Kinetic Effect on Pressure-Induced Phase Transitions of Room

Oct 14, 2015 - Room temperature ionic liquids (RTILs) have intriguing high-pressure phase behavior, and investigation of how pressure affects phase tr...
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Kinetic Effect on Pressure-Induced Phase Transitions of Room Temperature Ionic Liquid, 1‑Ethyl-3-methylimidazolium Trifluoromethanesulfonate Haining Li,†,‡ Zheng Wang,†,‡ Liucheng Chen,‡ Jie Wu,‡ Haijun Huang,*,† Kun Yang,‡ Yongqiang Wang,‡ Lei Su,*,‡,§ and Guoqiang Yang*,§ †

School of Sciences, Wuhan University of Technology, Wuhan, Hubei 430070, China Center for High Pressure Science and Technology Research, Zhengzhou University of Light Industry, Zhengzhou, 450002, China § Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100080, China ‡

ABSTRACT: Room temperature ionic liquids (RTILs) have intriguing high-pressure phase behavior, and investigation of how pressure affects phase transitions of RTILs might yield interesting results. We here present kinetically driven phase transitions of 1-ethyl-3-methylimidazolium trifluoromethanesulfonate ([Emim][CF3SO3]) at different rates of ∼0.3 and ∼1.2 GPa/h up to ∼5 GPa. Two crystalline phases formed at ∼1.3 and ∼1.7 GPa with increasing pressure at lower compression rate of ∼0.3 GPa/h; however, the amorphous phase solidified with superpressurized glass above ∼3.3 GPa at higher compression rate of ∼1.2 GPa/h. Notably, crystal polymorphism is discussed in view of the conformational isomerism of [Emim]+ cation and an unknown cation conformer is observed. These facts indicate that kinetic effect on pressureinduced phase transitions of [Emim][CF3SO3] might be dependent on compression rate, which needs to be considered as a nonnegligible factor for phase transitions of RTILs under high pressure. induced by releasing the pressure.10 In addition, decompressioninduced crystallization was also observed in 1-ethyl-3-methylimidazolium tetrafluoroborate ([Emim][BF4])11 and 1-octyl-3methylimidazolium hexafluorophosphate ([Omim][PF6]).12 Under high pressure, why is crystallization easy to occur for some RTILs, but generally hampered for some RTILs? Why is crystallization induced upon decompression? These phenomena are definitely related with their inherent liquid structure of each specific cation−anion combination. Moreover, kinetic effects might also play an important role. To gain a deeper understanding of how pressure affects the phase behavior of an RTIL, in this study, 1-ethyl-3-methylimidazolium trifluoromethanesulfonate ([Emim][CF3SO3]) was selected as an object. The unusual phase behaviors of [Emim][CF3SO3] have been investigated at different compression rate up to ∼5 GPa using in situ Raman spectroscopy. Furthermore, the conformational changes of [Emim]+ have been studied in relationship with crystal polymorphism, and an unknown cation conformer was observed under high pressure.

1. INTRODUCTION Room temperature ionic liquids (RTILs) have become an important class of solvents and soft materials over the past decades because of their unique properties, including nonvolatility, nonflammability, extraordinarily high chemical and thermal stability, good conductivity, etc.1 Being considered as “green solvents”,2 RTILs have great potential in academic research and industrial applications for increasing social pressure for new green technologies. Recently, pressure-induced phase transitions of RTILs have also led to growing interest in order to exploit their possible applications under extreme conditions. Some RTILs crystallize readily under high pressure. High pressure-induced crystallization, which might be a practical method of purifying and recycling RTILs,3 can be achieved through different courses. 1-Butyl-3-methylimidazolium hexafluorophosphate ([Bmim][PF6]) easily crystallized upon compression at room temperature.3 Furthermore, pressureinduced crystal polymorphism of [Bmim][PF6] was also identified, which was associated with conformational changes of the butyl side chain.4−6 However, many RTILs show a strong tendency to superpress and often solidify in the form of glasses under high pressure.7−9 It is interesting that unusual crystallization could be induced by releasing the pressure for some RTILs.10−12 N,N-Diethyl-N-methyl-N-(2-methoxyethyl)ammonium tetrafluoroborate ([DEME][BF4]) served as a superpressurized glass above ∼3.3 GPa, and unusual crystallization could be © 2015 American Chemical Society

2. EXPERIMENTAL SECTION [Emim][CF3SO3] was supplied by Lanzhou Institute of Chemical Physics, Chinese Academy of Science, whose purity Received: August 28, 2015 Published: October 14, 2015 14245

DOI: 10.1021/acs.jpcb.5b08384 J. Phys. Chem. B 2015, 119, 14245−14251

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The Journal of Physical Chemistry B

Figure 1. Raman spectra of [Emim][CF3SO3] at lower compression rate of roughly 0.3 GPa/h in (a) ν(CS) and ρ(SO3), (b) δs(CF3), (c) νs(SO3), and (d) ν(CH) of [Emim][CF3SO3] at room temperature (297 K). Right: Photographs of the sample under different pressures.

3. RESULTS AND DISCUSSION Raman spectral changes of [Emim][CF 3 SO 3 ] at lower compression rate of roughly 0.3 GPa/h are shown in Figure 1. Four spectral ranges were selected as representative Raman peaks of the cation and anion. According to the previous study,15,16 the Raman peaks at 313, 347, 756, and 1033 cm−1 are assigned to CS stretching of the [CF3SO3]− anion, ν(CS), rocking of the SO3, ρ(SO3), symmetric deformation of CF3, δs(CF3), and symmetric stretching of SO3, νs(SO3), respectively. And the Raman band at 1026 cm−1, which is the shoulder of 1033 cm−1, might originate from ring in-plane symmetric stretching of [Emim]+.17 The Raman signals ranging from 2800 to 3200 cm−1 are CH stretching of [Emim]+, ν(CH). A remarkable point was that the spectral profiles underwent dramatic changes above ∼1.3 GPa. The Raman peaks of ν(CS), ρ(SO3), and δs(CF3) became sharper. The peak of νs(SO3) and the original shoulder, which already merged into one peak, split into two sharp peaks again. Notably, Raman spectra of ν(CH) changed dramatically and some new peaks appeared. It could be predicted that [Emim][CF3SO3] underwent a phase transition leading to a crystalline phase (hereafter, designated as phase I) at ∼1.3 GPa. More remarkably, upon a further compression up to ∼1.7 GPa, another phase transition of phase I took place leading to a crystalline phase (designated as phase II) which, as it turned out, showed a completely different Raman spectrum. The peak intensity of νs(SO3) became weak evolving into a shoulder, and a new peak at 1020 cm−1 appeared. Meanwhile, ν(CH) exhibiting a completely different Raman spectrum from phase I was accompanied by the appearance of new peaks and disappearance of old peaks. In situ microscopic observation also provided circumstantial evidence on these successive phase transitions, which was also shown in Figure 1 (right). The sample changed from transparent liquid into a translucent phase I at ∼1.3 GPa, then to phase II at ∼1.7 GPa.

was more than 99.5 wt %. Before all the measurements, the sample was kept under vacuum at a moderate temperature (353 K) for at least 3 days to reduce the moisture content and volatile compounds to negligible values. Its molecular weight is 260.23 g/mol, and melting point is 264 K.13 A diamond anvil cell (DAC), with a diamond culet size of 500 μm, was used for generating pressures up to ∼5 GPa. The sample was contained in a 200 μm diameter hole in a T301 gasket which was preindented to a thickness of about 100 μm and clamped between two the diamond anvils. Two type Ia diamonds with low fluorescence were used for Raman measurements. Pressures were calculated from the shift of the ruby R1 fluorescence line.14 Raman experiments were carried out using a Renishaw inVia Raman microscope (Renishaw, United Kingdom) with 532 nm wavelength excitation. Raman spectra were collected in a backscattering geometry with a 2400 grooves/mm grating, and the slit width was selected as 65 μm corresponding to a resolution of ∼0.5 cm−1. The sample image could be collected through an achromatic lens and then focused onto a CCD detector for visual monitoring during experiments. The obtained Raman spectra were fitted with Gaussian−Lorentzian mixing function using the WIRE 3.3 software (Renishaw, United Kingdom) for analyzing the spectral data. All the Raman measurements were conducted as the sample was compressed or decompressed in steps at room temperature (297 K). The samples were held under each pressure for about 10 min until the equilibrium was established. In recording the data, pressure was increased at intervals of about 30 min, including duration times. Two compression processes with different compression rates were designed. One is compression at lower compression rate of roughly 0.3 GPa/h, and the other is compression at relative higher compression rate of roughly 1.2 GPa/h. 14246

DOI: 10.1021/acs.jpcb.5b08384 J. Phys. Chem. B 2015, 119, 14245−14251

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Figure 2. Changes in (a) Raman shift and (b) fwhm of ν(CS), ρ(SO3), δs(CF3), ring in-plane symmetric stretching of [EMIM]+, and νs(SO3) as a function of pressure at lower compression rate of roughly 0.3 GPa/h.

cation, especially around the alkyl chain of [Emim]+ cation, was more greatly perturbed than that around the anion at higher compression rate due to high structural flexibility of the cations. Figure 4a and Figure 4b showed changes in Raman shift and fwhm of representative Raman peaks as a function of pressure at higher compression rate of roughly 1.2 GPa/h. With increasing pressure, a slight change in slope occurred in Raman shifts and fwhm as a function of pressure at ∼3.3 GPa, which indicated that [Emim][CF3SO3] might experience a phase transition around this pressure at higher compression rate. Unlike compression at lower rate, [Emim][CF3SO3] did not crystallize at higher compression rate, which was consistent with visual inspections of no boundaries indicating crystal domains, shown in Figure 3 (right). To confirm the phase of [Emim][CF3SO3] at higher compression rate up to ∼5 GPa, the linebroadening method of the ruby R1 line by Piermarini et al. was adopted.18 The initiation point of ruby R1 broadening has been used as an approximate measurement of the glass transition pressure (pg) for some molecular liquids18 and RTILs.7−9 In the same manner, the change in fwhm of the R1 line relative to the 0.1 MPa line-width at higher compression rate of roughly 1.2 GPa/h was displayed in Figure 5. The initiation point of ruby R1 abruptly broadening was estimated at ∼3.3 GPa. Thus, it could be speculated that [Emim][CF3SO3] might be superpressed into a glassy state without crystallizing above ∼3.3 GPa. In this context, we should refer to the recent studies by Chang et al.19 who similarly investigated the phase transitions of pure [Emim][CF3SO3] and [Emim][CF3SO3]/nanogold mixture by infrared spectroscopy under high pressure. They reported that the spectral change of the imidazolium C−H absorption indicated a pressure-induced phase transition at ∼0.4 GPa for both pure

Raman shift and full width at half-maximum (fwhm) under different pressures upon compression at rate of roughly 0.3 GPa/h are shown in Figure 2a and Figure 2b. Two discontinuities in Raman shifts and fwhm as a function of pressure were observed at ∼1.3 and ∼1.7 GPa, which was coincident with Raman spectral changes and microscopic observation in Figure 1. These discontinuities might arise from pressure-induced phase transitions along with corresponding structural reorganizations. Therefore, it could be concluded that [Emim][CF3SO3] underwent two successive phase transitions at ∼1.3 and ∼1.7 GPa, and crystal polymorphism was observed at lower compression rate of roughly 0.3 GPa/h up to ∼5 GPa. On the other hand, decompression rate was difficult to control compared with compression rate. As shown in Figure 1, [Emim][CF3SO3] exhibited phase II by releasing pressure until ∼0.7 GPa with decompression rate of roughly 1 GPa/h. Raman spectra of the retrieved sample at atmospheric pressure were almost the same as the initial spectrum under ambient pressure, which meant that the whole process was reversible. Raman spectra of [Emim][CF3SO3] were investigated at higher compression rate of roughly 1.2 GPa/h, which are shown in Figure 3. Remarkably, at higher compression rate, the main characteristic peaks of the bands exhibited a blue shift accompanied by a broadening of the band, and no new peaks appeared. The peak of νs(SO3) and the original shoulder merged into one peak. Upon compression, Raman spectral changes of the ν(CH) of the cation were larger than those of the anion, and Raman spectral changes of the ν(CH) of the alkyl chain (2800− 3050 cm−1) were also larger than those of the ν(CH) of imidazolium ring (3050−3200 cm−1) for the [Emim]+ cation. These results might indicate that the environment around the 14247

DOI: 10.1021/acs.jpcb.5b08384 J. Phys. Chem. B 2015, 119, 14245−14251

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Figure 3. Raman spectra of [Emim][CF3SO3] at higher compression rate of roughly 1.2 GPa/h in (a) ν(CS) and ρ(SO3), (b) δs(CF3), (c) νs(SO3), and (d) ν(CH) of [Emim][CF3SO3] at room temperature (297 K). Right: Photographs of the sample under different pressures.

Figure 4. Changes in (a) Raman shift and (b) fwhm of ν(CS), ρ(SO3), δs(CF3), ring in-plane symmetric stretching of [EMIM]+, and νs(SO3) as a function of pressure at higher compression rate of roughly 1.2 GPa/h.

pressure infrared measurements to reduce the absorbance of the samples) or nanoparticles might facilitate the crystallization. A major surprise was that Raman spectra of [Emim][CF3SO3] changed dramatically upon a pressure decrease down to ∼0.4 GPa

[Emim][CF3SO3] and [Emim][CF3SO3]/nanogold mixture. These previous results might be considered as low compression rate, and the reason why the phase transition pressure is lower than our results might be that the CaF2 crystals (used in high 14248

DOI: 10.1021/acs.jpcb.5b08384 J. Phys. Chem. B 2015, 119, 14245−14251

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kinds of materials (such as elemental sulfur23 and isotactic polypropylene24) using a pressure-jump apparatus, which could generate compression rate between conventionally static and dynamic compression rate scales. Although DAC is generally high pressure apparatus for generating static compression, our study suggests that compression rate also plays an important role in phase solidification of [Emim][CF3SO3] in DAC, the crystalline phase formed at lower compression rate, and the amorphous phase solidified at higher compression rate. As to another example of the kinetically driven crystallization of RTILs, Yoshimura et al. found that [DEME][BF4] occasionally crystallized upon compression, but it usually formed a superpressed liquid.10 Notably, crystal polymorphism was observed in the decompression process. The authors proposed that compression rate, sample volume in the DAC, and subsequent cooperatively dynamics leading to the formation of the reaction seeds for the phases might play a fundamental role. Further, they investigated the decompression-induced crystallization on [Emim][BF4] and the effect of compression rate on pg.11 These results along with this study may indicate that compression rate is necessary to be considered for pressureinduced phase transitions of RTILs, which usually can be ignored for high pressure experiment using DAC. This phenomenon might be correlated with the unique and characteristic structure of RTILs. Upon compression, larger and less symmetric ions of RTILs require more time for crystal packing than some organic solvents with small molecular weight. Thus, pressure-induced phase transitions of RTILs might be sensitive to compression rate using DAC apparatus. Furthermore, the specific crystal polymorphism of [Emim][CF3SO3] at lower and higher compression rates is also discussed in relationship with the conformational isomerism of [Emim]+ cation. According to previous studies,25 [Emim]+ has two stable conformers for the CH2(N) bending, the planar (trans) form and the nonplanar (gauche) form, due to the rotational isomerism of the ethyl group. The Raman spectral changes ranging from 370 to 500 cm−1 with different compression rate are focused on to investigate the conformation equilibrium for the CNCC angle of the ethyl chain, which is shown in Figure 6a and Figure 6b. In this spectral region, Raman bands at 387, 430, and 448 cm−1 of [Emim]+ are assigned to the nonplanar, nonplanar, and planar conformers, respectively. It can be seen that two conformers coexist in the liquid state under ambient temperature and pressure. At lower compression rate, it can be seen that Raman band only exists at 448 cm−1 but no Raman band at 387 and 430 cm−1 at ∼1.3 GPa, which means phase I shows a planar conformer. Upon subsequent compression, two new peaks at 400 and 420 cm−1 appear which originate neither from planar conformers nor from nonplanar conformers. Thus, it can be speculated that phase II exhibits an unknown conformer of [Emim]+, which has not been reported. Quantum chemical calculations show that there are two stable conformations for [Emim]+, namely, planar and nonplanar. This unknown conformer has similar spectral characteristic of doublet to nonplanar conformer. Therefore, it can be predicted that phase II corresponds to distorted crystal containing the perturbed nonplanar conformer. The packing is believed to be due to cation−anion Coulombic attraction and weak C−H···F interactions for [Emim][PF6]26 or [Emim][BF4].27 Unlike these two [Emim]+ salts, the ions of [Emim][CF3SO3] may be packed in the crystal lattice via weak interionic C−H···O hydrogen bonds as well as a significant F···F interaction.28 C−H···O hydrogen bonds and short F···F contacts might coexist, and both influence

Figure 5. Pressure broadening of the sharp ruby R1 fluorescence line relative to the 0.1 MPa line-width at higher compression rate of roughly 1.2 GPa/h.

with decompression rate of roughly 1 GPa/h, as shown in Figure 3, which is totally different from that of phase I and phase II. The Raman peaks became sharper, and some new peaks appeared. Meanwhile, the sample turned into translucent from transparency. These results indicated that a phase transition took place leading to another crystalline phase (designated as phase III) around 0.4 GPa by releasing pressure of superpressurized glass. For pressure-induced crystallization and phase transitions, Fanetti et al.20 found that pyridine could either crystallize in phase II or give rise to a glass, and phase I was never obtained by compression of the fluid but only decompression of the sample brought to the formation of phase I. Fanetti et al. thought that the missed crystallization of phase I upon compression was more likely related to the kinetic effect. In this study, [Emim][CF3SO3] could crystallize in phase I and phase II successively at lower compression rate, while [Emim][CF3SO3] served as a superpressurized glass at higher compression rate and crystallization of phase III was induced by releasing the pressure of the glassy phase. These results suggested that this phenomenon was also induced by a kinetic effect, which was similar to Fanetti’s result. Furthermore, there were similar energies for the four phases and high energy barriers among them making the transformations kinetically driven. Compression rate plays a fundamental role in kinetic effect on pressure-induced crystallization and phase transitions of [Emim][CF3SO3]. For solidification of melts, as we know, the phase transitions from melt to crystal or from melt to metastable phase (e.g., amorphous) are dependent on cooling rates. For RTILs, 1-butyl-3-methylimidazolium chloride ([Bmim]Cl) shows crystal polymorphism through a different cooling process.21,22 By cooling liquid [Bmim]Cl down to 18 °C and keeping it for 48 h, two different types of crystals, crystal 1 and crystal 2, were obtained. Upon leaving crystal 2 for more than 24 h at dry ice temperature, crystal 2 was converted to crystal 1.22 That is to say, different crystal polymorphs of RTILs might be obtained by two different cooling rates. In fact, compression is equivalent to cooling in the way of melt solidification, which can also overcome the limit of thermal conductivity. Recently, Hong et al. found that rapid compression could effectively solidify metastable structures from melt and slow compression always induced stable crystalline phases for many 14249

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Figure 6. Raman spectral changes of [Emim][CF3SO3] in the region from 370 to 500 cm−1: (a) compression at rate of roughly 0.3 GPa/h; (b) compression at rate of roughly 1.2 GPa/h; (c) intensity fractions of the planar conformers of [Emim][CF3SO3] as a function of pressure. The triangles and circles represent the planar conformers at lower compression rate and higher compression rate, respectively.

equilibrium closely correlates with the liquid structure. Once superpressurized glass forms at higher compression rate, the disorder of liquid structure and conformational equilibrium near its pg might be both frozen in at high pressures. On the other hand, the intensity fractions of the planar conformers are almost the same below ∼1.3 GPa at two different compression rates. That is, the local structure of [Emim]+ cation is similar before the transition from superpressed liquid to phase I occurs. The liquid structure of RTILs resulted from a balance between long-range electrostatic force and geometric factor. Various studies had been made to elucidate the role of weak interactions, such as hydrogen bond, π−π stacking, van der Waals forces, and electrostatic forces in RTILs.29 Compression of RTILs not only reduced the ion−ion distances but also might change the weak interactions among them. At lower compression rate, more time is available to sample configuration, which might give rise to different weak interactions from that at higher compression rate. Different compression rates might lead to different weak interactions, which might be an important factor influencing the tendency for crystallization. Therefore, it could be concluded that compression rate might make great contribution to kinetically driven phase transitions. In summary, kinetic effects on pressure-induced phase transitions of [Emim][CF3SO3] have been investigated with different compression rates up to ∼5 GPa by Raman measurements. Two successive phase transitions took place at ∼1.3 and ∼1.7 GPa, and crystal polymorphism was observed at lower compression rate of roughly 0.3 GPa/h. And [Emim][CF3SO3] behaved as a superpressurized glass above ∼3.3 GPa at higher compression rate of roughly 1.2 GPa/h, which showed decompression-induced crystallization at ∼0.4 GPa. Meanwhile, the conformational changes of [Emim]+ have been studied in relationship with crystal polymorphism in detail, and an

crystal packing, which might induce this new conformer. Future high pressure synchrotron X-ray diffraction experiment and detailed MD simulation might confirm this proposal and provide a description of the detailed organization of crystal structure and corresponding conformational structure under high pressure. Additionally, at higher compression rate, two conformers are present in equilibrium up to ∼5 GPa, although the sample solidifies as a superpressurized glass above ∼3.3 GPa. Intriguingly, Raman band 387 and 430 cm−1 appeared abruptly in the decompression-induced crystalline phase, which implied that phase III presented a nonplanar conformer. To neglect the interference of Raman peaks of anion, 430 cm−1 as signatures of the nonplanar conformer and 448 cm−1 as signatures of the planar conformer were used to analyze the conformational change in this anion. The intensity fractions ( f) of the planar conformers were determined by fplanar =

Iplanar Iplanar + Inonplaner

(1)

where Iplanar and Inonplanar indicated Raman peak area of planar and nonplanar conformers of [Emim]+, respectively. As was shown in Figure 6c, the fraction of planar conformers increased at higher compression rate, which meant the planar conform was preferred with higher compression rate. It is interesting to refer that the superpressed glassy state of [Emim][BF4] at higher pressures also preferred the planar conformer.11 Thus, the local structure of [Emim]+ cation has a similar conformational preference for glassy state under high pressure. Besides, at higher compression rate, the tendency of f planar against pressure is basically concordant with Raman shifts and fwhm of some characteristic bands and the ruby R1 fwhm change, showing inflection point at ∼3.3 GPa (pg). Moreover, the increase of f planar against pressure became slow above pg, which implies the conformational 14250

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(12) Li, J.; Su, L.; Zhu, X.; Li, H.; Cheng, X.; Li, L. DecompressionInduced Disorder to Order Phase Transition in Low-Melting Ionic Liquid [OMIM][PF6]. Chin. Sci. Bull. 2014, 59, 2980−2986. (13) Bennett, M. D.; Leo, D. J.; Wilkes, G. L.; Beyer, F. L.; Pechar, T. W. A Model of Charge Transport and Electromechanical Transduction in Ionic Liquid-Swollen Nafion Membranes. Polymer 2006, 47, 6782− 6796. (14) Mao, H. K.; Bell, P. M.; Shaner, J. W.; Steinberg, D. Specific Volume Measurements of Cu, Mo, Pd, and Ag and Calibration of the Ruby R1 Fluorescence Pressure Gauge from 0.06 to 1 Mbar. J. Appl. Phys. 1978, 49, 3276−3283. (15) Burba, C. M.; Rocher, N. M.; Frech, R.; Powell, D. R. CationAnion Interactions in 1-Ethyl-3-Methylimidazolium Trifluoromethanesulfonate-Based Ionic Liquid Electrolytes. J. Phys. Chem. B 2008, 112, 2991−2995. (16) Huang, W.; Wheeler, R. A.; Frech, R. Vibrational Spectroscopic and ab initio Molecular Orbital Studies of the Normal and 13C-labelled trifluoromethanesulfonate anion. Spectrochim. Acta, Part A 1994, 50, 985−996. (17) Heimer, N. E.; Del Sesto, R. E.; Meng, Z.; Wilkes, J. S.; Carper, W. R. Vibrational Spectra of Imidazolium Tetrafluoroborate Ionic Liquids. J. Mol. Liq. 2006, 124, 84−95. (18) Piermarini, G. J.; Block, S.; Barnett, J. D. Hydrostatic Limits in Liquids and Solids to 100 kbar. J. Appl. Phys. 1973, 44, 5377−5382. (19) Chang, H. C.; Hung, T. C.; Wang, H. S.; Chen, T. Y. Local Structures of Ionic Liquids in the Presence of Gold under High Pressures. AIP Adv. 2013, 3, 032147. (20) Fanetti, S.; Citroni, M.; Bini, R. Structure and Reactivity of Pyridine Crystal under Pressure. J. Chem. Phys. 2011, 134, 204504. (21) Holbrey, J. D.; Reichert, W. M.; Nieuwenhuyzen, M.; Johnson, S.; Seddon, K. R.; Rogers, R. D. Crystal Polymorphism in 1-Butyl-3methylimidazolium Halides: Supporting Ionic Liquid Formation by Inhibition of Crystallization. Chem. Commun. 2003, 14, 1636−1637. (22) Hayashi, S.; Ozawa, R.; Hamaguchi, H. O. Raman Spectra, Crystal Polymorphism, and Structure of a Prototype Ionic-liquid [bmim]Cl. Chem. Lett. 2003, 32, 498−499. (23) Jia, R.; Shao, C. G.; Su, L.; Huang, D. H.; Liu, X. R.; Hong, S. M. Rapid Compression Induced Solidification of Bulk Amorphous Sulfur. J. Phys. D: Appl. Phys. 2007, 40, 3763−3766. (24) Wang, M. Y.; Liu, X. R.; Zhang, C. R.; Zhang, D. D.; He, Z.; Yang, G.; Shen, R.; Hong, S. M. Compression-Rate Dependence of Solidified Structure from Melt in Isotactic Polypropylene. J. Phys. D: Appl. Phys. 2013, 46, 145307−145311. (25) Umebayashi, Y.; Fujimori, T.; Sukizaki, T.; Asada, M.; Fujii, K.; Kanzaki, R.; Ishiguro, S. Evidence of Conformational Equilibrium of 1Ethyl-3-methylimidazolium in Its Ionic Liquid Salts: Raman Spectroscopic Study and Quantum Chemical Calculations. J. Phys. Chem. A 2005, 109, 8976−8982. (26) Fuller, J.; Carlin, R. T.; De Long, H. C.; Haworth, D. Structure of 1-Ethyl-3-methylimidazolium Hexafluorophosphate: Model for Room Temperature Molten Salts. J. Chem. Soc., Chem. Commun. 1994, 299− 300. (27) Choudhury, A. R.; Winterton, N.; Steiner, A.; Cooper, A. I.; Johnson, K. A. In situ Crystallization of Low-Melting Ionic Liquids. J. Am. Chem. Soc. 2005, 127, 16792−16793. (28) Choudhury, A. R.; Winterton, N.; Steiner, A.; Cooper, A. I.; Johnson, K. A. In situ Crystallization of Ionic Liquids with Melting Points below −25 °C. CrystEngComm 2006, 8, 742−745. (29) Dupont, J. On the Solid, Liquid and Solution Structural Organization of Imidazolium Ionic Liquids. J. Braz. Chem. Soc. 2004, 15, 341−350.

unknown cation conformer was observed. These present results show that compression rate plays an important role in kinetically driven crystallization and phase transitions of [Emim][CF3SO3]. The significance of this finding is to suggest that compression rate needs to be considered for pressure-induced phase transitions of RTILs because of their unique and characteristic structure. Our findings may provide useful insights into how pressure affects the phase behaviors of RTILs and imply that compression rates should be concerned in pressure-induced phase transition of RTILs using DAC apparatus.

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AUTHOR INFORMATION

Corresponding Authors

*H.H.: e-mail, [email protected]. *L.S.: e-mail, [email protected]. *G.Y.: e-mail, [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Grants 21273206 and 31201377), Key Research Project of Higher Education of Henan Province (Grants 15A140016 and 2010GGJS-110), and the School Scientific Research Fund Project (Grant 2013XJJ009).

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REFERENCES

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DOI: 10.1021/acs.jpcb.5b08384 J. Phys. Chem. B 2015, 119, 14245−14251