Phase behaviors of room temperature ionic liquid linked with cation conformational changes: 1-butyl-3-methylimidazolium hexafluorophosphate.

We have investigated phase-transition behaviors of a typical room temperature ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate ([C(4)mim]PF(6)), using calorimetric and Raman spectroscopic techniques. Although there was some confusion on its phase behaviors in previous reports, our measurements with a laboratory-made calorimeter at a slow scanning rate (5 mK/s) have definitely revealed that [C(4)mim]PF(6) has three crystalline phases. From the Raman spectroscopic study, the conformations of the butyl group for these crystalline phases are assigned to gauche-trans, trans-trans, and gauche'-trans conformations in lower-energy order. It has been also shown that these three conformers coexist in the liquid, supercooled liquid, and glass states. It is concluded that all of the phase transitions of [C(4)mim]PF(6) except the glass transition are associated with conformational changes of the butyl group.