Surface Structure of Ionic Liquids Determined by X-ray reflectivity and Sum-Frequency Generation Spectroscopy

X-ray reflectivity and surface sum-frequency generation spectroscopy were used to study the surface of [BMIM][X] ionic liquids (BMIM = 1-butyl-3-methylimidazolium, X = BF$_{4}$, PF$_{6,}$ and I). Sum-frequency signal strength from the terminal methyl groups of the cation at the surface indicates the topmost surface of these ionic liquids are occupied by polar-oriented hydrophobic butyl chains having $\sim $1/3 of the alkyl chain density of fully-packed hexadecanol Langmuir monolayer. X-ray reflectivity data could be fitted well by the assuming the first layer with low electron density followed by the electron-rich second layer on top of bulk ionic liquid. Detailed analysis of the reflectivity data in conjunction with the sum-frequency findings strongly suggests the molecules forming the top-most layer are on average oriented with their butyl chains (loosely packed) towards the gas/liquid interface while the core/anions (densely-packed) are in contact with the bulk liquid.