The relations between surface p H, ion concentrations and interfacial tension

Thep H of a surface phase cannot be estimated by the methods available for bulk phases: in this paper an attempt is made to estimate thep H of a surface phase by analysing the behaviour of carboxyl groups in an adsorbed interfacial film. The work of Donnan (1899), Jahrisch (1922), Hartridge and Peters (1922) and Peters (1931) has shown that the interfacial tension between solutions of long-chain acids, or amines, and aqueous solutions, is much lowered when thep H of the aqueous phases is such that the end-group of the acid or amine, which forms the interfacial film between the immiscible solvents, is electrolytically dissociated. The interfacial tension is low on the alkaline side and high on the acid side for the fatty acids, but low on the acid side and high on the alkaline for the amines. The change of tension takes place over thep H range 5-10, and the form of the curves relating interfacial tension top H is strongly reminiscent of the change of dissociation withp H, indicating that there is a close relationship between the interfacial tension and the degree of dissociation. Close inspection of Peters’s curves shows, however, that the range ofp H over which the i. t. change occurs with the fatty acids is about 3 units ofp H more alkaline than the range over which dissociation occurs in bulk phases; also that dilution of the buffer solutions, without change inp H, may cause a rise in i. t. Accepting the view that the changes in i. t. are due largely to changes in ionization of the carboxyl or amino groups in the surface film, these results strongly suggest that thep H in the surface layer may not be the same as in the bulk aqueous phase.