Abstract

Ion transport through oil-water interfaces is relevant to various phenomena in analytical and electrochemistry and other related areas. However, detailed understanding of the molecular mechanism of ion transport is still a challenging issue. A main difficulty in studying microscopic mechanism of ion transport lies in scarcity of experimental probe techniques with sufficient selectivity and sensitivity to the interface region. Detecting structural fluctuation is even more challenging by experimental techniques, though it is considered to be quite significant at liquid-liquid interfaces. Therefore, molecular dynamics (MD) simulation can play important roles in exploring the details of the ion transport. However, it is not straightforward to characterize very soft, dynamic interfaces by MD simulation. A new viewpoint is strongly desirable to understand liquid-liquid interfaces.

In the present work we discuss the catalytic effect on the ion transport.

It has been known that ion transport is facilitated by complex formation with ligand, called facilitated ion transfer (FIT). It is experimentally reported that even a trace amount of some ligand species can drastically facilitate interfacial ion transport of hard ions in a catalytic manner, though detailed molecular mechanism remained to be elucidated. In the present work we elucidate the mechanism by calculating multi-dimensional free energy surfaces that allow for treating the ion transport and complex formation simultaneously. The present work extend the concept of FIT to the system of catalytic shuttling involving transient formation of local complex at interfaces. The MD results clarify the condition whether the catalytic effect takes place or not, depending on the species of ions and ligands.