To understand the stability of the liquid phase of ionic liquids under high pressure, we investigated the phase behavior of a series of 1-alkyl-3-methylimidazolium tetrafluoroborate ([Cnmim][BF4]) homologues with different alkyl chain lengths for 2 ≤ n ≤ 8 up to ∼7 GPa at room temperature. The ionic liquids exhibited complicated phase behavior, which was likely due to the conformational flexibility in the alkyl chain. The present results reveal that [Cnmim][BF4] falls into superpressed state around 2-3 GPa range upon compression with an implication of multiple phase or structural transitions to ∼7 GPa. Remarkably, a characteristic nanostructural organization in ionic liquids largely diminishes at the superpressed state. The behaviors of imidazolium-based ionic liquids can be classified into, at least, three patterns: (1) pressure-induced crystallization, (2) superpressurization upon compression, and (3) decompression-induced crystallization from the superpressurized glass. Interestingly, the high-pressure phase behavior was relevant to the glass transition behavior at low temperatures and ambient pressure. As n increases, the glass transition pressure (pg) decreases (from 2.8 GPa to ∼2 GPa), and the glass transition temperature increases. The results indicate that the p-T range of the liquid phase is regulated by the alkyl chain length of [Cnmim][BF4] homologues.

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We explore the phase behavior of room-temperature ionic liquids (RTILs) compressed under high pressure to determine whether they crystallize or hold a liquid state. RTILs have attractive supercooling properties compared with ordinary molecular liquids, which easily become a glassy state without crystallizing at ambient pressure. Thus, phase behavior under extreme stress, such as pressure, might yield interesting results. Here, we show that N,N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium tetrafluoroborate ([DEME][BF4]) could be crystallized upon compression, but it usually formed a superpressed liquid. Alternatively, unusual crystallization could be induced by releasing the pressure on the superpressed liquid. Notably, crystal polymorphism was observed in the decompression process. These facts along with visual observations indicate the possibility of [DEME][BF4] serving as a superpressurized glass. Our findings may facilitate the development of a new range of applications for RTILs that have undergone high-pressure recrystallization.

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]).