Change in the Position of Equilibrium at Surfaces

In 1931 Peters (54) made what he describes as the first measurements of the strength of ionization at an interface. He measured the interfacial tension of long-chain acids and amines at the interface between benzene and water, the latter containing various buffering salts. The change, he found, of the interfacial tension due to appreciable ionization, in the case of the carboxyl head occurs at about 3.0 pH units on the alkaline side, and in the case of the amine some 4.0 pH units to the acid side of the point at which the corresponding water-soluble acid and base would commence to ionize in aqueous solution.  

The direct relation he assumed between ionization and interfacial tension seems reasonable in the light of more recent work, and the effect, now believed to depend on the ionic strength of the aqueous solution, was certainly real. The results of surface potential of fatty acid films, measured by Schulman (71), also confirm qualitatively his finding. 

If we consider in detail the ionization of a monolayer of acid, we realize that as soon as the first few molecules have become negatively charged, the interface takes up a potential different from that of the bulk of the aqueous solution. This electrical potential, as Peters (55) 

Mathematically, we may apply the Boltzmann eqnation to the hydrogen ions  

Similar equations apply to a base at the interface, when will be positive. 

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