Crystallization of ionic liquid [EMIM][NO3] under extreme conditions

Phase behavior of the ionic liquid 1-Ethyl-3-Methylimidazolium Nitrate [EMIM][NO3] has been investigated by using differential scanning calorimetry (DSC) and Raman spectroscopy. From the DSC results, it can be confirmed that [EMIM][NO3] crystallizes at Tc = 251 K and melts at Tm = 289 K with the cooling or heating rate of 10 K/min. Raman results demonstrate that the cation [EMIM] has a nonplanar-planar equilibrium of the ethyl group in the liquid state, while its solid structure obtained by cooling is formed with planar conformation. The high-pressure Raman spectra reveals that [EMIM][NO3] undergoes liquid to solid I phase transition at about 0.9 GPa and then transforms into solid II at about 2.2 GPa upon compression. Notably, another new phase transition is observed during decompression process. The nonplanar conformer is predominant under high pressures. In addition, Raman spectra of [EMIM][NO3] of the different phases are compared in detail, and the results show that the structure of the solid state at low temperature is different from these of the high pressure states.