Electrochemical Instability at Liquid/Liquid Interfaces

Abstract

When a liquid/liquid two-phase system contains ionic surfactants, the interface can exhibit a variety of intriguing features, such as spontaneous emulsification, kicking movements of the interface and oscillations of the interfacial tension and of phase-boundary potential [1-6]. These interfacial turbulences have been known for a long time in surface chemistry. A more recent interest in the interfacial turbulences originates in solvent extraction, in which the adsorption of ligands and metal-ligand complexes at the interface often results in the interfacial turbulences [7,8]. Such observations prompted theoretical studies of the surface instability, among which a seminal work by Sterling and Scriven resorting to the linear stability theory [9] has been an important basis of the theoretical studies of the Marangoni instability in liquid extraction and related systems[10]. On the other hand, the chaotic oscillation of electric current associated with the transfer of ions was known already at the beginning of the electrochemical studies of the electrified liquid/liquid interfaces [11,12]. To avoid such interfacial turbulences in recording current-potential curves, surfactant was added to the system in analogy with the suppression of the polarographic maxima by adding surface-active substances [13]. The interfacial turbulence in the voltammetry of ion transfer has been rather considered to be a nuisance than made use of as a phenomenon carrying rich information of the interfacial properties. As a matter of fact, the chaotic current oscillation appears in voltammograms with some regularity [14]. The current oscillation occurs at the potential in the vicinity of the standard iontransfer potential of the surface-active ions. Moreover, the potential where the interface becomes unstable is often limited to a certain region around the standard ion transfer potential. The origin of such interfacial turbulence in voltammetry of ion transfer across the liquid/liquid interface can be understood in terms