Influence of Water on the Orientation of Cations at the Surface of a Room-Temperature Ionic Liquid:  A Sum Frequency Generation Vibrational Spectroscopic Study

Sum frequency generation vibrational spectroscopy was used to determine the orientation of the cations in a room-temperature ionic liquid at the air−liquid interface. The ionic liquid that was studied was 1-butyl-3-methylimidazolium bis-trifluoromethylsulfonimide, [BMIM] + [imide] - . The orientation of the cation, [BMIM] +, was studied as a function of water pressure in the vapor over the range of 10 -5 to 200 Torr. At water pressures below 10 -4 Torr, the imidazolium ring was oriented parallel to the surface plane. On increasing the water pressure to greater than 10 -4 Torr, the imidazolium ring tipped up from the surface with an angle along the surface normal of 40−55°. No water signal was observed in the spectra obtained at the water pressures studied here.