Structural Transitions of 1-Butyl-3-methylimidazolium Chloride/Water Mixtures Studied by Raman and FTIR Spectroscopy and WAXS

Ionic liquid 1-butyl-3-methylimidazolium chloride (bmimCl) and its water mixtures were subjected to the temperature of −25 °C for various periods of time and then investigated by Raman and FTIR spectroscopy and WAXS. Neat bmimCl crystallized in a monoclinic lattice denoted M0. During a phase separation in samples with water concentrations of 0.44 and 2.90 wt %, a metastable monoclinic form denoted M was detected that was later converted into the monoclinic form M0 (0.44 wt %) or into an orthorhombic form (2.90 wt %). The form M existed only due to stresses imposed by hydrodynamic phenomena during the phase separation. According to Raman spectroscopy, the conformation of the butyl group in the bmim cation was identified as gauche–anti for the form M0 and as anti–anti for the orthorhombic form. Capillary forces and H-bonding affected the resultant conformation after the liquid–solid and solid–solid transitions. The ATR FTIR spectra in combination with DFT calculations unveiled further changes in the strengt...