Alkyl Chain Interaction at the Surface of Room Temperature Ionic Liquids: Systematic Variation of Alkyl Chain Length (R = C1-C4, C8) in both Cation and Anion of [RMIM][R-OSO3] by Sum Frequency Generation and Surface Tension

The gas-liquid interface of halide-free 1,3-dialkylimidazolium alkyl sulfates [RMIM][R-OSO3] with R chain length from C-1-C-4 and C-8 has been studied systematically using the surface-specific sum frequency generation (SFG) vibrational spectroscopy and surface tension measurements. From the SFG spectra, vibrational modes from the methyl group of both cation and anion are observed for all ionic liquid samples considered in the present study. These results suggest the presence of both ions at the gas-liquid interface, which is further supported by surface tension measurements. Surface tension data show a decreasing trend as the alkyl chain in the imidazolium cation is varied from methyl to butyl chain, with a specific anion. A similar trend is observed when the alkyl chain of the anion is modified and the cation is fixed.