Low-temperature and high-pressure phase changes of room-temperature ionic liquids

Abstract The solid states of crystalline and amorphous phases and their mixtures in room-temperature ionic liquids (RTILs) were distinguished both at low-temperature (LT) and high-pressure (HP) by X-ray diffraction. Various solidifications of the RTILs were investigated using three cations. These were 1-butyl-3-methylimidazolium, N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium and N-2-methoxyethyl-N-methylpyrrolidinium. The common anions were nitrate, tetrafluoroborate, hexafluorophosphate, [PF6]−, trifluoromethanesulfonate, and bis(trifluoromethanesulfonyl)imide. The solid states of the RTILs depended on the cation-anion combinations. Crystal polymorph formation, cold crystallization on heating, and decompression crystallization on pressure reduction derived from competition among the molecular orientational order, degrees of freedom of cation conformers, and molecular packing efficiency in addition to the anion effect. In [PF6]−-based RTILs, complex phase behaviors were observed both at LT and HP.