ELECTROKINETIC ASPECTS OF SURFACE CHEMISTRY. IV
THERATIOOF ELECTROOSMOSIS TO ELECTROPHORESIS LAURESCE S. MOYER’ Department of Botany, University of Minnesota, Minneapolis, &f innesota, and the Biological Laboratory, Cold Spring Harbor, New Yorlc Received November IS, 1997
Abramson (l),Daniel (6), Dummett and Bowden (7), and Bull (5) have found that the electrophoretic mobility, IJ, of inert particles coated with protein is the same as the electroosmotic mobility, u, of the solution past the wall of the electrophoresis cell when it, too, is coated with the same protein. White, Monaghan, and Urban (15, 16) have agreed that the ratio R (where R = u/v) is equal to unity in the presence of salts when a concentration of gelatin of about 0.01 per cent or more is present, but they contend that in the absence of added salt R assumes values between 2 and 2.3. Moyer and Abramson (13) investigated this reported value of R, using concentrations of gelatin down to 0.02 per cent at specific conohm-l-cm.-l, yet were unable to detect a ductances of about 4 X significant rise of the ratio from unity. Recently, however, White and Fourt (14) have again reported divergent ratios and maintain that the ctifference between the results of Moyer and Abramson and those of White, Monaghan, and Urban, in the absence of added salt, is due to the higher gelatin concentration used by Moyer and Abramson. It mas claimed that, although the coating was complete in all concentrations of gelatin employed, the effect of “an increase in gelatin concentration, as of increasing electrolyte concentration, is to bring the ratio t o unity”. As proof of complete coating of the cell and particles, the identity of their isoelectric points was presented. Abramson (2) has pointed out that identity of isoelectric points is a necessary but not a sufficient condition to establish identity of surfaces. (See in particular experiments performed by the writer (10) with aluminum oxide particles coated with egg albumin.) For their experiments Abramson and Moyer and Abramson hare always been careful to place both the cell and the particles in strong (I per cent) gelatin solutions until coating is established, and then to dilute the solutions to the gelatin concentration desired, using this device in order to have a low conductance with a complete film. John D. Jones Scholar a t the Biological Laboratory, Cold Spring Harbor, Kew York, during the summer of 1937. 391
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OH thr othrr hand, White et a1 ha\e contended that merely placing nall arid p:li.tiCtPs in contact mith 0 01 per cent gelatin solutions is sufficient to coat Ihe wrfaee.j completely and that the high values for the ratio are due t o other cause? Rilley and Hazel (17) have recently investigated the problem, using particles of colloidal ferric oxide in gelatin solutions. By exposing thc particles and cell walls to dilute gelatin solutions, they conc l u d d that t o 2c.t identical surfaces, other than a t the i5oelectric point and in tile a b m e r of added salts, about 0 2 per cent gelatin Itas required. Bel\ 7 i t h contentration divergent values for u and v nere reported. n of salt.; brought the ratio t o R = 1 0 Further details are i.d in Ihiw paper Similar results have been reported by van Glls state5, hox ever, that complete coating is attained first a t a concenof 0 05 per cent gelatin in the absence of salts. e expeiinierits now to be presented, quartz particles were placed in 1 pfI tent solutions of elpctrodialyzed “Silver 1,abel” gelatin (manufactured e1:can Carbon Work.) for fifteen minutes They were then Some of the sediment n of gelatin. Meanmhile i w ’ horizontal microelectrophoresis cell (a modification of a cell ed of one piece of glass)2 atin solution After the lapse of ? m ~ ,the ewes? gelatin %as remoted from the cell by rppeated riiiwig (at least four times) mith distilled aater. Precaution \vas taken to m a t t h c aalls of all meawnng vessels with gelatin in the same way. The sriymsion was then introduced into the cell and measurements perfoiined by our usual methods (9, 13). Figure 1 shows the observed mobility, V, of tilebe particles a t larious depth,, X, from the top of the cell. The specific conductances of these ohm-’-cm-’. The solutions of 0 001 per cent gelatin mere about 4 x smooth, unbroken curve has been draxT n from the usual parabolic equation j13), assuming R = 1.0. The two upper curves (dashed) have been calculated, taking Tz = 1.5 and R = 2.0. Other experiments in which Pyiex glass mas used instead of quartz gave the mine rpsults Under our condition9, it seems that the ratio is ~ e r ynear unity. These experiments indicate that once the surfacm are coated, they retain their coating even after great dilution of the gelatin. It appears probable that the exposure of particles t o initially dilute gelatin solutions in distilled xatcr may not produce a complete, uniform film The ratio of unity hardly permits one t o drairi conclusions (17) as t o the value of the constant rectangular cross section, n i t h non-polarizable electrodes, and 2 This cell has has been shown t o yield data which check quantitatibelq with results obtained from mensuiemenls in U-tubes ( 3 , 4 , 9 ) White et al and N’illey and Hare1 used cells for 4 hirh, as yet, no comparisons of this charxccer seem t o have been presented.
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of proportionality in the Helmholtz-Smoluchowski equations of electroosmosis and electrophoresis, except that it is the same in both equations when the coated surfaces of the particle and the wall are exposed to identical conditions, The agreement between mobilities of certain dissolved and adsorbed proteins suggests t h a t these adsorbed proteins confer their
2t
FIG.1. The mobility, V , of gelatin-coated particles a t various levels, z, of a microelectrophoresis cell. Both experiments were performed a t a final gelatin concentration of 1:100,000 without added salt, and a t a specific conductance of about ohm-*-crn-l. The smooth, unbroken curve (lowest) has been calculated, 4 X assuming R = 1.0. T h e two upper curves (dashed) have been calculated, taking R = 1 . 5 and R = 2.0. I t is evident t h a t , within the limits of error, the experimental points fit the curve for R = 1.0, indicating t h a t electrophoresis and electroosmosis are equal. T h e mobilities given in each figure have been reduced t o unit field strength. own effective radii on the particle and the wall surfaces (2, 11, 13). Hence the constant might assume varying values, depending on the ionic strength and effective radius (12), yet under any given conditions the proportionality constant would be the same for both wall and particle, when sompletely coated, and the ratio would remain a t R = 1.0. Gelatin is
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a protein of non-uniform particle size. I n certain cases this might hinder the attainment of identity of surfaces (by adsorption of slightly different components by wall and particle), yet for systems where R = 1.0 the surfaces under coinparison have probably adsorbed the same conipoiieiit (or mixture of components). Hence under these conditions 1' would he the mean radius and Kr would be statistically the same for both wall and particlc. SUhlMART
T h e ratio of electroosmotic t o elehrophoretic mobility of gelatin-coated surf:. c's IS very near unity, even in gelatin solutions as dilute as 0.001 per ten: a i d in the absence of added Salt, if care is taken to iiisurp B conipletib coating. ADDENDUM
Wliile this paper TTas in press, White and Fourt published their conit!: (J. Phys. Chem. 42, 29 (1938)). Two of their undialyaed parations (Eastman and Coignet) gave high ratios a t low conin distilled water but Alrfa gelatin yielded ratios af 1.00, even at ii.01 pc'~'rcnt gelatin In distilled water. This contradiction to be obserwti in thcir own data makes it necessary to scrutinize their experimentkd method for its precision. White and Fourt report that initial exposure of the mall of the cell to 1 per cent geiatin solutions, and subsequent dilution, failed to reduce the high ratios, but no measurements appear to be presented in which both wall and particles were initially c.xposed t o this concent'ration, as in our experiments. They mention the ratio of 1.11 obtained b y Moyer and dbrarnson with 0.02 per cent dgfa gelatin in distilled water. That. this is not significant'ly higher than 1.0 is showri by our control experiment (13) in M:/100 potassium chloride with 0.2 per cent gelatin for which the rat,io was 1.07. REFERESCES (I) ABI~AXSOS, H. A , : J. Gen. Physiol. 13, 657 (1930). ( 2 ; ABR.AM\~~OX, H. A , : Electrokinetic Phenomena. The Chemicai Catalog Co., Inc., S e w York (1934). (3) A B R A M S O S , H. A , . h 6 D M O Y E R , L. s.: Trans. Elertrochem. SoC. 71, 135 (1937). (I) ABRAXSOS,H. A,, MOYER,L. S.,.\XD VOET, A , : J . Am. Chem. So?. 68, 2362 (1936). (5) BCLL,€ B.: I.J. Phys. Chem. 39,577 (1935). (6) DAXIEL, J.: J. Gen. Physiol. 16, 157 (1933). (7) DUYIIETT, A , , AND ROWDEX,P.: Proc. ICoy. Soc, (London) A142, 382 (1933). (8) VAN GILS,G. E . : Electrophorese Illetingen. Utrecht (1936). (9) S f O Y E R , L. s.: J. h c t . 31, 531 (1936). (10) l f O Y E R , L. J. Phgs. Cheni. 42, 71 (1938).
s.:
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(11) (12) (13) (14)
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MOYER,1,. S.: J. Biol. L'hem. 122, 641 (1938). MOYER,L.S.,AND ABELS, J. (2.: J. Biol. Chem. 121, 331 (1937). MOI-ER,L. S . , . m u ABRAME,OX, H . A , : J. Gen. Phgsiol. 19,727 (1936). WHITE, H. L,, AND FOURT, I,.: Paper presented a t the Fourteenth Colloid
Symposium, Minneapolis, Minnesota, June 11, 1937 (quotation from mimeographed abstract). (15) WHITE,H. L., ~ ~ O S A G H . A S ,B., . ~ S D URBAS,F,: J. P h p Chem. 39, 611 (1935). s , J. Phys. Chem. 39, 925 (1935), (16) WHITE, H. L., . ~ N Dl I o s a c i ~ . ~B.: (17) KILLEY, 9. R., AND HAZEL, F.: J. Phys. Chem. 41, 699 (1937).
