Pressure-Induced Solidification of 1-Butyl-3-methylimidazolium Tetrafluoroborate

We have used Raman spectroscopy to investigate the high-pressure phase behavior of 1-butyl-3-methylimidazolium tetrafluoroborate (bmim]BF₄]), a representative ionic liquid, at pressures up to ~7.5 GPa. We have also studied how increasing pressure leads to conformational changes in the bmim]⁺ cation. We have found that liquid bmim]BF₄] undergoes pressure-induced solidification (freezing) into a superpressed (metastable) state at 2.5 GPa; another structural change probably occurs at ~6 GPa. Remarkably, conformational changes in the bmim]⁺ cation between trans and gauche conformers are concordant with the metastable structural changes of bmim]BF₄]. As the pressure is increased from ambient, the fraction of gauche conformers increases, but the gauche fraction decreases above the solidification pressure (2.5 GPa), and slope of the gauche/trans ratio changes again above 6 GPa. We interpret these results in terms of the fragility of the ionic liquid.