Electro-Responsive Surface Composition and Kinetics of an Ionic Liquid in a Polar Oil

The quartz crystal microbalance (QCM) has been used to study how the interfacial layer of an ionic liquid dissolved in a polar oil at low weight percentages responds to changes in applied potential. The changes in surface composition at the QCM gold surface depend on both the magnitude and sign of the applied potential. The time-resolved response indicates that the relaxation kinetics are limited by the diffusion of ions in the interfacial region and not in the bulk, since there is no concentration dependence. The measured mass changes cannot be explained only in terms of simple ion exchange; the relative molecular volumes of the ions and the density changes in response to ion exclusion must be considered. The relaxation behavior of the potential between the electrodes upon disconnecting the applied potential is more complex than that observed for pure ionic liquids, but a measure of the surface charge can be extracted from the exponential decay when the rapid initial potential drop is accounted for. The adsorbed film at the gold surface consists predominantly of ionic liquid despite the low concentration, which is unsurprising given the surtactant-like structures of (some of) the ionic liquid ions. Changes in response to potential correspond to changes in the relative numbers of cations and anions, rather than a change in the oil composition. No evidence for an electric field induced change in viscosity is observed. This work shows conclusively that electric potentials can be used to control the surface composition, even in an oil-based system, and paves the way for other ion solvent studies.