A COMPARISON OF THE ELECTROPHORETIC, ELECTROSMOTIC AND STREAM POTENTIAL ISOELECTRIC POINTS AT GLASS AND GELATIN SURFACES1 BETTY MONAGHAN, H. L. WHITE,
AND
FRANK URBAN
Department of Physiology and Department of Biological Chemistry, Washington University School of Medicine, Saint Louis, Missouri Received June 14, 1934
According to modern conceptions the ionic double layer responsible for electrokinetic phenomena consists of a sheet of ions containing an excess of either positive or negative charges rigidly attached to the solid wall, and a second layer of ions in the adjacent liquid containing an equal excess of charges of the opposite sign. The wall (inner Helmholtz) layer does not move under the influence either of an applied hydrostatic pressure or of an imposed E.M.F. The outer layer, according to Stern (13)) may be further divided into a diffuse (Gouy) component which i s moved by an applied hydrostatic pressure, and a second (outer Helmholtz) component immediately adjacent to the wall layer, which cannot be moved by hydrostatic forces. The potential drop across the diffuse, movable component is the electrokinetic or zeta potential. Urban, White, and Strassner (14) suggest that although the outer Helmholtz layer is not displaced by hydrostatic forces, it may nevertheless move in an electric field. They point out that, on this assumption, only the outermost or diffuse component contributes to stream potential, whereas both the diffuse and the outer Helmholtz layers contribute to electrosinotic velocity. If urn is the electrosmotic velocity in the center of a capillary, and uo and uh the velocity of the diffuse and Helmholtz layers, respectively, then :Um
= uo
ED($+ Uh = EDY -+ 477 47v
P)
- -ED+ 47v
where E = applied E.M.F. per unit length of capillary, D = dielectric constant, q = viscosity, 1 = potential drop across the diffuse layer, and $ = potential drop across the entire double layer. 1 Presented before the Eleventh Colloid Symposium', held at Madison, Wisconsin, June 14-16, 1934. 585
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MONAGHAN, WHITE AND URBAN
This leads to the conclusion that when b (as measured by the stream potential method) becomes zero, electrosmosis may persist, provided that 9 is not also zero. Further, if .t and 9 are of opposite sign, stream potential may reverse while electrosmosis continues in the original direction. The symbol $ here designates the potential difference across the entire double layer and does not refer to the potential measured by the glass electrode, the latter apparently depending, within the range of concentrations studied by us, only on the hydrogen-ion concentration. These deductions may be tested experimentally by determining the direction and velocity of electrosmosis when 5, as measured by stream potential, is zero. This paper is concerned only with a comparison of isoelectric concentrations for stream potential, electrophoresis and electrosmosis, respectively; a discussion of the absolute magnitude of the electrokinetic potentials at other concentrations will be deferred. The literature contains only one record of a direct comparison of the stream potential isoelectric point with electrosmotic or electrophoretic isoelectric points on the same material. Abramson and Grossman (3) found that the hydrogen-ion activity required to stop electrosmosis and electrophoresis in an albumin-covered electrophoresis cell with albumincovered particles was practically the same as that previously found by Briggs (4) to be isoelectric for stream potential through a diaphragm covered with the same protein. Isolated observations on isoelectric points of surfaces other than protein, however, do not show the same agreement with the twomethods. Elissafoff (7)found a concentration of 1.9 X M thorium nitrate to be isoelectric for electrosmosis in a quartz capillary and about 7 X M thorium nitrate for a Jena glass capillary; whereas Freundlich and Ettisch (8) and Kruyt and van der Willigen (9) both found that stream potential in a glass capillary was already reversed at concentrations of 3 to 4 X lo-’ M thorium nitrate. The possibility remained, of course, that differences in the kind of glass used might have been respolisible for this wide discrepancy. M thorium chloride Powis (11) found that a concentration of 3 X was required to reverse the electrophoretic movement of droplets of ”cylinder oil,” while Bull and Gortner (5) report that 1.2 X M thorium chloride is isoelectric for stream potential on Nujol. Here again, a difference in the kind of oil used might account for the results. EXPERIMENTAL
In the present paper, stream potential, electrosmotic, and electrophoretic isoelectric concentrations are obtained of (1)hydrogen ion with gelatin, and (2) thorium chloride, ferric chloride, and aluminum chloride with Pyrex glass. Electrosmotic and electrophoretic isoelectric concentrations were deter-
ELECTROKINETIC PHENOMENA
587
mined by observing the movement of Pyrex glass particles in a closed Pyrex electrophoresis cell of the flat Northrop-Kunitz type. Abramson (1) discusses the use of such cells for electrosmosis and electrophoresis. The observed velocity of the particles at any given depth in the cell is the algebraic sum of the true electrophoretic velocity plus the electrosmotic velocity at that depth. The true electrophoretic velocity, V,, is the observed velocity of the particles at 0.211 of the distance from the top or bottom of the cell. The true electrosmotic velocity, V,, may then be obtained either according to the formula V, = 2(V+ - V,), where V+is the observed velocity in the middle of the cell, or according to the formula V , = Vo - V,, where VOis the observed velocity at the wall of the cell. It was found in preliminary experiments that when the particle velocity was plotted against cell depth a symmetrical parabola was obtained; also that values of V , as calculated from the two formulas given above showed good agreement. In subsequent experiments, therefore, observations were made only a t the bottom of the cell and at 0.211 of the distance from the bottom, and the electrosmotic velocity calculated from the second formula. In the case of thorium chloride with Pyrex, the electrosmotic isoelectric point was also determined directly by applying an E.M.F. across a Pyrex capillary of 600y diameter containing varying concentrations of thorium chloride and observing the rise or fall of liquid in the recording arm of the apparatus due to electrosmotic transport across the capillary (Quincke (12)). The two methods gave the same results. Stream potential determinations were carried out on a Pyrex capillary of 300p diameter by the method described by White, Urban, and Van Atta (15). For all isoelectric point determinations on gelatin, solutions which were 4 x N with respect to potassium chloride plus hydrogen chloride and which contained 0.01 g. gelatin per liter were used. The pH was varied by changing the proportion of hydrogen chloride to potassium chloride, Measurements of pH were performed by the glass electrode method. Both particles and cell were allowed to stand in contact with the gelatin solution for one hour before measurements were begun, in order to allow the glass surfaces to become coated with the protein. It has been shown repeatedly (Abramson (2) ; Dummett and Bowden (6)) that many substances, including glass, adsorb a complete coating of gelatin from dilute solutions of this protein, and that such protein-covered surfaces no longer behave like glass surfaces, but rather show the properties of the adsorbed protein, That is, by treating the glass surfaces as described they have in effect been converted into gelatin surfaces, so that it is now possible to study the effect of ions (in this case hydrogen ions) on electrokinetic phenomena at a gelatin surface. To obtain reproducible results with such poorly buffered solu-
588
MONAGHAN, WHITE AND U R B A N
tions it was found necessary to increase the length of the side arms on the electrophoresis cell to about 20 cm. on each side in order to prevent diffu'sion of acid from the electrodes into the flat part of the cell during a series of readings. Stream potential measurements were made on gelatin-coated glas~capillaries, using the same solutions that were used for electrosmosis and electrophoresis. In order to secure complete coating of the capillary in tt short time, a more concentrated gelatin solution (0.1 g. gelatin per liter) was run through for a few minutes before the determinations began. At hydrogenion activities close to the isoelectric point the stream potential generally required several hours to reach a constant value. The isoelectric point data are summarized in table 1. A few words concerning the accuracy and reproducibility of these results may be inserted here. It should be pointed out that the figures listed are not the result of isolated observations near the isoelectric point, but are TSBLE 1 Isoelectric p H for gelatin and isoelectric ion concentrations with Pyrex glass ISOELECTB.IC
pH
FOR
QELATIN
ISOELECTRIC CONCENTRATIONS OF
ISOELECTRIC CONCENTRATIONS OF
PYREX
PYREX
THClr WITB
AlCla WITE
M
Stream potential.. . . . . . . . . . . . . . Electrosmosis.. . . . . . . . . . . . . . . . . Electrophoresis. ...............'
4.75 4.75 4.75
4 3 3
x x x
IBOE>ECTRIC CONCENTRATIONB OF FeClo WITH PYREX
M
M
10-7
1x
10-8
1
10-8
3 3
10-4 10-6
3 3
10-6
x x
x x x
10-6 10-6 10-8
taken from smooth curves representing electrokinetic potential over a wide range of concentrations. A typical set of curves is reproduced in figure 1. The absolute magnitudes of the electrokinetic potentials obtained by the three methods are not of quantitative significance; we were interested here only in the isoelectric points, and conditions essential to a quantitative measurement of electrokinetic potential were not adequately controlled. The isoelectric concentrations are given only to the nearest whole number, since the curves are not reproducible with greater accuracy than this. For example, on six experiments on the electrophoresis cell, the isoelectric concentration varied between 2 and 4 X M thorium chloride. In two experiments with the Quincke set-up the curve crossed the zero potential line between 2.5 and 3 X low6M thorium chloride. However, experimental variations in the isoelectric point (due probably to slight contamination of the surfaces or to differences in the time allowed for equilibrium, etc.) do not go beyond these limits; at a concentration of 1 X 10-6 M thorium chloride, electrophoresis and electrosmosis always
ELECTROKINETIC PHENOMENA
589
FIQ,1. ELECTROKINETIC POTENTIAL AS A FUNCTION OF ThCIil CONCENTRATION 0, electrosmosis; 0 , stream potential; A, electrophoresis A B
C
FIQ.2. SCHEMATIC REPRESENTATION OF THE COURSEOF
THE POTENTIAL CURVES VARYINQ CONCENTRATIONS OF ThCI4 A represents the solid wall, B the outer Helmholtz layer, C the outer boundary of the diffuse layer. The potential difference between A and C = $; that between B and C = Curve 1 represents the condition in water, where $ and 5 have the same sign; curve 2, 4 X 10-7M thorium chloride, stream potential isoelectric; curve 3, 8 X lo-' M thorium chloride, stream potential reversed; curve 4, 3 X 10-6 M thorium chloride, stream potential reversed, electrosmosis isoelectric; curve 5, 1X M thorium chloride, stream potential and electrosmosis the same, both reversed; curve 6, 1 X 10-4 M thorium chloride, both reversed, electrosmosis greater than stream potential.
IN
r.
590
MONAGHAN, WHITE AND URBAN
have the same sign as in water, while at a concentration of 5 X lo--’ M thorium chloride, stream potential is always reversed. With trivalent aluminum and iron the difference in the isoelectric concentrations for stream potential on the one hand and electrophoresis and electrosmosis on the other hand is not so great as with thorium chloride, but no less definite. On the gelatin surfaces all curves crossed the zero potential line at a pH between 4.7 and 4.8. DISCUSSION
From the experimental data on thorium-glass it is evident that even when the glass surfaces are the same2 (Pyrex) and the solutions identical, a great differenceexists in the isoelectric concentrations of thorium chloride for stream potential on the one hand and for electrosmosis and electrophoresis on the other hand. At concentrations such that stream potential is reversed (between 4 X lo-’ M and 3 X M thorium chloride, electrosmosis and electrophoresis continue in the original direction. This is interpreted to mean that a potential drop may exist across the outer Helmholtz layer when the {-potential is zero, and further that the potential of the outer Helmholtz layer may be of opposite sign from that of the diffuse layer. In dilute solutions of thorium chloride, for example, if some of the hydrogen-ions in the outer Helmholtz layer are replaced by tetravalent thorium ions, with their attendant anions in the diffuse layer, then there may actually be an excess of anions in the diffuse component, with a resultant reversal of stream potential. A t the same time, if the boundary between mobile and immobile components of the electrical layer, which might here be designated as a triple layer, lies closer t o the wall for electrosmosis and electrophoresis than for stream potential, the sign of the former remains unchanged. This sign will be reversed only when the adsorption process has proceeded to the point (with increasing thorium concentration) where the potential at the outer boundary of the diffuse layer is less than that at the wall, which apparently does not take place until the thorium concentration has been increased at least sevenfold above that required for isoelectric stream potential. Figure 2 illustrates schematically this concept of variations in the course of the potential-distance curves as the thorium concentration is increased. Similar considerations hold for the aluminum-glass and iron-glass systems. It is recognized that some colloidal thorium, aluminum, or iron is almost certainly present, but this has no bearing on the reality of a difference in the isoelectric concentration of stream potential on the one hand and of electrosmosis and electrophoresis on the other. 2 The surfaces for electrosmosis and stream potential are certainly the same, i.e., fused smooth surfaces. We have also shown (10) that the electrophoretic behavior of broken (particle) and fused (sphere) glaw surfaces is the same.
ELECTROKINETIC PHENOMENA
591
On the other hand, the isoelectric hydrogen-ion concentration for gelatin is the same for electrosmosis, electrophoresis, and stream potential. I n this case, when -t becomes zero, # is also zero, and the two are never of opposite sign. Without entering into an adequate discussion of the mechanisms involved in the two cases, it should be pointed out that the layers are probably quite different on the two types of surfaces under consideration. In the case of gelatin the double layer is produced by an ionization process which gives rise only to varying ratios of univalent anions and cations in the solution, the total concentration remaining the same. It is difficult to see how such a process could produce a difference in sign of the outer Helmholtz as compared with the diffuse layer. SUMMARY
The isoelectric concentrat,ion for electrosmosis and electrophoresis on Pyrex glass is found to be for thorium about seven and for aluminum and iron about three times as great as the isoelectric concentration for stream potential on the same kind of glass. The hydrogen-ion isoelectric concentration for gelatin, on the other hand, is the same for electrosmosis, electrophoresis, and stream potential. The results are explained on the assumption that only the diffuse component of the double layer is moved by hydrostatic forces, while both the diffuse and the outer Helmholtz layers move in an electric field. REFERENCES (1) ABRAMSON,H. A.: J. Phys. Chem. 36, 289 (1931). (2) ABRAMSON,H. A.: J. Gen. Physiol. 14, 563 (1931); J. Gen. Physiol. 16, 575 (1932). (3) ABRAMSON,H.A., AND GROSSMAN, E . B.: J. Gen. Physiol. 14, 563 (1931). (4) BRIQGS,D.R.: J. Am. Chem. SOC.60, 2358 (1928). R. A,: Proc. Nat. Acad. Sci. 17, 288 (1931). (5) BULL,H.B., AND GORTNER, (6) DUMMETT, A.,AND BOWDEN,P.: Proc. Roy. SOC.London 142A, 382 (1933). (7) ELISSAFOFF,G.: Z. physik. Chem. 79, 385 (1912). (8) FREUNDLICH, H.,AND ETTISCH,G.: Z. physik. Chem. 116,401 (1925). (9) KRUYT,H.R., AND VAN DER WILLIGEN, P. C.: Kolloid-Z. 46, 307 (1928). (10) MONAGHAN, B. R., AND WHITE,H. L.: J. Phys. Chem. 39 (in press). (11) POWIS,F.: Z.physik. Chem. 89, 186 (1914). (12) QUINCKE, G.: Pogg. Ann. 113, 513 (1861). Z.Elektrochem. 30, 508 (1924). (13) STERN,0.: (14) URBAN,F.,WHITE,H. L., A N D STRASSNER, E. A , : J. Phys. Chem. 39,311 (1935). (15) WHITE,H.L., URBAN,F., AND VANATTA,E. A. : J. Phys. Chem. 36, 1371 (1932).
