Superpressing of a Room Temperature Ionic Liquid, 1-Ethyl-3-methylimidazolium Tetrafluoroborate

Abstract

We have investigated the phase behavior of 1-ethyl-3-methylimidazolium tetrafluoroborate ([emim][BF4]) at 298 K under high pressure conditions. We found that [emim][BF4] can be superpressed without crystallization up to ∼7 GPa. We propose that [emim][BF4] behaves as a superpressurized glass above 2.8 GPa. In view of the results, the environment around the alkyl-chain (C6 and C7-C8) of [emim][BF4] is largely perturbed rather than that around the imidazolium-ring in the superpressed state. We also discussed the results in view of the conformational isomerism of [emim](+) cation. Remarkably, as an alternative to pressure-induced crystallization, we have found that such a metastable liquid shows crystal polymorphism around 2.0 and 1.0 GPa upon decompression. The behavior is in contrast with the earlier results of 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]).

High-pressure phase behavior of the room temperature ionic liquid 1-ethyl-3-methylimidazolium nitrate

“…As shown in Fig. 4a [9], which comprises a different anion, did not crystallize but adopted a superpressurized state upon compression. In addition, its estimated (glass) transition pressure (p g ) amounted to approximately 2.…”

“…Therefore, investigating their stabilities under high pressure as another extreme variable is intriguing. Previous studies [9] showed that the common RTIL, 1-ethyl-3-methylimidazolium tetrafluoroborate ([emim] [BF 4 ]), could be superpressurized without crystallization at elevated pressures reaching 8.6 GPa and behaved as a superpressurized glass above 2.8 GPa. This suggests that RTILs are useful materials for the study of glass transition [10].…”

“…So far, few studies have been conducted to understand the behavior of [emim]-based RTILs at high pressure [9,[11][12][13][14]. For example, Su et al [11] investigated the solid to liquid phase transition of 1-ethyl-3-methylimidazolium hexafluorophosphate ([emim][PF 6 ]) up to~1 GPa by high pressure differential thermal analysis and found that another crystalline phase exists between solid and liquid [emim][PF 6 ].…”

Low temperature and high pressure crystals of room temperature ionic liquid:N,N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium tetrafluoroborate

“…Upon compression process at room temperature, no crystallization occurred in pure decompression process, subsequent crystallizations were observed in the RTILs [10,11]. The nonequilibrium freezing upon compression might be relating to the conformational-driven free energy landscape [16].…”

“…A significant point is that monoclinic a-axis corresponds to the diagonal length on a-b plane of orthorhombic lattice without the lattice mismatch. Under high pressure and at room temperature, superpressurized phenomenon was observed by highpressure X-ray diffraction [10,11]. On compression process up to 7.6 GPa, there was no sharp Bragg reflections at all in [DEME] [BF 4 ].…”