacoustics. The effect of pH on the ESA values in a 40 vol% alumina suspension

were shown to follow the same general trend as in dilute suspensions (see Figure 10).

However, the pH range in which ESA values are high is broader than that for a dilute

suspension, indicating that concentrated alumina slurries can be stabilized at higher pH values

than the dilute suspensions.

233
1 1 1 1 ri i r • i i i r~rrr iiii —

i—r—

r

j—

Rank Bros.

o MATEC(G=1.0)

It .1. I_ i i i i i i i i t . i ii iiii. iiii

' iiii

3 4 5 6 7 8 9 10

pH

Figure 9. Comparison of electroacoustic and electrophoretic mobilities

for a polymer coated titania dispersion1141

234
235
CONCLUSIONS

Electrokinetic techniques for surface charge characterization have traditionally been used

to study surface chemical phenomena and arrive at correlations with bulk properties in

mineral processing applications. Recently developed electroacoustic technique have made it

possible to determine surface charge behavior of relatively concentrated suspensions.

Dynamic mobility is related to electrophoretic mobility and an excellent correlation has been

observed in zeta potential trends obtained by electroacoustic and electrophoretic techniques.

Electroacoustic methods have a distinct advantage over electrokinetic measurement

techniques.

Mineral suspensions are heterogeneous in nature and a range of particle size distribution

is encountered. Electrokinetic measurements, which require small sample volumes and can

be conducted only in dilute suspensions, have been adequate for most mineral processing

applications. However, such measurements are limited to suspensions which are optically

transparent and cannot be used in high viscosity media. Electroacoustic measurements, on

the other hand, require larger samples, although smaller volumes can be used with a parallel

plate sensor, and can be done in concentrated or flocculated suspensions. There are no

optical or viscosity limitations to the type of suspension used and measurements in stirred

and flowing systems are possible with a probe sensor.

Electroacoustic measurements for surface characterization are used in areas such as basic

characterization of the solid-liquid or liquid-liquid interface of disperse systems, screening raw materials for differences in surface properties, and monitoring processes in real-time.

Industrial applications of electroacoustic measurements include scale-up of flocculation

processes, preparation of coal-water slurries, dispersion of clays and other minerals,

evaluation of dispersing agents, and dewatering of clays.

ACKNOWLEDGEMENTS

The authors acknowledge the Minerals Resources Research Center, University of Florida

for their financial support towards this work.

236
REFERENCES

1. Kulkarni, N., M.S. Thesis, University of Florida, 1993.

2. Moudgil, B. M., Engg. Sci. D. Thesis, Columbia University, New York, 1981.

3. Moudgil, B. M. and Ince, D., Surfactants in solution, 10, 1989, Ed.K.L. Mittal,

Plenum Press, New York.

4. Somasundaran, P., Ph D. Thesis, University of California, Berkeley, California, 1964.

5. Mackenzie, J. M. W., Mineral Sci. Engg., 3(3), 25, 1971.

6. Debye, P., J.Chem. Phys., 1, 13, 1933.

7. Enderby, J., Proc. Phys. Soc., 207, 321, 1951.

8. O'Brien, R. W., J. Fluid Mech., 190, 71, 1988.

9. White, L. R., Trans. Faraday Soc., 1987.

10. Marlow, B. J., Fairhurst, D., Pendse, H. P., Langmuir, 4,611, 1988.

11. Babchin, A. J., Chow, R. S., Sawatzky, R. P., Adv. Colloid Interface Sci., 30, 11,

1989.

12. Scales, P. J. and Jones, E., Langmuir, 8, 385, 1992.

13. James, M., Hunter, R. J., O'Brien, R. W., Langmuir, 8, 420, 1992.

14. Miller, N. P. and Berg, J. C, Colloids and Surfaces, 59, 119, 1991.

15. Springgate, M. E., M.S. Thesis, University of Florida, 1990.

237
SURFACE CHARGE CHARACTERIZATION OF PORTLAND CEMENT IN THE

PRESENCE OF SUPERPLASTICIZERS

David C. Darwin* Roger Y. Leung, and R. Trevor Taylor

W.R. Grace & Co. - Conn.

Research Division

Washington Research Center

7379 Route 32

Columbia, MD 21044

'corresponding author

ABSTRACT

The surface charge characterization techniques of electroacoustics, microelectrophoresis,

and streaming potential are compared for the system of superplasticizer and portland cement.

All techniques were found to have limitations with respect to the absolute zeta potential

determination of realistic systems. Electroacoustic measurements were used to evaluate

interparticle effects in portland cement slurries. The electroacoustic measurements on

portland cement showed important differences from results obtained on other colloidal

systems.

INTRODUCTION

Over one billion tons per year of Portland cement is manufactured in all parts of the

world, and used as the primary binder in concrete for construction. Essentially, the

production process involves mining minerals containing mainly calcium carbonate and silicon

oxides. These are kiln fired, forming hard pellets called clinker. The clinker is then finely

ground with a few percent calcium sulfate, usually in a ball mill. The resulting Portland

cement is then a powder with a broad distribution of particle sizes, with mean radii typically

from 3 to 10 microns.

In its end use application, portland cements are combined with water, sand, and stones

to form concretes. Typical mass based water to cement ratios range from 0.3 to 0.8. The

238
cement particles and water form a paste or slurry. However, the mixture is not a typical

colloid as most of the particles are quite large and show effects of settling. Analysis of the

cement - water system is further hindered by the fact that the cement particles begin to react

with the water from the moment of first contact. This results in surface and solution

compositions that change with time.

Nevertheless, the cement - water system has received extensive study with respect to its

electrokinetic properties [1-10]. Other researchers have gone a step further and examined the

electrokinetic behavior of individual phases within the cement [11,12]. In the concrete

industry it is common to admix various chemicals to control the reaction chemistry and

rheology of hydrating cement suspensions. Many of these systems have been subject to

electrokinetic analysis [13-24]. Among the most interesting of these admixtures are anionic

polymers called superplasticizers. Superplasticizers are dispersants which reduce the yield

stress of concretes and allow increased flow of fresh concrete without addition of water,

which would serve to reduce the compressive strength of the hardened material if added. It is

widely believed that superplasticizers adsorb onto cement particles and induce negative

charge.

Past works generally used microelectrophoresis techniques to determine the zeta

potential of cement particles in dilute solutions. The advent of electroacoustic analysis which

can measure electrokinetic properties under concentrated slurry conditions has led us to

investigate the properties of concentrated portland cement slurries. In this study, we

compare the surface charging behavior of superplasticizers on portland cements using

electroacoustic techniques, microelectrophoresis, and streaming potential. We then use

electroacoustic methods to investigate the interparticle effects of electrokinetic measurements

at higher cement volume fractions.

EXPERIMENTAL

Two portland cements were used, referred to as "New York" and "Texas". The mean

particle radii were determined by laser light diffraction to be 5.5 and 4.9 microns,

respectively. A particle size distribution for the Texas cement is shown in Figure 1. A

239
compositional analysis of both cements is given in Table 1. The superplasticizer used was a commercial naphthalene sulfonate formaldehyde condensate (designated NSFC; Grace,

WRDA-19). The superplasticizer was supplied as a 40% solution in water and was diluted to

appropriate concentrations for use.

Electroacoustic measurements were made using a Matec Applied Sciences ESA-8000

System in ESA mode. The electroacoustic probe was calibrated to a LudoxTM silica

standard, as described in the Matec manual. Conductivity and pH electrodes were calibrated

using standard KCI solutions and standard pH buffers. For an electroacoustic measurement,

cement was weighed out corresponding to the desired volume fraction, and the proper

amount of deionized water was added. The resulting slurry was magnetically stirred for 4

minutes, and then put onto the ESA 8000. Magnetic stirring was used in the ESA-8000

during the measurements to avoid particle settling. Superplasticizer was either added after 3

minutes of stirring in water with an additional 1 minute stir (for single point measurements),

or auto-titrated into the cement slurry in small increments using a 30 second equilibration

time.

Microelectrophoresis measurements were made on a Pen-Kern System 3000 instrument.

For these measurements, 0.05g of cement was dispersed in 500g deionized water. The

particles were stirred for 3 minutes. Superplasticizer was then added and allowed to stir for

1 minute. The suspension was then loaded into the Pen Kern sample cell for zeta potential

determination.

Streaming potentials were measured in a Mutek PCD 02 particle charge detector. The

experimental setup consists of a cylindrical vessel and a displacement piston both constructed

of teflon. The reciprocal piston movement at 4 Hz forces the colloid to stream in a slit

between the cylindrical vessel and the piston, inducing a streaming potential. For streaming

potential measurements, 0.5g of cement was dispersed in 500g deionized water, and 10 cm3

solution was used in the experimental vessel. Superplasticizer was added to the vessel in

increments by an autotitrator. The value of the streaming potential was then recorded as a

function of the amount of superplasticizer added.

240
RESULTS AND DISCUSSION

I. Technique Comparison

The basic experiment used in this work to compare the surface charge characterization

techniques of electroacoustic analysis, microelectrophoresis, and streaming potential is the

titration of superplasticizer onto the New York cement. Past results of this type of

experiment using microelectrophoresis show that as higher dosages of superplasticizer are

added the particles acquire negative charge with an eventual plateau value of the zeta

potential [14,17,19,20]. This can be understood as the polyanionic polymer adsorbing onto

the cement surface until it has reached a limiting value.

A. Electroacoustic Analysis

Zeta potential measurements using electroacoustic measurements were performed at

various cement concentrations. Results shown in Figure 2 are for 200g cement in 200g

water, corresponding to a volume fraction of 0.24. These results were generated using

incremental titration of NSFC. Qualitatively, the expected negative charging of the particles

by superplasticizer is found. However, experiments done at lower volume fraction show

positive changes in zeta potential. These experiments are described in detail later. The y-axis of the plot is labelled "apparent zeta potential" because the true values of the

zeta potential in these experiments are uncertain. Two effects contribute to this uncertainty.

First, the broad particle size distribution causes an uncertainty in the calculation of zeta

potential from ESA signal. This is due to the Matec software's calculation assuming

monodisperse particle size distribution, where O'Brien's mathematical formulation holds

[25]. The effects of particle size distribution on electroacoustic measurements were

subsequently evaluated and discussed by Scales [26] and James [27]. The proper zeta

potential can be calculated in principle by weighted integration of the ESA signal over the

particle size distribution [28]. For the measurements presented here, the mass average

particle size was used as suggested by James [27]. Specifically for this study, particle radii

of 3.5 microns and 5.5 microns were used for the Texas and New York cements,

respectively. The densities of the cements were taken to be 3.15 g/cm3

. Secondly, there is a

241
limitation on the volume fraction range of validity for the ESA to zeta potential conversion

using these assumptions. An upper bound of volume fraction no greater than 0.05 has been

proposed [27], and in Figure 2 we have shown data acquired at 0.24, for reasons that will be

evident later.

B. Microelectrophoresis

Microelectrophoresis measurements were performed at various single concentrations of

NSFC. A particle concentration of 50 mg cement per 500g deionized water was found to be

optimum. Lower particle concentrations gave insufficient signal and higher concentrations of

particles resulted in no variation of zeta potential as a function of NSFC concentration,

presumably due to interparticle interactions. Results shown in Figure 3 indicate a negative

charging trend qualitatively similar to that found by numerous investigators [14,17,19,20].

Our results are in good quantitative agreement with those presented in the work by Singh

[20].

Limitations of the microelectrophoresis technique used here for cement included: rapid

particle settling inside the microelectrophoresis sample cell, which limits signal levels and the

ability to signal average over time, and also results in gathering signal from mainly the

smaller particles; fresh solutions of cement and superplasticizer had to be prepared for each

value of superplasticizer as titration of the reservoir cement suspension was found to be

irreproducible; poor reproducibility of single point results, necessitating experiments to be

performed on each solution 4 to 6 times to provide adequate statistics.

C. Streaming Potential

Streaming potentials as a function of superplasticizer concentration were measured in an

autotitration mode. 0.5g cement particles were dispersed in 500g deionized water, and NSFC

was incrementally titrated. Results of the experiments are shown in Figure 4. Changes in

streaming potential are reported on the figure because it was found that the absolute value of

the streaming potential was dependent on the initial cleanliness of the cell. Furthermore, we

found that in doing control experiments the ionic composition of the solution must be

equivalent to that of the cement which it serves as a reference for. This is expected because

242
the streaming potential depends on the conductivity. In these experiments we used a sodiumlime-gypsum solution [NaLG; 21. 3g Na2S04 , 30g calcium hydroxide, 5g reagent grade

gypsum, total volume of 1 liter, with particulate allowed to settle out], which simulates the

ionic environment generated by portland cement. The NaLG solution was used at a

concentration which matched the conductivity of the 0.01% cement suspension used for

streaming potential. The results show negative charging similar to results observed by

microelectrophoresis techniques. The two control experiments shown demonstrate that

neither dilution effects from adding water to cement nor effects on the streaming potential

from adding NSFC can be invoked to explain the negative streaming potential of cement

particles in the presence of NSFC.

Evaluation of Interparticle Effects by Electroacoustic Techniques

The advent of electroacoustic techniques for surface charge characterization now allows

one to investigate surface charge characteristics in concentrated slurries where interparticle

effects can become important. This is especially important for portland cement applications,

as typical use concentrations are w/c = 0.3 to 0.5. These concentrations are orders of

magnitude above the regimes where microelectrophoresis and streaming potential

measurements are applicable. Furthermore, ionic effects can be important in cements as

under typical use conditions, the concentration of ions will be near 0. 1 M, mostly in Na, K,

Ca cations, and 504 and OH anions. Although the particle concentration in

microelectrophoresis and streaming potential measurements cannot be increased to the

realistic regime, it may be advisable to adjust the ionic strength to match the realistic regime.

Nagele et. al. have performed many studies on the zeta potential of portland cement as

determined by microelectrophoresis [1-10], including the variation of cement zeta potential

over time [3-6,8,9]. Nagele's zeta potential measurements in deionized water starts initially

at -13 mV and rises to -10 mV after 100 minutes [Figure 3, Reference 8].

ESA time series data on the Texas cement is shown for various water to cement ratios

in Figure 5. The absolute values of the zeta potentials are uncertain as discussed above,

however, any integration scheme on a single cement will correct all of the numbers by the

same function. Thus, either interparticle effects or ionic effects are responsible for the

243
differences between the different concentrations. Clearly, the zeta potential of hydrating

cement particles decreases with concentration. Our data also differ from that of Nagele [8]

in that our zeta potentials decrease (became more negative) over the 30 minute measurement

period, while Nagele' s increase. This discrepancy could be due to the variations in ionic

content of water, especially in the microelectrophoresis experiment as observed by Nagele

[8]. Subsequently, Nagele used cement extract to simulate highly concentrated cement

dispersions [9]. This changed the values of the zeta potential found, but not the initial

positive trend with time. ESA is much less sensitive to the ionic strength of the water used

as the large amounts of cement used dominate the ionic strength.

The charging action of sulfonated superplasticizers (and other admixtures) on portland

cement has been extensively studied by microelectrophoresis [13-24]. The generally

observed behavior is as the superplasticizer concentration is increased, the zeta potential of

the cement particles becomes more negative until a plateau is reached. Our electroacoustic

measurements presented in an earlier section verified this behavior for cement slurries of

w/c = 1.

In order to investigate interparticle effects on superplasticized cements, we performed

measurements at volume fractions of 0.01 and 0.05, regimes where the zeta potential

conversion from ESA signals should be valid. Data from these experiments is shown in

Figure 6, where once again the y-axis label of "apparent zeta potential should be noted when

one evaluates the magnitude of these effects. Much to our surprise, the zeta potential of

these titrations showed an increase in cement zeta potential with increasing superplasticizer

concentration. These experiments were reproducible.

A number of effects might be expected to influence these results, including variations in

the zeta potential over the time of the titration, ESA phase errors, and ionic strength effects.

From the data in Figure 5 it is seen that the time variations should affect the 0.01 and 0.05

volume fraction experiments in the opposite direction to that observed, while the 0.24

volume fraction experiment would be relatively unaffected. To address phase and time

effects, each experiment was individually phased to 0 to ± 5 degrees phase angle before

measurement and single point measurements were made with mixing cycle identical to that

used in the microelectrophoresis experiments described above, yielding constant mixing time

244
for all measurements. The observed trends are plotted in Figures 7a and 7b, and the trends

are maintained as seen in the autotitration experiments. To investigate the effects of ionic

strength, we adjusted the solution of some 0.01 volume fraction experiments to the

conductivity observed at 0.05 volume fraction with the sodium-lime-gypsum solution

described in the section on streaming potential. The additional ions shifted the apparent zeta

potential from -53.7 mV to the unrealistic value of -173 mV. Thus, the effect of additional

ions is opposite to that we observed with cement, where higher concentrations of cement (and

thus higher concentration of ions) produces zeta potentials nearer to zero. The ESA signal is known to have non-linearities at volume fractions above 0.05 for

silica [27]. In Figures 8a and 8b we show the relationship for the Texas cement. As before,

we used the mean mass averaged particle size in the Matec software which performs the

ESA-zeta conversion. The data are not in line with that for silica, as a constant zeta

potential is not observed with volume fraction nor is there a linearly decreasing ESA signal

with increasing volume fraction. Figures 9a and 9b show the volume fraction dependencies

of the Texas cement dosed with 0.4% NSFC. Dependencies in this case are as expected with

a flat zeta potential and a linearly decreasing ESA signal from volume fraction 0.01 to 0.15.

Experiments were also conducted with the samples diluted in cement extract to simulate ionic

effects, and this was found to affect only the lowest volume fractions.

The unconventional dependencies of electroacoustically derived zeta potentials and ESA

signals for portland cement explain why anomalous charging effects are observed upon the

addition of superplasticizer, because the zeta potentials at the start of the experiment are

artificially more negative than expected. When NSFC is added, the zeta potentials tend

towards the same plateau for the 0.05 and 0.24 volume fractions. It is known that NSFC is

a dispersant for cement particles, so one explanation may be flocculated particles when little

superplasticizer is present. When observations are made at a volume fraction of 0.24, these

effects may be abated due to the sample being more like a slurry than a dispersion. Ionic

effects cannot be totally ruled out, but they require additional investigation. Also, it should

be noted that experiments such as these can be cement specific, thus the anomalies may not

occur in all portland cements.

245
CONCLUSIONS

In this work we have studied three methods of zeta potential determination, electroacoustics,

microelectrophoresis, and streaming potential as applied to portland cement. The three

techniques are in qualitative agreement regarding the generation of negative charge on cement

particles caused by superplasticizer. Each of the techniques has limitations in terms of absolute

zeta potential determination.

Electroacoustic techniques provide a method for evaluation of electrokinetic properties of

portland cement at near realistic conditions. Empirically, electroacoustic techniques provide an

ease of use advantage over the other techniques studied. For superplasticized cement slurries,

puzzling electroacoustic data on the charging of cement particles by superplasticizer as a function

of volume fraction remain unexplained.

246
REFERENCES

1. Nagele, E. and Ney, P., Cem. Concr. Res., 12, 535-536 (1982).

2. Nagele, E., Chem. Eng. Sci., 40, 2141-2148 (1985).

3. Nagele, E., Cem. Concr. Res., 15, 453-462 (1985).

4. Nagele, E., Cem. Concr. Res., 16, 853-863 (1986).

5. Nagele, E., Cem. Concr. Res., 17, 573-580 (1987).

6. Nagele, E. and Schneider, U., Cem. Concr. Res. 17, 977-982 (1987).

7. Nagele, E. and Schneider, U., Cem. Concr. Res., 18, 257-264 (1988).

8. Nagele, E., Chem. Eng. Sci., 44, 1637-1645 (1989).

9. Nagele, E. and Schneider, U., Cem. Concr. Res., 19, 978-986 (1989).

10. Nagele, E., Cem. Concr. Res., 21, 478-483 (1991).

11. Suzuki, K., Nichikawa, T., Kato, K., Hayashi, H., and Ito, S., Cem. Concr. Res., H

759-764 (1981).

12. Singh, N.B., Singh, A. K., and Singh, S.P., J. Amer. Ceram. Soc, 73, 3063-3068

(1990).

13. Daimon, M. and Roy, D.M., Cem. Concr. Res., 8, 753-764 (1978).

14. Daimon, M. and Roy, D.M., Cem. Concr. Res., 9, 103-110 (1979).

15. Collepardi, M., Monosi, S., Moriconi, G, and Corrardi, M., Cem. Concr. Res., 10,

455-462 (1980).

16. Massazza, F., Costa, U., and Barrila, A., J. Amer. Ceram. Soc, 65, 203-207 (1982).

17. Miyake, N., Ando, T., and Sakai, E., Cem. Concr. Res., 15, 295-302 (1985).

18. Andersen, P. J., Kumar, A., Roy, D. M., and Wolfe-Confer, D., Cem. Concr. Res.,

16, 255-259 (1986).

19. Andersen, P. J., Cem. Concr. Res., 16, 931-940 (1986).

20. Singh, N. B. and Singh, S. P., J. Mat. Sci., 22, 2751-2758 (1987).

21. Andersen, P. J., Roy, D. M., and Gaidis, J. M., Cem. Concr. Res., J7, 805-813

(1987).

22. Mangialardi, T. and Paolini, A. E., Cem. Concr. Res., 18, 351-362 (1988).

247
23. Andersen, P. J., Roy, D, M., and Gaidis, J. M., Cem. Concr. Res., 18, 980-986

(1988) .

24. Nawa, T. Eguchi, H., and Fukaya, Y., Third Int. Conf. on Superplasticizers, 405-424

(1989) .

25. O'Brien, R. W., J. Fluid Mech., 190, 71-86 (1988).

26. Scales, P. J. and Jones, E., Langmuir, 8, 385-389 (1992).

27. James, M., Hunter, R. J., and O'Brien, R. W., Langmuir, 8, 420-423 (1992).

28. Mann, R., Matec Applied Sciences, Private Communication.

248
Table 1

Composition of Portland Cements Expressed as Oxides (% by mass)

Component New York Texas

Na2 0 0.20 0.01

K20 0.50 C1.42 MgO 3.91 1.30

CaO 65.95 67.60

A£2 03 4.16 4.71

FejOj 2.88 4.20

Si02 20.61 21.60

Ti02 0.44 0 24

S03 1.19 1.99

L.O.I. 0.67 0.87

249
FIGURE CAPTIONS

Figure 1 . Particle size distribution for Texas cement as measured by laser light

diffraction.

Figure 2. Change in apparent zeta potential of the New York cement as a function of

superplasticizer concentration at volume fraction of 0.24 (w/c = 1), as measured by ElectroSonic Analysis.

Figure 3. Change in zeta potential of the New York cement as a function of

superplasticizer concentration as measured by microelectrophoresis.

Figure 4. Change in streaming potential of New York cement as a function of

superplasticizer concentration as measured by streaming potential. Effects of

dilution by water and addition of superplasticizer to water are included for

reference.

Figure 5. Time variation of zeta potential of Texas cement caused by cement hydration at

three different volume fractions.

Figure 6. Change in apparent zeta potential of the Texas cement as a function of

superplasticizer concentration for different volume fractions. The total time for

this titration is close to the time scale of Figure 5.

Figure 7. Single point measurements of apparent zeta potential change of Texas cement as

a function of superplasticizer concentration for a) high volume fractions, b) low

volume fractions.

Figure 8. Dependence of electroacoustically determined quantities as a function of volume

fraction for Texas cement; a) apparent zeta potential,, b) ESA signal.

Figure 9. Dependence of electroacoustically determined quantities as a function of volume

fraction for Texas cement with 0.4% NSFC added; a) apparent zeta potential,

b) ESA signal.

250
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01 LL 262
SESSION V

COATINGS

Session Chairman

N. Sanders

Specialty Minerals Inc.

ROLE OF PARTICLE SIZE AND ELECTROKINETIC SONIC AMPLITUDE IN

PERFORMANCE OF SOLVENT BASED GRAVURE INKS

R. Krishnan

Sun Chemical Corporation

631 Central Avenue

Carlstadt, New Jersey 07072

ABSTRACT

In Gravure printing, the destruction of chrome plating in the form of banding crops up

time and time again. While several factors are involved in this phenomena, from the

physico-chemical properties of ink, the particle size of pigments, and the Electrokinetic Sonic

Amplitude (ESA) seems to play a major role.

Maximizing ESA offers a solution to this problem. This is probably due to the reduction

in abrasiveness of the carbon black pigment as particle size decreases. Although these

particles are sterically stabilized, maximum in ESA seems to correspond to a minimum in

particle size in most cases.

INTRODUCTION

Of the three major printing processes, Lithographic, Gravure, and Letterpress, the

Gravure printing is the preferred type for long run publications because of its high quality

reproduction at high speeds. Gravure printing is also used in packaging materials such as

paper and board since line cutting and creasing operations are easier because of the drying

characteristics of inks. In addition, for printing on non-porous substrates such as aluminum

foils it is ideal, because lithographic printing is easier only on water absorbing substrates.

The printing image is engraved into a gravure cylinder in the form of cells which are filled

with ink. A gravure cylinder consists of copper plated steel tube. A gelatine coating is

exposed under a positive of the image and it hardens to a depth which depends on the light

received. After etching with ferric chloride solution, a cylinder which consist of square cells

of varying depth results. Darker tones correspond to deeper cells which carry more ink.

Finally, the finished cylinder is chrome coated.

263
A schematic of the gravure printing unit is shown in Figure 1 . The printing unit is an ink

duct in which the etched cylinder rotates in a low viscosity ink. Excess ink is scraped by

means of a metal doctor blade. Printing occurs at the nip between the etched cylinder and a

rubber covered impression roller. The printed web is then heated in a dryer and succeeding

colors are printed' 11

. Most publication gravure inks are toluene based in which the polymeric binder is usually

a calcium or zinc resinate. The pigments are Carbon Black, phthalo cyanine blue, diarylide

yellow, and Lithol Rubine, corresponding to the four process colors.

The problem discussed in this paper with respect to the ink's performance is described as

"banding". Banding is a scoring of the surface of the gravure cylinders which starts mainly

near the edges and progresses into printing areas. It occurs erratically, and predominantly on

the cylinders running with black ink; although some cases of blue ink banding have been

reported, very few of the other colors are involved. As the length of the run increases,

banding progresses into the printing areas of the cylinders causing spoilage of the printed

materials. Dechroming and rechroming of a banded cylinder sometime help to prolong the

life of the cylinder, but it usually reoccurs. The erratic nature of the occurrence of this

problem and the possible involvement of over twenty operating parameters make the analysis

of the situation extremely complex. .

Major Factors:

Major factors which lead to the destruction of chrome plating in the form of scaling:

a) Chromium:

Different chrome electrolytes, which have an effect on wear behavior with regard to

the respective chromium structure.

b) Wiper:

Wiper speed and wiper pressure are also factors which may have a considerable effect

on chromium wear.

c) Printing Ink:

Physico-chemical properties of ink, on the basis of their composition and method of

manufacture have wear a characteristic behavior.

264
RESULTS AND DISCUSSIONS

Optical photomicrographs (Figures 2 and 3) of two typical banded cylinder shells reveal a

network of surface cracking or reticulation in the chrome plated surface of the banded areas

on both specimens.

This network of defects extends ahead of the actual banded area for a small distance and

appears to be the initiating stage of the failure of the chrome plating.

A possible mechanism for the destruction which is very apparent in the center of each

band might be the flaking off of the small chips of chrome loosened by this cracking or

crazing phenomenon and getting trapped beneath the doctor blade. The hardness of the

chrome flakes would then cause a rapid erosion and scoring of the cylinder's surface.

The photomicrographs (Figures 2 and 3) of a cylinder showing the leading edge of the

band indicates an area extending beyond the heart of the band where the copper underlayer is

exposed, which is covered with the crazed or cracked chromium surface which has not yet

deteriorated to the point of flaking off and exposing the copper sub-surface. It also shows

the beginning of the non banded area which is essentially free of these hairline surface

defects in the chromed surface.

A doctor blade section taken from the press was also examined. Photomicrographs

(Figures 4 and 5) of the upper edge of this blade, in the vicinity of the band show an

increased amount of wear as evidenced by a low spot in the blade edge as well as a number

of areas where metal wear is shown. They show an area where a leaf of blade metal has

been pealed backward and is on the verge of breaking off and in addition, a wire edge on the

verge of separating from the blade. The excessive amount of wear in this area would be

expected if an unusual concentration of hard or abrasive materials, such as chrome flakes

which may have separated in the banded areas to expose copper, collected under the blade

while running.

265
Lubricity of Gravure Inks

It has been proposed that banding may be correlated with the lubrication ability of gravure

inks. Black ink (carbon black) was indeed found to be more abrasive than colored inks

(organic pigments). However, a black ink with increased aromaticity (higher ratio of

Toluene to Mineral Spirit) gave poor lubrication as compared to low aromatic ink. But in

actual printing the high aromatic ink reduced the banding problem significantly.

Particle Size of Pigments in the ink

A particle size analysis of the various inks involved were carried out using an Ultrafine

Particle Size Analyzer (UPA) from Microtrac. It monitors Brownian motion of particles

smaller than 5 microns as a means of measurement. Particles vibrating at various velocities

by means of collisions with surrounding solvent molecules cause a shift in incident light

frequency. Measurement of resulting frequency shifts (Doppler effect) allows a calculation

of particle size and particle size distribution. The results are shown in Table I. A sample of

the output corresponding to ink C is also attached (Figure 6). It shows a trend of lower

particle size leading to less incidence of banding.

Electrokinetic Sonic Amplitude of inks

In order to understand the reason for variations in particle size distribution,

electroacoustic characterization of these inks was carried out. 131 The formulations of these

inks were essentially similar except for small variations in the amount of zwitter ionic

surfactant used. The ratio of pigment to grinding (stability) resin was kept constant. The

Electrokinetic Sonic Amplitude was measured using a MATEC system. The results are also

shown in Table I. Whenever the ESA is less that 0.5/zPa/Volt Meter the incidence of banding was very

high.

These experiments help us to offer a hypothesis for the problem of cylinder wear with

solvent based inks. Although the solvent based inks are stabilized by steric stabilization the

charge seems to play a major role in obtaining smaller particle size. For the systems

studied, lower the ESA, higher is the particle size. In addition, under high shear conditions

266
such as between a doctor blade and a gravure cylinder, with inks having lower ESA the

particles may agglomerate which then become very abrasive creating cylinder banding.

REFERENCES

1. The Printing Ink Manual, Ed R.H. Lench, VanNostrand Reinhold (1988).

2. Microtrac Ultrafine Particle Analyzer Manual, Leeds & Northrup (1990).

3. Matec Applied Science ESA 8000 Systems Manual.

267
TABLE I

Mean Particle Size, ESA and Banding Tendency of Differenct Ink Samples

Sample

D (A + Zwitterionic

Surfactant)

Mean Particle Size

0.65

0.5

0.41

0.46

Electrokinetic

Sonic Amplitude

^Pa*m/V

0.27

0.62

1.24

0.75

Banding

Tendency

High

Medium

None

Very Low

268
Figure Captions

Figure 1. A schematic of gravure printing system.

Figure 2 and 3. Photomicrographs of typical banded cylinder shells showing a network of sruface

crakcing.

Figure 4 and 5. Photomicrographs of a cylinder showing the leading edge of the band in which a

copper underlayer is exposed.

Figure 6. Particle size distribution of ink C by using an instrument based Doppler effect.

269
FIGURE 1

Ink pump

?70
='>mm i

271
FIGURE 4

FIGURE 5

272
Outpat File: siegwerk

Percent in Channel Graph

Ultrafine Particle Analyzer

Version 2.0"

Saiple ID 1 : SIEGWERK

Mode: Pull Range Distribution Fomat: Intensity

Run Tine: 300 seconds

Run I: 1 of 2 runs Loading Index: 0.903 Reflected Power: 199 iv Above Residual: 0.00 X Fluid Desc: SPARE 3 Cell Teiperature: 25.1 C Transparent Particles: So Particle refractive Index: 1.50Suinary Data

ci = 101 = 50X = ,1254

.5053

li = 0.2216

la = n/a

cs = n/a

sd = 0.1409

Saiple ID 2: toluene

Date: 9/28/92 Tine: 10:54 Opper Channel Edge: 2.7500

Lower Channel Edge: 0.0054

Chan. Progression: standard Umber of Channels: 36 Residuals: Enabled

Below Residual: 0.00 X Fluid Viscosity: 1.000 cp

Spherical Particles: No Fluid Refractive Index: 1.33

5218

1205 7832

4994

8916

7497

4458 3749

3152 2651 2229

1874 1576

1325

1114

0166

0139

0111

Percent

971
ELECTROKINETICS OF PHOSPHORS

Arunava Dutta

Osram Sylvania Inc

Research and Development

100 Endicott Street

Danvers, MA 01923

ABSTRACT

Electrokinetic studies have been conducted on a variety of fluorescent lamp phosphors

using the ESA technique. In particular, the ESA vs pH and IEP of phosphors emitting in the

UV, red, green, blue and white regions of the spectrum are investigated. The effects of nonluminescent additives, multicomponent phosphor blends, surface coatings, wash treatments

and ionic polymers on the electrokinetic behavior of phosphors are studied.

INTRODUCTION

Phosphors are inorganic materials which produce luminescence when excited by high

energy radiation. The incident radiation can be in the form of UV/visible photons or

electrons and ions. Phosphors are used in a broad variety of applications which include

general purpose fluorescent lamps, photocopying lamps, suntanning and medical lamps, high

pressure mercury lamps, sign tubes, tagging and identification, cathode ray tubes,

electroluminescent displays and X-ray intensifying screens. Every phosphor consists of a

host crystal and an activator center which plays a key role in the luminescence mechanism.

This paper will focus on lamp phosphors in general and on fluorescent lamp phosphors

in particular. A new generation highly energy efficient compact fluorescent lamp may have

as many as four to five phosphors deposited on the inner curved glass surface of the lamp.

Each phosphor has a particular emission spectrum in the visible range when excited by 254

nm UV photons which constitute the bulk of the radiation generated in a low pressure

mercury discharge. The overall spectrum of visible light generated by such a lamp is an

274
appropriate summation of the individual contributions from the various phosphors.

The phosphor coating on the interior surface of a fluorescent lamp is achieved by

contacting the glass with an aqueous suspension of phosphors in a manner so as to cover the

entire surface of the glass. Excess suspension is drained under gravity, following which the

wet coating is dried by passing hot air over it, after which the dried coating is baked at an

elevated temperature in air. In addition to the phosphors, the aqueous coating suspension

may contain one or more of the following ingredients: very fine particle size alumina, metal

salts, polymeric binders, surfactants, defoamers, cross-linking agents, plasticizers and dyes.

The organic components of the suspension are removed from the coating film during the

baking step, leaving behind primarily the phosphors and small amounts of non-luminescent

metal oxides.

The texture of the phosphor coating in a finished lamp is very sensitive to the degree of

dispersion of the phosphors in the coating suspension. Flocculation of the phosphor particles

leads to unacceptable lamp texture and the conditions of the coating suspension have to be

optimized to prevent this undesirable phenomenon. Flocculation may occur between particles

of the same phosphor or between particles of different phosphors.

The stability of a phosphor coating suspension depends on a balance between

electrostatic repulsive forces and the van der Waals attractive forces. The former are a result

of the electrical double layer around the phosphor particles. The iso-electric points (IEP) of

the various solids in the suspension and the dependence of the ESA (electrokinetic sonic

amplitude) on pH constitute a key information that is necessary for an understanding of the

stability of phosphor coating suspensions.

ESA OR ZETA POTENTIAL?

Electrokinetic studies were conducted using a Matec ESA-8000 system. In this

apparatus [1] an alternating high frequency electric field is applied to a

dispersion between two parallel electrodes, and the particles vibrate in this field producing a

coherent sound wave. ESA, or electrokinetic sonic amplitude, is the pressure amplitude of

this sound wave per unit of applied electric field. The units of ESA are mPa-m/V. The

275
relevant equations [2] describing the operation of the apparatus are listed below:

ESA = <j)(Ap) C\l d (1)

VL d

= (€C/ti)G(a) (2)

a = wr 2 /v (3)

In the equations listed above, jud is the dynamic mobility of the phosphor particles, <j> is

the volume fraction of the solids in the dispersion, Ap is the difference in density between

the phosphor and water, c is the speed of sound in the suspension, e is the dielectric

permittivity of the suspension, f is the zeta potential of the phosphor and r\ is the dynamic

viscosity of water. It is seen from Equation (1) that the magnitude of the ESA is directly

proportional to the dynamic mobility of the phosphor. Equation (2) shows that the dynamic

mobility is proportional to the zeta potential of the material. It follows that ESA is

proportional to the zeta potential of the phosphor.

In this paper changes in f" of the phosphor particles in the dispersion are followed by

tracking the magnitude and polarity of the ESA signal. The reason that this investigation

focuses on ESA which is a direct instrument output rather than f* which is a computationally

derived parameter is the uncertainty surrounding the applicability of the expression for G(a),

which is used to compute from ESA, to the phosphor samples at hand. This makes the

calculated value of f questionable.

G(a) is an inertial correction which accounts for the decrease in dynamic mobility with

particle size. The maximum value of G(ot) is unity which is attained by very fine sized

particles. In Equation (3) w is the angular frequency of the input electrical signal, v is the

kinematic viscosity of water and a is dimensionless. The assumptions [2] which are used in

the derivation of G(a) include a dilute suspension (< 10 vol % solids), large particle radius

276
(r) relative to the thickness of the double layer (/cr > 50) and a narrow particle size

distribution (a < 20% of mean size).

The double layer thickness (1/k) varies from about 3 nm to 96 nm [3] for a change in

ionic strength from 10~2 mol/1 to 10"5 mol/1. While electrolyte is not deliberately added in

this work, the specific conductivity of the phosphor dispersion prior to potentiometric

titration is influenced by a small degree of dissolution of ions from the phosphor surface.

The anions and cations which leach out of the surface of the phosphors are polyvalent entities

which significantly increase the ionic strength beyond what is expected for a 1:1 electrolyte

of the same concentration. The specific conductivity of the phosphor dispersions is in the

range of 30 to 50 jxS/cm. For the sake of comparison, a 10"3 M KC1 solution has a

conductivity of about 145 ^iS/cm. Based on this comparison, it is estimated that the ionic

strength is at least 10^ mol/1 in this work which leads to a maximum value of about 30 nm

for 1/k. It follows that the phosphor particles must be larger than about 3 fim in diameter

for the criterion kt > 50 to be satisfied. This is usually the case. The solids loading assumption used in the derivation of G(a) is also satisfied since the

maximum concentration of phosphors used in this study is about 2 vol%. It is the third

assumption which is violated by commercial fluorescent lamp phosphors. The particle size

distribution is very broad: there are particles from about 2 fim to about 20 /zm in diameter.

No single particle size can uniquely describe the population for the purposes of calculation of

S*. Any computed value of f using the expression for G(a) is, therefore, questionable. It is

for this reason that ESA rather than f is tracked in this research.

It follows from the nature of G(a) that a larger particle size phosphor would produce a

smaller ESA signal than a smaller size phosphor with the same surface chemistry. This is

caused by the larger inertia of the bigger particles which cannot track the rapidly alternating

electric field as fast as the smaller particles. The magnitude of the ESA signal is also

sensitive to the phosphor density and concentration: the signal increases if one changes to a more dense phosphor or uses a more concentrated dispersion of the same phosphor.

277
E KPERIMENTAL TECHNIQUE AND SCOPE

Table 1 lists the various phosphors which have been studied and their densities. The

effect of a non-luminescent material like alumina on the electrokinetics of phosphor

suspensions is also investigated. It is easier to refer to most phosphors by their acronyms

and this practice is adopted in this paper. The BAM, CAT and YOE are rare earth (RE)

activated phosphors which luminesce in the blue, green and red portions, respectively, of the

visible spectrum when the phosphors are excited by 254 nm photons in a fluorescent lamp.

Good color rendition lamps may have this blend of RE activated phosphors in certain

proportions by weight. The CW phosphor has a white light emission spectrum and may be

used in conjunction with RE phosphors or by itself. The remaining two phosphors in Table

1 do not have common acronyms and are categorized by their dominant emission wavelength.

Thus, the strontium borate phosphor emits in the UVA region (320 to 400 nm) of the

spectrum while the cerium aluminate phosphor emits in the UVB region (260 to 320 nm).

These UV phosphors, which are referred to as UVA and UVB phosphors from now on, are

used in sun tanning lamp applications.

The pH and conductivity probes of the Matec ESA-8000 system are calibrated prior to

the measurement of every new phosphor dispersion. The ESA probe is calibrated with a Ludox TM colloidal silica dispersion, as recommended by Matec [1]. A specified mass of

phosphor ranging from 10 to 15 g is added to 200 cm3 distilled water in a Teflon sample cell

which contains a cross shaped Teflon magnetic stirrer. The cell is then inserted in the Matec

apparatus. Water is circulated in a plexiglass jacket positioned around the cell to maintain

the temperature of the contents of the sample cell at 25°C ± 0.3°. In addition to the

magnetic stirrer, the contents are also agitated by a motor driven propeller. The pH,

conductivity and ESA are monitored every minute for about 72 h using the Time Series mode

of data acquisition. This length of time is sufficient to stabilize a phosphor dispersion in

terms of ESA and pH.

Arrangements are made for potentiometric titration after the dispersion has stabilized.

The automated titration system is first washed with acid/base as appropriate. Phase

calibration is then conducted with a knowledge of the polarity of the sample at the end of the

278
time series study. This information is derived from chemical considerations of the surface or

by observing the changes in ESA vs pH during the time series. Titration is either done with

0.1 M HN03 or with 0.1 M NaOH. Specific adsorption of ions is not a problem with either

of these titrants. IEP measurements are reproducible to within 0.15 pH units.

TABLE 1 PHOSPHOR TYPES

PHOSPHOR (ACRONYM) DENSITY (g/cc)

BaMg2 Al 16027 :Eu(BAM) 3.70

(Ce,Tb)MgAl1] 019 :Ce,Tb(CAT) 4.15

Y2 03 :Eu(YOE) 5.15

LaP04 :Ce,Tb (LAP) 5.20

Ca5 (F , CI) (P04) 3: Sb : Mn(CW) 3.10

SrB407 :Eu(UVA) 3.40

Ce(Sr,Mg)Al n O, 7 :Ce(UVB) 4.30

A£2 03 (AOC) 3.75

RESULTS

CW Phosphor and Effect of AOC

Figure 1 shows the electrokinetic results obtained for a dispersion of 12.65 g of CW in

200 cc of water (a 2 vol% solids dispersion). The stable pH of this dispersion prior to

titration is about 6.4. The IEP of this phosphor is about 2.3 which reflects the presence of

strongly anionic sites on the surface of this material. XPS (X-ray photoelectron

spectroscopy) of the surface of this phosphor reveals the following atomic composition: Ca

17%, P 17%, O 62%, F 3.8% and the balance is Sb.

279
Figures 2 through 5 are data for dispersions containing mixtures of CW and alumina in

various proportions. The alumina is Aluminum Oxide C (AOC) from Degussa. AOC has a

primary particle size of about 20 nm and a specific surface area of about 90 m

2 /g as compared to about 0.55 m

2/g for the CW phosphor. Four distinct CW/AOC blends are

tested ranging from an AOC loading of 1.25% to 10% by mass of the phosphor. The total

surface area presented by unaggregated AOC in water is about twice that of the CW

phosphor when the AOC loading is 1.25%. The stable pH of these CW/AOC suspensions is

only weakly sensitive to the concentration of AOC and is about 7.2. The total mass of the

mixture is maintained at 12.65 g in 200 cm3 of water for all four CW/AOC blends.

Prior to potentiometric titration, every CW/AOC dispersion is referenced to itself for

the purposes of phase calibration. There is a dramatic change in IEP from 2.3 to about 7.1

with the addition of only 1.25 % AOC, as concluded from Figures 1 and 2. The

measurements shown in Figures 3 through 5 indicate that the IEP of the CW/AOC blend

progressively increases from 7. 1 to about 8.5 as the concentration of AOC is raised from

1.25% to 10%. There is a discontinuity in the ESA curve around the IEP in Figure 4. Such

discontinuities have been discussed in the literature [4] and are usually attributed to ionic

background interference. No electrolyte is, however, used in this work. The discontinuity

in the ESA curve in Figure 4 is, therefore, not well understood and warrants further study.

In Figure 5, where the AOC concentration is 10% compared to the 5% in Figure 4, there is

also a discontinuity in the ESA curve but this is less prominent than that in Figure 4. The IEP of AOC, as shown in Figure 6, is about 9.15. The AOC dispersion contains

6 g alumina in 200 cm3 water. The very small size of the AOC particles is primarily

responsible for the large ESA signals associated with this material. The IEP of AOC and

CW being 9.15 and 2.3 respectively, these materials will not remain individually dispersed at

intermediate pH values. In particular, electrokinetic considerations would dictate that the

AOC be positively charged and the CW be negatively charged at these pH values, leading to

inter-particle flocculation. Given the much larger surface area of the AOC relative to CW,

this would translate to a portion of the surface of the CW phosphor being covered by the

AOC. This reasoning is upheld by the following analysis of the electrokinetic data.

280
The IEP of the CW/1.25% AOC system is 7.1 which means that the net ESA is zero at

this pH. At this pH, AOC and CW would individually have positive and negative surface

charges respectively. Let us assume that at this pH the components are individually

dispersed with no inter-particle flocculation due to electrostatic attraction. In other words,

we are assuming that the CW surface is not covered by the AOC to any degree at this pH.

The ESA then can only be zero at this pH if the positive ESA contribution from the AOC is

matched by an equal negative ESA contribution from the CW phosphor. These contributions

are calculated using the information in Figures 1 and 6 which has to be adjusted for the

appropriate concentration of the solids in the CW/1.25% AOC blend. Under these

conditions it can be shown that the net ESA would not be zero but rather a sizable negative

quantity due to the strong negative ESA of the CW phosphor at this pH. It follows that the

net ESA can only be zero at pH 7. 1 if the effect of the AOC is to coat a portion of the

surface of the CW phosphor thereby masking some of the negative contribution to the ESA.

The extent of coverage of the CW surface by the AOC is a function of the pH and the

relative total surface areas of the two solids exposed to the liquid.

Effect of a Thin Film Coating on the cw Phosphor

Figure 7 shows the results of an electrokinetic study for a dispersion of CW phosphor,

12.65 g in 200 cm3 water, which has a thin film (~ 400 A°) coating of alumina on its

surface. XPS shows a 95% attenuation of the Ca signal from the phosphor substrate

indicating a fair degree of surface coverage by the alumina coating. This coating may be

applied by any one of a variety of known methods. A comparison of Figures 1 and 7 proves

that the IEP of the strongly acidic CW may be changed to a basic value by depositing an

alumina film on its surface. This has implications as regards the degree of stability of the

overall coating suspension.

Effect of YOE. CAT and BAM Phosphors

Figure 8 shows the electrokinetic behavior of a red YOE phosphor dispersion which

has 15 g of the powder in 200 cm3 water. The basic nature of the surface is reflected in the

281
IEP of 8.75. XPS of the surface of the YOE phosphor reveals the following atomic

composition: Y 24%, O 74% and the balance is Europium. The O/Y ratio is almost three,

like in yttrium hydroxide, which may indicate a highly hydroxylated surface. The green

CAT phosphor, 10 g in 200 cm3 water, has an even more basic surface as indicated by an IEP of 9.3 in Figure 9. XPS of the CAT phosphor yields the following surface atomic

composition: Al 35%, O 61%, Mg 2.1%, Tb 1.3% and Ce 0.94%. The blue BAM

phosphor, 10 g in 200 cm3 water, is the most basic of the three RE phosphors as observed in

Figure 10 which displays an IEP of 9.85 for this material. XPS of the blue BAM phosphor

shows that the surface atomic composition is as follows: Al 37%, O 60%, Mg 1.5%, Ba

1.1% and the balance is Europium.

The effect of YOE and CAT on the IEP of a phosphor blend with CW is also

investigated. A three component blend of YQE, CAT and CW phosphors is prepared in 200

cm3 distilled water with 3.75 g each of the first two components and 7.5g of the CW

phosphor. This composition results in about a 2 vol % total solids dispersion. In addition,

the total surface area exposed to the water by the CW in this blend is comparable to the sum

of the surface areas of the YOE and CAT phosphors. The electrokinetic characteristics of

the blend is shown in Figure 11. The blend IEP is 7.65 which is much greater than the IEP

of CW (2.3) and somewhat smaller than the IEP of either YQE (8.75) or CAT (9.3).

Effect of Wash Treatment

Various wash treatments are given to different phosphors at some final stage of the

manufacturing process. While these treatments are meant to improve the brightness and/or

lumen maintenance of these materials, these washes also influence the electrokinetic behavior

of the phosphors which has to be accounted for while considering the stability of the coating

suspension. Figure 12 shows the electrokinetic behavior of a green LAP phosphor, 10.5 g of

powder in 200 cm3 water or a 1 vol % solids dispersion, which has not undergone any surface

wash treatment. The IEP of this phosphor is 6.6. XPS of this phosphor provides the

following surface atomic composition: La 5.8%, P 14%, O 74%, Ce 4.6% and Tb 2.3 %.

There is more phosphorus on the surface than is required for the amount of lanthanum

present. The same LAP phosphor after a KOH base wash is shown in Figure 13. XPS of

282
the surface of this washed phosphor shows the following atomic composition: La 6.3%, P

13%, O 73%, Ce 4.8% and Tb 2.2%. The IEP of the washed phosphor has increased to

8.9. The increased IEP of the base washed phosphor will in turn influence the stability of

any multicomponent phosphor blend in which the washed LAP is introduced.

Effect of Ionic Polymers

A dispersion of CW, 12.65 g in 200 cm3 water, is prepared in the Matec sample cell.

At the end of 2.5 h of aging, the ESA is -0.171 mPa mN while the pH is 6.25. The polarity

of the ESA indicates that the surface is negatively charged at this pH, which should be the

case given that the IEP for CW is about 2.3. At this point, 1 cm3 of a strongly cationic, low

molecular weight polymer (Betz 1190) is rapidly introduced into the dispersion. The addition

of this polymer causes the ESA to change very quickly to -I- 0.415. This change in the

polarity and magnitude of the ESA is not accompanied by any significant change in pH.

The positive ESA and its large magnitude proves that the cationic polymer adsorbs

strongly on the initially negative CW surface, transforming it to a positively charged surface.

In order to have changed polarity, the surface must have gone through a zero ESA.

Flocculation at this juncture is, however, avoided by the rapid increase in the magnitude of

ESA. This rapid increase may be attributed to two critical parameters: the amount of

polymer which is introduced and the short time scale over which the introduction is made.

Changes in the polarity and magnitude of ESA can have significant impact on the

degree of dispersion of a coating suspension containing one or more phosphors. In

particular, a cationic polymer treated surface will have a more basic IEP than a non-treated

surface. In addition, the quality of thin film coatings deposited from a solution onto the

phosphor surface can be greatly influenced by such changes in phosphor surface charge

characteristics.

Having changed the initially negatively charged CW surface to a positively charged

surface, 1 cm3 of a highly anionic polymer (BASF Sokalan PA30) is slowly introduced into

the dispersion. The polarity of the ESA is now reversed going from + 0.415 to a final value

of - 0.385. The pH increases by about 2 units. The extent of change in the ESA cannot be

explained by this amount of movement in pH. Flocculation is visible when ESA decreases to

283
about zero. The suspension disperses again as the ESA, now negative, increases in magnitude

with gradual addition of the anionic polymer. Adsorption of an anionic polymer onto the

surface of a cationic polymer treated CW phosphor can, therefore, change the surface

polarity from positive to negative.

UV Phosphors

The electrokinetic behavior of an UVA strontium borate phosphor, 15 g in 200 cm3

water, is shown in Figure 14. The extremely basic nature of the surface of this phosphor is

reflected in an IEP of about 11.2. This is one of the highest measured IEPs of any

fluorescent lamp phosphor. XPS of the surface of this UVA phosphor gives an atomic

composition as follows: Sr 6.8%, B 36%, O 56%, Eu 0.37% and Ba 0.78%. A suntanning

lamp is required to have the proper relative amounts of UVA and UVB emission. The

electrokinetics of a strontium magnesium aluminate UVB phosphor, 15 g in 200 cm3 water,

is shown in Figure 15. This phosphor has an IEP of 9.9. XPS of this phosphor provides

surface composition as follows: Al 36%, O 60%, Sr 2.5%, Mg 0.34% the balance being

other atoms. The surface composition of a phosphor does not necessarily reflect its bulk

composition. Cerium, for example, is not detectable on the surface of this phosphor.

Rheological testing of an aqueous suspension of a blend of the UVA and UVB phosphors

shows strong pseudoplasticity at pH between 10 and 11.5. This is a result of interparticle

flocculation.

CONCLUSIONS

The electrokinetic behavior of a large variety of fluorescent lamp phosphors has been

investigated. These luminescent materials have a broad range of iso-electric points: about

2.3 for CW, 6.6 for LAP, about 8.75 to 10 for the YOE, CAT and BAM rare earth

phosphor and about 10 to 11 for the UV emitting phosphors. Addition of a very small

amount of very fine size alumina causes a significant increase in the effective IEP of

CW/AOC blends relative to CW. A thin film coating of alumina on CW increases its IEP by

about 6 pH units. A tricomponent blend of YEO, CAT and CW phosphors compared to

284
CW, has an effective IEP of 7.65. Potassium hydroxide base wash of the LAP green

phosphor increases its IEP from almost neutral pH to about 9. Adsorption of cationic and

anionic polymers onto the surface of the CW phosphor leads to charge reversal.

REFERENCES

1. Matec Applied Sciences, ESA-8000 System Operating Manual, 1992.

2. O'Brien, R. W., J. Fluid Mech., 190, 71 (1988).

3. Hunter, R. J., Zeta Potential in Colloid Science . Academic Press, 1981.

4. Hackley, V. A., Premachandran, R. S. and Malghan, S. G., Ceramic Transactions

"Characterization Techniques for Solid-Solution Interfaces in Ceramic Systems," Ed. J.

Adair, and J. Casey, American Ceramic Society: In Press, 1993.

285
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300
ADVANCES IN LIQUID TONER CHARGING MECHANISM

ELECTROACOUSTIC MEASUREMENT OF LIQUID TONER CHARGE

James R. Larson

Webster Research Center

Xerox Corporation

Webster, NY 14580

ABSTRACT

Methods for determining the electrical properties of liquid toners are critiqued with an emphasis on electroacoustic measurements. Advances in understanding the mechanism are

discussed.

INTRODUCTION

Particles dispersed in a hydrocarbon liquid become charged by the addition of a charge

director, a hydrocarbon soluble ionic surfactant, which aggregates into inverse micelles in

non-polar media. This effect is utilized in liquid electrostatic developers for imagewise

particle deposition which enables electrostatic printing. 1 The charge director micelles ionize

to form both positive and negative micelles which can compete with particles of the same

sign for image charge during the image development process. Several techniques are

available for the measurement of particle charges. These include methods by which particles

are electrophoretically developed and the transient current monitored2 ' 3

, one method where

micelle counter ions are stripped from the particles by fluid flow4

, and particle charge can be

related to particle mobility which can be measured by a number of techniques. 5 7 All of these

methods have some utility but their effectiveness will depend on how well they account for

all of the charged species in the particle dispersion and other factors which effect charge on

the particle. In this report a discussion of the influence of micelle ionization and applied

electric fields on the particle charge and its measurement are presented. Electroacoustic

methods7 are identified as being very effective in assessing particle charges because they

operate at relatively large fields and are independent of the charged micelle activity.

301
EXPERIMENTAL

The liquid toner dispersion was prepared by a hot melt blend-cold particle size reduction

process which resulted in 1-2 micron polymer particles. The hydrocarbon dispersant used

was Isopar® L (Exxon Chemical Co., Houston, TX). The charge directors studied were

Basic Barium Petronate® and Neutron Barium Petronate® (Witco Corporation, New York,

NY). Viscosities were determined with the use of Cannon-Fenske Viscosimeter Tubes

(Cannon Instrument Co., State College, PA). The conductivity of the liquid toner

dispersions and charge director solutions were determined with a Scientifica 627 Conductivity

Meter (Scientifica, Princeton, NJ). The measurement signal for this meter is a low distortion

18 hz sine wave with an amplitude of 5.4 to 5.8 volts rms. Toner particle mobilities were

determined with a MBS-8000 electrokinetic sonic analysis (ESA) system (Matec Applied

Science, Hopkinton, MA). The system was calibrated in the aqueous mode per

manufacturers recommendation to give an ESA signal corresponding to a zeta potential of -26

mv for a 10% (v/v) suspension of Ludox-TM (DuPont). The system was then set up for

non-aqueous measurements. The electrophoretic mobilities obtained are converted to zeta

potentials using O'Brien's method.7

PARTICLE CHARGING MECHANISM

Acid-base interactions between the particles and the charge director micelles, equations

(1) and (2), are believed to be an important mechanism for particle charging. A large body

of evidence has been published supporting the acid-base mechanism. 8

" 13 In the present

discussion it is assumed that charging occurs from the transfer of an ion between a site on

the particle surface and a micelle. The charging site on the particle may be a strongly acidic

or basic site inherent to the particle composition or may be a site where ionic material is

adsorbed onto the particle surface.

302
Negative Charging Particle

particle-H + micelle

acid base

(particle-) 4- (micelle-H+) (1)

Positive Charging Particle

particle + micelle-H (particle -H+) + (micelle-)

base acid

(2)

PARTICLE CHARGE AND MICELLE IONIZATION

Particle charging in a liquid toner system is assumed here to occur by ion exchange

between the particle and a charge director micelle. The charge director micelles (M°) can

self ionize to form both positive and negative micelles (M+ and M-). The micelle charges

have long been recognized to compete with particles for image charge in electrostatic printing

systems.

The linear relationship between the conductivity of charge director solutions and

concentration, as seen in Figure 1, combined with the fact that only a small fraction of the

micelles are ionized 14 suggests that charge director ionization is a second order reversible

reaction. This is consistent with the proposed mechanism for the ionization of Aerosol-OT in

non-aqueous media1516 and with the increase in conductivity of charge director solutions with

increasing temperature that cannot be accounted for by viscosity changes alone. 14 Toner

particle charging has also been proposed to be a reversible second order process involving a

charging site on the particle S° and a neutral micelle producing a charged site on a particle

(S-).

17 These processes are depicted in equations (3) - (6) for negatively charged particles.

In equations (3) - (6): [M°] = number of neutral micelles/dispersion volume; [M+] =

number of positively charged micelles/dispersion volume; [M-] = number of negatively

charged micelles/dispersion volume; [S°] = number of uncharged sites/particle volume and

303
Figure 1.

Conductivity of Mautral Barium Petronate in Isopar L

Mmutrai Barium Pmtronato %Sol!d»

[S-] = number of negatively charged sites/particle volume. Examination of equations (4)

and (6) leads to the conclusion that the level of micelle ionization will be influenced by the

presence of the particles and vise versa. The interaction of charging sites on the particles

(S°) and micelles (M°) forming positive micelles (M+) will force the micelle ionization

equilibrium, equation (4), to the left, i.e., causing a reduction of micelle ionization. This is

a significant point as attempts to account for the level of micelle ionization in a particle

dispersion often require the measurement of the electrical properties of charge director

solutions in the absence of particles and assume that the values obtained will be

representative of the micelles in the particle dispersion.

304
Micelle Ionization

M° + M° M+ + M" (3)

K; = [M (4) +][M]/[M°][M°]

Particle Charging

S° + M° ^ M+ + S" (5)

K,, = [M (6) +][S]/[S°][M0

]

TEMPERATURE DEPENDENCE OF PARTICLE CHARGING

AND MICELLE IONIZATION

Support for the conclusion that micelle ionization and toner charging cannot be dealt with

separately is obtained by examining the effect of temperature on the two processes. The

ionization of micelles in charge director solutions is governed by equation (4) and is expected

to increase as K; increases. The effect of temperature on micelle ionization has been studied

and Kj has been found to increase with temperature as expected for an endothermic

processes.

17 An example of this is found in Figure 2 where it is shown that the temperature

dependence of conductivity of charge director solutions is greater than the temperature

dependence of the viscosity.

The effect of temperature on particle charging has also been studied. 17 If particle

charging and micelle ionization are independent then the effect of temperature on particle

charging could be predicted by examination of equation (6) alone. In this case it would be

reasonable to expect that Kq would behave similarly to and that particle charging would

increase with increasing temperature. However, if micelle ionization and particle charging

are coupled then the expected effect of temperature on particle charging will be more

complex.

The examination of the effect of temperature on particle charging is simplified by

assuming that there is sufficient charge director concentration so that [M+

] — [M ]. Without

particles [M+

] = [M], however, with particles [M+] > [M ]. Under conditions of high

charge director concentration [M+

] = [M ]:

then [S ]/[S°] « Kq //K; (7)

305
Figure 2.

Neutral Barium Petronate and Isopar L

Conductivity and Viscosity vs Temperature

0.00300 0.0O325 0.00350

1/T (Kelvin)

Taking [S] to be the concentration of surface sites, both charged and uncharged, and 9 to be

the fraction of surface sites that are charged.

[S ]/[S°] = [S-]/([S] - [S ]) = 0/(1-0) (8)

Combining [7] and [8] and rearranging gives

0 = Kq/(Kq + /K,) (9)

Examination of equation (9) leads to the conclusion that regardless of the relative magnitudes

of Kq and K; that the effect of temperature on particle charging will be moderated by the fact

that the rate of increase of K,, with temperature will likely be similar to that of Kq -I- /K; .

306
Particle charging could increase or decrease with temperature depending on the relative

temperature effects on Kq and /Kj.

This expectation is consistent with the experimental evidence available. Kohler, et.al.,

1

reported that the temperature dependence of particle surface charge for a liquid toner

dispersion to be relatively invariant with temperature between 10°C and 50°C. The zeta

potential and charge/mass ratio of liquid toners using two different barium petroleum

sulfonates as charged directors were measured by electroacoustic methods (ESA) and found

to be relatively independent of temperature compared to the temperature dependence of the

conductivity and micelle ion concentration found for the charge director solutions alone. 17

This data is presented in Figures 3-4.

Figure 3.

E

C

o

o

280 -|

260 -

240

220 -

200 ~

180 -

160 "

140

Conductivity v* Temperature

for Charge Directors in Isopar L

20

NBP

a

BBP

T T

30 40

Temperature (C)

50

307
Figure 4.

£54 Zeta Potential vs Temperature

BBP Charged Toner

I

i

NBP Charged Toner

" 1 I «- 1 '

1

1 1

i 20 25 30 35

Temperature (C)

FIELD DEPENDENCE OF PARTICLE CHARGING

Particle charges can be calculated from the electrophoretic mobility of the particles with

a knowledge of the particle size and the viscosity of the fluid medium. However, it has been

observed that the particle electrophoretic mobilities are not constant with applied electric field

as would be expected for particles of constant charge. The field dependence of mobility was

first reported by Stotz

19 for polystyrene spheres dispersed in an aliphatic hydrocarbon.

Mobility increases as great as 10 fold were reported for field variations in the range of 10 to

1000 V/mm. Similar phenomena were observed by Novotny20' 21 for titanium dioxide

dispersed in p-xylene where a four fold increase in the mobility was observed when the field

was increased from 100 V/mm to 1000 V/mm. This effect has also been reported for liquid

toner dispersions by Kohler18

, Niv6

, and Larson. 22

308

E

! 7 b

160 -

140 -

120 -

100 -

80 -

60 -

40 -

21}
The increase in particle charge is explained by the stripping of counter charges from the

double layer. As the double layer is removed the field dependence is expected to decrease

and eventually at very high electric fields the mobility is expected to be field independent. 19

Field independent mobility has been observed at high fields in a number of cases.

61819

MICELLE IONIZATION AND FIELD DEPENDENT CHARGING:

CHARGE MEASUREMENT METHODS

A common approach to measure the charge on particle dispersions of the type being

discussed here is to electrophoretically develop the particles in a parallel cell arrangement

and measure the mass developed and calculate the charge deposited by monitoring the

transient electrical current.

2,3 This method is effective in determining the total number of

charges in the system, charges obtained from both particle charging and micelle ionization,

relative to the mass of the particles.

However, if the interest is in the charge on the particles then the charges from micelle

ionization and those from particle charging must be separated. This is traditionally done by

measuring the charges in the charge director solution in the absence of particles and then

subtracting that from the total charge measured for the particle dispersion. It is clear from

the discussion above that this will lead to an incorrect evaluation as the level of micelle

ionization in the particle dispersion is moderated by the presence of particles and will be over

estimated by the measurement in the absence of particles.

A novel approach to obtaining the particle charge independent of the micelle charges was

recently reported. 4 This method obtains the charge on a given mass of particles by removing

the micelle counter ions on the particles by filtration and having the filtrate flow into a

Faraday cup. The measured current is caused only by the charges associated with the

particles as the positive and negative charges caused by micelle ionization are equal in

number and cancel.

Particle charge can also be obtained from measurement of the electrophoretic mobility of

the particles. With the mobility and a knowledge of the particle size and fluid viscosity the

particle charge can be calculated. 5 7,23 However, these methods often empty electric fields

309
which are well below the fields used in particle application. The mobilities measured under

these conditions and the particle charge calculated from the mobility may not be

representative of the real system. Methods to determine particle mobilities at higher fields

have been reported. One method which utilizes electroacoustic effect (ESA) has proven

particularly effective. 7

MEASUREMENT OF LIQUID TONER PARTICLE MOBILITY USING ESA

ESA has been used by a number of workers studying the charging mechanism of liquid

electrostatic toners.

13 ' 17 ' 22 ' 24 27 In several cases the ESA measurement was found to correlate

well with liquid toner performance. 22,24

In one study it was demonstrated that the improved image quality that is produced by the

addition of amino alcohol compounds to the formulation is due to a higher toner particle

negative charge as measured by ESA.24 The aminoalcohols were seen to be more effective

than the corresponding amines and even amines that were considerably more basic. These

results conflict with a simple acid-base picture of liquid toner particle charging. It was

proposed that the improved charging produced by the aminoalcohols is due to improved

solubility of the counter ion in the micelle. It was also shown in a surface IR study that the

effect of the amino alcohols is moderated by their absorption onto the particle surface. The

effect of adsorption onto the surface of particles was also demonstrated in the case of acidic

additives. 13 22 25 Again ESA measurements were critical as they showed which additives were

most effective in increasing the particle charge and this data was correlated to the tendency

of the additives to adsorb onto the particle surface or remain in the hydrocarbon phase. In

one case

22 work also included a study of the effect of charge director concentration and the

acid additive concentration on liquid toner conductivity and ESA mobility. These studies

revealed that there is a threshold level of charge director required to obtain the maximum

mobility and that the mobility of the particles is essentially independent of conductivity and

charge director concentration above that threshold level.

In other studies the ESA has been used to explore the use of metal carboxylates as

charge directors and aluminum complexes as charge additives to the toner particles. 26- 27

310
It is clear from an examination of this work that the ESA measurement is an effective

method for the assessment of liquid toner particle charge and can be used to discern the

details of the charging mechanism and to facilitate the selection of materials to improve toner

performance.

SUMMARY

A model for micelle ionization and particle charging was presented which shows these

phenomena to be inextricably coupled. The observed increase for micelle ionization with

temperature, in the absence of particles, compared to the insensitivity of particle charges to

temperature is seen as support for the model. This phenomenon and the increase in particle

charge with increasing field impacts the reliability of methods to evaluate particle charge.

Particle charge is best measured by a method which is independent of the charged micelle

activity and which is compatible with high fields. ESA mobility is such a method. A

literature review of the use of ESA for the study of liquid toner particles showed that this

method has been successful for determining particle mobilities.

ACKNOWLEDGMENTS

The author would like to thank Ian D. Morrison for his collaboration and many

discussions on the ionization of micelles and temperature effects on particle charging. The

assistance of Terri S. Robinson is also acknowledged.

311
REFERENCES

1. For a review see: S. P. Schmidt, J. R. Larson, and R. Bhattacharya, "Liquid Toner

Technology," in Handbook of Imaging Materials, edited by A. Diamond, Marcel

Dekker, New York, pp. 227-252, 1991.

2. P. E. Staples, S. A. Koop, Proceedings of IST's 7

th International Congress on NonImpact Printing Technologies, Vol. 1, IS&T, Springfield, VA, pp. 561-569, 1991.

3. V. Novotny and M. L. Hair, Journal of Colloid and Interface Science, 71, pp. 273-282,

1979.

4. I. D. Morrison, A. G. Thomas, and C. J. Tarnawskyj, Langmuir, 7, pp. 2847-2852,

1991.

5. R. E. Kornbrekke, I. D. Morrison, and T. Oja, Langmuir, 8, pp. 1211-1217, 1992.

6. Y. Niv, Y. Adam, and Y. Krumberg, Abstracts of IST's 3

rd International Congress on Non-Impact Printing Technologies, IS&T, Springfield, VA, pp. 57-60, 1986.

7. T. Oja, G. L. Peterson, and D. W. Cannon, U.S. Patent 4,497,208, 1985; R. W.

O'Brien, J. Fluid Mech., 190, pp. 71-86 (1988).

8. F. M. Fowkes, H. Jinnai, M. A. Mostafa, F. W. Anderson, and R. J. Moore, Colloids

and Surfaces in Reprographic Technology, M. Hair and M. D. Croucher, eds., Am.

Chem. Soc, Washington, D.C., 1982.

9 M. D. Croucher, S. Drappel, J. Duff, K. Lok, and R. W. Wong, Colloids and Surfaces,

11, pp. 303-322, 1984.

312
10. F. M. Fowkes, H. Jinnai, M. A. Mostafa, F. W. Anderson, and R. J. Moore, ACS

Symposium Series, No. 200, pp. 282-306, 1984.

11. J. R. Larson, The 4

th International Congress on Advances in Non-Impact Printing, IS&T,

pp. 142-145, 1988.

12. G. A. Gibson and R. H. Luebbe, The 7

th International Congress on Advances in NonImpact Printing Technologies, IS&T, pp. 533-550, 1991.

13. K. A. Pearlstine and J. R. Swanson, Journal of Colloid and Interface Science. 151, pp.

343-350, 1992.

14. T. G. Davis, G. A. Gibson, R. H. Luebbe, and K. Yu, Proceedings of the 5

th

International Congress on Advances in Non-Impact Printing Technologies, IS&T, p. 404,

1989.

15. H. F. Eicke and V. Arnold, Journal of Colloid and Interface Science, 46, p. 101, 1974.

16. Z. Randriamala, A. Denat, J. P. Gosse, and B. Gosse, Transactions on Electrical

Insulation, 20, p. 167, 1985.

17. J. R. Larson, I. D. Morrison, and T. S. Robinson, The 8

th International Congress on

Advances in Non-Impact Printing Technologies, IS&T, pp. 193-197, 1992.

18. R. Kohler, D. Giglberger, and F. Bestenreiner, Society of Photographic Scientists and

Engineers Journal, 22, pp. 218-227, 1978.

19. S. Stotz, Journal of Colloid and Interface Science, 65, pp. 118-130, 1978.

20. V. Novotny, Journal of Applied Physics, 50, pp. 324-332, 1979.

313
21. V. Novotny, Journal of Applied Physics, 50, pp. 2782-2794, 1979.

22. J. R. Larson, W. A. Houle, K. A. Pearlstine, J. R. Swanson, L. A. Page, T. J. Trout,

L. El-Sayed, The 6

th International Congress on Advances in Non-Impact Printing

Technologies, IS&T, pp. 585-597, 1990.

23. I. D. Morrison and C. J. Tarnawskyj, Langmuir, 7, pp. 2358-2361, 1991.

24. J. R. Larson, G. A. Lane, J. R. Swanson, T. J. Trout, and L. El-Sayed, Journal of

Imaging Technology, 17, pp. 210-214 (1991).

25. K. A. Pearlstine and L. Page, Journal of Imaging Science, 35, pp. 55-58, (1991).

26. K. A. Pearlstine, Journal of Imaging Science, 35, pp. 326-329, (1991).

27. G. A. Lane, Hard Copy and Printing Materials, Media, and Processes, SPIE, Vol. 1253,

p. 29 (1990).

314
SESSION VI

SUBGROUPS DISCUSSIONS

AND

IDENTIFICATION OF ISSUES

SESSION VI - SUBGROUPS DISCUSSIONS AND IDENTIFICATION OF ISSUES

This section contains a summary of problems and research needs. Technical problems/issues were discussed and research opportunities were identified during the workshop under five

major areas: 1) non-aqueous ESA measurements, 2) measurement considerations,

3) coatings, 4) coal, cements and clays, 5) fundamentals. The discussion leaders summaries

are presented below.

NON-AQUEOUS ESA MEASUREMENTS

CONDUCTIVITY < 20,000 ps/cm

Discussion Leader: J. R. Larson, Xerox Corporation

1. Calibration Standard

Dispersion of carbon black and Lubrizol charging additive in dodecane would be

supplied to non-aqueous users for ESA measurement. A comparison of these results

(available to all participants) would establish the reliability of the aqueous calibration

(with Ludox) for non-aqueous measurements. This effort may eventually result in a primary calibration standard for non-aqueous ESA measurements.

2. Mixing Effects On ESA Measurements

It was noted that in some cases the rate of mixing during measurements effects the

magnitude of the ESA signal. It is recommended that the exact mixing process be

described when reporting results so that experiments could be reproduced.

3. Needed Software Changes

Current software is inadequate for temperature correction for viscosity and density and

requires that some experiments (time-series runs) be discontinuted if recalibration is required. It was also noted that the system shuts down if a viscosity value greater

than 25 cps is entered into the software.

4. Non-aqueous Acoustosizer

Great interest was expressed for an Acoustosizer for non-aqueous measurements. It was thought this might be available in 3-5 years.

315
5. Measurement Of Small Samples

It was noted that small samples of 20-30 cm3 (too small for the probe) could be

measured with the flow through cell.

6. Temperature Resistant Probe

A probe that can withstand temperature up to 150°C is being developed. The current

probe should not be heated above 35 °C. The high temperature probe is expected to be available in 1994.

MEASUREMENT CONSIDERATIONS

Discussion Leader: V. A. Hackley, NIST

This group discussed various factors, problems, needs and limitations regarding the

electroacoustic measurement of aqueous systems. The following summary is based on these

discussions.

A. Calibration Standard

The general consensus of the group was that there is a great need for a generally

accepted calibration standard for determining the necessary instrument constant for

conversion of esa measurements in aqueous suspensions and for user related needs.

The suggestion was made that possibly a primary standard could be developed by

NIST, for instance, and dispersed to the instrument companies, while a secondary

standard would be calibrated against the primary and dispersed for general application

by end users.

Next we discussed the desired properties for a primary standard. The following is a

list of these properties, where an asterisk indicates those properties also required for a secondary standard.

Spherical

Monodisperse (or relatively narrow distribution)

Non porous*

Chemical stability* (should be insoluble, surface stabilized, etc.)

Kinetic stability* (easily dispersed and not readily flocculated)

Significant density difference, Ap*

High surface charge density*

Low inertial effect, G(ua/v) (this means that the particle size must be

below about 0.5 /xm so that wa/p is small)

316
In addition to these attributes listed above, we thought that it may be desirable to have

a standard that showed a well-characterized pH dependence, while pH-independence is desirable for calibration of the instrument constant. Another consideration was that

the standards could be chosen to be compatible with other (dilute) techniques such as microelectrophoresis. The Geltec silica currently used by Matec may be acceptable as a primary standard material, although long-term availability is a question.

Measurement Procedures

1. Analysis and Interpretation of Multicomponent Systems.

There appears to be a lot of uncertainty involved with the interpretation of suspensions

containing more than one phase. We feel that more work is needed in this area, including a general protocol for carrying out such measurements.

2. Aggregation Effects

The influence of changes in the effective mass of particles due to aggregation is not

clear. How might aggregation effect the ESA signal and determination of mobility or

zeta potential?

3. Temperature Effects on Phase Angle

In the new Matec system, where the accurate determination of phase angle is necessary for measurement of size, what are the potential problems associated with

temperature fluctuations?

4. Development of improved sample handling and mixing for ESA measurement of

highly loaded suspensions. Some problems with mixing have been encountered.

The concept of a continuous flow measurement system is also of general interest.

5. Development of on-line capabilities for quality control and intelligent processing of

materials. Further developments in this area will require the design and

testing of more robust measurement systems.

317
COATINGS

Discussion Leader: N. Sanders, Specialty Minerals Inc.

1. Background Electrolyte

- Corrections

- Relevance to actual process

Strong (and sometimes relatively "strange") supporting electrolytes are commonplace

in coating formulations. The influence of changes in these needs to be separable from

changes in particulate properties.

2. Electrokinetics vs Viscosity (rheology)

- What are relationships?

We need something better than the dilute system theories (Von Smoluchovski, etc.) to

tell us if electrokinetic measurements can, in fact, help us to understand complex

rheological phenomena such as dilatancy in high solids coating colors.

3. Effect of shear on Electroacoustic measurements

One of the great potential benefits of the electroacoustic methods is the ability to measure charge under shear where many of the corresponding coating properties

(rheology) are being assessed.

4. What is the correct particle size to use for interpreting ESA data?

- Effect on determination of IEP

Coating colors need to be evaluated not only under conditions of dispersion but also of

partial or total flocculation - how does flocculation affect the results?

5. Need comparison of ESA methods and classical electrokinetics for adsorbed

multilayers (of polyelectrolytes, e.g.)

It's currently uncertain whether the known effects of adsorbed layers in dilute

suspensions carries over to ESA methods or not. Does binding strength of adsorbed

layer enter?

6. How does ESA method handle "liquid" to "solid" transition?

- Immobilization detector?

318
Does ESA method care whether a coating color is "flowable" or not (in theory)? If it does, perhaps an extremely useful new application is in determining immobilizaton

conditions (critical for controlling dry coating structure).

7. How do we interpret ESA data on pigment mixtures?

Almost all real coating colors contain pigment mixtures. Ultimately the ESA method

must be able to interpret mixture data: hopefully theoretical models can be generated

to cover at least limiting cases.

8. How do we calibrate a dyamic coating slurry?

If the materials we must deal with are inherently unstable - what should we use to calibrate the ESA instrument to get dynamic information?

9. Need more measurements at high solids.

Most data available so far is at 10 vol% solids or less. Most coating colors are at 30+ vol%.

10. Need flow through cell that works.

High solids, higher viscosity slurries used for coating are difficult to handle in the

static equipment. Flow cells currently available have not performed satisfactorily at high solids.

11. Work needed to define needs for on-line process ESA system.

Ultimately a need will develop for application of ESA methods on-line in coating

color pigment manufacturing and/or coating color formulation/make-down. The

requirements to impose on such new technologies in traditionally demanding

environments must be scoped out.

12. Titration hysteresis effects.

More careful experiments need to be performed to determine where titration hysteresis

is likely to be observed and why.

13. Is it possible to obtain a charge distribution from ESA data?

It is well worth some theoretical effort to see just how much data can be pulled out of

ESA devices. If we can get particle size distribution, how about particle charge

distribution?

319
COAL, CEMENTS AND CLAYS

Discussion Leader: B. M. Moudgil, University of Florida

* Calibration Standards

- Ludox TM/Geltech Si02 /Ti02 . Aging effects should be studied

* Mixed Components

- Size and mobility distributions are major concerns

* Correction Factors (e.g. electrolyte, particle size)

- Provide addendum to manual of all correction factors

* Response Time

- Option to have a quicker response (at a higher noise to signal ratio)

* Cell Design

- Torque meter to monitor dispersion /aggregation

* On-line Instrument

- Ruggedness

- Minimum cleaning

- Quick response to changes

- Handle wide solids loading (20-60 wt.%)

* Upgrades/Technical Support

- Software upgrades

- Strong technical support

320
FUNDAMENTALS OF ELECTROACOUSTICS

Discussion Leader: R. O. James, Eastman Kodak Company

During the workshop it was seen that the successful application of electroacoustic

characterization techniques to the evaluation of the properties of colloidal dispersions depends on several factors. The first of these factors is the ability to reliably and accurately measure

the ESA signal amplitude and phase shifts that arise from the interaction of colloidal particles

with an electric or acoustic field. The second important factor is the availability of

quantitative theoretical models to relate the raw data to dynamic electroacoustic mobility to the familiar characteristic quantities such as the (static) electrophoretic mobility, the zeta

potential and the diffuse layer charge. Third, it is important to have suitable materials that

could be used as model colloids for calibration purposes or to check for correlations with

related experimental techniques, e.g., microelectrophoresis and dielectric spectroscopy.

In terms of the theories available to connect dynamic electroacoustic mobility to the static

mobility a number of related theoretical studies have been recently published and more are in

press. The modern theories begin treating the particle as a hard sphere with a smeared

uniform surface charge balanced by a diffuse double layer with an equal and opposite charge

density. The paper by O'Brien (J. Fluid Mech., 1988, 190, 71) provides an approximate

analytical expression for the thin double layer case, i.e., /ca> > 1. O'Brien's patent (US

Patent 5 059 909, 1991) and the study by Sawatzky and Babchin (J. Fluid Mech., 1993, 246,

321) provide the relationship for arbitrary /ca and for small values of the zeta potential or

surface charge for the same hard sphere, diffuse EDL model. Concurrent with developments

in ESA, progress in the related field of dielectric spectroscopy of colloids has raised

questions about the applicability of the smeared surface charge/diffuse EDL model to the

interfacial structure in which a more detailed microscopic model of bound and mobile ions in

the Stern or inner layer may have a significant influence on the comparison of experimental

observations and the model predictions. More recently than the workshop, White and

Mangelsdorf at the University of Melbourne reported comparison of exact and approximate

theoretical computations of high frequency electrokinetics of colloidal particles (paper 44,

67th Colloid and Surface Science Symposium, University of Toronto, 1993). In comparison

with O'Brien's computations the numerical solutions showed agreement at for thin double

layers (high /ca) and for low zeta potentials. It was reported that the paper had been

submitted to JOS and that it built on an earlier report by DeLacey and White (JCS Faraday

Trans 2 (1981) 79, 2007) via the inclusion of the particle inertial terms in the Navier-Stokes

and colloidal force balance equations. For thicker double layers and high zeta potential,

significant differences did occur between the "exact" and approximate methods as a function

of frequency.

Thus it appears that at the time of the workshop much of the theoretical background for

electroacoustic phenomena had been established. Progress continues with further attention

needs to be allocated to:

321
a) arbitrary /ca and large zeta potential and

b) the effect of microscopic structure, ionic binding and dynamics with in the

hydrodynamic shear zone.

Currently, the O'Brien model («:a > > 1) is most easily applied to insulating particles (low e) where the electric field factor, f, in the equation

u^co) =(2eC/3n) dt/) G(o>a 2/v)

is approximated to 0.5 so that \Ljtu>) = eC/"n • G (g>£

2/v) = \i(0) • G(wa 2/v).

At the workshop, Prof. R. J. Hunter agreed to develop a spreadsheet that would compute the

factor f for various conductivities, particle sizes, and frequencies. In addition, the

computation of an equivalent spherical volume diameter for use in the inertial term G(coa2

/»>) from a particle size distribution was another task that Prof. Hunter volunteered to cast into a

spreadsheet macro. The above discussion applies to the low volume fraction range from

about <j> = 0 to 0.05 in which the ESA response is linear with 0. There was some

discussion of the use of cell models to help describe the phenomema at higher volume

fractions, e.g., <f> = 0.1 to 0.4 where the ESA response becomes non-linear and maxima in

the signal may be seen. At this moment this area of electroacoustics needs further research

and development. The major difficulty is to establish robust methods to obtain appropriate

methods to relate the static and dynamic mobilities in the high <j> regime.

Particles with Complex Geometry and Structure

Colloidal dispersions are prepared in a wide variety of particle geometries and interparticle structures e.g., floccs, gels, colloidal crystals etc. The particles may also have a wide range

of thickness of adsorbed polymeric layers or coatings where polymer chains or other matter

may penetrate the hydrodynamic shear plane. Also other particles and adsorbed layers may

be in contact in the form of loose or elastic floccs of dimension much larger than the single

particle or solvent displacement during ESA oscillations. Even other particles may be

crystalline but have surface and internal pores. The question that arises is how we should

treat these types of geometries to evaluate dynamic and static mobilities and surface charge,

or, is the structure too complex for detailed analysis and description? All of the complicated

geometries and surface structures and reactions provide many challenges to researchers in the

field.

Particle Size Distribution

In ESA measurments and transformations, the mass or volume distribution of sizes and the

relative density of particles compared to the medium determines the inertial term in the

conversion to static mobilities. There was some discussion of the best suited techniques for

providing this data. X-ray detection of sedimentation or centrifugation provides one direct

322
route to this type of data, but other methods of detection with appropriate accuracy and

correction factors, e.g., Mie theory correction for optical density or turbidity detection and

other separation /detection schemes can be used.

Acoustic Absorption

Two papers at the workshop discussed acoustic absorption and acoustophoresis for evaluation

of particle size and particle surface charge. This is separate but perhaps complimentary to ESA and the "Acoustosizer" apparatus that was also described during the meeting. Both

electroacoustic and acoustic absorption method can yield signal amplitude and phase

information about colloidal dispersions as a function of frequency and solution properties.

The methods may offer some redundancy and also complementary information, but the

deconvolution of the frequency dependent electroacoustic data to yield dynamic mobility and

size distribution seems more robust at present.

Further Research and Development

There was some discussion about ways to promote further understanding and use of

electroacoustic techniques and methods. Presently, instrument makers, university researchers

and industrial researchers are involved in various aspects of the field e.g., including

hardware and software system development, theoretical modelling, calibration and application

of methods, etc. The question was asked whether more activity could be sponsored or encouraged through efforts by bodies such as NSF, IFPRI, AIChE and NIST. No firm

recommendations were proposed, although the workshop type forum did encourage

communication between numbers of researchers might otherwise meet only rarely. The

workshop did provide a more public view of the achievements and challenges in the field.

Reactive Sites and Kinetics

With increased experience using ESA apparatus it has become evident that not all measurements are made at equilibrium or steady state. In fact, since ESA is a fairly rapid

technique, e.g., measurements on the time scale of seconds, ESA could be used to follow the

slow kinetics of surface charge reactions. Most simple ionic reactions are fast compared to

this, but mixing and surface re-equilibration are often slower in aqueous systems. In non- aqueous colloids with low conductivity, the surface reactions may be considerably slower.

In view of this it was recommended that users measure ESA amplitude and phase as a

function of time in at least part of any epxerimental study to evaluate the importance of

instrument stability or drift and to obtain some idea of rates of equilibration.

323
VII. LIST OF PARTICIPANTS

Al Behan Bob Gill

UOP R&D Matec Applied Sciences

Rolf Braunig Philip Goetz

Eupenen Strasse 150 Pen Kem, Inc.

David Cannon Vincent A. Hackley

Matec Applied Sciences NIST

Subhash Chander Anne Hardy

Penn State Univ. Saint Gobain/Norton

Hans Coll Yan C. Huang

Eastman-Kodak International Paper

David Darwin Martin A. Hubbe

W.R. Grace & Co. International Paper

Nikolaas De Jaeger Robert Hunter

Parcotec Univ. of Sydney

Fred Desai Robert 0. James

Univ. of Michigan Eastman Kodak Co.

Andrei Dukhin Ralph Kornbrekke

Pen Kem, Inc. The Lubrizol Corp.

Arunava Dutta Ramasamy Krishnan

GTE Electrical Products Sun Chemical

Amjad Farooq James Larson

Colgate Palmolive Xerox

Ellen Forbus Roger Leung

Engelhard Corp. W.R. Grace & Co.

George Gibson Jie Li

Xerox Binney & Smith, Inc.

324
Subhas G. Malghan

NIST

Berislav Markovic

Columbia University

W. Morris Mobley

NYSCC at Alfred Univ.

Ian Morrison

Xerox Corp.

Brij Moudgil

Univ. of Florida

Trevor S.B. Sayer

ICI Paints

Richard Slepetys

Engelhard Corp.

Stuart Soled

Exxon

Krishna Kumar Subramanyan

Columbia University

Jorge Valdes

AT&T Bell Labs.

Raymond Mountain

NIST

Tonis Oja

Matec Applied Sciences

Boudewijn Van Lent

Bayer AG

Bruce Weiner

Brookhaven Instruments

Patrick Pei

NIST

Hemant Pendse

Univ. of Maine

Nancy Phillips

Pen Kem, Inc.

John Pollinger

Allied Signal Ceramic

Kevin J. Roth

Specialty Minerals, Inc.

Dale Russell

Hewlett Packard

Nigel Sanders

Specialty Minerals, Inc.

325

ir U.S GOVERNMENT PRINTING OFFICE: 1993— 341-832/ 83091



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