Adsorbed films between two immiscible liquids

An a-hydroxyacid showed similar changes in interfacial tension, commencing at a lower pu an ester showed a fall beginning at a much higher Put perhaps owing to some hydrolysis occurring at the interface.4 

Displacement of equilibria in adsorbed layers. If an equilibrium exists in solution between two or more constituent substances, and one of these is adsorbed more strongly than another, that one will be more concentrated in the surface and the equilibrium in the surface layer will be shifted in the direction of that constituent. It often happens, owing to electrolytic dissociation or to hydrolysis, that a single pure substance when dissolved in water consists of such an equilibrium mixture, and if the bulk solution alone were under consideration, an aqueous solution of such a substance would naturally be treated, according to the phase rule, as a two-component system. But when surfaces enter into consideration, unless the ease of adsorption of both the constituents of the equilibrium mixture in solution is identical, the adsorption of each has to be considered separately and consequently the system must be regarded as consisting of three components at least, not two.5 

One such case has already been met with, in 8 undissociated fatty acids are concentrated at the air-liquid surface of soap solutions made by 

Pays-Bae, 46, 549 (1927) (further references here). 

Bartell and Fu1 find a similar hydrolytic adsorption of organic acids, from their salts, on silica. 

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Adsorbed films between two immiscible liquids. The question of the meaning of the term pn in the surface layer has been raised by Crax-ford, Gatty, and Teorell,2 without, however, coming to any very clear decision. Danielli s estimate was a very rough one, based on the application of the Donnan equilibrium between the surface layer and the interior, and suffers from the difficulties always attending an attempt to consider concentrations in the surface layer in a similar way to concentrations in a bulk phase the surface layer is not homogeneous. pH is closely related to, and is determined by, the electrochemical potential (see Chap. VIII, pp. 304 ff.), and this depends on the electrostatic potential, which varies rapidly at different levels near to the surface it appears possible that the only satisfactory definition of pa in the surface may be one which varies rapidly at different depths. The question appears one which would repay... 

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