Understanding Thermal Phase Behaviors of PF₆^－-Paired Imidazolium-Based Ionic Liquids at the Molecular Level

Abstract

Here we review our recent works on thermal phase behaviors of ionic liquids, 1-alkyl-3-methylimidazolium hexafluorophosphates ([C_nmim]PF₆, n=1~4). Their complex thermal phase behaviors observed in calorimetric measurements were investigated at the molecular level using combined techniques of Raman spectroscopy, quantum chemical calculations, X-ray analyses, and nuclear magnetic resonance spectroscopy. It was demonstrated that the conformational flexibility of the side chain in the cation played a key role for thermal phase behaviors of some ILs as represented by [C₄mim]PF₆, nevertheless, which was not always the case for others as evidenced in [C₁mim]PF₆ results.