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

Article pubs.acs.org/JPCB

Superpressing of a Room Temperature Ionic Liquid, 1‑Ethyl-3methylimidazolium Tetrafluoroborate Yukihiro Yoshimura,*,† Hiroshi Abe,‡ Takahiro Takekiyo,† Machiko Shigemi,† Nozomu Hamaya,§ Ryoichi Wada,⊥ and Minoru Kato⊥,¶ †

Department of Applied Chemistry and ‡Department of Materials Science and Engineering, National Defense Academy, 1-10-20 Hashirimizu, Yokosuka, Kanagawa 239-8686, Japan § Graduate School of Humanities and Sciences, Ochanomizu University, 1-1-2 Otsuka, Tokyo 112-8610, Japan ⊥ Graduate School of Science and Engineering and ¶Department of Pharmacy, College of Pharmaceutical Sciences, Ritsumeikan University, 1-1-1 Nojihigashi, Kusatsu, Shiga 525-8577, Japan 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]).

1. INTRODUCTION We can trick liquids into supercooling below their crystallization temperatures or superheating above the boiling points at ambient pressure. The behavior on metastable liquids is noteworthy, as it may open up the possibility of a new area in liquid chemistry and/or physics. The most particular feature of room temperature ionic liquids (RTILs) is “room temperature molten salts” consisting of only cations and anions.1 We can supercool RTILs below their melting points leading to a formation of the glasses. Then, it is intriguing to know the stabilities of RTILs under high pressure conditions as another extreme variable end. Recently, there have been gathering reports2−10 on the high pressure phase behavior of imidazolium-based RTILs, as typical RTILs, together with the studies by ourselves.4,5,7,9,10 The imidazoliumbased RTIL with PF6− anion easily crystallizes (at ∼0.1 GPa)2−4 rather than that with BF4− anion5,6 upon compression. Su et al.6 showed successive phase transitions in 1-butyl-3methylimidazolium tetrafluoroborate ([bmim][BF4]) up to 30 GPa through where the crystallization does not take place. The authors insisted that pressure-induced amorphization of [bmim][BF4] probably occurs at 21 GPa. Therefore, the high pressure phase transition of RTILs is dependent on the anionic property.7 On the other hand, the same authors8 reported that the phase transition behavior (from solid to liquid) of 1-ethyl-3methylimidazolium hexafluorophosphate ([emim][PF6]) under high pressures (∼1 GPa) is slightly different from that of 1butyl-3-methylimidazolium hexafluorophosphate ([bmim]© 2013 American Chemical Society

[PF6]) using a high-pressure differential thermal analysis (DTA) method. Then, we expected that the phase behavior of 1-ethyl-3-methylimidazolium tetrafluoroborate ([emim][BF4]) with shorter alkyl-chain length may be different from that of [bmim][BF4] under high pressure. The [emim][BF4] is known to show a conformational equilibrium between the planar and nonplanar conformers for the CNCC angle of the [emim]+ cation,11 whose optimized structures are shown in Figure 1. To further extend our preliminary studies9,10 and gain more information at much higher pressures, we have investigated the phase behavior of [emim][BF4] up to 7 GPa. Here, we present a topic about a superpressed metastable liquid under high pressure conditions; [emim][BF4] could not crystallize upon compression but formed a superpressed liquid. Remarkably, we found that crystallization of [emim][BF4] was induced by releasing pressure on the superpressed liquid, following that crystal polymorphism was observed in the processes. For a comparison, we provide results on the supercooled glassy state of [emim][BF4] at 77 K and 0.1 MPa.

2. EXPERIMENTS We used [emim][BF4] (Kanto Chemical Co., Cl− < 0.005%, Br− < 0.005%, Na+ < 0.002%, Li+ < 0.002%, H2O < 0.05%) as a Received: June 5, 2013 Revised: August 13, 2013 Published: September 10, 2013 12296

dx.doi.org/10.1021/jp4055507 | J. Phys. Chem. B 2013, 117, 12296−12302

The Journal of Physical Chemistry B

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Figure 1. Optimized structures of the planar (P) and nonplanar (Np) conformers of [emim]+ cation (calculated by B3LYP/6-311G+(d) level) with atom numbering scheme.

RTIL in this study. Since the as-received sample may contain a small amount of water, we checked the concentration to be less than 100 ppm using a Karl−Fischer titration method. We used Raman spectroscopy to explore phase changes and/ or bonding structures of liquids, because it provides information on the local structure in the liquid state. High pressure Raman spectra were typically measured at room temperature (298 K) by a JASCO NR-1800 Raman spectrophotometer equipped with a single monochromator and a charge-coupled device detector combined with a diamond anvil cell (DAC). The 514.5 nm line of argon ion laser excitation (∼ 350 mW) was typically used. X-ray diffraction measurements using a focused synchrotron beam were carried out at room temperature with the DAC at Photon Factory (BL-18C) in the High Energy Accelerator Research Organization, Tsukuba, Japan. The incident X-ray beam was collimated down to 100 μm in diameter. The incident wavelength was estimated to be 0.061991 nm calibrated with a CeO2 standard. An imaging plate system was selected to obtain two-dimensional Debye rings.12 To reduce the preferred orientation on the Debye rings, twodimensional data were reduced into one-dimensional diffraction patterns. The observed X-ray diffraction patterns were analyzed by FOX, which is characterized by ab initio crystal structure determinations.13 All the Raman and X-ray diffraction data were collected as the sample was compressed/decompressed in steps up to ∼7 GPa. In recording the data, compression and decompression rates were roughly 1 GPa/h and 0.5 GPa/h, respectively. The pressures were determined from the spectral shift of the R1 fluorescence line of the ruby ball in the sample chamber of the DAC.14,15 The sample preparations were done in a drybox to avoid atmospheric H2O and CO2.

Figure 2. Raman CH stretching spectra of [emim][BF4] as a function of pressure. The numbers represents the spectral assignments of the CH stretching modes of the alkyl-chain (C6, C7, and C8) and the imidazolium ring (C2, C4, C5): (1) CH ss (C8); (2) CH ss (C6, C7, C8); (3) CH ass (C7, C8); (4) CH ass (C8); and (5) CH ass (C6); (6) CH ass (C6); (7) new peak; (8) CH ass (C4, C5); (9) CH ss (C2); and (10) CH ss (C2, C4, C5), respectively. The p−T conditions were 0.1 MPa to 7 GPa and 298 K.

3050 cm−1 is the CH stretching (νCH) mode of the alkyl-chain and that from 3050 to 3200 cm−1 is the νCH of the imidazoliumring of [emim]+ cation. Remarkably, upon compression, the resolution of each peak becomes unclear and the spectra show a single broad bandlike feature. All the frequency changes upon compression in Figure 2 are displayed in Figure 4a. We determined the center positions of all the peaks in the spectra of Figure 2 by the second derivative analysis. The peak components in the νCH bands show a linear frequency shift against pressure without any discontinuous jump. However, around 2 GPa a small shoulder centered at ∼3050 cm−1 (unknown band) appears, whereas the bands due to C6−C8 and C7−C8 almost disappear above ∼5.5 GPa. These results suggest that the environment around the alkyl-chain of

3. RESULTS AND DISCUSSION 3.1. Superpression of [emim][BF4] at 298 K. Figure 2 shows Raman CH stretching spectra of [emim][BF4] as a function of pressure. Photomicrographs showing the transformations are shown in Figure 3. The observation of the C−H stretching vibration from RTILs can serve as a useful probe to reflect the structural change.16 The detailed vibrational assignments of the Raman CH stretching band of [emim][BF4] were well established by Heimer et al.17 The typical assignments are indicated as numbers (1−10, see the details in the caption) on the spectrum at 0.1 MPa. Using these assignments, we discuss the pressure-induced Raman spectral changes of [emim][BF4]. The frequency region from 2800 to 12297



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Figure 4. The Raman CH stretching-frequency shifts of [emim][BF4] as a function of pressure (a) compression, (b) decompression. The numbers 1−10 correspond to the peaks of the [emim]+ cation in Figure 2. (1●) CH ss (C8), (2○) CH ass (C6, C7, C8), (3▲) CH ass (C7, C8), (4△) CH ass (C8), and (5 ■) CH ass (C6), (6□) CH ass (C6), (7▼) new peak, (8 blue filled circle) CH ass (C4, C5), (9, blue open circle) CH ss (C2), and (10, blue filled triangle) CH ss (C2, C4, C5).

Figure 3. Optical images in the sequence of elevated (left line) and deelevated (right line) pressures. We note that apparently boundaries indicating crystal domains were not observed in the sample texture and the sample remained in the (probably metastable) liquid state across the pressure range from ambient to 7 GPa. In situ crystal growths (phase I and II) of [emim][BF4] in the DAC are conceivable upon decompression stages.

[emim][BF4] is largely perturbed rather than that around the imidazolium-ring upon compression. We think that a kind of structural change was locally starting around ∼2 GPa, which will be discussed below. A remarkable point is that a crystallization of this material did not occur in this investigation even in the 7 GPa range at room temperature which is consistent with the visual inspections in the sequence of elevated pressures as shown in Figure 3 (left). We can make sure of the phase of [emim][BF4] under the pressure of 7 GPa using the line-broadening method of the ruby R1 line by Piermarini et al.18 The line-broadening will occur, if nonhydrostatic stress states exist on the sample in DAC. That is, we can know the glass formation induced by pressure, particularly with respect to the variation of glass transition point as a function of pressure for measuring local stresses.18 The change in the full width at half-maximum (fwhm) of the R1 spectra relative to the 0.1 MPa line-width with pressure (●) is displayed in Figure 5. We can see that the increase of Δfwhm with increased pressure is larger above the initiation point, which is estimated to be around 2.8 GPa. The initiation point is interpreted as an approximate measurement of the glass transition pressure (pg).18 Therefore, a straightforward conclusion is that [emim][BF4] can be superpressed (over-

Figure 5. Pressure broadening of the sharp ruby R1 fluorescence line (full width at half-maximum, fwhm) relative to the 0.1 MPa line width, ●, compression, ○, decompression. The value of the Δfwhm (○) decreases with decreasing pressure down to around 3 GPa. Below 3 GPa, the Δfwhm shows zigzag behavior probably due to the crystallizations of the glassy solid with respect to the decreasing pressure process.

pressurized) into a metastable (glassy) state without crystallizing. More remarkably, if we further compress the superpressed liquid above pg, we observed another large change in the Δfwhm around 4−6 GPa region as shown in Figure 5. These may be explained by the following: homogenization of the glassy solid first occurred leading to minimize the Δfwhm 12298



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the phase I. This behavior is also shown to be caused by a kinetically driven phenomenon. Moreover, it is interesting to point out that [bmim][BF4], which has a different cation with longer alkyl chain from [emim][BF4], showed complete reversibility of the pressure effects if the pressure is released from 30 GPa down to 0.1 MPa where the decompression-induced crystallization did not occur.6 In the same way, N,N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium bis(trifluoromethylsulfonyl)imide, [DEME][TFSI] did not crystallize when the pressure was released from the superpressed state at 5.5 GPa down to 0.1 MPa.24 Thus, these phenomena are dependent on both cation and anion. Of course, before providing a comprehensive view, we need further experimental studies. Figure 6a,b reveal X-ray diffraction patterns of the decompression process. The calculated diffraction patterns are

values but the local stresses on the sample emerged again with respect to the increasing pressure processes. As a result, Δfwhm shows a kink around 4−6 GPa region. The results may indicate that [emim][BF4] takes another (densified) structure induced by higher pressure, in which the identification of the absolute nature of the possible phases (whether they are structurally and thermodynamically distinct phases) requires further continuous studies with, for example, pressure−volume equation of state. More or less, these observations remind us of structural changes in a well-documented case of a densification of silica glass in the literature:19,20 The densification of silica glass showing about 20% increase in density was observed when compressed beyond 25 GPa.19 The microscopic details and mechanism of the densification process are not fully understood, but using molecular dynamics simulation methods Trachenko and Dove20 proposed that if pressure causes an increase of the average coordination numbers, patches of the glass structure become locally unstable, with atomic relaxations occurring in the form of large-amplitude atomic displacements. These involve the breaking of original bonds and forming of new bonds.20 3.2. Decompression-Induced Crystal Polymorphism in [emim][BF4]. The surprise of the behavior is that, upon pressure decrease down to ca. 2 GPa, a phase transition of the metastable phase took place leading to a crystalline phase which, as it turns out, shows a completely different Raman spectrum as shown in Figure 2 (hereafter designated as phase II). More remarkably, upon further decrease in pressure, we observed another phase transition to crystal phase (designated as phase I) at ∼1 GPa. As a glass can be regarded as a frozen state of the liquid structure near its pg, we interpret as that [emim][BF4] went through two crystalline states and entered the superpressed liquid stated. Thus, the original crystalline states which might be thermodynamically more stable than the superpressed state around these pressure ranges appeared in the course of decompression processes. These observations suggest very similar energies for the phases making the respective changes kinetically driven. We suppose that the specific behavior in the phase transitions of [emim][BF4] observed is directly correlated with the conformational isomerism of [emim]+ cation, as will be discussed in section 3.3. The phenomenon of decompression-induced crystallization may be similar to a “cold crystallization”, which is observable in the relaxation from the glassy sample upon heating at 0.1 MPa.21,22 We can understand that the frozen state under low temperature is relaxed upon heating, thereby the crystalline phase which is energetically favorable at the corresponding temperatures would appear. As to other example of the decompression-induced crystallization on RTILs, we have recently found the case of an aliphatic quaternary ammonium-based ionic liquid in which the cation has a flexible methoxyethyl group on the nitrogen atom, N,N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium tetrafluoroborate (denoted as [DEME][BF4]).23 Additionally, we tried some other compression rates (0.5 GPa/h, 0.8 GPa/h, and 1.0 GPa/h) and decompression rates (0.5 GPa/h, 0.8 GPa/h, and 1.5 GPa/h). We found that a slower compression rate (0.5 GPa/h) seems to give a bit lower pg value (pg = ∼2.5 GPa). On the other hand, decompression of the crystal phase II normally brought the formation of phase I around 1 GPa. However, if we decrease the pressure quickly (>0.8 GPa/h), we have experienced that we could not obtain

Figure 6. X-ray diffraction patterns of the decompression-induced crystal polymorphism in [emim][BF4] at 298 K: (a) phase II at ∼1.2 GPa, (b) phase I at 0.6 GPa. The calculated results are represented by red curves. The calculated peak positions are in good agreement with the observed ones (blue color), though the peak intensities have minor discrepancies due to the preferred orientations on the Debye rings.

expressed by red curves in the figures. Crystallographic data analyzed using FOX are listed in Table 1. Both high pressure crystals were determined to be monoclinic lattices (Z = 8 for phase II and Z = 4 for phase I, where Z is the number of molecules per unit cell). It is interesting to refer that Matsumoto et al.25 reported a single crystal structure of frozen [emim][BF4] at ambient pressure determined by a low temperature X-ray diffraction method. [emim][BF4] crystallizes in the monoclinic space group P21/c with a = 8.653(5) Å, b = 9.285(18) Å, c = 13.217(7) Å, β = 121.358(15) Å, Z = 4 at 100 K. Thus the space group of phase I is the same as that of low temperature crystal at ambient pressure. Intriguingly, [emim][BF4] exhibits a unique structure wherein [emim]+ cations form one-dimensional pillars facing the imidazolium ring to the next ring linked by H(methylene)···π electron interactions. The BF4− anion also forms one-dimensional pillars along the same direction with the nearest F···F contact distance of 3.368(3) Å. This kind of structure contributes to the stability of crystal packing. 3.3. Conformational Isomerism of [emim]+ Cation at High Pressures. Next, we investigated the local structure of [emim]+ cation when [emim][BF4] falls into the superpressed state. Here, we focus on the CH2(N) bending band (νCH2(N)b) reflecting the change of the planar (P)−nonplanar (Np) 12299



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Table 1. Crystal Data of the High Pressure Phases along with the Low Temperature Phase25 at 0.1 MPaa crystal low-temperature phase phase II (1.2 GPa) phase I (0.6 GPa)

P21/c monoclinic P21/c

a (Å)

b (Å)

c (Å)

β

Z

ρ(g/cm3)

wR

R

8.653 8.238 8.849

9.285 11.316 9.063

13.217 18.883 13.334

121.358 100.052 123.129

4 8 4

1.450 1.518 1.468

0.1706 0.2438 0.1056

0.0577 0.2070 0.0948

a The weighted reliability and conventional factors are expressed as wR and R, respectively. Z is the number of molecules per unit cell. Unfortunately we could not determine the space group for the phase II probably due to the preferred orientation of the Debye rings, but considering the liquid density at 298 K and 0.1 MPa (ρ = 1.294 g/cm3), the crystal structures at high pressures seem to be well optimized. It is important to note that an entirely different high pressure phase (crystal polymorphism) appeared on decompression processes.

results are shown in Figure 8. The CNCC angle of [emim]+ cation prefers the Np conformer at 0.1 MPa. But the population

equilibrium for the CNCC angle of the ethyl-chain in the region from 400 to 450 cm−1. The conformational equilibrium is affected by environmental conditions such as temperature, pressure, and solvent, and closely correlates with the liquid structure.26 Figure 7 shows the representative Raman spectral changes as a function of pressure. The Raman bands at 430, and 448 cm−1 were assigned to the Np and P conformers, respectively.

Figure 8. Intensity fractions of the conformers of [emim][BF4] as a function of pressure (a) compression, (b) decompression. The open and closed circles represent the nonplanar and planar conformers of [emim]+ cation. The red open and closed circles represent the values for nonplanar and planar conformers of [emim]+ cation in the glassy state at 77 K and 0.1 MPa. Figure 7. Raman spectral changes of [emim][BF4] in the region from 300 to 500 cm−1.

of P conformer increases with increasing pressure and finally it becomes a major conformer above ∼5 GPa region (Figure 8a). Interestingly, we can see that the behavior of the intensity fraction against pressure is basically concordant with the ruby R1 fwhm change in Figure 5. On the other hand, on decompression, the intensity fraction of P conformer decreases, but at 2 GPa it suddenly increases again reaching to the value of 1.0 (i.e., takes all P conformer) where the transitions from superpressed liquid to crystal II and subsequently to crystal I phases occur (Figure 8b). Then, finally the intensity fractions revert back to the original values in the liquid state at 0.1 MPa. Though results on the correlation between the conformational isomerism and the metastable (glassy) state of aqueous salt solutions at 0.1 MPa were reported,31 we believe that the kind of present results concerning superpressed state under high pressure conditions might be one of the unique properties of

To show the detailed conformational change in the [emim]+ cation, we determined the intensity fractions ( f) of the conformers. The observed Raman band intensity (I) is proportional to the product of the Raman cross section (σ) and the concentration of the conformer (c) (I ∝ σc). Assuming that the ratio of the Raman scattering cross sections between the conformers for the same vibrational mode is independent of a pressure,27−30 the f value of each conformer is given by INp IP , fNp = fP = IP + INp IP + INp (1) where IP and INp indicate the relative Raman intensity of planar and nonplanar conformers of [emim]+ cation, respectively. The 12300



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completely different. However, whether the two glassy phases are thermodynamically distinct remains an open question for further experimental studies.

RTILs: The phase transition behavior of RTILs has a relationship with the conformational changes.22 As to the liquid state at ambient pressure, Kanzaki et al.32 reported the structure at 298 K by large angle X-ray scattering (LAXS) experiments. Higher probability of the anion was found nearby the ring proton such as the protons connected with C2, C4, and C5, while the lower probability distributes above and below the imidazolium plane toward the 1-ethyl and 3-methyl groups. Interestingly, as to a connection with the conformational equilibrium, the anion probability toward the direction facing the C2 proton on the imidazolium ring plane is very small for the nonplanar cation which is the dominant conformer in the liquid state, while significant anion probability toward the same direction can be found for the planar cation. As discussed in Figure 8, in the superpressed state, the planar conformer becomes predominant where the conformational stabilities of [emim]+ cation may be induced due to the void volume contribution,9 which relates to the molecular packing and represents the empty space of a hard sphere if we assume RTILs were treated as hard spheres. 3.4. Comparison with the Results at 77 K and 0.1 MPa. Finally, one might be interested in the phase behavior at lower temperatures at 0.1 MPa. [emim][BF4] eaily forms the glass at low temperatures upon cooling. Actually, in a separate measurement, we observed that the [emim][BF4] shows a glass transition at around ∼174 K (determined by a conventional, simple DTA measurements), then transforms to probably a metastable supercooled state (and crystallizes at cold crystallization temperature Tcc ≈ 197 K) upon heating. Finally the complete melting of the crystal to a normal liquid occurred at around 288 K. The results are good in accordance with the reported results.33 Comparison of the spectra between the low temperature glassy state (77 K, 0.1 MPa) and the superpressed state (298 K, 7.2 GPa) is shown in Figure 9. In the glassy state at 0.1 MPa, the population of the Np conformer is dominant (f Np = 0.83), whereas the superpressed glassy state at higher pressures prefers the P conformer. Thus, importantly the local structures of [emim]+ cation in the two glassy states are

4. SUMMARY We have investigated the phase behavior of [emim][BF4] under high pressure conditions by Raman and X-ray measurements. We have shown that [emim][BF4] can work as a superpressurized glass, in which the local structure is totally different from the quenched glass made by quick cooling at 0.1 MPa. We have provided spectral information that the superpressurization of [emim][BF4] occurs at much lower pressures than [bmim][BF4] reported at 21 GPa.6 Besides, in contrast to the results of [bmim][BF4] at high pressures, the unusual crystallization of [emim][BF4] could be induced by releasing the pressure on the superpressurized glass. We believe that the present study has revealed important findings in the physics of liquids and provided new insights into the phase transition behavior of RTILs, except for the liquid to solid or solid to solid transitions upon compression/cooling. We feel that these kinds of unveiled characters in liquid may facilitate the search for a new range of applications for RTILs.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +81-468-41-3810. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to Mr. N. Hatano for experimental supports. Part of this work was performed under the approval of the Photon Factory Program Advisory Committee (Proposal No. 2010G546).

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REFERENCES

(1) Welton, T. Room-Temperature Ionic Liquids. Solvent for Synthesis and Catalysis. Chem. Rev. 1999, 99, 2071−2084. (2) Su, L.; Li, M.; Zhu, X.; Wang, Z.; Chen, Z.; Li, F.; Zhou, Q.; Hong, S. In Situ Crystallization of Low-Melting Ionic Liquid [BMIM][PF6] under High Pressure up to 2 GPa. J. Phys. Chem. B 2010, 114, 5061−5065. (3) Russina, O.; Fazio, B.; Schmidt, C.; Triolo, A. Structural Organization and Phase Behavior of 1-Butyl-3-methylimidazolium Hexafluorophosphate: A High Pressure Raman Spectroscopy Study. Phys. Chem. Chem. Phys. 2011, 13, 12067−12074. (4) Takekiyo, T.; Imai, Y.; Hatano, N.; Abe, H.; Yoshimura, Y. Pressure-Induced Phase Transition of 1-Butyl-3-methylimidazolium Hexafluorophosphate [bmim][PF6]. High Press. Res. 2010, 31, 35−38. (5) Imai, Y.; Takekiyo, T.; Abe, H.; Yoshimura, Y. Pressure- and Temperature-Induced Raman Spectral Changes of 1-Butyl-3-methylimidazolium Tetrafluoroborate. High Press. Res. 2010, 31, 53−57. (6) Su, L.; Zhu, X.; Wang, Z.; Cheng, X.; Wang, Y.; Yuan, C.; Chen, Z.; Ma, C.; Li, F.; Zhou, Q.; Cui, Q. In Situ Observation of Multiple Phase Transitions in Low-Melting Ionic Liquid [BMIM][BF4] under High Pressure up to 30 GPa. J. Phys. Chem. B 2012, 116, 2216−2222. (7) Yoshimura, Y.; Takekiyo, T.; Imai, Y.; Abe, H. High Pressure Phase Behavior of Two Imidazolium-based Ionic Liquids, [bmim][BF4] and [bmim][PF6]. In Ionic Liquids: Classes and Properties; Handy, S. T., Ed.; InTech Publishing: Croatia, 2011; section 8, pp 187−202. (8) Su, L.; Li, L.; Hu, Y.; Yuan, C.; Shao, C.; Hong, S. Phase Transition of [Cn-min][PF6] under High Pressure up to 1.0 GPa. J. Chem. Phys. 2009, 130, 184503 (1−4).

Figure 9. Comparison of the Raman spectra of [emim][BF4] at various states. (a) the CH stretching vibrational region and (b) the CH2(N) bending band (νCH2(N)b) reflecting the change of the planar (P)−nonplanar (Np) equilibrium. Temperature-induced Raman spectral changes at 0.1 MPa were controlled with LINKAM THMS600 (Japan Hightech Co.). 12301



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

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