Role of image charges in ionic liquid confined between metallic interfaces

Abstract

The peculiar properties of ionic liquids in confinement have not only become essential for energy storage, catalysis and tribology, but still pose fundamental questions. Recently, an anomalous liquid–solid phase transition has been observed in atomic force microscopy experiments for 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF₄]), the transition being more pronounced for metallic surfaces. Image charges have been suggested as the key element driving the anomalous freezing. Using atomistic molecular dynamics simulations, we investigate the impact of image charges on structure, dynamics and thermodynamics of [BMIM][BF₄] confined between gold electrodes. Our results not only unveil a minor role played by the metal polarisation, but also provide a novel description of the interfacial layer. Although no diffuse layer can be defined in terms of the electrostatic potential, long range effects are clearly visible in the dynamical properties up to 10 nanometers away from the surface, and are expected to influence viscous forces in the experiments.