FACTORS INFLUENCING ELECTROPHORETIC MOBILITIES AND APPARENT CRITICAL POTENTIALS H. L. WHITE AND BETTY MONAGHAN Department of Physiology, Washington University School of Medicine, St. Louis, Missouri Received January 6, 1986
Various observers have reported that electrophoretic mobility is increased above the water value by the addition of uni-univalent salts. Powis (16) found the mobility of oil droplets to increase with added potasM and then to fall off. He also found (17), sium chloride up to 2.5 X according to his table 1, that clouding of an oil emulsion after two days M potassium chloride, where the zeta potential is 52 begins in 5 X mv. or electrophoretic velocity is 3 . 7 ~per second per volt per centimeter, the zeta potential in water being 46 mv. With barium chloride, clouding after two days begins in 5 X M , with a zeta potential of about 37 mv.; with aluminum chloride in 1 X M with a zeta potential of 38 mv.; and with thorium chloride between 5 X and 1 X M with a zeta potential between 40 and 7 mv. However, Powis rather arbitrarily takes a break in the clouding-concentration curve as the criterion of the critical condition, this occurring in all cases a t a higher concentration than that required to initiate clouding. At these critical concentrations so defined, the zeta potential is about the same with all the salts investigated, that is, close to 30 mv., which is taken as the critical potential. Powis (18), working with arsenic trisulfide sol, took the conditions obtaining in 5 X M barium chloride as critical. Here coagulation is rapid and the zeta potential is 26 mv. The same degree of coagulation was M with a zeta potential of first obtained in aluminum chloride a t 5 X 25 mv. and in 7 X M thorium nitrate with a zeta potential of 26 mv. The zeta potential with the “pure” colloid was not determined, but he concludes that it would be less than 100 and probably less than 60 mv. With potassium chloride, however, the critical concentration was 4 X 10-2 M and the zeta potential 44 mv. Powis ascribes the higher critical potential in potassium chloride to a salting-out effect due to the higher concentration required with univalent salts, i.e., the sol would still have been stable at a zeta potential of 44 mv., but for the salting-out effect. Kruyt and van der Willigen (9), working with arsenic trisulfide, selenium, and mercuric sulfide sols, do not carry the mobility-concentration curves 925 T a l JOURNAL OF PHYSICAL CHIUYISTRY, V O L . XXXIX. NO.
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H. L. WHITE AND BETTY MONAGHAN
quite out to the flocking concentrations, but i t is evident that with all the sols mobility at flocking concentration of uni-univalent salts is greater than for salts with bivalent cations. With the former salts the mobility usually increases continuously almost up to the flocking concentration. They conclude that electrophoretic mobility is not a true measure of zeta. They ascribe the increasing mobility with increasing concentration of univalent salts up t o nearly the flocking concentration and the fact that mobility a t flocking concentration is higher with univalent than with multivalent cations to (1) variations in dielectric constant produced by the relatively high univalent salt concentration required for flocking (3 to 8 X 10-2 M ) and to (2) the probable incorrectness of the assumption that the mean distance between the electrical double layers equals the distance from the wall a t which fluid velocity becomes constant. As to the first of these points, considerable difference of opinion exists regarding the effect of added electrolytes on the dielectric constant of water; the effect postulated by Kruyt may exist but cannot at present be evaluated quantitatively (10). As to the second point, Koenig (6) has pointed out that this assumption is not essential to the derivation of the electrokinetic equations. The above results were obtained with an ultramicroscopic method. Kruyt and Tendeloo (8) also find the mobility of lyophilic sols increased on addition of electrolytes, using a moving boundary method. However, in a later paper Bungenberg de Jong, Kruyt, and Lens (2) question the earlier results, showing increased mobility in presence of electrolytes, and state that unpublished results of Kruyt and de Haan with a microscopic method show no such increase, the implication of this last paper apparently being that electrophoretic determinations properly carried out are a true index of zeta. Pennycuick (14) found the mobility of platinum sols to increase slightly in low sodium chloride concentrations and then to fall off slowly, being a t the coagulaking concentration about 80 per cent as great as in water. The critical mobility, i.e., that a t the coagulating concentration, was about the same for barium chloride, barium hydroxide, and hydrochloric acid and was about half as great as the critical mobility in sodium chloride; the critical concentrations for barium chloride were much lower than for SOdium chloride, the latter being about 3 X 2M.I Similar results with various sols have been reported by Ivanitzkaja and Proskurvin (5),2 by Briggs (l),and by various other workers. Freundlich and Zeh (3), however, find only a decrease of mobility of arsenic trisulfide 1The concentration figures of figure 2 of this paper are apparently all too low by tenfold, evidently a typographical error; compare Pennycuick (Z. physik. Chem. 148, 419, 424 (1930)). * The results in their very dilute solutions, as with 1 X 10-1' M potassium chloride added, can hardly be of any aignificance.
INCREASING ELECTROPHORETIC MOBILITIES
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sol on adding potassium chloride or other univalent salts; they used the moving boundary method. Rabinowitsch and Fodimann (19) find that addition of potassium chloride decreases mobility of arsenic trisulfide sols if the acid content of the sol is low and increases it if the acid content is high; their results seem surprising in that after adding 7 X M hydrochloric acid, additional 2 X lou5M potassium chloride exerts a significant effect. Various explanations for the increasing electrophoretic mobility on addition of uni-univalent electrolytes have been proposed. Powis (18) believes that it is due to an actual increase in zeta, effected presumably by an increase in the charge density by adsorption of added anions. The later fall in mobility in the higher concentrations would then be ascribed to a lessened diffuseness of the positive layer with a resultant lowering of zeta. That the higher critical mobility with univalent inorganic salts is dependent on the high concentrations required for coagulation is shown by the results of Briggs (1) with highly adsorbed univalent organic cations. With these coagulation is produced in low concentrations, and the critical mobility is that common to all polyvalent cations. With the monovalent inorganic potassium cation Briggs agrees with other workers in finding an M. initial increase in mobility which may be back to normal in 5 X Briggs accepts the.view of Kruyt and van der Willigen ( 7 ) that the maintained or increasing mobility in relatively concentrated uni-univalent inorganic electrolyte solutions is due to an increase in dielectric constant rather than in zeta. Rabinowitsch and Fodimann could not consistently verify Freundlich and Zeh’s prediction, which was based upon a concept developed by Hevesy (4)that if the original zeta potential is greater than 70 mv., addition of electrolytes will lower it to 70, while if the original value is less than 70, electrolytes will raise it to 70. Rabinowitsch and Fodimann’s explanation of the salt effect appears to be self-contradictory. They first say that the chief factor lowering zeta is the replacement of hydrogen ions in the electrical layer by potassium ions; this occurs in their non-acidified sols with a resultant lowering of mobility. This replacement is inhibited by an increased acidity of the medium, and therefore with an acidified sol the addition of potassium chloride does not lower the zeta potential or mobility. I n the next paragraph, however, to explain the actual increase in mobility often seen in acidified sols on addition of potassium chloride, they assume that potassium ions form a more diffuse layer than hydrogen ions with a resultant increase in zeta, Pennycuick (14, 15) assumes that the strong affinity of the platinum surface for hydroxyl groups causes a hydrolytic cleavage of the salt, the basic cleavage product then reacting a t the surface to form new ionogenic spots. It is difficult to accept this explanation, however, since the addition of a neutral salt to water does not increase the hydroxide-ion activity.
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H. L. WHITE AND BETTY, MONAGHAN
I n any case Pennycuick makes no attempt to explain the finding that the critical mobility with bases and neutral monovalent salts is higher than with salts of polyvalent cations. In a previous publication (White, Monaghan, and Urban (22)) it was shown that in water and dilute solutions the ratio of electrosmotic velocity on a plane Pyrex surface to electrophoretic velocity of Pyrex particles 1to 3 p in diameter was always greater than unity. The ratio decreased as the concentration of added salt increased, becoming unity a t a concentration between loF3and lo-* M potassium chloride and a t all higher concentrations. Although values for the absolute magnitude of electrokinetic velocity were not obtained a t that time, it was pointed out that the discrepancy in the ratio was undoubtedly t o be ascribed t o retardation of the particles in dilute solutions. Several factors were assumed to be responsible for this retardation. Two of these, the decrease of (1) double layer thickness and (2) charge density, with decreasing particle radius, retard the particle by lowering the zeta potential. This effect is present whether or not an external voltage is applied; it will therefore contribute to the instability of the particle. A third factor, the distortion of the electric field by the particle, is of doubtful significance in the case of the conducting particles of metallic sols. The fourth factor, however, the polarization of the diffuse double layer by the external field, which was considered most important in causing deviation of the ratio from unity, applies to all pasticles in dilute solutions. Zeta, calculated from electrophoretic velocity, will therefore be considerably lower than the electrosmotic zeta, lower also than the true zeta of the particle. Furthermore, it will appear t o go through a maximum on the addition of salt when no maximum exists in the electrosmotic or true zeta-concentration curves. The bearing of these findings on the problem of critical potentials of lyophobic sols is obvious. Those salts which coagulate a t low concentrations will appear to have a lower critical potential (measured by electrophoresis) than the monovalent salts and bases which require much higher concentrations to precipitate. This apparently lower critical potential is, however, an artifact inherent in the electrophoretic method. It is difficult to obtain electrosmotic measurements on surfaces identical with the colloidal sols used in coagulation studies. On the other hand, both electrosmotic and electrophoretic data may easily be obtained on glass and protein-coated glass surfaces. The present paper is concerned with the absolute magnitude of electrosmotic and electrophoretic velocities on glass and protein-coated surfaces as a function of electrolyte concentration. EXPERIMENTAL
Both electrophoretic and electrosmotic velocities were obtained by observing microscopically the movement of particles at stated levels in a
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INCREASING ELECTROPHORETIC MOBILITIES
Pyrex electrophoresis cell of the type described by Mattson (11). It follows from the formula expressing electrosmotic flow of the liquid a t different levels in a cylindrical cell (Mattson (12)) that true electrophoretic velocity in such a cell is the observed velocity of the particles a t 0.147 of the diameter from the wall. True electrosmotic velocity is the velocity of the water immediately adjacent t o the wall, or (with reverse sign) that of the water in the center of the cell. It may be obtained by subtracting algebraically the true electrophoretic velocity from the observed velocity of the particles TABLE 1 Potassium chloride-Pyrex at 87°C. __
vw
__
-
p/SEC. (Viniddle
v w p/SEC. AVERAGE
62.9 57.8 48.1 37.2 22.6
61.7 57.5 48.3 36.8 23.0
APPAR-
VP V./CkI.
rw
ENT I p V./CM. IP -~ vp! __ -- -- --
-
molar
HzO -44.6 $78.8 10-6 -41.7 $73.3 10-4 -29.8 +66.7 10-3 -13.7 $59.9 10-2 0 $46.1
$15.9 $15.5 +18.6 +22.7 $23.5
60.5 57.2 48.4 36.4 23.5
3.49 13.6 3.41 12.6 4.09 10.6 4.99 8.09 5.16 5.05
-
35 34 40 49 51
134 124 105 80 50
3.8 3.7 2.6 1.6 0.98
TABLE 2 Potassium chloride-Pyrex AT CONCENTRATION OF 1
