Effects of Water on Mica-Ionic Liquid Interfaces

A growing body of work shows that water can affect the structure and properties of the ionic liquids near solid surfaces, which has rich ramifications in applications of ionic liquids such as lubrication and energy storage. Using molecular dynamics simulations, we investigate how water affects the three-dimensional structure of ionic liquids [BMIM] [Tf2N] near mica surfaces with two different charge densities. We show that water can alter not only the layering of ions near the mica surface but also their lateral and orientation ordering and the aggregation of cations' hydrophobic tails. Water often, but not always, weakens the structuring of interfacial ionic liquids. The multifaceted impact of water on the interfacial structure of ionic liquids can be traced back to the fact that water is both a dielectric solvent and a molecular liquid. Based on the additional observations that the adsorption of water at mica ionic liquid interfaces is enhanced by ionic liquids and surface charge, we suggest that the structure of ionic liquids near solid surfaces is governed by the three-way coupling between the self-organization of ions, the adsorption of interfacial water, and the electrification of the solid surfaces.