Local structures of water in 1-butyl-3-methylimidazolium tetrafluoroborate probed by high-pressure infrared spectroscopy.

We have investigated the aggregation behaviors of water molecules in 1-butyl-3-methylimidazolium tetrafluoroborate/water mixtures using high-pressure methods. Under ambient pressure, the IR spectra indicate that two types of O-H species: free O-H and bonded O-H, existing in ionic liquid/water mixtures. As samples were compressed, a continuous loss of the free O-H band intensity was observed. This observation may have arisen from changes in the local structures of water molecules, and the geometrical properties of the hydrogen-bond network are likely to be perturbed as the pressure is elevated. A complementary insight of the O-H spectral features is obtained by measuring the concentration-dependent variation in the mid-infrared spectra under high pressure. A sharp O-H stretching band was observed in a diluted mixture corresponding to the high order in an ice VII-like structure. Nevertheless, a very broad O-H feature was measured in a concentrated mixture, which may be attributed to the presence of more than one stable cluster.