Article pubs.acs.org/JPCB

Stability of the Liquid State of Imidazolium-Based Ionic Liquids under High Pressure at Room Temperature Yukihiro Yoshimura,*,† Machiko Shigemi,† Mayumi Takaku,‡ Misaho Yamamura,‡ Takahiro Takekiyo,† Hiroshi Abe,§ Nozomu Hamaya,‡ Daisuke Wakabayashi,∥ Keisuke Nishida,∥ Nobumasa Funamori,∥ Tomoko Sato,⊥ and Takumi Kikegawa# †

Department of Applied Chemistry, National Defense Academy, 1-10-20, Hashirimizu, Yokosuka, Kanagawa 239-8686, Japan Graduate School of Humanities and Sciences, Ochanomizu University, Tokyo 112-8610, Japan § Department of Materials Science and Engineering, National Defense Academy, 1-10-20, Hashirimizu, Yokosuka, Kanagawa 239-8686, Japan ∥ Department of Earth and Planetary Science, University of Tokyo, Tokyo 113-0033, Japan ⊥ Department of Earth and Planetary Systems Science, Hiroshima University, Higashi-Hiroshima 739-8526, Japan # Institute of Materials Structure Science, High Energy Accelerator Research Organization, Tsukuba 305-0801, Japan ‡

ABSTRACT: To understand the stability of the liquid phase of ionic liquids under high pressure, we investigated the phase behavior of a series of 1-alkyl3-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.

1. INTRODUCTION Ionic liquids (ILs) are molten salts comprising only organic cations and organic/inorganic anions, and usually have melting points below 373 K.1 ILs have received considerable attention as novel solvents, e.g., environmentally benign solvents, catalysts, and lubricants, because of their appealing properties, such as their negligible vapor pressure and thermal stability.2−4 In contrast to conventional salts, ILs can remain in a liquid state near room temperature despite the fact that the governing interactions are the Coulomb forces among constituent ions.4,5 One typical feature of ILs is that they have a unique local liquid structure, which has been studied using X-ray 6,7 and neutron8−10 diffraction techniques, and molecular dynamics simulation studies.11−14 In the case of imidazolium-based IL, which is one of the most studied ILs to date, alkyl tails longer than the butyl chain aggregate, causing the nanoscale phase separation (nanostructural organization) of polar and nonpolar domains,11−15 with electrostatic interactions between polar groups and van der Waals forces from the nonpolar alkyl chains contributing to the aggregation. Interestingly, ILs have © 2015 American Chemical Society

hierarchical structures over short- to long-range regions (0.1 to 10 nm), where each segment of the structure may have a distinct relaxation time.9,16 Thus, the alkyl chain plays an important role in determining the morphology of ILs. All these features are most likely keys to preventing ions from crystallizing. Aside from their possible applications, the study of ILs can benefit other research areas, such as glass transition, since most ILs are glass-forming liquids that can be easily supercooled.17 Many studies guided by this have been conducted by several groups using different methods.18−24 Notably, in their pioneering work, Holbrey and Seddon25 showed the preparation and thermal properties of a series of 1-alkyl-3-methylimidazolium tetrafluoroborates, hereafter abbreviated as [Cnmim][BF4], at ambient pressure as a function of chain length (n = 0−18, here n = 0 is Hmim). The [Cnmim][BF4] (n = 2−9) shows marked Received: January 8, 2015 Revised: May 13, 2015 Published: May 19, 2015 8146

DOI: 10.1021/acs.jpcb.5b03476 J. Phys. Chem. B 2015, 119, 8146−8153

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spectra using GRAMS/386 software (Galactic Industries Corp.). The pressure-loading device was a screw-type diamond anvil cell (DAC, SR-DAC-KYO3-3d, Kyowa) with anvil culet 0.6 mm in size. We used a T301 stainless steel gasket with a 0.3 mm hole. The sample was normally compressed and then decompressed up to ∼7 GPa in steps of approximately 1 and 0.5 GPa/h, respectively, including the duration time. [C8mim][BF4] was studied by synchrotron radiation (SR) small-angle X-ray scattering (SAXS). A sample was encapsulated into a hole of 0.3 mm diameter drilled in the T301 gasket and loaded on a modified Mao-Bell-type DAC with 0.6 mm culet anvils. SR-SAXS measurements were carried out up to ∼4 GPa at beamline BL18C in Photon Factory, KEK, Japan. Incident X-rays at a wavelength of 0.08177 nm were focused and collimated to 0.1 mm in diameter. An imaging plate, used as a detector, was placed at a distance of 1274 mm from the sample in a vacuum chamber with a polyimide (Kapton)-film window. Acquisition time for each scattering pattern was 900 s. The pressure was determined from the spectral shift of the R1 fluorescence line of several ruby balls (0.015 mm diameter) put in the sample chamber of the DAC.32 The error of the estimated pressure was ±0.05 GPa. The sample was prepared in a glovebox filled with argon gas to avoid contamination with water in air. All the Raman and SR-SAXS measurements were done at room temperature. For comparison, simple differential thermal analysis (DTA) measurements upon the quick cooling of the sample were performed at ambient pressure. The precooling procedure was done by inserting the entire sample directly into liquid nitrogen (estimated quenching rate of ∼500 K/min). After removing the sample from the liquid nitrogen, the DTA traces were recorded at a heating rate of ∼2 K/min from an initial temperature equaling that of the liquid nitrogen (77 K). Other details were essentially the same as in our previous study.33 As a reference material for the measurements, benzene (Wako Pure Chemical Co.) was used. The glass transition Tg and cold crystallization Tcc temperatures upon heating were determined by the onsets of the exothermic and endothermic peaks, respectively. The reproducibility of the transition temperature was ±1 K.

tendencies to supercool before crystallizing and to form glasses with no indication of crystallization, even during slow cooling (1 K/min). Calorimetric studies by Yamamuro et al.23,26 also revealed that most imidazolium-based ILs exhibit glass transition behavior around 200 K and their glass transition temperatures Tg depend on anion size: the larger the anion, the lower the Tg value. Based on the results of previous studies, as described above, the manner by which pressure affects the phase behavior of these unique liquids is an important factor to explore. Studies on the physical properties and phase behaviors of ILs under high pressure would contribute to a basic understanding of ILs that would allow their application under stressed conditions, such as in lubricants and pressure media, and also make it possible to perform recycling and purification by high-pressure crystallization.27,28 In a previous study,29 we found that [C2mim][BF4] can be superpressed without crystallization up to ∼7 GPa. Remarkably, as an alternative to pressure-induced crystallization, we have found that such a metastable liquid shows crystal polymorphism upon decompression. This behavior is in contrast with the results of [C4mim][BF4], wherein the crystallization of [C4mim][BF4] did not take place through the compression and decompression processes.30,31 To expand our knowledge of ILs, it is important to gain a fundamental understanding of the effect of the alkyl groups on their phase behavior under high pressure. In this study, to elucidate the mechanisms determining the stability of the liquid phase of ILs, we investigated the phase behavior of [Cnmim][BF4] as a function of alkyl chain length (n = 2−8) up to ∼7 GPa. Chemical structure of [Cnmim][BF4] is shown in Figure 1. Competition among the different types of

3. RESULTS AND DISCUSSION First we describe the typical results of pressure-induced Raman spectral changes in the C−H stretching vibrational modes of the imidazolium cation, which are commonly used as an indicator of crystallization (Figure 2). We found that the pressure-induced crystallizations of [Cnmim][BF4] did not occur, whereas [C2mim][BF4] and [C8mim][BF4] were found to crystallize during the decompression process from the superpressed state. The results for [C2mim][BF4] are in an agreement with ref 29, where [C2mim][BF4] shows crystal polymorphism at approximately 2.0 (phase II) and 1.0 GPa (phase I) upon decompression. Unfortunately, because the pressure range is beyond the coverage of direct thermodynamic measurements, such as DSC/DTA, in a fashion similar to previous studies,29,34 we used the line broadening of the sharp ruby R1 fluorescence line to distinguish high-pressure vitrification from crystallization. As an example of the results, we provide the spectral change of [C6mim][BF4] in Figure 3. Figure 4 shows the variations in the full width at half-maximum (fwhm) of the ruby R1 line relative to the 0.1 MPa line on [Cnmim][BF4] for 2 ≤ n ≤ 8 during the compression process. The pressure where the line broadening begins is defined as the glass transition pressure pg.35 We can see that the increase of

Figure 1. Chemical structure of 1-alkyl-3-methylimidazolium tetrafluoroborate ([Cnmim][BF4] ; n = 2−8).

interactions in ILs most likely determines the phase transitions and affects the behavior of the ILs. Here, we determine a stability region of the liquid phase of [C nmim][BF 4] homologues under high pressure and present a classification model for this behavior in the patterns of superpressurization upon compression and decompression-induced crystallization from the superpressurized glass.

2. EXPERIMENTAL SECTION [Cnmim][BF4] was used in this study with n values ranging from 2 to 8 (Kanto Chemical Co. for n = 2 and 4; Ionic Liquids Technologies Inc. for n = 3, 5, 6, 7; and Wako Pure Chemical Co. for n = 8). Raman spectra were measured by a JASCO NR-1800 Raman spectrophotometer equipped with a single monochromator and a liquid nitrogen cooled charge-coupled device (CCD). The spectra were excited by the 514.5 nm line of an Ar+ laser (Lexel) with a power of 350 mW. The Gaussian−Lorentzian mixing function was used to analyze the observed Raman 8147

DOI: 10.1021/acs.jpcb.5b03476 J. Phys. Chem. B 2015, 119, 8146−8153

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n = 2−8 can be superpressed into a metastable (glassy) state without crystallizing. The effect of altering the alkyl chain length on the transition behavior is seen to be continuous. Remarkably, when the superpressed liquid was further compressed above pg, another change in the Δfwhm occurred in the 4−6 GPa range, as shown in Figure 4. Here, we tentatively call the pressure where this change occurs p1. We note that for n ≥ 5 one more kink appears to materialize in the Δfwhm, which we call p2, though we experienced that the kinks in Δfwhm for p1 and p2 were sometimes shallow and difficult to define precisely. These results can be interpreted to indicate that other structural changes may be generated in [Cnmim][BF4], with other (densified) structures being induced by higher pressures. In this regard, it is interesting to note that the multiple glass transition behavior related to nanophase separation in poly(n-alkyl methacrylate) homopolymers was reported by Beiner et al.,36 who observed three qualitatively different glass transitions at 0.1 MPa. In an effort to explain this unusual glass transition behavior, two empirical mechanisms were suggested: a static nanophase separation between main and side chains and a dynamic heterogeneity with two intrinsic length scales. Observed pg, p1, p2 values are plotted in Figure 5A, including their experimental uncertainties based on several runs. The variations in pg and p1 against pressure yield a sigmoid-like curve. We find that solid and dotted lines in panel A can be fitted with the equations of state (n > 2) as below. pg = [a /1 + e−{(n − 3.825)/ b}] + 2.133 ± 0.04619; a = 0.6574 ± 0.01132 and b = −0.4029 ± 0.2617 Figure 2. Typical example of the results of the pressure-induced Raman CH stretching spectral changes in [Cnmim][BF4] (n = 2, 4, 6, and 8) as a function of pressure.

(1)

p1 = [a /1 + e−{(n − 4.853)/ b}] + 3.753 ± 0.07036; a = 2.032 ± 0.1147 and b = −0.2114 ± 0.1817

(2)

As the alkyl chain length increases from n = 2 to 8, pg shifts from 2.8 GPa to lower pressures of ∼2 GPa. In other words, the [Cnmim][BF4] with longer alkyl chains tend to solidify at rather low pressures. In addition, a drastic decrease in p1 is observable at n ≥ 5, but p2 is almost constant for n = 5−8. Since the ruby R1 line reflects the non-hydrostatic condition, the shear and uniaxial stresses in [Cnmim][BF4] for n ≥ 5 are assumed to be non-negligible, despite being in the glassy solid state. We speculate that the differences in the extents of the line broadening for different values of n correlate with the divergence of the hierarchy structure. It appears that n = 5 is the crossover point, above which the applied pressure affects the inherent structure in [Cnmim][BF4] to a different degree. As mentioned above, we sometimes experienced that longer alkyl chains correspond to a less defined kink in the Δfwhm of p1 and/or p2. This results in difficulties in obtaining welldefined p1 (and p2) values at higher pressures. We suspect that this is due to the dynamic heterogeneity with the intrinsic hierarchy structure relaxing on different time scales. Interestingly, Faria et al.37 proposed that the Raman spectrum of crystallized [C3H7NH3][NO3] shows microheterogeneity resulting from different local structures under high pressures; this means that different parts of the same sample exhibit different Raman profiles. Structural rearrangements take place in both the nonpolar and polar domains, which can be independent of each other.

Figure 3. An example of the results for variations in the spectra ([C6mim][BF4]) of the ruby R1 line relative to the width of the 0.1 MPa line.

Δfwhm with increased pressure drastically increases above the initiation point pg. Thus, all the [Cnmim][BF4] homologues for 8148

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Figure 4. Pressure broadening of the sharp ruby R1 fluorescence line (full width at half-maximum, fwhm) relative to the 0.1 MPa line width. The lines are guides to the eyes.

relationship between liquid structure and the low-Q peak.41 For example, it was suggested that the position of the prepeak corresponds to a spatial correlation length, such as the distance between neighboring shells in the liquid structure12 and/or intermolecular correlations between anion pairs, adjacent tails, and cation tail/anion pairs.42 Hardacre et al.43 proposed that the low-Q peak arises from spatial correlations between imidazolium rings at the second coordination shell separated through the alkyl groups. Figure 6 shows a pressure dependence of SAXS intensities for [C8mim][BF4] at room temperature. As far as the authors know, this is the first report on pressure-induced changes in the low-Q peak for ILs. An intense peak around 4 nm−1 is identified as Kapton films for X-ray windows. The observed intensities were normalized by the Kapton peak. At ambient pressure, the position of the prepeak for [C8mim][BF4] was almost the same as the one reported by Triolo et al.39 With increasing pressure, the prepeak position shifted gradually to higher Q and the peak intensity decreased. A significant finding is that the prepeak almost disappeared above ∼2 GPa, at which [C8mim][BF4] falls into superpressed state shown by the Raman results. It should be noted that the peak intensity reverts back when we retrieve the sample at 0.1 MPa. The molecular origins of prepeak remain somewhat controversial, and thus we do not go

Here we need to consider the interactions of ions due to strong Coulomb forces and of the alkyl chains due to weak van der Waals forces, which involves the compact packing of the alkyl chains with increasing pressure. Tokuda et al.38 proposed ionicity as a useful parameter to characterize the properties of ILs. Changes in the alkyl chain length cause changes in the interactive forces, which result in decreased electrostatic attraction between the cation and anion. On the other hand, the increase in the number of hydrocarbon units enhances the van der Waals interactions.38 Therefore, the balance between these two opposite factors is altered depending on the extent of applied pressure, besides the alkyl chain length. The present remaining ambiguities include the change in the nanostructure domains of [Cnmim][BF4] with applied pressure. One idea is to perform SAXS experiments. As a first report on the SAXS mearements for the imidazolium-based ILs at ambient pressure, Triolo et al.39 showed that a characteristic peak at the low-Q region (2−4 nm−1) appeared. They proposed that this characteristic prepeak is direct evidence of the structural heterogeneity (nanophase separation) of ILs and the value of Bragg spacing D is the measure of an average size of the domain (the alkyl tail domain size + the thickness of the surrounding charged shell).40 However, there have been some debates on the molecular origin of the low-Q peak, and the 8149

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Then, we consider how the local structure changes under high pressure from other factors such as a conformational change of [C8mim]+ cation. The [Cnmim]+ cation has conformational equilibria between the trans and gauche conformers for the main long alkyl chain. The ∼625 and ∼600 cm−1 peaks in the CH2 rocking mode are assigned to the trans and gauche conformers, respectively.44,45 We determined the intensity fractions ( f) of gauche and trans conformers by curve fitting spectra in the CH2 rocking mode (Figure 7A). The f value of each conformer is given by fgauche =

Igauche Igauche + Itrans

,

ftrans =

Itrans Igauche + Itrans

(3)

where Igauche and Itrans indicate the relative Raman intensity of gauche and trans conformers of [C8mim]+ cation, respectively. The results are shown in Figure 7B. Figure 7B shows that the [C8mim]+ cation prefers the gauche conformer in the liquid state at 0.1 MPa. The population of gauche (trans) conformer increases (decreases) by ∼15% upon compression up to when the system falls into the glassy sate. After the pg point the gauche fraction becomes constant against pressure and then starts to decrease after p1. After p2, the slope of fgauche seems to change again. These results indicate that the local structure around the [C8mim]+ cation is changing with pg → p1 → p2. Interestingly, we can see that the behavior of the f against pressure is basically concordant with the ruby R1 Δfwhm change in Figure 4. Taken together, a plausible interpretation is that p1 (and p2) is the result of the smaller dynamic heterogeneity of the main chains with two intrinsic length scales, i.e., n = 2−4 and the longer alkyl chain segments. In this study, we found that all [Cnmim][BF4] homologues for n = 2−8 exhibit glass transition behaviors at 0.1 MPa, and that with n = 2 exhibits cold crystallization, where a similar phenomenon is observable in the relaxation from the glassy sample of polymers upon heating at 0.1 MPa.46,47 The frozen state at low temperatures is understood to be relaxed upon heating, thereby causing the crystalline phase, which is energetically favorable at these higher temperatures, to appear. The summarized glass transition temperature Tg is plotted in Figure 5B, which shows that the Tg increases as the chain length n increases. This is simply due to the higher viscosity coupled with the elongation of the alkyl chains for glass formation upon cooling. It is interesting to point out that Hamaguchi et al.48 reported the increment of the population of trans conformers with larger n values of [Cnmim][BF4] at 0.1 MPa and 298 K. The results in Figure 5B indicate that, the larger the population of trans conformers in the initial liquid phase, the higher the Tg of [Cnmim][BF4] where the crystallization phenomenon is bypassed by supercooling. We find that the solid line in Figure 5B can be fitted with the equation of state (n > 2) as below.

Figure 5. Changes in the (A) glass transition pressure pg (●), p1 (○), and p2 (▲) and (B) glass transition temperature Tg (●) for [Cnmim[BF4] with the crystallization pressure pc (□) upon decompression process and cold crystallization temperature Tcc (▲) as a function of alkyl-chain length (n). The pg values for n = 2 and 4 are in good agreement with those in the previous results.29,30 The solid and dotted lines in panel A are fitted with the equations of state (see text.) Dashed line is a just guide to the eye. The solid line in panel B is fitted with the equation of state (see text). Data on glass transition pressures for [C4mim][BF4] (1.9 GPa) and [C8mim][BF4] (2.1 GPa) were recently presented by Ribeiro et al.49 We suspect that the slight differences in the values as compared with the present studies are likely due to differences in experimental conditions, such as compression rate and measurements of the apparatus used (gasket size, diamond culet size, etc.).

Tg = an2 + bn + 167.4 ± 1.391; a = −0.1595 ± 0.06051

and

b = 4.563 ± 0.6141 (4)

Figure 6. Pressure-induced changes in the SAXS patterns for [C8mim][BF4] at room temperature.

We also found that [Cnmim][BF4], which crystallizes upon decompression, tends to crystallize by thermal treatment at ambient pressure, although the two are not perfectly matched. Comparing the high-pressure (pg) and the low-temperature (Tg) phase diagrams reveals a kind of mirror image in the behavior, suggesting that the high-pressure phase behavior of

much further. But, in view of the SAXS results, the causes for the structural changes at pg and p1 are probably different from each other. 8150

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Figure 7. (A) Effects of pressure on changes in Raman spectra of [C8mim][BF4] for the CH2 rocking mode. The dotted line indicates the gauche conformer; dashed line indicates the trans conformer. (B) Intensity fractions ( f) of the conformers of [C8mim][BF4] as a function of pressure. The open and closed circles represent the trans and gauche conformers of [C8mim]+ cation, respectively. Dashed vertical lines are just guides to the eye.



ILs can be anticipated from the results of the low-temperature phase behavior during crystallization or glass formation to some extent. We infer that the movement of a moiety of the alkyl chain is likely key to the phase transition behavior and triggers crystallization. As we mentioned in our earlier reports,29,34 the conformational behaviors of low-temperature (quick-cooled) and highpressure glasses are completely different, although whether the two glassy phases are thermodynamically distinct remains an open question. Hence, it is important to note that, recently, Ribeiro et al.49 proposed the equations of state for ILs estimating molar volume under pressures on the order of GPa and showed the relationship between two states’ pg values at 295 K and Tg values at 0.1 MPa, introducing a scaling parameter γ. The present results may be valuable for testing this assumption. In summary, based on the present study, we have provided a high-pressure phase diagram for [Cnmim][BF4] homologues with different alkyl chain lengths for 2 ≤ n ≤ 8. We have found that [Cnmim][BF4] homologues for 2 ≤ n ≤ 8 fall into the glassy state with applied pressures of 2−3 GPa. There is an implication of multiple phase or structural transitions to ∼7 GPa. These pg values are lower than those of commonly used pressure media, such as a 4:1 mixture of methanol and ethanol (9.8 GPa) and 2-propanol (4.2 GPa), but are slightly higher than or comparable to those of glycerol (1.4 GPa) and silicone oil (0.9 GPa).50 If we consider the result of [C4mim][PF6], which crystallizes upon compression,51 the results of [Cnmim]based ILs can be separated into, at least, three distinct groups on the basis of their behavior: (1) pressure-induced crystallization upon compression due to the reduction of lattice energies through disruption of packing efficiencies,51 (2) superpressurization upon compression, and (3) decompression-induced crystallization from the superpressurized glass. More advanced experiments, however, are required to present a thorough discussion.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS Part of this work was performed under the approval of the Photon Factory Program Advisory Committee (Proposal No. 2011G127 and No. 2014G151). This work has been financially supported by Grants-in-Aid for Scientific Research 23654156 and 25287111 from Japan Society for the Promotion of Science.



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DOI: 10.1021/acs.jpcb.5b03476 J. Phys. Chem. B 2015, 119, 8146−8153

Article

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DOI: 10.1021/acs.jpcb.5b03476 J. Phys. Chem. B 2015, 119, 8146−8153

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-meth...
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