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.

The direct evidence of confined water ("water pocket") inside hydrophilic room-temperature ionic liquids (RTILs) was obtained by complementary use of small-angle X-ray scattering and small-angle neutron scattering (SAXS and SANS). A large contrast in X-ray and neutron scattering cross-section of deuterons was used to distinguish the water pocket from the RTIL. In addition to nanoheterogeneity of pure RTILs, the water pocket formed in the water-rich region. Both water concentration and temperature dependence of the peaks in SANS profiles confirmed the existence of the hidden water pocket. The size of the water pocket was estimated to be ∼3 nm, and D2O aggregations were well-simulated on the basis of the observed SANS data.

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]).

We have found that the room-temperature ionic liquid (RTIL) reveals outstanding pressure-induced phase changes from a liquid state to a crystal polymorph and finally to a glass form upon compression by up to 8 GPa. The RTIL is 1-butyl-3-methylimidazolium hexafluorophosphate, [C4mim][PF6], which offers the opportunity to investigate a variety of fluctuations in one system and can be completely recovered without dissociation or polymerization, even after decompression. Similar to charge frustration, spin ice-like frustration, and geometric frustration in high potential spintronics/multiferroic materials, the RTIL frustrations are classified into charge (scalar), orientation (vector), and coordination number (topology). Degrees of freedom at each state of [C4mim][PF6] are described by charge balancing, molecular orientational order/disorder, molecular conformations of the C4mim(+) cation, and the coordination number. Here, we show a novel "conformation glass" induced by high pressure.

The conformational stability of 1-alkyl-3-methylimidazolium tetrafluoroborate ([Cmim][BF], n = 3-8) under high pressure was investigated using Raman spectroscopy to reveal the preferential role of the alkyl-chain length (n) in high-pressure glass transition. To evaluate this, we determined the intensity ratio (r) and differences in the partial molar volume (ΔV) between the whole trans and gauche conformers of the [Cmim] cation using Raman intensities. Interestingly, both values were classified into a two alkyl-chain length region at the border of n = 5. The coulombic interaction (cation-anion interaction) for the conformational stability is the predominant factor below n = 5 (the cation-head portion: alkyl carbon number C < 5), and the alkyl-chain packing effect (cation-cation interaction) is the predominant factor above n = 5 (the cation-tail portion: C > 5). In combination with the conformational preference of the [Cmim] cation under a high-pressure glassy state, the alkyl chain displays a preferential role, i.e., an increase in the gauche conformer of [Cmim][BF] adjusts to avoid crystallization (the conformational adjustment effect). In the presence of the coulombic interaction, the preferential role of the flexible alkyl chain is an important key to elucidate the mechanism of the complicated high-pressure phase transition behavior of ionic liquids.