Electrocapillary phenomena at oil-water interfaces

Summary

The interfacial tension, between the oil phase containing an ionic surface active agent and the aqueous phase containing an inorganic electrolyte, is a function of the potential difference applied to the two phases from an outer circuit. This phenomenon, the electrocapillarity at oil-water interfaces, was used by the present authors to study the structure of electrical double layers and the interaction between surface active agent ions and counterions. In the present study an amphoteric material, the lecithin, was used as the surface active agent, and its adsorption and dissociation behaviours as well as its interaction with counterions were examined. It was found that the lecithin dissolved in the “oil” phase behaved as the cationic or anionic surface active agent according as the p_H of the “aqueous” phase was acidic or alkaline. This showed that the ionic groups of the lecithin molecule are immersed into the aqueous side of the interface. Since the interfacial tension was independent of the applied potential at the interfacial isoelectric point, this point could be measured with a high sensitivity. The ionic species of the inorganic electrolyte in the aqueous phase had a marked influence on the isoelectric point. The reversal of charge spectra thus obtained were found to agree fairly well with that obtained by other authors using electrophoretic measurement.