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]I, 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 40-55degrees. No water signal was observed in the spectra obtained at the water pressures studied here.