Here we review our recent works on thermal phase behaviors of ionic liquids, 1-alkyl-3-methylimidazolium hexafluorophosphates ([C
nmim]PF
6, 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 [C
4mim]PF
6, nevertheless, which was not always the case for others as evidenced in [C
1mim]PF
6 results.
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