ADSOEPTION POTENTIALS AND ELECTROKINETIC PHENOMENA 241 



face of two adjacent phases, the isolation of the resulting compound in 

 a pure state is not possible, and, therefore, its stoichometric relations 

 cannot as yet be established. 



At this juncture we must return to the extended definition of 

 adsorption given on page 234. When the substance of the non- 

 aqueous phase does not display a marked molecular attraction 

 either for, H- or OH-ions, then it behaves towards a solution con- 

 taining H+- or OH~-ions exactly as a mass-free space, and in such 

 a case the accumulation of the H- or OH-ions at the surface must 

 follow simply in accord with Gibbs' principle. It is not possible to 

 predict whether the H- or OH-ions will show more surface-activity. 

 But experience tells us that numerous non-aqueous phases which are 

 chemically neither acids nor bases assume a negative charge when 

 in contact with pure water. This is a partial aspect of Coehn's law 

 (see below, page 288). The common basis of all these phenomena 

 renders it probable that the non-aqueous phase does not participate 

 in the H+- OH "-distribution, and that this distribution occurs as 

 if the aqueous phase were in contact with an empty space. The 

 negative charge on the non-aqueous phase in respect to the aqueous 

 phase leads one to postulate that the OH-ions are more strongly sur- 

 face-active than the H-ions, that, therefore, the OH-ions accumulate 

 at the surface of the water phase, and that consequently the interior 

 of the water phase becomes positive in respect to its own surface layer 

 and hence also in respect to the adjacent phase. If we are to apply 

 here our extended definition of adsorption, we must suppose that 

 such chemically indifferent substances, as ethyl ester, benzonitrile, 

 oils, hj'drocarbons, etc., have a greater tendency to adsorb OH-ions 

 than H-ions, and are, therefore, to be considered acidoids. But even 

 substances not chemically indifferent are more frequently negatively 

 charged towards water rather than positively. Thus Freundlich and 

 Gyemant^ have found that anihne is negatively charged towards 

 water or towards water saturated with anihne. It could perhaps be 

 expected that aniline, being a base, must adsorb H-ions (C6H5NH2 

 -f- H+ = C6H0NH3+), and that at all events it should adsorb H-ions 

 much better than OH-ions. But this is quite erroneous. Those few 

 aniline molecules which have combined Avith H-ions have become 

 water-soluble aniUne-ions, and we could scarcely expect any signifi- 



* H. Freundlich and A. Gyemant, Zeitschr. f. physikal. Chem. 100, 182 

 (1922). 



