Surface Orientation of 1-Methyl-, 1-Ethyl-, and 1-Butyl-3-methylimidazolium Methyl Sulfate as Probed by Sum-Frequency Generation Vibrational Spectroscopy

The orientation of the ions at the surface of ionic liquids has been determined by the surface-sensitive technique sum-frequency generation vibrational spectroscopy. The results indicate that both ions are present at the first layer of the gas-liquid interface. Furthermore, the alkyl chains are found to be extended toward the gas phase and away from the liquid phase. The proposed models for the preferred orientation of the ions are in good agreement in comparison with the results obtained from the recent MD simulation studies. The salts considered here are 1,3-dimethylimidazolium methylsulfate, 1-ethyl-3-methylimidazolium methylsulfate, and 1-butyl-3-methylimidazolium methylsulfate.