Water at Ionic Liquid Interfaces

摘要

Water is known to affect bulk properties of ionic liquids (ILs) including density, viscosity, conductivity and gas absorption. It is also becoming increasingly recognized that water gives rise to significant changes at IL interfaces. In this chapter we review the surface sensitive analytical techniques and molecular dynamic (MD) simulations utilized to probe the IL-vacuum, IL-gas and IL-solid interfaces in the presence of water. An overview is first given from the perspective of surface science experiments, followed by a focus on results from MD simulations. Experimental studies in most cases examined the IL in the melted state, while a few studies examined the IL-vacuum interface while the IL transitioned between frozen and melted states. Over the past two years there has been a significant increase in atomic force microscopy (AFM) studies probing the effects of water on the IL-solid interface of both neutral surfaces and electrified surfaces. Experimental and MD simulation studies that vary the amount of water often reveal water mole fraction (x_w) dependent structural changes and IL layering at IL-gas and IL-solid interfaces. Under low x_w conditions the concentration of water at the interface can be significantly different than in the bulk. While water is often viewed as a ubiquitous contaminant, it is also possible to envisage utilizing water as a chemical knob to influence x_w dependent interfacial IL chemistry and structure with significant implications in gas absorption, electrochemical and surface catalytic studies.