Orientation and Structure of Ionic Liquid Cation at Air/bmimBF₄ Aqueous Solution Interface

The water-miscible room temperature ionic liquid 1-butyl-3-methylimidazolium tetrafluorob-orate (bmimBF₄) is a model system for studying the interactions between ionic liquid and water molecules. In this work the orientational structure of the low concentrated aqueous solution of bmimBF₄ at the air/liquid interface was investigated by sum frequency gener-ation vibrational spectroscopy. It has been found that at very low concentrations, the butyl chain exhibited a significant gauche defect, indicating a disordered conformation; and the cation ring oriented with a fairly small tilting angle at the surface. When the concentration increased, the cation ring tended to lie flat at the surface, and the gauche defects of the butyl chain decreased due to the intermolecular chain-chain interactions and the consequent more ordered interfacial molecular arrangement. Additionally, the anti-symmetric stretching mode in the PPP and SPS spectra exhibited a peak shift, showing that there exists more than one kind of orientation or chemical environment for the butyl CH₃ group. These results may shed new light on understanding the surface behavior of water-miscible ionic liquids as well as the imidazolium based surfactants.