Structural Change of the Mixtures of Ionic Liquid and Water Studied by Infrared Absorption Spectroscopy

Infrared absorption spectra of the mixtures of ionic liquid and water (1-butyl-3-methylimidazolium tetrafluoroborate, [BMIM]BF$_{4})$ with varying concentrations were obtained by Attenuated Total Reflection (ATR) method. Investigation of the spectra in the OH-stretch vibration range indicated the structural change of the water with the change in the concentration. At very low concentration of water, two peaks around 3600cm$^{-1}$ were assigned to the monomeric form of water molecules weakly hydrogen bonded to the BF$_{4}^{-}$ anions. With the increase in the water concentration, the broad feature at $\sim $3460cm$^{-1}$ corresponding to the bulk water took over the above monomeric peaks, which gradually redshifted with the increased water concentration. In the range from 2800 to 3200cm$^{-1}$ for the various CH-stretch vibration modes in the cation, the peaks in this ranged blueshifted with the increase in the water concentration. This blueshift was as much as $\sim $7cm$^{-1}$ for the CH$_{3}$ vibration modes of butyl chain while it hardly changed for the modes for the CH attached to the imidazolium core, suggesting varying degree of interactions between the carbon-bonded hydrogen and the water molecules.