Ionic Liquid|Water Interface: A New Electrified System for Electrochemistry

Abstract

A room-temperature ionic liquid (RTIL) consisting of hydrophobic cations and anions forms a two-phase system with water (W). The interface between the RTIL and W is inevitably electrified because of the dissolution of cations and anions constituting the RTIL into the W phase and the two-phase system should, hence, be considered as a new electrochemical system where the phase-boundary potential across the RTIL|W interface plays a key role in determining physicochemical properties of the two-phase system, notably the interfacial charge transfer and the structure of the electrical double layers at the interface. When the solubility of the RTIL in W is on the order of a few mmol dm⁻³ or higher, the RTIL|W interface behaves as an electrochemically nonpolarized interface; the phase-boundary potential is determined by the composition of the two phases. At the opposite extreme where the solubility is a few tens µmol dm⁻³ or lower, the interface can be taken as an electrochemically polarized interface, since the phase-boundary potential can be controlled by externally applying a voltage across the two terminals of the cell that comprises the RTIL|W interface. This electrochemical polarizability allows us to employ a variety of electrochemical techniques for studying the RTIL|W interface. Electrochemical view points as well as electrochemical techniques are powerful in clarifying the unique properties of this new electrified interface. Conversely, this two-phase system poses us intriguing problems, which can in turn widen the scope of electrochemistry.