Effects of water and methanol on the molecular organization of 1-butyl-3-methylimidazolium tetrafluoroborate as functions of pressure and concentration

The structural organization in mixtures of 1-butyl-3-methylimidazolium tetrafluoroborate $\left(\left[\mathrm{bmim}\right]\left[\mathrm{B}{\mathrm{F}}_4\right]\right)$ /water or methanol was studied by infrared spectroscopy. No drastic change in the concentration dependence of the alkyl C–H band frequency was observed at high concentration of the ionic liquid. This behavior indicates a clustering of the ionic liquid in alkyl regions. Nevertheless, the presence of methanol significantly perturbs the ionic liquid–ionic liquid associations in the imidazolium region. On the basis of the responses to change in pressure and concentration, two different types of O–H species, i.e., free O–H and bonded O–H, were observed in the O–H stretching region. For $\left[\mathrm{bmim}\right]\left[\mathrm{B}{\mathrm{F}}_4\right]$ /water mixtures, the compression leads to loss of the free O–H band intensity. It is likely that free O–H is switched to bonded O–H as high pressures are applied. For $\left[\mathrm{bmim}\right]\left[\mathrm{B}{\mathrm{F}}_4\right]$ /methanol mixtures, the free O–H is still stable under high pressures.