Understanding the Effect of the Adsorbed Water on the Mica Surface on Extended Layering of Ionic Liquids

Understanding the effect of the adsorbed water on the mica surface on extended layering of ionic liquids

Xiao Gong, Andrew Kozbial and Lei Li

Department of Chemical & Petroleum Engineering

University of Pittsburgh

Pittsburgh, PA 15261

Ionic liquids (ILs) have attracted extensive interests due to their exceptional physicochemical properties and many promising applications. Some important applications such as catalysis and lubrication involve nanometer-thick IL flms confined to solid surfaces. Therefore, it is critical to understand the molecular-level structure of ILs at the IL/solid interface. Previously, several groups have reported the extended solvation layers of ILs on the mica surface. It has been generally believed that the electrostatic interaction at the IL/mica interface is the key to the observed extended layering. Our recent experimental results indicate that, indeed, water adsorption on the mica surface is the key to the extended layering of ionic liquids. The atomic force microscopy (AFM), attenuated total reflectance-fourier transform infrared spectroscopy (ATR-FTIR) and contact angle (CA) results show that ionic liquids form extended layering on a mica surface under ambient conditions when water is adsorbed on the mica surface under such conditions. However, when some hydrocarbon contaminants in the ambient replace the water on the mica surface after a simple oven heating at high temperatures, instead of layering, ionic liquids show droplet structure, i.e., dewetting. Based on the experimental results, we propose that water enables ion exchange between K+ and the cations of ILs on the mica surface and therefore triggers the ordered packing of cations/anions in ILs, resulting in extended layering.