12 THEORY OF COLLOIDAL BEHAVIOR 



which will cause them to " adsorb" most readily those ions of an 

 electrolyte which have the opposite sign of charge from the 

 colloidal particle. This adsorption is supposed to annihilate the 

 charge of the particles causing them to coalesce. The higher the 

 charge of the ion the more readily it is adsorbed; and this is 

 presumed to explain why the flocculating action of ions increases 

 with their valency. 1 



The hypothesis that the electrical charges of micellae of proteins 

 are diminished or annihilated by the preferential adsorption of 

 the ions of a salt rests on no measurements and the hypothesis 

 has never advanced beyond the stage of vague qualitative specu- 

 lation. Such speculations would never have been accepted or 

 considered if it were not for the fact that there existed no direct 

 measurements of the charges of suspended protein particles. 

 The writer found a method of directly measuring the P.D. between 

 protein particles and surrounding liquid, and was thus able to 

 follow minutely the influence of the hydrogen ion concentration 

 and of the addition of salts on the P.D. 2 The quantitative data 

 thus gained made it possible to investigate the origin of the 

 P.D. and it was found that this P.D. is due to the fact that proteins 

 form ionizable salts with acids and bases. Whenever protein 

 ions are prevented from diffusing through membranes or gels 

 permeable to crystalloidal ions, peculiar equilibrium conditions 

 are established resulting in an unequal distribution of the 

 oppositely charged crystalloidal ions between colloidal particle 

 and surrounding liquid. This unequal distribution of oppositely 

 charged ions leads to the P.D. at the boundary of micellae and 

 surrounding liquid. It is possible to explain mathematically, 

 from Donnan's equation for such membrane equilibria, the influ- 

 ence of acids, alkalies, and neutral salts on the charges of the 

 micellae, and it can be shown that the observed P.D. agrees 

 quantitatively with that calculated from the equilibrium equation. 

 It thus turns out that the explanation of the annihilation of the 



1 For a full presentation of the adsorption theory the reader is referred to 

 BANCROFT, W. D., " Applied Colloid Chemistry," New York, London, 1921, 

 in this series, and to LEWIS, W. C. McC., "A System of Physical Chemistry," 

 2nd ed., vol. 1, p. 346, London, New York, Bombay, Calcutta, and Madras, 

 1920. 



a LoEB, J., J. Gen. PhysioL, vol. 3, p. 667, 1920-21; vol. 4, p. 351, 1921-22. 



