Takahiro Takekiyo

Pressure-induced phase transition of 1-butyl-3-methylimidazolium hexafluorophosphate [bmim][PF6]

We have investigated the pressure-induced Raman spectral change of 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) using Raman spectroscopy. The relative Raman intensity at 590 cm−1 of the CH2 rocking band assigned to the gauche conformer of the NCCC dihedral angle of the butyl group in the [bmim]+ cation increases when the pressure-induced liquid-crystalline phase transition occurs, while that at 610 cm−1 assigned to the trans conformer decreases. Our results show that the high-pressure phase transition of [bmim][PF6] causes the increase of the gauche conformer of the [bmim]+ cation.

Pressure- and temperature-induced Raman spectral changes of 1-butyl-3-methylimidazolium tetrafluoroborate

In the present study, we have investigated the pressure- and temperature-induced Raman spectral changes of 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]). [bmim][BF4] did not crystallize at high pressures below 1.4 GPa. Upon compression, spectral changes in the band due to the CH stretching mode of the alkyl chain are larger than those due to the imidazolium ring, indicating that the environment around the alkyl chain is significantly perturbed. On the other hand, the Raman spectral change of [bmim][BF4] upon cooling at ambient pressure is very small and no crystallization occurs down to 113 K.

Stability of the Liquid State of Imidazolium-Based Ionic Liquids under High Pressure at Room Temperature

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.

Decompression-Induced Crystal Polymorphism in a Room-Temperature Ionic Liquid, N,N-Diethyl-N-methyl-N-(2-methoxyethyl) Ammonium Tetrafluoroborate

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.

Deuterated water effect in a room temperature ionic liquid: N,N-diethyl-N-methyl-N-2-methoxyethyl ammonium tetrafluoroborate.

A deuterated water effect in the room temperature ionic liquid (RTIL) N,N-diethyl-N-methyl-N-2-methoxyethyl ammonium tetrafluoroborate, [DEME][BF(4)], is observed in its crystal domain structures; crystallization temperature, T(c); crystal superstructures; and volume contractions. The above effect, seen in [DEME][BF(4)]-0.9 mol % H(2)O mixtures, is reduced in 1.0 mol % 0.75 H(2)O x 0.25 D(2)O and 0.9 mol % 0.5 H(2)O x 0.5 D(2)O mixtures and is completely suppressed in 1.3 mol % D(2)O mixtures. Interestingly, T(c) decreased systemically with D substitutions of water. In contrast to the crystal state, it was found that there is no difference between H(2)O and D(2)O mixtures in the liquid state, on the basis of X-ray diffraction patterns. At around 80-90 mol % H(2)O, the intermolecular correlation of [DEME][BF(4)] as a local structure changes to that of bulk water.

Ionic liquid-induced formation of the α-helical structure of β-lactoglobulin.

Structural modification of bovine milk β-lactoglobulin (β-LG) in aqueous 1-butyl-3-methylimidazolium nitrate ([bmim][NO3]) and ethylammonium nitrate ([EAN][NO3]) solutions has been investigated by Fourier transform infrared and circular dichroism spectroscopy. Remarkably, high ionic liquid (IL) concentrations (>15 mol %IL) caused formation of a non-native α-helical structure of β-LG and disruption of its tertiary structure. Furthermore, while [bmim][NO3] promoted protein aggregation, [EAN][NO3] inhibited it probably owing to differences in the unique solution structure (nanoheterogeneity) of the ILs by the different cationic species. The IL-induced α-helical formation of β-LG shows a behavior similar to the alcohol denaturation, but a disordered structure-rich state was observed in the β-α transition process by adding IL, in contrast to the case of an aqueous alcohol solution of protein. We propose that the molten salt-like property of aqueous IL solutions strongly support α-helical formation of proteins.

Superpressing of a room temperature ionic liquid, 1-ethyl-3-methylimidazolium tetrafluoroborate.

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]).

Pressure-induced Raman spectral changes of N,N,diethyl-N-methyl-N-(2-methoxyethyl) ammonium tetrafluoroborate

We have measured Raman spectral changes in one of the aliphatic series of ionic liquids, N,N,diethyl-N-methyl-N-(2-methoxyethyl) ammonium tetrafluoroborate, [DEME][BF4], under high pressure. Although the [DEME] cation has mainly two stable conformers of TG +−tg + t and TG +−g + g − t in the liquid state, the population of the minor conformer, i.e. TG +−g + g − t, decreases with increasing pressure. The difference in the partial molar volume between the two conformers was determined from the pressure dependence of the relative Raman intensity ratio.

Raman spectroscopic study on the hydration structure of tetraethylammonium chloride in water

The hydration structure of tetraalkylammonium chloride (Et4NCl) in aqueous solution has been studied by Raman Spectroscopy as functions of pressure and concentration. The hydration volume due to the hydrophobic hydration around trans-trans. trans-trans (tt.tt) conformer is larger than that around trans-gauche. trans-gauche (tg.tg) conformer. The hydration structures are closely correlated to the conformational equilibrium in the solution.

Anomalous Conformational Change in 1-Butyl-3-methylimidazolium Tetrafluoroborate–D2O Mixtures

We have investigated the effect of deuterated water on the conformational equilibrium between the gauche and trans conformers of the [bmim] cation in mixtures of water and 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF(4)]), an ionic liquid, at room temperature. A comparison of the results obtained from solutions made with H(2)O and with D(2)O highlights an anomalous conformational change in the D(2)O solution showing an extended N-shaped behavior. The gauche conformer of the [bmim] cation in D(2)O increased up to x = ~50 (D(2)O mol %); however, it decreased up to higher water concentrations of x = ~85 before again increasing drastically toward x = ~100. We provide spectroscopic evidence that the anomalous conformational dynamics of the [bmim] cation in D(2)O is directly related to the H/D exchange reaction of the C-H group at position 2 of the imidazolium ring.