ELECTROKINETICS.
XIV
A CRITICALCOMPARISON OF ELECTROPHORESIS, STREAMING
POTENTIAL, AND ELECTROSMOSIS~ HENRY B. BULL
Division of Agricultural Biochemistry, University of Minnesota, Minneapolis, Minnesota Received June 1.4, 193.4
There have been various attempts to check the different methods of measuring electrokinetic potentials against one another. Thus Saxen (18) working with clay plates found good agreement between streaming potential and electrosmosis. Kanamaru (16), on the other hand, was unable to find agreement between electrosmosis and streaming potential for diaphragms made of cellulose. Briggs ( 5 ) attempted to check with streaming potential technique the values obtained by Abramson (l), using quartz particles coated with egg albumin, Excellent agreement was found. Since, however, both of these workers used relatively impure samples of egg albumin from completely different sources and also different buffer systems, the agreement appears to be fortuitous, This conclusion is borne out by subsequent observations by Abramson and Grossman (3). Abramson (2) has found good correspondence between electrosmosis and electrophoresis, using protein-covered surfaces. It appeared worth while to the author to reexamine the whole question and compare systems as nearly alike and under as similar conditions as possible. EXPERIMENTAL
The streaming potential apparatus used was the same as that reported in a previous communication by the author (7) and was arranged in the same fashion, except that two capillaries were used in series in order to increase the streaming potential. The electrophoretic cell was a radical modification of the one reported by Buzhgh (12), and a further modification of the one used by Bull and Sollner (11) and also by Bull, Ellefson, and Taylor (8). No complete description 1 Paper No. 1280, Journal Series, Minnesota Agricultural Experiment Station. Presented before the Eleventh Colloid Symposium, held a t Madison, Wisconsin, June 14-16,1934.
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HENRY B. BULL
of this apparatus has been published. It therefore seems worth while to describe the cell, since it combines the virtues of being inexpensive, simple in operation and construction, and easily cleaned. It consisted simply of a strip of plate glass into which a groove 1 mm. deep and 1.5 cm. wide had been cut, ground and polished lengthwise on its face. Small holes were ground through the strip of glass about 2 cm. from each end. These holes had glass tubes sealed into them and served as inlet and outlet for the cell. The electrodes consisted of copper wire imbedded in plaster of Paris made with half-saturated potassium chloride and inserted in the ends of the groove. The plaster was smoothed off, a thin cover glass obtained from an old photographic plate was laid over the groove, and the joint between the cover glass and the plate glass was sealed with melted paraffin wax. Melted wax was also poured behind the electrodes. The wax was allowed to solidify and the cell was ready for calibration. (The complete cell is shown in figure 1.) The fitting together of the cell requires only a few minutes so that the cell may be taken apart, cleaned, and put together
iicm
'
FIQ.1. THEMODIFIED B U Z ~ QELECTROPHORESIS H APPARATUS
again with no great trouble. The cell was placed upon the stage of a microscope equipped with a 12.5 X ocular and an 8 mm. objective which combined working distance with sufficient magnification. The thickness of the chamber when filled with water was measured by means of the screw micrometer of the microscope. The copper electrodes were connected to the D.C. source of current. The potential gradient was obtained by measuring the distance between the electrodes, and dividing this into the total potential drop as determined by an accurately calibrated voltmeter. The potential across the cell was usually around 200 volts. In accordance with the formulation by Smoluchowski (21), measurements were made at the stationary levels. These stationary levels lie at 0.21 and 0.79 of the total depth of the cell. A series of four readings was made in one direction and a like number in the opposite, until a t least sixteen measurements had been obtained at each level. Velocity was measured with a stop watch by determining the time required for a particle in sharp focus to travel between two lines of the ocular micrometer. The electrosmotic velocity was determined by making use of the technique employed
ELECTROPHORESIS, STREAMING POTENTIAL, ELECTROSMOSIS
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by Abramson (2). Sixteen determinations of mobility were made a t the half depth of the cell. The electrophoretic mobility was subtracted from these and multiplied by 2. The result is the electrosmotic velocity of the solution. Both the electrophoretic and electrosmotic mobilities are expressed in p per second per volt per centimeter. Conductivity was determined with an A.C. galvanometer. This instrument had been previously checked against a 1000-cycle vibrator and earphones. The usual bridge arrangement was used. The 1000-cycle frequency was used to determine the cell constant. The method and values of Parker and Parker (17) were used for the calibration with N/10 potassium chloride. The particles used in the electrophoretic measurements were Pyrex glass particles of about 1 p in diameter, Pyrex glass was selected instead of quartz to insure the same surface as that of the capillaries which were used in the streaming potential measurements. Two proteins were used to coat the particles and capillary walls. They were electrodialyzed Bacto gelatin prepared by Sinclair and Gortner with an ash content of 0.05 per cent, and egg albumin made from fresh eggs and crystallized three times from ammonium sulfate, according to Sorensen (19). The egg albumin was subsequently electrodialyzed. The protein solutions were made up the day before the experiment was to be conducted. This was to allow the solution to come completely to equilibrium. The electrophoretic and streaming potential measurements were always made on the same day and usually within two hours of each other. The temperature was 25" =t 1°C. Hydrochloric acid was used to bring the protein solutions to the desired hydrogen-ion concentration. The use of the ordinary buffer systems was avoided, owing to the desire to keep the conductivity as low as possible and thus obtain a higher streaming potential. The hydrogen-ion concentrations were determined with a quinhydrone-platinum electrode, using Clark's values for the electrodes. The streaming potential measurements were conducted first. The entire protein solutions were streamed through the capillaries several times . before readings were taken. In all cases at least twenty-one readings were made at different pressures and in reverse directions. Upon the completion of the streaming potential measurements the protein solution was removed to a flask, a portion poured out, and powdered Pyrex glass added. This was allowed to stand fifteen minutes, and the electrophoretic velocities determined. The ratio between the zeta potential as determined by streaming potential and electrophoresis was obtained by use of the following formulas:
t* = 4n~Hq -(streaming potential) DP '
580
HENRY B. BULL
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666
wxx
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.
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ELECTROPHORESIS,
fc
STREAMING POTENTIAL,
ELECTROSMOSIS
581
=4 5 (electrophoresis) v
DX
where K is the conductivity in reciprocal ohms, H the streaming potential, q the coefficient of viscosity, P the pressure forcing the liquid through the capillaries, X the potential gradient across the electrophoretic cell, D the dielectric constant, and V the velocity of the particle. Dividing equation 1 by equation 2
and introducing the proper constants to convert the ordinary units into electrostatic units we have
sb = 7.57 x bE
H KX
108 - -
P V
(4)
This equation was used to calculate the values given in table 1. It was thought that it might also be interesting to compare the two methods using surfaces which were not covered with protein. Accordingly, some of the same quartz used by Bull and Gortner (10) in their study on particle size was suspended in 2 X 104M sodium chloride and its electrophoretic velocity determined. The true streaming potential was assumed to be given by the quartz where the potential no longer varied with the particle size, that is, at the largest size of particle. No decided trend in speed was noted between the various particle sizes. The electrophoretic speed of cellulose fibers was also measured. The cellulose was the same as that used by Bull and Gortner (9), and the streaming potential data was taken from their paper. DISCUSSION
Although there are only fifteen ratios reported for the relation between streaming potential and electrophoresis for protein surfaces, actually these represent a great many measurements. There have been 480 electrophoretic measurements and 294 streaming potential determinations, so the whole takes on the color of a statistical analysis. The ratio comes out to be 0.97, which within experimental error may be considered equal to 1. The ratio of the electrophoretic potential to the electrosmotic potential is very close to 1, which is in good agreement with the results of Abramson (2). There is also satisfactory agreement between the electrophoretic potential and the electrosmotic potential. Thus it is seen that with protein-coated surfaces there is good correspondence between the three methods of determining the electrokinetic potentials. This lends support to the customary theoretical treatment of these phenomena, and would
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H E N R Y B. BULL
seem to support the factor of 4 in the equation for electrophoresis as derived by von Smoluchowski (20) and Henry (14).2 The values for quartz and cellulose are quite disturbing. There is indeed very little resemblance between the electrophoretic values and the streaming potential results. It is not certain that the surface potential on the larger quartz particles is the same as that for the very small ones used in the electrophoretic study, though there was no evidence of any very decided trend in the potential as determined with electrophoresis. The results obtained from these measurements seem to bear out the factor of 6n, instead of 4n as proposed by Debye and Huckel (13, 15). The values obtained for cellulose by the streaming potential method are much too low. If the results of Kanamaru (16) be accepted for the streaming potential on cellulose, the agreement between the streaming potential and electrophoresis becomes much better; however the electrosmotic results then fall out of line. It may be that the small values for the streaming potential with cellulose are due to the diminished pore size in the diaphragm packed tightly with cellulose. This is in keeping with the experimental results of White, Urban, and Van Atta (22) and also of Bull and Gortner (10) and with the theoretical considerations of Bull (6) and also of Bikerman (4). More investigation of surfaces which derive their charge by adsorption rather than by dissociation, as with proteins, is badly needed. SUMMARY
1. A new electrophoretic cell has been described. 2. The electrophoretic, electrosmotic, and streaming potential methods
* Henry proposes the equation for the electrophoretic mobility, where p is the specific conductivity of the dispersions medium, p' is the apecific conductivity of the particle, and the other letters have the same significance as previously noted. It can be seen that when dealing with nonconducting particles the factor for the equation becomes 47r. With highly conducting particles the mobility should approach zero. This has not been generally observed. Henry explains this by assuming that the particles become polarized and are in this fashion rendered non-conducting. Bull and Sollner (11) have published mobility data obtained with mercury particles in a mercurous nitrate solution. These particles should be non-polarizable. Substituting approximate values in the Henry equation for the specific conductivities of the mercury and a 0.001 M mercurous chloride solution, we find that the equation becomes approximately
This indicates that the mobility should be in the order of 1/100,00Oth of that observed. This represents a large discrepancy between the theoretical and experimental results.
ELECTROPHORESIS, STREAMING POTENTIAL, ELECTROSMOSIS
583
of measuring the electrokinetic potentials have been compared, using the same solution and surface. 3. Good agreement was found when dealing with protein-coated surfaces. 4. Poor correspondence was found between electrophoresis and streaming potential for quartz and cellulose surfaces. REFERENCES
(1) ABRAMSON,H. A.: J. Am. Chem. SOC.60, 390 (1928). H.A.: J. Phys. Chem. 36, 289 (1931);J. Gen. Physiol. 16, 1 (1932). (2) ABRAMSON, E. B.: J. Gen. Physiol. 14, 563 (1931). (3) ABRAMSON,H.A., AND GROSSMAN, (4) BIKERYAN, J. J.: Z. physik. Chem. 163A, 378 (1933). (5) BRIGGS,D. R.: J. Am. Chem. SOC.60, 2358 (1928). (6) BULL,H. B.: Kolloid-Z. 60, 130 (1932). (7) BULL,H. B.: Kolloid-Z. 66, 20 (1934). (8) BULL,H. B., ELLEFSON, B. S., AND TAYLOR, N. W.: J. Phys. Chem. 38, 401 (1934). (9) BULL,H.B., AND GORTNER, R. A.: J. Phys. Chem. 36, 456 (1931). (10) BULL,H.B., AND GORTNER, R. A.: J. Phys. Chem. 36, 111 (1932). (11) BULL,H.B., AND SBLLNER, K.: Kolloid-Z. 60, 263 (1932). (12) BUZAQR,A.: Kolloid-Z. 48, 33 (1929). (13) DEBYE,P., AND HUCKEL,E.: Physik. Z. 26, 49 (1924). (14) HENRY,D. C.: Proc. Roy. SOC.London L33A, 106 (1931). L , Physik. Z. 26, 204 (1924). (15) H ~ C K E E.: (16)KANAMARU, K.: Cellulose Ind. 7, 3 (1931); Chem. Abstracts 26, 3895. (17) PARKER, H. C., AND PARKER, E. W.: J. Am. Chem. SOC. 46,312 (1924). (18) S A X ~ NU.: , Wied. Ann. 47, 46 (1892). S.P. L.: Compt. rend. trav. lab. Carlsberg 12, 12 (1917). (19) S~RENSEN, (20) VON SMOLUCHOWSKI, M.: Anz. Akad. Wiss. Krakau, p. 182 (1903). (21) VON SMOLUCHOWSKI, M.: Handbuch der Elektrieitat und der Magnetismus, Vol. 11, p. 366 (1921). (22) WHITE,H.L.,URBAN,F., AND VANATTA,E. A.: J. Phys. Chem. 36,3152 (1932).
