Electrode screening by ionic liquids

Abstract

In this work we are concerned with the short-range screening provided by the ionic liquid dimethylimidazolium chloride near a charged wall. We study the free energy profiles (or potentials of mean force) for charged and neutral solutes as a function of distance from a charged wall. Four different wall charge densities are used in addition to a wall with zero charge. The highest magnitude of the charge densities is ±1 e nm⁻² which is close to the maximum limit of charge densities accessible in experiments, while the intermediate charges ±0.5 e nm⁻² are in the range of densities typically used in most of the experimental studies. Positively and negatively charged solutes of approximately the size of a BF⁻₄ ion and a Cl⁻ ion are used as probes. We find that the ionic liquid provides excellent electrostatic screening at a distance of 1–2 nm. The free energy profiles show minima which are due to layering in the ionic liquid near the electrodes. This indicates that the solute ions tend to displace ionic liquid ions in the layers when approaching the electrode. The important role of non-electrostatic forces is demonstrated by the oscillations in the free energy profiles of uncharged solutes as a function of distance from the wall.