Interfacial Restructuring of Ionic Liquids Determined by Sum-Frequency Generation Spectroscopy and X-Ray Reflectivity

Surface sum-frequency generation spectroscopy and X-ray reflectivity were used to study the surface of [BMIM]][X] ionic liquids (BMIM = 1-butyl-3-methylimidazolium cation, X = BF4-, PF6-, and I- Sum-frequency signal strength from the terminal methyl groups of the cation at the surface indicates that the topmost surface of these ionic liquids is occupied by polar-oriented hydrophobic butyl chains having approximately 1/3 of the in-plane density of fully packed alkyl chains as observed by the same method for the hexadecanol Langmuir monolayer. X-ray reflectivity data reveal a layer with density larger than that of bulk. However, the reflectivity is not sufficiently sensitive to the exact location of this layer either at the vacuum interface or sandwiched between the bulk and-the low-density alkyl chain, as observed in the sum-frequency measurements. Analysis of the reflectivity data in conjunction with the sum-frequency spectra strongly suggests the molecules forming the topmost layer are on average polar-oriented with their (loosely packed) butyl chains toward the gas/liquid interface, while the (densely packed) imidazolium cores/anions are in contact with the bulk liquid.