Gas-liquid interface of hydrophobic and hydrophilic room-temperature ionic liquids and benzene: Sum frequency generation and surface tension studies

Surface investigation of room-temperature ionic liquid and benzene mixtures has been performed using sum frequency generation vibrational spectroscopy and surface tension measurements. Specifically, the study looked into the effect of benzene on the cation orientation of room-temperature ionic liquids at the gas-liquid interface. 1-Butyl-3-methylimidazolium hexafluorophosphate, [BMIM][PF6] and 1-butyl-3-methylimidazolium tetrafluoroborate, [BMIM][BF4] were considered for hydrophobic and hydrophilic ionic liquids, respectively. The cation orientation was investigated as a function of benzene concentration in ionic liquids ranging from 0.0 to 0.25 mol fraction. For concentrations > 0.01 mol fraction of benzene, the benzene seems to affect the conformation of the butyl chain and only slightly for the imidazolium ring. This is evident with the presence of the methylene symmetric and antisymmetric modes at 2850 and 2911 cm(-1), respectively, and the absence of the aromatic ring modes. Benzene molecules appear to be present at the surface for all concentrations > 0.01 mol fraction for both ionic liquids. In addition, the intensity of benzene peak in the hydrophobic ionic liquid seems to be constant while it is noticed to increase continuously for the hydrophilic one, for concentrations > 0.01 mol fraction. The surface tension data for the specified range of concentration showed a decreasing trend as the amount of benzene is increased for both ionic liquids studied.