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NIST Special Publication 945

Guide to the Nomenclature of Particle Dispersion

Technology for Ceramic Systems

Vincent A. Hackley

!57 (0.945

•000
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'At Boulder, CO 80303. ^Some elements at Boulder, CO.
NIST Svecial Publication 945

Guide to the Nomenclature of Particle

Dispersion Technology for Ceramic Systems

Vincent A. Hackley

Ceramics Division

Materials Science and Engineering Laboratory

National Institute of Standards and Technology

Gaithersburg, MD 20899-8520

February 2000

U.S. DEPARTMENT OF COMMERCE

William M. Daley, Secretary

TECHNOLOGY ADMINISTRATION

Dr. Cheryl L. Shavers, Under Secretary of Commerce for Technology

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

Raymond G. Kammer, Director
National Institute of Standards and Technology Special Publication 945

Natl. Inst. Stand. Technol. Spec. Publ. 945, 24 pages (February 2000)

CODEN: NSPUE2

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 2000

For sale by the Superintendent of Documents, U.S. Government Printing Office

Washington, DC 20402-9325
Guide to the Nomenclature of Particle Dispersion Technology

Preface

It is generally agreed that measurements and standards play an integral role in the manufacture of

ceramic materials. They enhance reliability by providing a basis for quantifying and comparing

material properties during each phase of the manufacturing process, from raw materials to the

finished product. Equally important is the establishment of a uniform and widely accepted

nomenclature to permit the efficient exchange of scientific and technical information, and to provide a sound basis on which to standardize measurement methods and data reporting

practices.

In June 1997, NIST established the Ceramic Processing Characterization Council (CPCC)"^ to assist the U.S. ceramics industry in the development of measurements and standards

infrastructure. The CPCC is a voluntary, cooperative program among NIST, universities, private

companies, other government research laboratories and private sector laboratories. A pnncipal

goal of the consortium is to develop guidelines and recommended practices for the

implementation of process measurements. Within the CPCC, the Dispersion & Rheology

Working Group (DRWG) is responsible for addressing those measurement issues associated with

particulates in liquid media. DRWG members quickly recognized the fundamental importance of

terminology to any measurement-based strategy, and subsequently initiated a project to develop

a uniform nomenclature for dispersion systems.

An ad hoc committee was formed, whose members included J.H. Adair (Penn State), J.A. Casey

(IBM), R.A. Condrate (Alfred), M. Frey (3M), V.A. Hackley (NIST), W. Hunt (3M), A.

Jillavenkatesa (NIST), M.J. Mayo (Penn State), G.Y. Onoda (NIST), P. Somasundaran

(Columbia), and T. Wood (3M). The committee was charged with reviewing the technical

content and language of the nomenclature, which I had volunteered to compile. Several revisions were passed through the committee before a draft version was circulated for comment by CPCC

members at large. In addition to the committee review, two distinguished authorities in the

particle science field, Bob Hunter at the University of Sydney and George Scherer at Princeton, were invited to review the nomenclature and provide input according to their respective areas of

expertise. Their help is gratefully acknowledged.

In compiling this nomenclature, every effort was made to maintain a degree of uniformity with

existing standards and conventions. To this end, the cooperation of the American Chemical

Society and the International Union on Pure and Applied Chemistry (lUPAC) were instrumental.

To help put this nomenclature into practice, I have prepared this special publication. It is my

general hope that it will serve as a resource to those working in particle dispersion applications,

particularly in the ceramic sciences. It is my further hope that this document will serve to promote the establishment of a commonly recognized system of terminology throughout the

ceramics community.

^ Formally known as the Ceramic Processing Characterization Consortium.

in
Guide to the Nomenclature of Particle Dispersion Technology

Table of Contents

PREFACE Hi

I. INTRODUCTION 1

2. DEFINITIONS 1

2.1 Physical Description of Dispersed Systems 1 Related Terms 2

Recommendations 3

2.2 States of Subdivision (dispersed phase) 3

Definitions Based on Size 3

Definitions Based on Structure 3

Related Terms 4

Recommendations 5

2.3 Association and dissociation Processes 5

Association Processes 5

Dissociation Processes 6

Related Terms 6

Recommendations 7

2.4 Dispersion Stability 7

States of Stability 7

Stability Mechanisms 7

Interaction Terms 8

Related Terms 9

Recommendations 10

2.5 Interfacial and Electrokinetic Properties 10

The Interface 10

Adsorption Processes ' 11 Electrical Properties 12

Electrokinetic Effects 13

Related Terms 14

3. BIBLIOGRAPHY 16

3.1 Primary Sources 16

3.2 Secondary Sources 16

ACKNOWLEDGEMENTS 17

LIST OF REVIEWERS AND CONTRIBUTORS 17

INDEX TO DEFINED TERMS 18

iv
Guide to the Nomenclature of Particle Dispersion Technology

GUIDE TO THE NOMENCLATURE OF PARTICLE DISPERSION

TECHNOLOGY FOR CERAMIC SYSTEMS

1. INTRODUCTION

The use of nomenclature for identifying

dispersed particle systems, along with their

associated properties and components, is often inconsistent and subject to misinterpretation in the ceramic and

materials science literature. For example,

terms for describing the state of association

of particles in suspension (e.g., aggregate or agglomerate) often carry specific

connotations that vary among different

authors. In an attempt to standardize usage of

dispersion terminology, and as a resource for

researchers, engineers and students, an

internally consistent system of nomenclature

has been developed. In compiling definitions, a variety of sources were examined,

including lUPAC' recommendations, current

and draft ASTM^ and ISO^ standards, and

published materials drawn from both the

ceramic and the more general colloid and

interface science literature (see the

bibliography for a complete list of primary

and secondary sources).

Definitions have been written first and

foremost to serve those people in the

ceramics field, but should be of equal utility to those involved in a broad range of

dispersion-based non-ceramic applications.

This is not, nor is it intended to be, an

exhaustive compilation. Rather, this

document focuses on commonly encountered

terms, and endeavors to provide a consistent

' International Union on Pure and Applied

Chemistry

^ American Society for Testing and Materials

International Organization for Standardization

framework for improved technical

communication.

The terms are organized into five

subsections, each dealing with a specific

aspect of dispersion: (1) physical description

of dispersed systems; (2) states of

subdivision; (3) association and

disassociation processes; (4) dispersion

stability; and (5) interfacial and electrokinetic

properties. Acceptable alternative

expressions and common abbreviations are shown in parentheses adjacent to the defined

term. In general, the American Chemical

Society's formalism for abbreviations has

been adopted here."^

In some cases, a definition may have two

parts; one more general in context and the

other more specific to ceramic applications.

Recommendations are also provided, for

instance, regarding the discontinuance of

terms deemed ambiguous or obsolete, and are presented at the end of each subsection.

Within each definition, separately defined

terms are identified in italics at their first occurrence. In addition, an alphabetized

index to defined terms is provided.

2. DEFINITIONS

2.1 Physical Description of Dispersed

Systems

Aerosol

Droplets or particles dispersed in a gaseous phase.

^ The ACS Style Guide, 2"" Edition, 1997.

1
Guide to the Nomenclature of Particle Dispersion Technology

Continuous Phase

Constituting the dispersion medium, a phase that exhibits continuity throughout

the dispersion; e.g., the Hquid in a suspension.

Dispersed Phase (Discontinuous Phase)

In a dispersion, the phase that is distributed in the form of discrete

discontinuities (particles, droplets or bubbles), in a second immiscible phase

that is continuous.

Dispersion

In general, a two-phase system in which

discontinuities of any kind (solid, liquid,

gas) are dispersed in a continuous phase

of a different composition or state; more

specifically in the field of ceramics, the

term dispersion is used to describe a suspension of solid particles in a liquid

medium.

Emulsion

A dispersion consisting of two or more

liquid phases.

Hydrosol

A sol in which water forms the

dispersion medium.

Liquid Phase

Consisting of a condensed fluid; e.g., the

dispersion medium in a suspension.

Organosol

A sol in which an organic liquid forms

the dispersion medium.

Particulate Phase (Solid Phase)

The particles in a suspension, gel, or

aerosol.

Slip

A term that refers to a suspension

prepared for the expressed purpose of

consolidating the solid phase (e.g., by

slip-casting, tape-casting, or spray

drying).

Slurry

A concentrated ceramic particulate

suspension.

Sol

A liquid dispersion containing particles

of colloidal dimensions.

Suspension

A liquid in which solid particles are dispersed.

Related Terms

Heterodisperse

Describes a polydisperse particulate

system in which more than one discrete

size distribution mode occurs; e.g., bimodal, trimodal, etc.

Monodisperse

Realistically, all dispersions exhibit a

finite spread in their particle size

distribution. In practice, the term

monodisperse can be used to identify a dispersed system in which all particles are of nearly the same size, forming a narrow (unimodal) distribution about an average value. Numerically, a dispersion

may be considered monodisperse if 90%

of the distribution (1.645a, where o is the

standard deviation of the size

distribution) lies within ±5% of the

average size, (d):

l^<0.05

id)

Polydisperse

Describes a dispersed system in which

many particle sizes occur. In practice, a system may be considered polydisperse if

2

i
Guide to the Nomenclature of Particle Dispersion Technology

less than 90% of the size distribution

(1.645a, where a is the standard

deviation of the size distribution) Hes

within ±5% of the average size, (d):

Well-Dispersed

A term used to describe a stable

suspension in which the minimum

particle size has been achieved.

Recommendations

Relative Concentration Terms

It is recommended that relative

descriptive terms relating to particle

concentration in suspension (e.g., dilute or concentrated) be defined in such a manner that the reader has a clear

understanding of their relevance to the

measurement or application at hand. For

instance, in a light scattering

measurement, "dilute" may infer the

absence of multiple scattering, whereas in an ultrasonic measurement this term may

imply a linear response with

concentration. These conditions may vary

by several orders of magnitude with

respect to particle concentration.

Concentration can also by defined on a more fundamental basis, taking into

consideration the relative dominance of

thermal, hydrodynamic, or surface forces

in controlling suspension properties.

2.2 States of Subdivision (Dispersed

Phase)

Deflnitions Based on Size

Nanosize (Nanophase)

A special state of subdivision implying

that the particles (or atomic clusters)

have average dimensions smaller than

roughly 100 nm, and exhibit properties

not normally associated with the bulk

phase (e.g., quantum optical effects).

Colloid

State of subdivision implying that the

particles have at least in one direction a dimension roughly between 1 nm and 1

|Lim. Colloids are significantly affected by

Brownian motion when suspended in a

liquid.

Ultrafine

State of subdivision implying that the

particles have in any given direction a maximum dimension lying roughly

between 1 \im and 10 ]xm.

Fine (Subsieve Range)

State of subdivision implying that the

particles have in any given direction a maximum dimension less than roughly

37 [im.

Coarse (Sieve Range)

State of subdivision implying that the

particles have in any given direction a dimension greater than roughly 37 [im.

Granule

State of subdivision generally referring to dry particulates with dimensions lying

roughly in the 50 |Lim to 200 |Lim range;

typically, granules are aggregates of finer

particles produced by spray-drying.

Granulation is performed for ease of

handling during subsequent consolidation

operations.

Deflnitions Based on Structure

Particle

Any condensed-phase tridimensional

discontinuity in a dispersed system may

generally be considered a particle; e.g., droplets in an emulsion or solids

5
Guide to the Nomenclature of Particle Dispersion Technology

dispersed in a liquid. The term is normally used in reference to solid

materials. An aggregate may also be

regarded as a particle.

Droplet

Liquid-phase particle in an emulsion or

aerosol.

Particulate

Composed of distinct particles.

Primary Particle

Smallest identifiable subdivision in a

particulate system. Primary particles may

also be subunits of aggregates.

Microsphere

Refers to a spherical particle in the

micrometer size range.

Aggregate

A cohesive mass consisting of particulate

subunits.

Hard-Aggregate

An aggregate that cannot be easily

redispersed by the application of

moderate mechanical agitation (shaking,

stirring, or ultrasonication) and/or mild

chemical treatment. Hard-aggregates

consist of subunits that have been

chemically bonded or fused.

Agglomerate

In a suspension, an aggregate held

together by physical or electrostatic

forces.

Coagulate (Coagulum

)

In a suspension, an aggregate formed by

the addition of electrolyte.

Floe

In a suspension, an aggregate formed by

the addition of a polymer. Floes are

generally characterized by a loose

structure (low density).

Powder

A relatively dry, undispersed

accumulation of particulate matter with a macroscopic consistency.

Gel

Bicontinuous structure with a solid and a liquid component. The solid network may

consist of particles or polymers, held

together by covalent, ionic, or dispersion

(physical) forces. The network may be

elastic, viscoelastic, or plastic. The scale

of the mesh of the network (distance

between cross links) is of colloidal

dimensions.

Aerogel

A porous solid produced from a gel in

such a way that very little shrinkage

occurs. Typically, the term refers to materials made by supercritical extraction

of the solvent, although structurally

equivalent materials can be made under

ambient conditions by increasing network

stiffness and/or elastic recovery, and by

reducing interfacial tension.

Alcogel

A gel containing an alcoholic hquid

phase.

Hydrogel

A gel containing an aqueous liquid phase.

Xerogel

Porous solid made by drying a gel under

subcritical conditions.

Related Terms

Average Agglomeration Number (AAN)

An estimate of the degree of

agglomeration in a suspension. AAN is

4
Guide to the Nomenclature of Particle Dispersion Technology

the average number of primary particles

contained within an agglomerate. AAN is calculated from the ratio of the median

particle size, as determined by, for

example, light scattering, sedimentation

or electrical zone sensing techniques, to the average equivalent spherical volume

(Vbet) given by the BET gas adsorption

method, such that: AAN^ 50 _ ( D^^ SSA p

BET

where V50 is the equivalent spherical

volume calculated from the median

diameter, D50 in jim, SSA is the specific

surface area in m^/g and p is the particle

density in g/cuv'.

Equivalent Spherical Diameter

The diameter of a sedimenting particle

determined from Stokes' law and

assuming a spherical shape. The term is sometimes used in conjunction with other

measurement techniques and theoretical

constructs, where spherical geometry is assumed.

Fractal

A structure that has an irregular geometry

under all scales of observation (i.e., it is non-Euclidian). The fractal dimension of

a species, Df, is the exponent to which a

characteristic length scale must be raised

to obtain proportionality with the overall

size of the species. Destabilized

suspensions tend to form aggregates with

fractal structures. In this case, Df has a value lying between 1 and 3, where Df=3

is a fully dense object.

Minimum Particle Size (MPS)

An experimental quantity operationally

defined as the minimum particle size that can be achieved by a particular

dispersion process as determined by an

appropriate measurement technique. The

characteristic dimension used in

determining the MPS should be clearly

noted (e.g., mean size, median size,

modal size, etc.).

Ultimate Working Unit

An individual particle or group of

particles that retains its structure throughout a dispersion process and

subsequent application. See also

minimum particle size.

Recommendations

Floccule

It is recommended that this term not be

used in the ceramic literature, (see Floe)

Hard, Soft

With the exception of the defined term

hard-aggregate, it is recommended that

such adjectives be avoided in the context

of dispersed phase structure. If their use

is deemed necessary to convey materialspecific information, then the author

should make a clear statement that

defines the meaning and extent of usage.

2.3 Association and Dissociation Processes

Association Processes

Aggregation

A general term defined as any process by

which particles collect to form a cohesive mass or cluster; the resulting structure is called an aggregate.

Agglomeration

Formation of aggregates in a suspension

through physical (van der Waals,

hydrophobic) or electrostatic forces. The

resulting structure is called an agglomerate.

5
Guide to the Nomenclature of Particle Dispersion Technology

Coagulation

A specific type of agglomeration in

which formation of aggregates is induced

by the addition of electrolyte to a suspension. The resulting structure is termed the coagulate or coagulum, while

the electrolyte additive is termed the

coagulant.

Flocculation

Formation of aggregates in a suspension

mediated by polymeric species, that are

either attached to the particles or exist

freely in the suspending medium. The

resulting structure is called a floe, while

the polymer additive is termed a flocculant. Polymer bridging is a

flocculation process.

Gelation

Formation of a continuous (space-filling)

solid network characterized by a finite

static shear modulus (stress/strain ratio);

results from percolation of bonds

between particles or polymers. The

resulting structure is termed a gel.

Fusion

Process by which particles form

irreversibly bonded structures; often

characterized by the appearance of

interparticle necks, (see also hardaggregates)

Heteroagglomeration,

Heterocoagulation, Heteroflocculation

Generally refers to the aggregation of

dissimilar particles; in ceramic

applications, the formation of aggregates

by the cohesion between particles of

different materials (e.g., alumina and

silica).

Orthokinetic Aggregation

The process of aggregation induced by

hydrodynamic motions, such as stirring.

sedimentation, or convection.

Perikinetic Aggregation

The process of aggregation induced by

Brownian motion.

Sol-gel

Process for making a gel from colloidal or molecular precursors.

Dissociation Processes

Deagglomeration

Reversal of agglomeration, i.e., the

dispersion of agglomerates to form a suspension.

Deflocculation

Reversal of flocculation, i.e., the

dispersion offloes to form a suspension.

Comminution

Breaking down large pieces to the

required size; term commonly used in

association with milling of ceramic

slurries.

Peptization

Refers to the reversal of agglomeration

by the addition of a strong acid or strong

base, such as HCl or NaOH.

Related Terms

Diffusion-Limited Rate

Refers to a rate of aggregation

corresponding to the frequency of

encounter (collision rate) of the particles.

Each collision results in particle

adherence (i.e., a sticking probability

equal to 1). The rate of encounter is controlled by the diffusion rate, which

depends on the viscosity of the medium,

the dimensions of the particles, and the

concentration of the particles.

6
Guide to the Nomenclature of Particle Dispersion Technology

Reaction-Limited Rate

Refers to a rate of aggregation that is controlled by the reactivity of the

panicles (i.e., the frequency of collisions

resulting in particle sticking). Usually

characterized by a sticking probability

much less than 1. A low sticking

probability results from the presence of an energy barrier.

Syneresis

Spontaneous shrinking of a gel with

exudation of hquid.

Ultrasonication

Application of high-energy, highfrequency sound to a suspension in order

to disperse aggregates. Dispersion is thought to arise from the energy released

during fluid cavitation.

Recommendations

Sonication, Sonification

It is recommended that these terms not be

used in the ceramic literature, (see

Ultrasonication)

2.4 Dispersion Stability

States of Stability

Colloidal Stability

A physical state that characterizes the

relative ability of colloids to remain

dispersed in a liquid; suspensions that do

not aggregate at a significant rate are said

to be colloidally stable. The precise

connotation depends on the time frame

under consideration. Colloidal stability is a form of kinetic stability, and is therefore considered a metastable

thermodynamic state. From this

perspective, aggregation may be

described as a transition from a metastable to a stable state, occurring at

rates that depend on the magnitude of the

activation energy barrier that separates

them.

Kinetic Stability

Most dispersed systems are thermodynamically unstable, relative to

their separate bulk phases; however, a dispersion may exist for an appreciable

length of time and therefore exhibit

kinetic stability. The term may be used in

reference to various destabilizing

processes, e.g., aggregation, coalescence,

or sedimentation.

Stable Suspension

A suspension that has sufficient kinetic

stability to prevent the occurrence of

significant aggregation as measured over

a relevant time frame. Stability may be

ascertained by suitable experimental

means, such as particle size, turbidity, or sedimentation measurements.

Unstable Suspension

A suspension that lacks kinetic stability as measured over a relevant time frame; a highly unstable suspension is one that is subject to rapid {diffusion-limited)

aggregation.

Stability Mechanisms

Electrostatic Stabilization

Mechanism in which aggregation is inhibited by the presence of a mutually

repulsive electrostatic potential that

surrounds each particle.

Steric Stabilization

Mechanism in which aggregation is inhibited by the presence of an adsorbed

polymer layer that is firmly anchored to the particle surface so as not to desorb

during collisions. In general, a steric

stabilizing agent has one portion of its

7
Guide to the Nomenclature of Particle Dispersion Technology

structure that exhibits low solubihty in

the dispersion medium and/or high

affinity for the particle surface, and the

other portion is soluble in the medium.

Electrosteric Stabilization

Mechanism in which aggregation is inhibited by the combined effects of

electrostatic and steric stabilization.

Usually associated with the adsorption of

polyelectrolytes onto the particle surface.

Depletion Stabilization

Mechanism in which aggregation is inhibited by the presence of free (nonadsorbed) polymer due to the creation of

high-energy depletion zones (i.e., depleted of polymer compared to the bulk

solution) between closely interacting

particle surfaces.

Interaction Terms

DLVO

An abbreviation for a theory of the

stability of colloidal dispersions

describing the pair-wise interaction

between charged particles in a dielectric

medium. The theory, derived

independently by Derjaguin and Landau,

and by Verwey and Overbeek, calculates

the opposing effects of attractive van der

Waals forces and repulsive electrostatic

forces on the interaction potential.

Interaction Potential

The potential free energy between two

surfaces, typically presented as a function

of separation distance. By convention, a positive potential is mutually repulsive

and a negative potential is mutually

attractive.

Primary Maximum

The first appearance of a maximum in the

interaction potential energy curve with

increasing separation distance. The

primary maximum results from the fact

that repulsive and attractive forces decay

at different rates as a function of

separation distance. In DLVO theory, a large primary maximum acts as an energy

barrier, preventing aggregation of

particles into the primary minimum.

Primary Minimum

The first appearance of a minimum in the

interaction potential curve with

increasing separation distance. The

primary minimum results from the fact

that repulsive and attractive forces decay

at different rates as a function of

separation distance. In DLVO theory, a deep (negative) primary minimum acts as an energy well, allowing particles to adhere and resulting in a loss of colloidal

stability.

Secondary Minimum

A shallow energy minimum (usually of

the order of a few kT) in the interaction

potential curve occurring at relatively

large separation distances beyond that of

the primary maximum. In the presence of

such an energy well, secondary minimum

aggregation may occur. Because of the

shallow nature of the secondary

minimum, the aggregates formed are held together weakly and as such tend to be unstable toward rather small energy

inputs such as stirring.

Born (Hard Core) Repulsion

As two surfaces are brought into close

contact, the attractive van der Waals

force between them increases

continuously. At some point in their

approach, the electron clouds of the two

surfaces begin to overlap, giving rise to a repulsive force termed the Bom or hard

core repulsion. This results in a steep

increase in the interaction potential curve

8
Guide to the Nomenclature of Particle Dispersion Technology

at very small interatomic separation

distances, becoming effectively infinite

when interpenetration occurs.

Solvation (Structural) Forces

Non-DLVO forces that occur at extremely

small separation distances (typically a few molecular diameters) when particles

interact through an intervening fluid

medium. These forces arise whenever

liquid molecules are induced to order into

quasi-discrete layers between surfaces,

and can result in a monotonically

increasing (repulsive), monotonically

decreasing (attractive), or oscillatory

interaction potential. In aqueous solvents

these forces may be referred to as hydration forces.

Hamaker Constant

In the case of panicle interactions, a

material constant that measures the

relative strength of the attractive van der

Waals forces between two surfaces.

Particles interacting through an

intervening fluid medium will experience

a reduced attractive potential due to the

presence of the third component.

Hard Sphere Interaction

A largely theoretical construct in which

the interaction potential between

approaching particles is assumed to equal

zero, except upon contact where it goes

abruptly to infinity (i.e., no

interpenetration occurs).

Noninteracting

If no barrier to particle approach, contact

and adherence exists, the particles are said to be noninteracting. If the primary

minimum is sufficiently deep, every

collision will result in particles sticking

together. The rate of aggregation will

then be kinetically controlled (diffusionlimited rate).

Related Terms

Critical Coagulation Concentration

(ccc)

The molar concentration of electrolyte,

Co, necessary to induce rapid (diffusionlimited) aggregation. Experimentally

determined by extrapolation of In W

versus In Co to In W=0, where W is the

stability ratio.

Coagulant

An electrolyte additive that induces

coagulation in a suspension.

Schulze-Hardy Rule

An empirical rule summarizing the

general tendency of the critical

coagulation concentration to vary

inversely with the sixth power of the

counterion charge number of added

electrolyte.

Lyotropic Series

An ordered series of ions indicating, in

decreasing order, their effectiveness in

influencing the behavior of colloidal

dispersions. Typically associated with an

ion's relative propensity to coagulate a

dispersion.

Stability Ratio

Ratio of the diffusion-limited to reactionlimited rate constants for aggregation. A

large ratio indicates a high degree of

colloidal stability, whereas a ratio of

unity indicates that diffusion-limited

conditions prevail and the system is colloidally unstable. The rate constants

are determined experimentally from the

initial rates of aggregation. Usually

denoted by the symbol, W.

9
Guide to the Nomenclature of Particle Dispersion Technology

Defoaming (Antifoaming) Agent

A surfactant that when present in small

amounts prevents the formation of a foam

or aides in the coalescence of bubbles.

Dispersing Agent (Stabilizing Agent,

Dispersant)

A substance that when present in small

amounts facilitates the dispersion of

aggregates and improves the kinetic

stability of particles. For example,

polyelectrolytes are often used as dispersing agents in ceramic processing.

Surface Active Agent (Surfactant)

A substance that lowers the interfacial

tension between the solution in which it

is dissolved, and other phases which are present (e.g., solid particles in a suspension), and, accordingly, is positively adsorbed at the interface.

Polyelectrolyte

A macromolecular substance that, on

dissolving in water or other ionizing

solvent, dissociates to give polyions

(polycations or polyanions) - multiply

charged ions - together with an

equivalent amount of counter ions. A

polyelectrolyte can be a polyacid, a polybase, a polysalt, or a polyampholyte.

Frequently used as dispersing agents in

ceramic slurries.

Sedimentation

The settling of suspended particles or droplets due to the influence of gravity or an applied centrifugal field.

Sedimentation Volume

The volume of particulate sediment

formed in a suspension. If the sediment is formed in a centrifugal field, the strength

of this field should be explicitly

indicated, otherwise normal gravity is understood.

Brownian (Thermal) Motion

Random fluctuations in the density of

molecules in a liquid, due to thermal

energy, cause other molecules and small

dispersed particles to move along

random pathways. This random motion is termed Brownian motion, and is most

noticeable for colloidal particles.

Coacervation

When a colloidal suspension loses

stability, a separation into two liquid

phases may occur. This process is termed

coacervation. The phase that is more

concentrated in the colloid is the

coacervate, and the other phase is the

equilibrium solution.

Recommendations

Deflocculant, Dispersing Aid

It is recommended that these terms not be

used in the ceramic literature, (see

Dispersing Agent)

2.5 Interfacial and Electrokinetic

Properties

The Interface

Interface

A boundary between two immiscible

phases, at least one of which is condensed. Experimentally, the portion

of the sample through which the first derivative of any concentration versus

location plot has a measurable departure

from zero. In a suspension, the region of

contact between the particle surface and

the suspending medium.

Interfacial Region (Interphase)

The region that exists between two

phases where the properties vary from

those in the bulk.

10
Guide to the Nomenclature of Particle Dispersion Technology

Surface Region

The tridimensional region, extending

from the free surface of a condensed

phase towards the interior, where the

properties differ from the bulk.

Electrical Double Layer (EDL)

The term describes the non-random array

of ions at an interface in which two

oppositely charged layers coexist. For

particles dispersed in a fluid, the EDL

consists of the surface charge and the

solution charge. The solution charge may

be further subdivided into Stem and

diffuse layers, which is often referred to as the triple layer model.

Double Layer Thickness

Length characterizing the decrease of

potential with distance from a charged

interface. Typically defined as 1/k, where

K is the Debye-Hiickel parameter. For

low potentials it represents the distance over which the potential falls to lie, or about one third, of the value of the

surface potential.

Diffuse Layer

The region surrounding a suspended

particle in which non-specifically

adsorbed ions are accumulated and

distributed by the opposing action of the

electric field and thermal motion.

Stern Layer (Compact Layer)

Counter and co-ions in immediate

contact with a surface are said to reside in

the Stem layer, and form with the fixed

surface charge a molecular capacitor.

Often equated with the immobile portion

of the electrical double-layer that exists

inside the shear plane.

Inner Helmholtz Plane (IHP)

At a charged interface, an imaginary

plane representing the distance of closest

approach of desolvated ions to the

surface, and containing the ions or molecules that are specifically adsorbed.

Outer Helmholtz Plane (OHP)

At a charged interface, an imaginary

plane representing the distance of closest

approach of solvated (hyrdrated) ions to the surface. Often equated with the

position of the shear plane.

Shear Plane (Plane of Shear, Surface of

Shear)

In calculating the electrokinetic potential

from electrokinetic phenomena it is often

assumed that a sharp plane separates the

liquid adhering to the solid surface from

the mobile liquid. This imaginary plane is considered to lie close to the solid

surface.

Adsorption Processes

Adsorption

The process by which a substance is accumulated at an interface or in an

interfacial region. Should not be

confused with absorption, which denotes

accumulation inside a material or phase.

Adsorbate

A substance that is adsorbed at the

interface or into the interfacial region of

a substrate material, or adsorbent.

Adsorbent

The substrate material onto which a substance is adsorbed.

Adsorption Isotherm

The relationship between the equilibrium

quantity of a substance adsorbed and the

composition of the bulk phase, at constant temperature.

11
Guide to the Nomenclature of Particle Dispersion Technology

Specific Adsorption

Ions are specifically adsorbed when they

are present in the Stem layer in amounts

that exceed those expected from simple

electrostatic considerations. Empirically,

ions that are specifically adsorbed have a noticeable effect on the isoelectric point.

Non-Specific Adsorption

Ions are non-specifically adsorbed when

they are kept in the interphase only by

long-range coulombic interactions. They

are believed to retain their solvation shell

and in the position of closest approach to the interface they are separated from it by

one or more solvent molecular layers.

Empirically, ions that are non-specific

(indifferent) have no measurable effect on

the value of the isoelectric point.

Chemical Adsorption (Chemisorption

)

Molecules are chemically adsorbed when

they exist within the Stem layer and form

bonds with the surface groups, which

have a significant valence contribution.

Empirically, ions that are chemically

adsorbed have a noticeable effect on the

isoelectric point of a suspension and

exhibit a significant enthalpy (heat of

adsorption).

Physical Adsorption (Physisorption)

Adsorption in which the forces involved

are intermolecular (i.e., van der Waals,

hydrogen bonding) of the same kind as those responsible for the non-ideality of

real gases and the condensation of

vapors, and which do not involve a

significant change in the electronic

orbital patterns of the species involved.

Monolayer Adsorption

Adsorption in which only a single layer

of molecules becomes adsorbed at an

interface. In monolayer adsorption, all

adsorbed molecules are in the position of

closest approach to the substrate surface.

Multilayer Adsorption

Adsorption in which more than a single

layer of molecules is adsorbed at the

interface. Molecules adsorbed in excess

of monolayer adsorption are not in the

position of closest approach to the

substrate surface.

Coadsorption

The simultaneous adsorption of two or more species.

Desorption

The process by which the amount of an adsorbed substance is reduced.

Electrical Properties

Isoelectric Point (iep)

For many ceramic systems, the pH at which dispersed particles show no

electrophoretic mobility and the zeta potential has a value of zero. More

generally, the pl value at which zeta is zero, where I is the potential determining

ion.

Point of Zero Charge (pzc)

A particle carrying no fixed charge. The

precise identification of pzc depends on

the definition adopted for surface charge.

Typically for ceramic systems, the pH at which hydroxyl and proton adsorption is just balanced to cancel net charge; here,

the hydroxyl and proton are defined as the charge determining species.

Surface Charge Density

The quantity of electrical charge

accumulated at a particle-solution

interface, expressed per unit area; usually

represented by the symbol oq.

) 12
Guide to the Nomenclature of Particle Dispersion Technology

Shear Plane Potential

The potential difference across the

mobile part of the electrical double layer

at a charged solid-liquid interface;

potential at the shear plane during an

electrokinetic measurement. Synonymous

with zeta potential for the case of

particles suspended in a liquid.

Streaming Potential

When a liquid under a pressure gradient

is forced through a capillary or porous

plug, excess charges (ions) near the wall

are swept along by the liquid creating an accumulation of charge downstream. An

electric field is also created, which

opposes this accumulation. After a steady

state has been established, the measured

potential difference across the capillary or plug is called the streaming potential

and is related to the pressure gradient and

to the shear plane potential.

Zeta Potential (Electrokinetic Potential,

Shear Plane Potential)

The potential drop, ^, across the mobile

part of the electrical double layer, that is responsible for the electrokinetic

phenomena. C, is positive if the potential

increases from the bulk of the liquid

phase towards the shear plane. Certain

assumptions or estimations regarding the

double layer properties must be made in

order to calculate ^ from experimental

data. It is therefore essential to indicate in

all cases which equations have been used

in the calculation of t,. It can be shown,

however, that for the same assumptions

about the double layer properties, all electrokinetic phenomena must give the

same value for the electrokinetic

potential.

Potential Determining Ions

Those species of ions that by virtue of

their equilibrium distribution between

two phases determine the difference in

Galvani potential between these phases.

They are often, but not always, identical

with the ions that stabilize a colloidal

suspension formed from these phases.

Co-ions

In systems containing large ionic species

(e.g., colloids), co-ions are those that,

compared to the large ions, have low

molecular mass and the same polarity.

For instance, in a suspension of

negatively charged particles containing

sodium chloride, the chloride ions are coions and the sodium ions are counterions.

Counterions

In systems containing large ionic species

(e.g., colloids), counterions are those that,

compared to the large ions, have low

molecular mass and the opposite polarity.

For instance, in a suspension of

negatively charged particles containing

sodium chloride, the sodium ions are counterions and the chloride ions are coions.

Charge Reversal

The process wherein a charged particle is caused to assume a new charge of the

opposite polarity. Such a change can be

brought about by oxidation, reduction,

dissociation, adsorption, or ion exchange.

Electrokinetic Effects

Electrokinetics

Referring to the relative motions of

charged species in an electric field. The

field may be applied, or it may be created

by the motion of a liquid or adjacent solid

phase.

Electro-osmosis

When a liquid moves in response to an

applied electric field, while an adjacent

13
Guide to the Nomenclature of Particle Dispersion Technology

solid phase remains stationary (e.g., in a

capillary or porous plug), this is called

electro-osmotic flow. Fluid motion is due

to the reaction of charged species within

the fluid, usually dissolved ions, to the

applied field.

Electrophoresis

The motion of charged particles in an

applied electric field.

Electrophoretic Mobility (Static,

Dynamic)

The electrophoretic velocity per unit field

strength, symbol |ie=v/£'.; \x,q is positive if the particle moves toward lower potential

and negative in the opposite direction.

When measured in a d.c. electric field, |ie is referred to as the static mobility. When

measured in a high-frequency field it is referred to as the dynamic mobility, and

given the symbol i^d or |i(a)). Dynamic

mobility may be a complex quantity at high frequencies.

Electroacoustics

Referring to the electric-acoustic

coupling in a fluid containing charged

colloids or ions; an effect that is responsible for the electrokinetic sonic

amplitude, colloid vibration potential and

ion vibration potential.

Acoustophoresis

The induced motion of particles

subjected to an acoustic field. Charged

particles will generate an electric field as a result of this motion (see ultrasonic

vibration potential).

Ultrasonic Vibration Potential (UVP)

When a sound wave propagates through a

fluid containing charged particles (ions or colloids), coherent acoustophoretic

motion of the particles creates alternating

dipoles that generate a macroscopic

potential difference termed the ultrasonic

vibration potential.

Ion Vibration Potential (IVP)

The ultrasonic vibration potential of an

ionic solution; also known as the Debye

effect.

Colloid Vibration Potential (CVP)

The ultrasonic vibration potential of a

colloidal suspension. The resulting

potential difference is related to the

dynamic mobility of the particles;

reciprocal effect to electrokinetic sonic

amplitude.

Electrokinetic Sonic Amplitude (ESA)

When a high-frequency alternating

electric field is applied to a dispersion of

charged colloids, the oscillatory

electrophoretic motion of the particles

relative to the surrounding medium

results in a measurable acoustic field

whose amplitude is related to the

dynamic mobility; reciprocal effect of

colloid vibration potential. The phase

difference between the applied field and

the resulting acoustic response can also

be used to estimate the particle size

distribution.

Related Terms

Amphoteric

Refers to a type of surface in which the

same surface group (reactive site) is able

to function as both an acid and a base.

That is, the site may dissociate to release a proton or accept a proton.

Zwitterionic

Refers to a type of surface in which two

distinct surface groups (reactive sites) are

present. One is capable of dissociating to release a proton (acid group), and the

14
Guide to the Nomenclature of Particle Dispersion Technology

other is capable of accepting a proton

(base group).

Hydrophilic

May be used to describe the character of

interaction of a particular atomic group

(or substance) with an aqueous medium.

In this usage the term has the relative

qualitative meaning of "water loving."

The more general term, lyophilic

("solvent loving"), is used to distinguish a class of colloidal systems.

Hydrophobic

The tendency of hydrocarbons (or of

lipophilic hydrocarbon-like groups in

solutes) to form intermolecular

aggregates in an aqueous medium, and

analogous intramolecular interactions. In

this usage the term has the relative

qualitative meaning of "water fearing."

The more general term, lyophobic

("solvent fearing"), is used to distinguish a class of colloidal systems.

Indifferent (Supporting) Electrolyte

An ionic solution, whose constituents are not electroactive (i.e., they have no

significant effect on the surface potential

of the material under study; no oxidative

or reductive capacity) in the range of

applied potentials being studied, and

whose ionic strength (and, therefore,

contribution to the conductivity) is usually much larger than the

concentration of an electroactive

substance to be dissolved in it. The ions

constituting an indifferent electrolyte are

said to exhibit no specificity for the

particle surface.

Ionic Strength

A measure of electrolyte concentration

given by /=y2^c, Zi', where c, are the

concentrations, in moles per liter, of the

individual ions, /, and Zj are their ion

charge numbers.

Debye-Hiickel Parameter

A parameter in the Debye-Hiickel theory

of electrolyte solutions, denoted as k. For

aqueous solutions at 25 °C, k=3.288//

in reciprocal nanometers, where / is the

ionic strength. See Thickness of the

Electrical Double Layer.

Double Layer Compression (Screening)

Increasing ionic strength causes the

electrical potential near a charged surface

to fall off more rapidly with distance.

This is referred to as double layer

compression or screening, because the

double layer thickness shrinks as the

Debye-Hiickel parameter increases with

increasing ionic strength.

Electroviscous Effects

For dispersions of charged particles,

these are those components of the

viscosity connected with the charge on

the particles.

Suspension Effect

The Donnan e.m.f. between a suspension

and its equilibrium liquid. The effect is most commonly encountered with pH

measurements in colloidal suspensions.

Potentiometric Analysis

Analysis based on the measurement of

electrical potential using, for example, a pH or ion-selective electrode.

Potentiometry is often combined with

titrimetric analysis in the determination

of particle surface charge.

Titrant

The solution containing the active agent

with which a titration is made.

15
Guide to the Nomenclature of Particle Dispersion Technology

Titration

The process of determining the amount of

a substance A by adding increments of

substance B with provision for some

means of recognizing the point at which

all of A has reacted. This allows the

amount of A to be found from the known

amount of B added up to this point.

Titrimetric Analysis

Analysis of test sample properties based on titration.

3. BIBLIOGRAPHY

3.1 Primary Sources

Compendium of Chemical Terminology - lUPAC Recommendations, Compiled by

V. Gold, K.L. Loening, A.D. McNaught,

and P. Sehmi, Blackwell Scientific

Publications, Oxford, UK, 1987.

L.L. Schramm, The Language of Colloid

and Interface Science - A Dictionary of

Terms, American Chemical Society,

Washington, DC, 1993.

R.J. Hunter, Foundations of Colloid

Science, Volume 1, Oxford University

Press, Oxford, UK, 1989.

P.C. Hiemenz and R. Rajagopalan,

Principles of Colloid and Surface

Chemistry, 3^^ Edition, Marcel Dekker,

NY, 1997.

G.Y. Onoda and L.L. Hench, "Physical

Characterization Terminology," Chapter

5, pp. 35-37 in Ceramic Processing

Before Firing, Edited by G.Y. Onoda and

L.L. Hench, John Wiley & Sons, NY,

1978.

3.2 Secondary Sources

Adsorption of Inorganics at Solid-Liquid

Interfaces, Edited by M.A. Anderson and

A.J. Rubin, Ann Arbor Science

Publishers, Ann Arbor, MI, 1981.

Chemical Processing of Ceramics, Edited

by B.I. Lee and E.J. A. Pope, Marcel

Dekker, NY, 1994.

Sample Preparation for the

Determination of Particle Size

Distribution of Ceramic Powders,

ISO/DIS 14703, 1998.

Particle Size Analysis: Dispersing Agents

for Powders in Liquids, ISO Working

Document 14887, 1997.

Surface and Colloid Chemistry in

Advanced Ceramics Processing, Edited

by R.J. Pugh and L. Bergstrom, Marcel

Dekker, NY, 1994.

Ultrasonic and Dielectric

Characterization Techniques for

Suspended Particulates, Edited by V.A.

Hackley and J. Texter, American

Ceramic Society, Westerville, OH, 1998.

C.J. Brinker and G.W. Scherer, Sol-Gel

Science, Academic Press, NY, 1990.

D.F. Evans and H. Wennerstrom, The

Colloidal Domain, VCH Publishers, NY,

1994.

E. K. Fischer, Colloidal Dispersions, John

Wiley & Sons, NY, 1950.

R.J. Hunter, Zeta Potential in Colloid

Science - Principles and Applications,

Academic Press, NY, 1981.

16
Guide to the Nomenclature of Particle Dispersion Technology

D. Myers, Surfaces, Interfaces, and

Colloids - Principles and Applications,

Chapter 10, "Colloids and Colloidal

Stability," VCH Publishers, NY, 1991.

D.H. Napper, Polymeric Stabilization of

Colloidal Dispersions, Academic Press,

NY, 1983.

R.H. Ottewill, "Stability and Instability in

Dispersed Systems," Journal of Colloid

and Interface Science, 58, 357-373

(1977).

Th.F. Tadros, "Control of the Properties

of Suspensions," Colloids and Surfaces,

18, 137-173 (1986).

Acknowledgements

A number of terms were included

courtesy of the American Chemical

Society from the book by L.L. Schramm.

Some definitions contained in the

Compendium of Chemical Terminology

were used, in part or in whole, courtesy

of the International Union on Pure and

Applied Chemistry.

List of Reviewers and Contributors

James H. Adair, Merrilea J. Mayo

The Pennsylvania State University,

Particulate Materials Center

Jon A. Casey

IBM, Materials Technology

Robert A. Condrate

Alfred University, New York State

College of Ceramics

Matthew Frey, William Hunt,

Thomas Wood

3M Company, Ceramic Technology

Center

Vincent A. Hackley, Ajit Jillavenkatesa,

George Y. Onoda (retired)

National Institute of Standards and

Technology, Ceramics Division

Robert J. Hunter

University of Sydney, School of

Chemistry, Australia

George W. Scherer

Princeton University, Department of

Chemical Engineering

Ponisseril Somasundaran

Columbia University, Langmuir Center

for Colloids and Interfaces

17
Guide to the Nomenclature of Particle Dispersion Technology

Index to Defined Terms

A

AAN. See Average Agglomeration Number

Acoustophoresis , 14 Adsorbate, 1

1

Adsorbent, 11 Adsorption, 11 Chemical, 12 Monolayer, 12

Multilayer, 12 Non-Specific, 12

Physical, 12

Specific, 12 Adsorption Isotherm, II Aerogel, 4

Aerosol, 1

- Agent

Antifoaming, 10 Defoaming, 10

Dispersing, 10 ' Stabihzing, 10

Surface Active, 10 Agglomerate, 4

Agglomeration, 5 Aggregate, 4

Aggregation, 5

Orthokinetic, 6

Perikinetic, 6

Alcogel, 4

Amphoteric, 14 Average Agglomeration Number, 4 B

Born, 8 Brownian Motion, 10 c

ccc. See Critical Coagulation Concentration Charge Reversal, 13 Chemisorption, 12 , Coacervation, 10 Coadsorption, 12 Coagulant, 9 Coagulate. See Coagulum

Coagulation, 6 Coagulum, 4

Co-ions, 13

Co/tow/, 3 Comminution, 6

Concentration. See Relative Concentration Terms

Counterions, 13

Critical Coagulation Concentration, 9 CVP. See Potential, Colloid Vibration

D

Deagglomeration, 6 Debye effect. 5ee Potential, Ion Vibration Debye-Hikkel Parameter, 15 Deflocculant. See Agent, Dispersing

Deflocculation, 6

Desorption, 12 Dispersant, 10 Dispersing Aid. See Agent, Dispersing

Dispersion, 2 DLVO, 8 Double Layer Compression, 15 Droplet, 4 E

i?Z)L. See Electrical Double Layer

Electrical Double Layer, 1

1

Electroacoustic, 14

electroactive. See Indifferent Electrolyte

Electrokinetic, 13 Electrokinetic Sonic Amplitude, 14 Electro-osmosis, 13 Electrophoresis, 14

Electroviscous Effects , 15 Emulsion, 2 Equivalent Spherical Diameter, 5 £Si4. See Electrokinetic Sonic Amplitude

F

F/oc, 4

Flocculation, 6

Floccule, 5 Fractal, 5 Fusion, 6 G

Ge/, 4

Gelation, 6

Granule, 3 H

Hamaker Constant, 9 Hard Core. See Bom Repulsion

//arrf Sphere Interaction, 9 Hard-Aggregate, 4

Heteroagglomeration, 6

Heterocoagulation, 6

Heterodisperse , 2 Heteroflocculation, 6

hydration forces. See Solvation Forces
Guide to the Nomenclature of Particle Dispersion Teclinology

Hydrogel, 4

Hydrophilic, 15 Hydrophobic, 15 Hydrosol, 2

I

iep. See Isoelectric Point IHP. See Inner Helmholtz Plane

Indifferent Electrolyte, 15

Interface, 10

Interfacial Region, 10

Interphase, 10

Ionic Strength, 15

Isoelectric Point, 12 TVP. See Potential, Ion Vibration

L

Layer

Compact, 11 Diffuse, 1

1

lyophilic. See Hydrophilic

lyophobic. See Hydrophobic

Lyotropic Series, 9 M

Microsphere, 4 Minimum Particle Size, 5 Mobility

Dynamic, 14

Electrophoretic, 14

Static, 14 Monodisperse, 2 MPS. See Minimum Particle Size N

Nanoparticle, 3 Noninteracting, 9 o

OHP See Plane, Outer Helmholtz

Organosol, 2 P

Particle, 3 Coarse, 3

Fine, 3 Primal y, 4

Ultrafine, 3

Particulate, 4

Peptization, 6 Phase

Continuous, 2

Discontinuous, 2

Dispersed, 2

Liquid, 2

Particulate, 2

Solid, 2 Physisorption, 12

Inner Helmholtz, 1

1

Outer Helmholtz, 1

1

Plane of Shear, 11 Point of Zero Charge, 12 Polydisperse, 2

Polyelectrolyte, 10

Potential Colloid Vibration, 14

Electrokinetic, 13

Interaction, 8 lonVibration, 14 Shear Plane, 13 Streaming, 13

Ultrasonic Vibration, 14

Zeta, 13 Potential Determining Ions, 13 Potentiometric Analysis, 15 Powder, 4

Primary Maximum, 8 Primary Minimum, 8 pzc. See Point of Zero Charge

R

Rate

Diffusion-Limited, 6

Reaction-Limited, 7 Relative Concentration Terms, 3 s

Schulze-Hardy Rule, 9 Screening, 15 Secondary Minimum, 8 Sedimentation, 10 Sedimentation Volume, 10

S/iear P&rne, 1

1

SiVwc Range. See Particle, Coarse

Slip, 2

Slurry, 2

Sol, 2

Sol-gel, 6

Solvation Forces, 9

Sonication, 7

Sonification, 7

Stability

Colloidal, 7

Kinetic, 7

Stability Ratio, 9

Stabilization Depletion, 8

Electrostatic, 7

Electrosteric, 8

Steric, 7 Structural Forces, 9 Subsieve Range. See Particle, Fine Supporting Electrolyte , 15

19
Guide to the Nomenclature of Particle Dispersion Technology

Surface Charge Density, 12 Surface of Shear, 1

1

Surface Region, 1

1

Surfactant, 10 Suspension, 2

Stable, 7 Unstable, 7 Suspension Effect, 15

Syneresis, 1 T

Thermal Motion, 10

Titrant, 15

Titration, 16

Titrimetric Analysis , 16

triple layer model. 5£e Electrical Double Layer

u

Ultimate Working Unit, 5 Ultrasonication, 1 UVP. See Potential, Ultrasonic Vibration w

Well-Dispersed, 3 X

Xerogel, 4 z

Zwitterionic, 14

2(9
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