Surface acid groups, water effects

The interfacial surface tension may be markedly affected by a surface reaction occurring in the presence of traces of impurities. A good example of such an effect is to be noted in the alteration of the interfacial surface tension between an oil containing a fatty acid and water containing acid or alkali. For acid solutions the interfacial surface tension remains constant and almost independent of the Ph of the aqueous phase. As the solution becomes more alkaline the carboxyl groups of the acid commence to dissociate and the interfacial surface tension falls rapidly. (Hartridge and Peters, Proc. Boy. Soc. A, Cl. 348, 1922, see p. 252.)... 

Since, on the acid side, the protein contains free basic groups, and on the alkaline free acidic groups, the assistance to spreading is possibly due to salt formation between the ions in solution and the protein in the film the reason why the least hydrated ions are the most effective may be that they come to the surface most easily, having less affinity for the water than the more hydrated. 

---