/ 6 ' & &

NATIONAL BUREAU OF STANDARDS REPORT

7448

THE ELECTROPHORETIC MOBILITY OF ASPHALTENES IN NITROMETHANE

by

James R. Wright and Richard R. Minesinger

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

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NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT

1004- 1 2-101 41 February 21, 1962

NBS REPORT

7448

THE ELECTROPHORETIC MOBILITY OF ASPHALTENES IN NITROMETHANE

©

James R. Wright and Richard R. Mine singer

Organic Building Materials Section

Building Research Division

Sponsored by

National Bureau of Standards

NATIONAL BUREAU OF STA

Intended for use within the C to additional evaluation and re listing of this Report, either li the Office of the Director, Nat however, by the Government i to reproduce additional coplei

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<QJBS>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

THE ELECTROPHORETIC MOBILITY OF ASPHALTENES IN NITROMETHANE

ABSTRACT

The microscope method of electrophoresis was employed to determine

the electrophoretic mobility of asphaltenes in a nonaqueous system.

Nitromethane was used as the suspending liquid. The electrophoretic

mobilities of asphaltenes from 24 air-blown asphalts were measured

by this method. All asphaltenes were found to bear a positive

electrical charge, although the magnitude of charge varied as denoted

by different electrophoretic mobilities. The presence of an electrical charge on asphaltenes supports the colloid theory of asphalt.

1 . INTRODUCTION

The colloidal nature of asphalt was first recognized by Nellensteyn [ 1

]

who proposed that asphalt contains three principal fractions; (a) the

medium or disperse phase (oils), (b) the lyophile part or protective

bodies (resins), and (c) the lyophobe part (asphaltenes) which he believed

to be elementary carbon. While many investigators accept Ne 1 lensteyn ’

s

colloid theory of asphalt, the elementary carbon nucleus concept has

been abandoned [2], Instead, the asphaltenes, which were shown by

elemental analysis to contain C, H, N, 0, and S [3], are considered to be

dispersed in the continuous oil phase, with the dispersion being maintained

by the resins [4]

.

Many factors have been cited in the literature to prove that asphalt is a colloidal system. Pfeiffer and Saal [5] proposed that the dispersed phase

consists of asphaltenes complexed with high molecular weight components

of the maltenes (oils and resins). They called this complex an asphaltene

micelle. The continuous phase, or intermicellar phase, consists of the

low molecular weight constituents of the maltenes. The asphaltenes are peptized to different degrees due to the aromatics and resins content

of the maltene phase. Eilers [6] refers to a number of cases which

showed that the rheological behavior of most asphalts is that of a colloidal disjpersion. In the same paper, this author presents data on

the voluminosity of asphaltenes in solutions, which he cites as proof of

the colloidal properties of asphalt.

1/

Numbers in brackets refer to literature references at the end of this

report

.
2

In an investigation of crude oils by ultra-centrifuging, at 80,000 times

gravity, Eldib, et al [7] found that the material which moved to the

bottom of the tube was colloidal in nature and was rich in asphaltenes.

Swanson [4] has shown by dielectric constant experiments that asphaltenes

are not homogeneous, and are polar in character. Csanyi and Bassi [8]

found that the conductivity of a benzene-methanol solution was increased

significantly by the addition of asphaltenes.

While many references may be cited which attest to the colloidal nature

of asphalt, direct experimental evidence to support this theory is

limited. For example, in the literature there is little experimental

proof, by a direct method of measurement, that asphaltenes are electrically

charged. However, if asphalt is truly a colloidal system, the dispersed

phase (asphaltenes) probably would carry an electrical charge because one

of the basic properties of colloidal particles is that they move in an electric field, i.e. exhibit electrophoresis [9, 10].

The purpose of the present work was to determine by a method of direct

observation, microelectrophoresis, what charge, if any, was present on asphaltenes. Since it was not possible to disperse asphaltenes in water,

the adaptation of the microelectrophoresis method [11] to a suitable

nonaqueous system was an important part of the research.

2. APPARATUS, MATERIALS, AND PROCEDURES

2.1 Electrophoretic Mobility Apparatus

2.1.1 Description of the Apparatus

The glass cell used for the electrophoretic mobility measurements was of

the Northrup-Kunitz type [12]— . This is a rectangular- type cell with

dimensions of 4.5 cm between the platinum electrodes, 1.25 cm wide, and

0.056 cm deep. The cell was mounted on a specially designed lucite base

which was positioned on the microscope stage by machined positioning pegs.

This insured using the same part of the cell for every sample, and eliminated

the problems of vibration and support due to the rubber connections as observed by Thompson [13],

2 /

Obtained from A. H. Thomas Company, Philadelphia, Pennsylvania.
3

Reversible zinc electrodes were placed in the end vessels and connected

in series through a reversing switch to direct current power supply (0 - 300 volts). Saturated ZnSO/,. was used as the electrolyte. A Spencer

microscope with a 30X eyepiece containing a cross-hatch graticle and

both 10X and 20X objectives, was used for observing particle movement.

The source of illumination was a detached microscope lamp, with a heat

filter, focused on a reflecting mirror. This minimized temperature

changes in the cell while measuring particle movement. Figure 1 is a schematic drawing of the electrophoresis apparatus.

Particle movement was measured by an electric timer—/ which could be

read directly to 0.01 second. The timer was equipped with a microsecond

delay switch which completed the circuit when held closed manually. This

permitted the operator to time the movement of a given particle with two

motions instead of four as would be required with a stopwatch.

The microscope, electrophoresis cell, and auxiliary cells were mounted

on plywood. This in turn was supported by cork and sponge rubber.

These precautions eliminated ordinary building vibrations. The entire

apparatus was used in an air-conditioned laboratory.

2.1.2 Calibration of the Apparatus

The ratio of applied EMF to the potential difference between the platinum

electrodes in the electrophoresis cell was determined by the means of a quadrant electrometer.it/ The applied voltage was measured by a volt-ohm

meter attached directly to the terminals of the zinc electrodes. This

eliminated any error due to voltage drop between the power supply and the

zinc electrodes, (Subsequently, a calibration curve was made for the

voltage applied by the power supply and that obtained at the zinc electrodes.

Applied voltages used in calculating particle mobility were taken from

this curve)

.

Potentials ranging from 40 to 175 volts were applied at the zinc electrodes

and the potential difference between the platinum electrodes was measured.

Twenty measurements were made in each direction. An average value was used to calculate the ratio of applied voltage to the potential difference

between the platinum electrodes. The value obtained was 5.86.

H

Obtained from the Standard Electric Time Company, Springfield, Mass.

4/ Method described in "Directions for Use for Cataphoresis Apparatus No.

2873" furnished by A. H. Thomas Company, Philadelphia, Pa.
Sample

Reservoir

ELECTROPHORESIS

APPARATUS

FIGURE

1.

SCHEMATIC

DIAGRAM

OF

ELECTROPHORETIC

MOBILITY

APPARATUS.
- 4

-

The cell depth was measured with the microscope. Dry asphaltenes,

1 - 5p diameter, were sifted into the cell before assembly and used as focusing points on the top inside and bottom inside of the cell. The

cell depth, as measured by the microscope fine adjustment screw, was

565p. However, as pointed out by Thompson [13], the apparent depth of

the cell when filled with water, for example, is less because of refraction

by the contents unless a water-immersion objective is employed. The

apparent depth of the present cell when filled with water was 424p which

agrees well with the value of 423p as calculated by the method of

Northrup [14]. Apparent depths were determined experimentally for the

organic liquids used in measuring asphaltene mobility. Top and bottom

stationary levels were 0.211 and 0.789 times the measured cell-depth

values, respectively [13],

The cross-hatch graticle in the microscope eyepiece was calibrated with

a stage micrometer. Each square was 50p across at 300X and 22p. across

at 600X.

The direction of positive particle velocity was determined for the

apparatus assembled as shown schematically in Figure 1. The cell was

filled with a dilute oil/water cationic emulsion. With the reversing

switch in the "up" position, the particles, as viewed through the microscope, moved from left to right. Thus, positive velocity or the presence

of a positive particle charge was shown by left to right movement in the

apparatus. These results were verified by the migration of cationic and

anionic asphalt emulsions, respectively.

As a final check on the present microelectrophoresis apparatus, a celldepth profile was made with a dilute cationic o/w type asphalt emulsion.

Particle velocity measurements were made at each eighth of the cell

depth. A plot of particle velocity versus cell depth resulted in a parabolic curve. According to Thompson [13], this indicated that only

electrophoretic effects are being observed.

2.2 Materials

2.2.1 Asphaltenes

The asphaltenes used in this research were obtained from twenty-four airblown roofing asphalts. The properties of the asphalts were previously

described by Greenfeld and Wright [15] and Wright and Campbell [16], The

asphaltenes were separated from the maltenes by the method of Kleinschmidt

[17], i.e. by extracting the maltenes with n-pentane.
5

Normally, in the separation of asphaltenes from n-pentane, each asphalt

is digested for a fixed period of time. It seemed likely that if the

degree of association of asphaltenes and maltenes varied among asphalts,

the degree of separation would vary aJ.so for a given time of pentane

digestion. To check this possibility, asphaltenes from four asphalts of

different accelerated weathering durabilities were soxhlet extracted with

n-pentane for eight hours at which time the extracting liquid was essen- tially colorless. Evaporation of the pentane resulted in maltenes

amounting to 5.8 to 10,2% by weight of the original asphaltenes. These

results showed that "purification” of the asphaltenes was necessary

before attempting to measure electrophoretic mobility.

Subsequently the asphaltenes from all 24 asphalts were "purified" by

the following procedure: The particle size of the asphaltenes was reduced to a maximum of ca 10p by pulverizing in a Wiley Mill (Laboratory

Intermediate Model, 60 mesh screen). Five grams of the pulverized

asphaltenes were soxhlet extracted with 400 ml of n-pentane for 8 hours.

The extracted asphaltenes were vacuum dried for 19 hours at 35° C and 10 mm

(Hg) pressure. All extractions were made with Phillips Pure-Grade n-pentane.

A gas chromotograph of this product showed 99.887, n-pentane. The only

impurity was iso-pentane.

The pulverized asphaltenes remained finely divided after extraction, and were easily dispersed in the organic suspending liquid for electrophoretic mobility measurements.

2.2.2 Dispersion Medium for Asphaltenes

Since asphaltenes cannot be wet by or dispersed in water, it was necessary

to use a non-aqueous system for electrophoretic mobility determinations.

The factors to be considered in selecting a non-aqueous suspending liquid

are suggested by the Helmholtz-Smoluchowski equation [11], which is:

e$E

u = 4TT

electrophoretic mobility

dielectric constant of the suspension

zeta potential

voltage drop between the platinum electrodes

viscosity of the suspending liquid

In addition to these factors, it was desirable that the asphaltenes be insoluble or sparingly soluble in the organic suspending liquid.

whereu

eS

E

’l
6

Van der Minne and Hermanie [18] in their extensive research on electrophoretic mobility in non-aqueous systems have shown that a suspending

medium must have a minimum dielectric constant in the range of 10 to 15

in order to measure particle mobility normally and in the same way as in aqueous systems. Preliminary experiments in this laboratory with a series of alcohols as suspending liquids showed that a viscosity of

2.8 centipoises at 25°C represented the maximum, if high applied potentials

were to be avoided. With these limits as guides, i.e. e mi n = 14 and ^ max = 2.8 centipoise, the International Critical Tables^/ were reviewed

for organic compounds which were potential suspending liquids for

asphaltenes. A total of 82 compounds was found.

The compounds were screened with respect to toxicity, volatility, availability, etc., and reduced to 20 in number. These were procured in the

highest purity commercial grades and evaluated as suspending liquids for

asphaltenes. N, N-Dimethy lacetamide and N, N-Dimethylformamide were

also included. The compounds are listed in Table 1.

A spectrographic method was used to measure the relative solubility of

the asphaltenes in each suspending liquid. Ten milligrams of asphaltenes

were dissolved in 100 ml of chloroform, and the absorbance measured in the

400 to 600 mp range with a Beckman DU Spectrophotometer. Thus, the

absorbances in all of the organic liquids evaluated were relative to

those in chloroform.

Ten milligrams of asphaltenes were mixed with 10.0 ml of each of the

22 compounds being evaluated, and allowed to stand overnight. In most

cases the asphaltenes settle to the bottom of the container. Where

asphaltenes were observed to be suspended, they were removed by

filtration. Absorbances were determined and the percent asphaltenes

dissolved, relative to chloroform, was calculated for each liquid. The

results are given in Table 1. Liquids dissolving more than 6.07>

asphaltenes were eliminated from further consideration.

Dielectric constant data from Volume VI; Viscosity data from Volume

VII of the International Critical Tables, respectively.
TABLE 1. COMPOUNDS EVALUATED AS

FOR ASPHALTENES

SUSPENDING LIQUIDS

Dielectric

Constant Viscosity

Asphaltene

Solubility

No. Name (20°C) (centipoise @20°C) (7o)—

/

1 Propionitrile 27.7

14. 2^/

0.41^ 1.03

2 n - Propyl nitrate — 10.8

3 N, N - Dimethylacetamide b/ — 10.3

4 iso - Butyronitrile 20 . 8

-'

30.0^ — 1.68

5 Propionaldehyde 0o41 6.0

6 Nitroethane — 2.06

7 Nitrobenzene 36.0 2.03 60.0

8 Acetonitrile 38.8 0,36 .

' 2.22&'

0.22

9 2 - Propanol 26 1.41

10 Cyclohexanone 18.3 2o3C&/ 55.0

11 Methanol 33.7 0,59 0.26

12 N, N - Dimethylformamide 2.55

13 2 - Butanone 18.5 0.43 9.4

14 Ethanol 25.9 ‘ 1,7

£/ 0.62L/

0.87

15 Nitromethane 39.4 0.24

16 Mesityl oxide 15 - 4

b/ — 17.0

17 2 - Chloroethanol 25. 8^' — 0.69

18 O' - Toluntrile 18*3 3.5

19 Ethyl cyanoacetate 27 . 7 ®/ 0.16

20 2, 4 - Pentanedione 23.1 — 5.1

21 Citraconic anhydride 40.3 —

2.6

22 0 - Nitroanisole 24.2 —

7.1

a./ Based on solubility in chloroform as 100%.

b/ 24°C 0 c/ 17 °C.

d/ 18 °C.

e/ 21 °C.

f/ 25°C.

£/ 22°C.
7

The remaining compounds were evaluated as suspending liquids for asphaltenes

in the apparatus described previously. Mixtures of 70 mg of asphaltenes and

70 gm of each liquid were made and the preliminary mobility rates determined.

A number of compounds were not satisfactory due to high viscosity, excessive

settlement, low mobility rates, etc. Nitromethane was selected as the best

suspending liquid for asphaltenes (e = 39.4, asphaltene solubility = 0.24%),,

Eastman Spectro Grade nitromethane was fractionated through a packed WhitmoreFenske column. The infrared spectrum of the fractionated product was com- pared with that of API Scan, Serial No. 1751.

2.3 Procedure for Electrophoretic Mobility Measurements

The asphaltene samples were prepared by dispersing 70 mg of asphaltenes

in 70 grams of nitromethane (0.1 0% concentration), and the mixture allowed

to stand about 30 minutes. The apparatus was adjusted to top stationary

level position, and the electrophoresis cell filled with nitromethane,

care being taken to avoid the presence of air bubbles. The asphaltene

suspension was poured into the sample reservoir and a portion of it introduced into the electrophoresis cell. The stopcocks were turned to the

ZnS 04 - Zn junctions and the asphaltene particles checked to insure that no movement or drift was taking place under zero potential.

Sufficient potential was applied to the zinc electrodes to produce particle

movement of such a velocity as to eliminate any diagonal drift (usually not

less than 75 volts). Particles of about 5p diameter were selected for

mobility rate measurements over a distance range of 50 to 150p, and their

movements timed.

The field was reversed about every 30 seconds to prevent electrode

polarization [19]. The potential was reduced to zero when readings were

not being made. Fresh samples of asphaltenes were introduced into the

electrophoresis cell periodically or when too few particles were in focus

for the desired number of readings. When the readings for a given sample were completed, the asphaltene suspension was drained from the system.

The apparatus was prepared for the next sample by flushing with chloroform

(to dissolve -residual asphaltenes), methanol, and nitromethane. The ZnSO^-

Zn electrode compartment was always inspected for gas bubbles which could

be removed by the addition of electrolyte from the ZnS04 reservoir. The

presence of any gas in this chamber sometimes caused drift of the particles

with no applied potential; presumably due to unequal pressures in the

closed system.
8

For each asphaltene sample, 20 to 30 particle velocity readings were made

in each direction. A single average time value was used to calculate

the electrophoretic mobility (u) by the following equation:

u = ° . X . d

t V

where

D = distance particle travels during timing period

t = time required to travel distance D

r = ratio applied potential to potential difference between the

platinum electrodes

V = voltage applied to zinc electrodes

d = distance between the platinum electrodes

By this equation, electrophoretic mobilities are expressed in p/sec/volt/cm.

In the apparatus used in the present work, r = 5.86 and d = 3,25 cm.

3 . RESULTS

Electrophoretic mobility measurements were made on asphaltenes from 24

asphalts. The number of samples measured for asphaltenes from each

asphalt range from one to three. The results are presented in Table 2.

In Table 3 the electrophoretic mobility data on asphaltenes from six

asphalts are compared to various properties of the original asphalts.

Comparisons are made with accelerated weathering durability, carbonyl

index, filtration time, solubility parameter and asphaltene content [15].

The statistical data on the electrophoretic mobility of asphaltenes from

six asphalts are presented in Table 4. For each asphalt, three separate

samples of asphaltenes in nitromethane were evaluated and the results

reported. The data are presented as average time values in each direction, and a final mobility value in p/sec/volt/cm.

4. DISCUSSION

4.1 General

By subjecting a dispersion of asphaltenes in nitromethane to an electric

field, as described previously, it was soon established that asphaltenes

carry an electric charge. It was further established that this charge

is of sufficient intensity to produce particle mobility by ordinary

microelectrophoresis techniques.
TABLE 2 . ELECTROPHORETIC MOBILITY DATA ON

ASPHALTENES FROM TWENTY-FOUR ASPHALTS

Asphalt

Asphaltene

Samples

Asphaltene Migration

Timei/ Electrophoretic

Direction

No. Source (number)— Left Right Avg. u/sec/volt/cm

1 Tia Juana 3 . 4.31 4.22 4.26 4.61

2 Tia Juana-Lago Colon 1 3.62 3.70 3.66 5.36

3 Calif. Coastal (catalyzed) 2 5.07 4.94 5.00 3.93

4 Talco -Hankins 1 5.10 5.10 5.10 3.85

5 Kansas -Indiana 1 5.01 4.51 4.75 4.13

6 Talco 2 5.12 5.04 5.08 3.86

7 Talco-Heidleberg 1 5.28 5.21 5.24 • . 3.75

8 Talco 3 4.92 4.94 4.93 3.98

9 Calif. Coastal (fluxed) 2 4.41 4.34 4.38 4.48

10 Lagomar 3 4.24 4.35 4.30 4.56

11 Kansas -Lyons 1 3.80 3.81 3.80 5.16

12 Tia Juana Medium 1 4.43 4.34 4.38 4 .48

13 Venezuela (fluxed) 1 5.38 4.67 5.02 3.91 -

14 Tia Juana C 1 3.76 3.81 3.78 5.19

15 Arkansas Blend 1 5.07 4.99 5.03 3.90

16 Ill-Kan-Okla 3 5.14 4.97 5.06 3.89

17 Venezuela-Mid East 1 6.17 5.87 6.02 3.26

18 Los Angeles Blend 1 5.04 4.34 4.69 4.18

19 Kansas A 1 5.07 4.39 4.73 4.15 . 20 Santa Maria 3 4.28 4.05 4.16 4.70

21 South Mountain 1 . 4.73 4.68 4.70 4.18

22 Lagunillas 1 5.07 4.93 5.00 3.93

23 Talco 1 5.01 4.72 4.87 4.03

24 Baxterville, La. 3 4.78 4.58 4.68 4.20

a/ Designates number of duplicate asphaltene-nitromethane dispersions used in average

time values for each mobility direction.

b/ Seconds per 100p distance at 97 applied volts. Values reported for each direction

are averages of 20 readings per sample.
TABLE 3. COMPARISON OF THE ELECTROPHORETIC MOBILITY OF ASPHALTENES TO

VARIOUS PROPERTIES OF THE ORIGINAL ASPHALTSAsphalt

No.

Electrophoretic

Mobility^-7 (u/ sec/volt/cm)

Durability

(daysil/)

Carbonyl

Index

- CM)

Filtration

Time

(seconds)

Critical

Solubility

Parameter

Asphaltene

Content

a)

20 4.70 52 0.225 150 8.08 41.6

1 4.60 54 0.066 203 7.85 42.0

10 4.56 59 0.058 305 7.97 38.7

24 • 4.20 48 0.061 284 7.99 38.9

8 3.98 46 0.064 216 7.90 45.0

16 3.89 93 0.038 642 7.73 39.8

aJ Source: S. H. Greenfeld and J. R. Wright, "A Study of Several Methods fot

Predicting the Durability of Roofing Asphalts," Materials and Standards

Research (in press).

•

'

I . bJ The values given here are averages of those shown in Table 2.

c/ ASTM D 529-597., Tentative Recommended Practice for Accelerated Weathering of

Bituminous Materials.
TABLE 4 . STATISTICAL DATA ON THE ELECTROPHORETIC

MOBILITY OF ASPHALTENES

a/

Asphaltene Migration Time- Electrophoretic

Asphal t Asphaltene Direction _____ Mobility

No, Source Sample Left Ripht Left Ri^ht Avr. u/ sec/volt/cm

1 Tia Juana a 4.43 4.23 4.34 4.32 4.33 4.53

b 4.28 4.10 4.08 4.15 4.15 4.73

c 4.30 4.22 4.44 4.32 4.32 4.54

8 Talco a 4.94 4.99 4.87 4.95 4.94 3.98

b 5.00 5.01 4.90 4.83 4.94 3.98

c 4.70 4.93 5.13 4.94 4.93 3.98

10 Lagomar a 4.09 4.07 4.31 4.51 4.25 4.62

b 4. 16 4.49 4.32 4.38 4.34 4.52

c 4.33 4.33 4.26 4.33 4.31« 4.55

16 Ill-Kan-Okla a 5.46 5.10 4.71 4.73 5.00 3.92

b 5.00 4.92 5.10 4.75 4.94 3.98

c 5.03 4.86 5.53 5.45 5.22 3.77

20 Santa Maria a 4.21 3.97 4.24 4.03 4.11 4.77

b 4.32 4.10 4.33 4.01 4.19 4.67

c 4.31 4.13 4.29 4.05 4.20 4.67

24 Baxterville, La. a 4.65 4.71 4.77 4.77 4.73 4.15

b 4.93 4.58 4.90 4.39 4.70 4.18

c 4.72 4.50 4.72 4.50 4.61 4.26

a/ Expressed as seconds per lOOp. distance at an applied potential of 97 volts. Each

entry is an average of 20 readings in a given direction.
9

The charge appears to be an inherent property of the asphaltenes, and is

not due to such phenomenon as streaming potential, static effects, etc.

This was proven experimentally by the facts that (a) no particle migration was observed unless a potential was applied, (b) the particles

reversed direction upon reversal of the electric field, (c) the electrophoretic mobility of a given sample of asphaltenes in nitromethane did

not decrease upon standing several weeks in the laboratory, and (d)

asphaltenes exhibited positive velocity in a number of organic suspending

liquids

Further evidences that only real electrophoretic effects were observed

in the asphaltene-nitromethane system were (a) the motion of the

particles was uniform and rectilinear between electrodes, (b) the

velocity was independent of the position in the observation area,

(c) the velocity was proportional to the field strength, and (d) no evidence of particle chain formation indicating dielectric polarization

was observed. According to Van der Minne and Hermanie [18], these are

the criteria for real electrophoresis free from any other effects

(e.g. electro-osmosis) which can cause particle mobility,

4.2 Sign of Asphaltene Charge

The results presented in Table 2 for the electrophoretic mobilities of

asphaltenes from 24 asphalts, showed that all asphaltenes had positive

velocity, i,e. were charged positively., While the charge intensity

varied among asphaltenes from different asphalts, the sign of the charge

did not.

These results are in agreement with the work of Swanson [4] who reported

that asphaltenes show high adsorptivity on a glass surface which is

negatively charged. The presence of a positive charge also supports

the findings of Csanyi and Bassi [8] who, in their attempts to separate

asphalt by electrical means, found that the asphaltenes from 13 out of

the 15 asphalts studied deposited on the negative electrode.

The experimental work of Van der Minne and Hermanie [20] is strong

evidence that the sign and magnitude of the charge is not due to the

difference in magnitude of the dielectric constants of the two phases

as indicated by Coehn's rule.
10

4.3 Source of Asphaltene Charge

The presence of an electrical charge on asphaltenes as determined by

microelectrophoresis , provides essentially no information as to the source of the charge. Obviously, additional experimental work must be

carried out to establish the charge source. It is postulated, however,

that the charge may be due to any of three posibilities which are:

(a) A charged molecular complex formed by adsorption of aromatic

maltenes by asphaltenes. The aromatic molecules act as an electron

source for positively charged carbon atoms adjacent to electronegative

nitrogen and oxygen atoms in the asphaltenes. This results in a deficiency of electrons in the adsorbed molecule and a net positive

surface charge. (b) Interaction of the asphaltenes with the suspending

liquid (nitromethane) in some way to produce a charge through ion formation, and (c) the presence of trace metals in the asphaltenes which may

exhibit charge under the experimental conditions employed.

4.4 Relationship Between Asphaltene Charge and Various

Properties of the Original Asphalt

The presence of an electrical charge on asphaltenes means that potential

electrostatic—^ bonding forces exist in asphalt. The electrostatic

bonds may be both intramolecular and intermolecular . In the latter case,

electrostatic bonds could be formed between two asphaltene molecules or an asphaltene and a maltene molecule. Furthermore, if the electrostatic

bonding forces between maltenes and asphaltenes were of sufficient order

of magnitude, a direct relationship should exist between the electrophoretic mobility rate of the asphaltenes and those properties of the

original asphalt, such as natural weathering durability, which are due

to asphaltene-maltene association as proposed by Kleinschmidt and

Snoke [21] and Greenfeld [22, 23],

Table 3 was compiled in order to make a comparison of the electrophoretic

mobility values of asphaltenes and various properties of the original

asphalt; namely, accelerated weathering durability, carbonyl index,

critical solubility parameter, and asphaltene content. Asphalt No. 20

has the highest asphaltene - electrophoretic mobility of the six asphalts.

It also has the highest carbonyl index, the lowest filtration time and the

largest solubility parameter value. The asphaltene content is above the

40% limit established for more durable asphalts [24], and the accelerated

weathering durability is in the low-intermediate range. Conversely,

asphalt No. 16 which has the lowest asphaltene - electrophoretic mobility

“^The term electrostatic is used here to indicate some form of polar bonding

forces. Conversely, all of the bonding forces in asphalt are not of the

covalent type.
11

also has the lowest carbonyl index, the highest filtration time, the

smallest solubility parameter value, and the longest accelerated

weathering durability. The asphaltene content is in the range of the more durable asphalts. For the remaining four asphalts whose properties

are intermediate between those of asphalts 20 and 16, no pattern can

be found relating asphaltene - electrophoretic mobility to these

properties. Since there appears to be no correlation between

asphaltene - electrophoretic mobility and other properties of the

asphalt, it is concluded that electrostatic bonding forces play a minor role in asphaltene-maltene association or asphalt durability.

This is in agreement with the work of Heithaus [25] who found that

asphaltene peptizability, as determined by flocculation ratio, played

a minor role in asphalt weathering life.

4.5 Reproducibility of Results

The data in Table 4 were used to determine the reproducibility of results

for the electrophoretic mobility measurements of asphaltenes in nitromethane. A statistical analysis of these data, with one instrument, one observer, and a constant voltage, for six asphalts with three samples

each, showed that the computed standard deviation for the mobility

values is 0.077 p/ sec/vo It/ cm, based on 10 degrees of freedom.

Equivalently, the standard error of the average value of three samples

is 0.077 = 0,045, Correspondingly, the 957, confidence limits for the

average of three samples are:

Average of three samples ± 2,228 x 0,045 =

average of three samples ± 0,10 p/sec/vclt/cm„

This compares favorably with the results of Gittens and James [19] who

report that the confidence limit for a single mean at p = 0,05, is ±37,,

5 . SUMMARY

The electrophoretic mobilities of asphaltenes from 24 asphalts were measured

by the microelectrophoresis method in a nonaqueous system using nitromethane

as the suspending liquid. The experimental results showed that real

electrophoretic effects were being observed, and that in the asphaltenenitromethane system electrophoresis can be measured normally and in the same way as in the aqueous system. All asphaltenes were found to carry a positive electrical charge. The presence of an electrical charge on asphaltenes supports the colloid theory of asphalts.
12

The relationship between asphaltene electrophoretic mobility and various

properties of the original asphalt is not clear. It is concluded that

while electrostatic bonding forces may be present, these are not predominant in asphaltene-maltene association or asphalt weathering durability.

The reproducibility of the experimental method was found to be ±0.1 p/sec/

volt/cm at the 95% confidence level.

6 . ACKNOWLEDGMENT

The authors wish to acknowledge the assistance of H, H„ Ku for the

statistical treatment of the data,.

7 , REFERENCES

[1] Nellensteyn, F 0 J„, J, Inst. Petrol., Technol,,, H) ? 31 1 (1934).

[2] Traxler, R» N„, "Asphalt, Its Composition, Properties, and Uses",

pp„ 72-89, Reinhold Publishing Corp . , New York, 1961

„

[3] Winniford, R„ S., "The Influence of Asphalt Composition on Its

Rheology", Preprint, ASTM Committee D-4 Meeting, Atlantic City,

June (i960).

[4] Swanson, J. M, , J. Phys „ Chem„, 46, 141 (1942).

[5] Pfeiffer, J c P. H„ and Saal, R„ N (. J„, J. Phys „ Chem0 , 44, 139

(1940).

[6] Eilers, H., J. Phys„ & Col. Chem„, 53, 1195 (1949).

[7] Eldib, I. A., Dunning, H„ N„ and Bolen, R„ J D , J. Chem, & Eng.

Data, 5, 550 (i960).

[8] Csanyi, Ln H. and Bassi, B. S„, "Analysis of Asphalts by Electrical

Means", Preprint, Assoc, Asphalt Paving Tech. Meeting, Montreal,

February (1958).

[9] Kruyt, H, R„, "Colloid Science", pp 0 3, 56, Elsevier Publishing Co„,

New York (1 952)

.

[10]

Glasstone, S„, "Textbook of Physical Chemistry"^ p, 1268, D„ Van

Nostrand Co., New York (1946).
13 -

[11] Bier, M,, "Electrophoresis, Theory, Methods, and Applications",

Chapter 10, Academic Press, Inc,, New York ( 1 959) 0

[12] Northrup, J„ H„ and Kunitz, Me , J. Gen, Physiol,, ]_, 729 (1 925)

[13] Thompson, M, R,, Trans, Electrochem, Soc,, 8J_, 147 (1942),

[14] Northrup, J, H,, Gen, Physiol,, 4, 629 (1921-22),

[15] Greenfeld, S, H„, and Wright, J„ R„, Materials and Standards

Research (in press) ,

[ 16 ] Wright, J, R„ and Campbell, P, G„, J, Appl„ Chem, (London),

(in press) „

[17] Kleinschmidt, L, R,, J, Research, NBS 54, 163-166 (1955),

[18] Van der Minne, J,, L, and Hermanie, P. H. J., I., J, Colloid Sci

600 (1952).

[19] Gittens, G. J. and James, A, M. , Anal. Biochem. , _1, 478 (1960).

[20] Van der Minne, J L, and Hermanie, P. H„ J., II, J, Colloid Sci

8, 38 (1953).

[21] Kleinschmidt,

(1959),

L. R. and Snoke, H. R„, J. Research, NBS 63C, 31

[22] Greenfe Id, S. H., J. Research, NBS 64C, 299 (1960)

[23] Greenfeld, S. H., J. Research, NBS 64C, 287 (1960)

[24] Wi lkinson, C. E., Striker, L, and Traxler, R . N., ASTM Bull. No

230, 42 (1958).

[25] Heithaus, J. J., ACS Symposium on Fundamental Nature of Asphalt

September 11-16, 1960 (pp. 23-37 of preprints).

US COMM-N BS - DC
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NATIONAL BUREAU OF STANDARDS

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