Ions at the Surface of a Room-Temperature Ionic Liquid

The orientation and interfacial location of both the cation and anion of the ionic liquids 1-butyl-3-methylimidazolium methyl sulfate and 1-butyl-3-methylimidazolium methanesulfonate have been determined by sum-frequency generation vibrational spectroscopy and X-ray crystallography. The results indicate that both the cation and anion occupy the first layer at the gas−liquid interface. Further, the methyl groups of the cation and anion are directed away from the liquid phase into the gas phase.