Monolayer Properties and Ion-transportation Phenomena of Crown Ether Compounds Differing in Crown Size at the Oil/Water Interface

Abstract

To evaluate the stability of crown ether compounds with branched hydrocarbon chains differing in crown size (18-crown-6, 15-crown-5, and 12-crown-4) at the oil/water interface, monolayer properties (especially π-A curve) of the compunds were studied using various salts. A stable monolayer of each crown compound was noted to form at the interface under metal complexing conditions. However, in a system including a compound with small crown size such as 12-crown-4, ion selectivity and ability of complex formation decreased, due possibly to the high heat of hydration of small-size ions. This was also observed in the electrokinetic behavior of the O/W emulsion stabilized by the same crown ether compound.Ion transport of crown ether compounds was studied with different crown size compound-salt combinations. Ion selectivity by crown ethers in the ion transport was higher than that in complex formation. Only the specific metal ion, fitting to crown size among cations in the system was transported through the oil phase. The transportation rate of specific ions was strongly influenced by the number of counter ions picrate ions, i.e., the more picrate anions for a fixed number of specific cations, the faster was this rate by enhanced ion-pairing with the crown ether complex and metal cations.