ELECTROPHORESIS IN NONAQUEOUS MEDIA
1527
(17) HUIZENQA, J . R . , GRIEQER, P. F . , A N D WALL,F. T . : J . Am. Chem. SOC.72,2636 (1950). (18) KATCHALSKY, A., AND GILLIS,J . : Rec. trsv. chim. 88, 879 (1949). (19) KATCHALSKY, A., KUNZLE,O., A N D KUHN,W.: J. Polymer Sci. 6, 283 (1950). (20) KATCHALSKY, A., AND SPITNICK, P . : J . Polymer sci. 2, 432 (1947). (21) KERN,W.: Z. physik. Chem. A M , 249 (1938). (22) MARKOWITZ, H . , A N D KIMBALL, G. E.: J . Colloid Sci. 6, 115 (1950). (23) OVERBEEK, J. TH.G . : Bull. soc. chim. Belges 67, 252 (1948). (24) PALS,D. T. F . , AND HERMANS, J . J . : J. Polymer Sei. 9, 897 (1948).
ELECTROPHORESIS IN NONAQUEOUS MEDIA’ MASON HAYEK Jackson Laboratory, E . I . du Pont de Nemours and Company, WiZmington, Delaware Received October 8 , 1960 INTRODUCTION
During the course of some fundamental studies on detergents for lubricating oils it was found desirable to study the mechanism of suspending action in nonaqueous systems. Little is known of peptizing processes in nonconducting systems, and it was the purpose of the studies reported here to learn whether or not dispersed particles can be electrically charged and whether or not such charges as may exist are responsible for the stability of the sols. The method selected to determine the nature of peptizing action was a study of the electrophoretic behavior of the sols. Electrokinetic phenomena in nonconducting systems have received relatively little attention and the results of the few studies are difficult to explain. Significant zeta potentials have been observed in organosols, even though no satisfactory explanation has been given for their origin (3, 4, 5, 6, 7, 8, 9). The only report dealing with the effect of surface-active agents on the disperse phase is that of Stubblebine (lo), dealing with dry-cleaning assistants. Although Stubblebine believed that his results were “purely qualitative and difficult to reproduce within large limits,” he showed that suspended particles in the organic sols were charged, and he believed that the charge was affected by the presence of dispersants. EXPERIMENTAL
Preliminary studies were carried out with an electrophoresis cell such as that described by Gemant (4). It consisted of two loops of copper wire placed 1-2 mm. apart on a microscope slide and attached through a double-pole double-throw switch to the terminals of a 45-v. B batteqy. The slide supporting the wires was mounted on a microscope stage, and a drop or two of the sol to be studied was placed between the mires. Contribution No. 89 from the Jackson Laboratory, E. I . du Pont de Nemours and Company.
1528
MASON HAYEIC
The cell uaed for quantitative measurements is shown in figure 1. Several types of cells were tested before this one was designed. The greatest di5culties in designing a suitable cell arose from the need for a high field strength to obtain significant rates of motion of the charged particles and the necessity for motion of charged particles in almost horizontal planes (at least within the limits of depth of focus of the microscope), even though the particles would follow the lines of force between the electrodes. These requirements were satisfied by the choice of dimensions for the cell. The cell consisted of two stainless-steel elec-
COPPER LEADS.
E/
ELECTRODES
‘SPACER FIG.1. Diagram of microelectrophoresis cell
trodes, each 64 mm. long, 8.5 mm. wide, and 3.21 mm. in height, with faces and bottom surfaces ground flat. These were mounted on a strip of Lucite acrylic resin the size of a microscope slide and separated from one another with spacers made from Lucite or glass, 0.2-1.3 mm. in thickness. The space between the electrodes was covered with a strip of Lucite. The height of the electrodes was not as critical as the dimension might indicate, but it had to be known accurately to permit electrophoretic measurements to be made at a “stationary level.” The dimensions of the electrodes were chosen sc ‘,hatthe effect of the side walls would be negligible (1). The electrodes were not permanently attached to the Lucite plate, but were held with a moderately heavy fluorocarbon grease, which was insoluble in the organic solvents used. This arrangement permitted the elec-
ELECTROPHORESIS IN NONAQUEOUS MEDIA
1529
trodes to be any desired distance apart for a particular experiment, and the cell could be thoroughly cleaned between experiments. A tight seal between the metal and glass could be made quite easily after a little practice. The use of a Lucite support and cover for the cell was in accord with the recommendation of Gemant (4)that a material be used which would have a conductivity lower than that of the organic medium. Since the spacers were coated with the nonconducting grease before being placed between the electrodes, glass spacers could be used. A Bausch and Lomb microscope, Model BA, fitted with a 12 X eyepiece and a 10 x objective and equipped with an adjustable stage, was used throughout the experiments. The eyepiece was fitted with a calibrated micrometer scale. The depth of the cell was calibrated in terms of divisions on the fine adjustment screw and found to be 3208 units, corresponding to 3.21 mm. The upper stationary level was then 0.211 times this depth, or 0.677 mm., 677 divisions on the microscope scale below the top of the cell. Although all measurements were made at this level, the precaution may not have been necessary because electroijsmosis was not significant in the organic media. The sols used consisted of one of the carbon blacks, Thermax or Spheron CC, dispersed in kerosene or cetane, with or without the aid of a dispersant. The Spheron CC was used without purification, and the Thermax was only washed with C.P. acetone and then held at 250-300°C. at 0.5 mm. for 30 min. The purity of the organic medium was not uniform throughout the studies, but it is described with the data. The redistilled kerosene was collected over the range 51-75°C. at 1.7 mm. The cetane was purified by extraction with concentrated sulfuric acid and sodium hydroxide solution and then washing with water, filtration through silica gel, and finally distillation at 118-120°C. at 2 111111. The suspensions were prepared by milling the carbon black in the liquid for 16-24 hr. and then diluting a small portion of this relatively concentrated suspension with a solution of the dispersant in the same liquid, or, in the case of the controls, with the liquid itself. Concentrations of carbon black as low as O.OOO1 per cent were used, low enough so that only one or two particles were in the entire field of vision and no influence could be exerted by one particle on @other. The dispersants used were of various efficiencies for suspending carbon black in organic systems, and all were prepared in this laboratory. Each petroleum sulfonate was prepared by a metathesis between sodium petroleum sulfonate, a mahogany acid salt obtained under the name Petronate 30 from L. Sonneborn Sons, Inc., and a chloride of the desired metal. For example, a reaction between sodium petroleum sulfonate and calcium chloride yielded calcium petroleum sulfonate and the insoluble sodium chloride. The barium alkylphenoxyacetate, the alkylphenols, and the alkylbenzenesulfonate were obtained by alkylation of phenoxyacetic acid, the phenol, or benzenesulfonic acid with a polypropylene fraction in the presence of anhydrous hydrogen fluoride, according to the procedure of Calcott, Tinker, and Weinmayr (2), and conversion of the products to the barium salts. The polyester was prepared by esterification of a mixture of hydroxystearic acid, sebacic acid, 2,5-hexanedioI, and 2-ethylhexanol, in the molar proportions 1.0, 0.66, 0.68, and 1.44, respectively.
1530
MASON HAYElC
When an electrophoretic measurement was to be made, the electrodes were mounted on the Lucite plate and the cell was placed on the microscope stage. The copper leads from the electrodes were connected through a double-pole doublethrow switch and avoltmeter to the terminals of a battery which furnished 3-22.5 v. The cell was carefully filled with the sol to be studied and covered with the Lucite cover. For each suspension, the width of the cell and the potential between the electrodes were measured and the sign of the charge, if any, and the velocity of the particles were determined. All measurements were made a t the upper stationary level. If the particles in the cell showed any uniform motion before the field was applied, the cell was leaking and had to be repaired. One criterion of a suitable microelectrophoresis cell is the reversibility of motion of the particles when the polarity is reversed (4), and that reversibility existed for all experiments reported here. The sign of the charge, if any, was first determined by reversing the polarity several times, and then the velocity of the particles was measured by determining the time, in seconds, for a given particle to move a certain distance in the field. The rate of motion was measured as often as necessary to permit a complete determination of the behavior of the particles. In those cases where satisfactory reproducibility was obtained, the rates were measured enough times BO that a good average could be taken. RESULTS AND DISCUSSION
Preliminary experiments, for which the electrophoresis cell consisted of two copper wires mounted on a microscope slide, showed that charged particles could exist in nonconducting systems. In cases where the particles were strongly charged, a movement of the entire mass of particles across the field to one of the electrodes was observed; when the polarity was reversed, the entire mrtss moved to the other electrode. In some cases where the particles were only weakly charged, particles started moving toward one electrode immediately after the field was applied, but before the entire mass could reach that electrode, the particles arranged themselves along the lines of force, as shown in the photomicrographs of Lantz and Pickett (8). A third type of phenomenon occurred in the absence of any additive. When the particles dispersed in kerosene alone were brought under the influence of an electric field, the individual particles could be seen to rush first to one electrode and then the other. That is, the particles appeared to be charged, to move to one electrode where they acquired the opposite charge, and then to return to the other electrode. The results of studies in the electrode assembly pictured in figure 1 are summarized in table 1. The first nine entries are for carbon suspensions in the absence of added surface-active agents. The first four suspensions were as nearly identical as possible and the data for these indicate the reproducibility of the measurements. As far as kerosene and cetane as media are concerned, the amount of water present, the range of purities used, and the choice of Thermax or Spheron CC as dispersed phase were of little significance. The mobilities in all cases, except in the presence of acetone, were of the order of 2 X lo-* p/sec./v./cm. The relatively high mobility of carbon particles in cetane which had been saturated with acetone must have been due to the increase in conductivity of the
1531
ELECTROPHORESIS I N NON.4QUEOUS MEDIA
system caused by the acetone. In the absence of acetone, the conductivity of the liquid was smaller than that of the carbon particles, and then the lines of force TABLE 1 Charaes on carbon blacks i n the presence
__
DISPEPSANI
CTIVE IP.
PEDIEN
>ISPEPSION MEDIUM
r/rcc./v./cm.
248 217 248 248 189
Thermax
209
+ + + + + +
Thermax Thermax
713 703
+ +
2.3 X lo-’ 1.9 x lo-’
Thermax
615
+
1.2 x 10-1
Thermax
220
+
5.1 X lo-’
Kerosene
Spheron CC
156
2.1
Cetane
Thermax
214
-
1.0
Kerosene
Spheron CC
177
-
1.5 X lo-*
1.3
Kerosene
Spheron CC
117
+
1.4 x lo-’
0.7
Cetane
Thermax
221
None
0
2.2
Cetane
Thermax
695
-
1.6
Kerosene
Spheron CC
113
+
1.0
Cetane
Thermax
127
?
1.0 1.0
None
Ferric petroleum sulfonate.. , , , , , Barium alkylphenoxide.. , . .
YOBILITY OY PARTICLES
Spheron CC Spheron CC Spheron CC Spheron CC Thermax
None None
Cobalt petroleum sulfonate.. . . . , . Magnesium petrc leum sulfate. . . Barium alkyl. phenoxyacetate Barium alkyl. benzenesulfonat
‘HAPGE
?h/cm.
Kerosene Kerosene Kerosene Kerosene Redistilled kerosene Redistilled kerosene, dried Cetane Cetane saturated with water Cetane saturated with acetone Cetane
None.
Polyester, . . . . . . . Calcium petroleum sulfonate Barium dodecyl o-cresoxide . . . ,
__
CARBON BLACK
per ccnl
None: Suspension A . . Suspension B . . Suspension C . . Suspension D . . None. . . . . . . . . . . .
dispersants
3.1 X 3.7 x 2.2 x 2.0 x 2.2 x
lo-’ 10-2
10-2 10-2
10-
2.9 X lo-*
1.8 x lo-’ Variable bility
Variable bility
mo-
mo-
5.2 x lo-* Variable mobility; alight indication of negative charge
did not act in a direction tangent to the surface of the particles; in pure cetane the lines of force must actually bend in from the medium to pass through the carbon particles preferentially and the effect of the field would be significantly
1532
MASON HAYEK
less than expected from the equation of Helmholtz (1). If the conductivity of the medium were increased by the addition of acetone, the equation would more nearly hold (4)and the mobility of the particles would increase. This explains the observation of Stubblebine (10) that acetone increased the mobility of suspended particles. The dispersants used in the studies are arranged in table 1 approximately in the order of increasing effectiveness in suspending carbon black, as measured by following the rate of sebtling of carbon particles in cylinders containing the sols. The mobilities found for carbon particles in the presence of these surface-active agents show that the particles may be charged and the charges may be different from the charge in the absence of the agents, but there is no relation between the charge produced and the suspending action of the surface-active agent. While the charges may exist, they are incidental to, not responsible for, the peptization. All the compounds tested except the polyester suspended carbon black to some extent, and most of the agents were good dispersants. However, no correlation existed between mobilities and sol stability. The particles in some sols were positive, in others negative, and in some cases no charged particles could be detected. I n the presence of ferric petroleum sulfonate the particles had approximately the same mobility as in the absence of the sulfonate, even though the additive was a good peptizing agent. The second reason for the conclusion above is that the mobilities in all cases in the presence of additives were of the same order as the mobilities in the pure liquid medium. Even though the sign of the charge was affected, the absolute magnitude in the presence of a dispersant would have been higher than in its absence if the charges were really responsible for the dispersion. Finally, there were cases where different particles in the same sol carried charges of different magnitude. A frequent observation during the course of the experiments was an erratic motion of particles under the influence of the electric field. In most sols that did not contain surface-active agents the mobilities were reproducible if they were measured within 10 min. after the suspensions were placed in the cell, hut with longer standing the mobilities gradually decreased. Similar observations had been reported by Lantz and Pickett (8). Thus, in the case of the suspension of Thermax in dry kerosene, in which the particles showed a mobility of 3.2 X lo-* p/sec./v./cm. for times below 10 min., the mobility for some particles fell as low as 4 x at the end of 15 min. In the case of sols containing dispersants, the mobility did not decrease continuously with increasing time, but often a uniform mobility could not be found for all the particles in the field. This fact substantiates several reports in the literature (3, 5, 6, 7,9). Going farther, it was not only found that a uniform mobility might not exist, but the charge on a particle could vary while the particle moved in the electric field. In many instances as the circuit was closed, particles were seen to move very rapidly at first and then almost, if not entirely, stop. When the polarity was reversed, the same kind of motion, but in the opposite direction, was noted. In such cases the velocity of a particle was taken as its initial velocity. This kind of motion could not be attributed to a shielding of the electrodes by particles which
ELECTROPHORESIS I N NONAQUEOUS
MEDIA
1533
were deposited immediately after the circuit was closed, because the concentration of particles was too low to permit shielding. I n other cases, the particles would move toward one electrode when the field was applied and then, as they approached that pole, siddenly reverse their direction and return to approximately the mme distance from the second electrode, where the reversal of motion would be repeated. If the circuit was held closed, an elliptical motion approximately in horizontal planes was followed by the particles. A possible explanation for this is a loss of charge from the irregularly shaped particles as they neared the electrodes. I t was interesting that in most instances particles which'reached an electrode when the circuit was closed would leave that electrode and move to the other when the polarity was reversed ; that is, when the polarity was reversed, particles which had been deposited on what was the positive electrode carried a charge to the new positive electrode. SUMMARY
A microelectrophoresis cell was constructed and a technique developed for the measurement of mobilities of suspended particles in nonconducting media in the presence or absence of dispersing agents. Variable and nonuniform charges on particles in such systems were observed. Particles in nonconducting media may be charged, but the charges are incidental to, not responsible for, the stability of the sols. (1)
(2) (3) (4)
(5) (6) (7) (8) (9) (10)
REFERENCES H. A . , ~ ~ O Y E RL. , s., A N D GORIN,M. H . : Electrophoresis of Proteins. Reinhold Publishing Corporation, New York (1942). CALCOTT, W. S., TINKER, J. M., A N D WEINMAYR, V . : J . Am. Chem. SOC.61,1010 (1939). G E M A N T , A . : Ind. Eng. Chem. 31. 1233 (1939). GEMANT,A.: J . Phys. Chem. 43, 743 (1939). HATSCHEK, E., A N D THORNE, P . C . L . : Kolloid-Z. 38, 1 (1923). HUMPHRY, R . H . : Kolloid-2. 98, 306 (1926). HUMPHRY, R . H., A N D JANE,R. S.: Trans. Faraday Soc. I, 420 (1926). LANTZ,E. A., ASD PICKETT, 0. A . : Ind. Eng. Chem. 22, 1309 (1930). J . A . : Ind. Eng. Chem. 29, 565 (1937). REISINC, STUBBLEBINE, W . : Ph. D. Thesis, University of Michigan, 1942. University Microfilms, Ann Arbor, Michigan (1912). ABRAMSON,
