Thickness-Dependent Molecular Arrangement and Topography of Ultrathin Ionic Liquid Films On a Silica Substrate

Thickness-dependent molecular arrangement and topography of ultrathin ionic liquid films on a silica substrate

Xiao Gong, Scott Frankert, Yongjin Wang and Lei Li

Department of Chemical and Petroleum Engineering, University of Pittsburgh

Pittsburgh, PA 15261

Room temperature ionic liquids (RTILs), due to their extremely low volatility, non-flammability and other remarkable physicochemical properties, have become a very promising candidate as next-generation green chemicals. Understanding the molecular arrangement and topography of RTILs at RTIL-solid interfaces is critical to many important applications, including catalysis, lubrication, electrochemistry and photovoltaic power generation. To gain this understanding, an effective approach is to investigate the solid-supported ultrathin RTIL films with the thickness around one monolayer or below, where the otherwise buried interface can be more easily characterized. In the current presentation, we report our experimental study of the molecular arrangement and topography on a silica-supported ultrathin film of ionic liquid, 1,2-dimethyl-3-propylimidazolium bis (trifluoromethylsulfonyl) imide (DMPIIm). Angle-resolved X-ray photoelectron spectroscopy (ARXPS) results indicated a transition from an “anion/cation” layered structure to a “drop-on-layer” structure when the DMPIIm film gets thicker. AFM results agreed to the ARXPS results and showed a transition from a smooth to a “rough” surface with the increase of DMPIIm thickness. The observed phenomena have been attributed to the competition between solid/IL attraction and anion-cation cohesion.