Effects of Cationic and Anionic Species on Monolayer Properties of Crown Ether Compound with Long-Chain at Oil/Water Interface

Abstract

The effects of metal salts added to the water phase on the properties of the monolayer of a surface-active crown ether compound with long-chain formed at the oil/water interface were examined by surface pressure measurements, and the ζ-potentials of an O/W emulsion and crown ether compounds were also examined under various salt conditions.Once crown ether compounds have complexed with metal ions at the O/W interface, they become more capable of remaining there owing to the increment in the hydrophilicity of their headgroups. With increase in the degree of complexing the stability of the monolayer becomes greater. The stability of monolayer was found to be influenced not only by the metal cation, but by its counter-anion in the water phase as well; i.e., in the case of potassium salts, this stability is controlled by the degree of the hydration free energy of the anion. Especially, when picric ion, a lipid soluble anion, is used, the monolayer becomes unstable with respect to desorption of a crown ether compound from the O/W interface. That is, crown ether compound complexes including the picric anion are easily transferred from the water phase to the oil phase by compression of the monolayer.