Conformational adjustment for high-pressure glass formation of 1-alkyl-3-methylimidazolium tetrafluoroborate

Abstract

The conformational stability of 1-alkyl-3-methylimidazolium tetrafluoroborate ([C_nmim][BF₄], n = 3–8) under high pressure was investigated using Raman spectroscopy to reveal the preferential role of the alkyl-chain length (n) in high-pressure glass transition. To evaluate this, we determined the intensity ratio (r) and differences in the partial molar volume (ΔV^{trans→gauche}) between the whole trans and gauche conformers of the [C_nmim] cation using Raman intensities. Interestingly, both values were classified into a two alkyl-chain length region at the border of n = 5. The coulombic interaction (cation–anion interaction) for the conformational stability is the predominant factor below n = 5 (the cation-head portion: alkyl carbon number C < 5), and the alkyl-chain packing effect (cation–cation interaction) is the predominant factor above n = 5 (the cation-tail portion: C > 5). In combination with the conformational preference of the [C_nmim] cation under a high-pressure glassy state, the alkyl chain displays a preferential role, i.e., an increase in the gauche conformer of [C_nmim][BF₄] adjusts to avoid crystallization (the conformational adjustment effect). In the presence of the coulombic interaction, the preferential role of the flexible alkyl chain is an important key to elucidate the mechanism of the complicated high-pressure phase transition behavior of ionic liquids.

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Introduction

The high-pressure (HP) phase transition of liquids, such as alkanes, acids, and alcohols, has been studied through the years to investigate the HP polymerization and HP metallization of liquids and the origin of the intermolecular interactions.^1–9 It is well known that many molecular liquids (MLs) crystallize under high pressure, with further compression causing amorphous or disordered phases.^1–9 Specific interactions, such as molecular packing, electrostatic interactions, aromatic interactions, and hydrogen bonding, in MLs have been discussed to understand the HP phase transition mechanism of liquids.^1–11

In contrast, ionic liquids (ILs) comprising organic cations and anions remain in the liquid state below 373 K and have attracted much attention because of their attractive properties, such as conductivity, viscosity, and thermal stability, making them suitable for use as environmentally benign solvents.¹² These properties are related to the static and kinetic hierarchy structure, which produces the nanostructure, ionic portion diffusion, alkyl-chain conformation, and methyl-group rotation of ILs.^13,14 In addition to the coulombic interaction, ILs can introduce various interactions, such as alkyl-chain packing (via selection of the cation and anion). Thus, information on the HP phase transition of ILs with a hierarchy structure might be expected to yield a new class of materials for HP applications, such as in HP synthesis techniques, lubrication, catalysis, and bioscience fields.

In recent years, there have been studies on the HP phase transition of ILs using optical spectroscopy, X-ray scattering, and molecular dynamics (MD) simulations.^15–29 On the whole, high pressure (up to 3 GPa) causes at least three IL phase transition patterns: the HP glassy state, compression crystallization, and decompression crystallization. These complicated phase transitions in this pressure range are characteristic of ILs and are hardly observed for most MLs, e.g., alkanes and haloalkanes.^1,5–7 According to our Raman and X-ray studies,^18,23 1-alkyl-3-methylimidazolium tetrafluoroborate ([C_nmim][BF₄], n = 2–8) with a short alkyl chain (n = 2) and a long alkyl chain (n > 8) tends to exhibit compression or decompression crystallization.^18,23 On the other hand, [C_nmim][BF₄] with an intermediate alkyl chain (n = 3–7) tends to form a HP glassy state.²³

Related to these effects, a conformational change of the alkyl chain of [C_nmim][BF₄] occurred, accompanying the phase transitions under high pressure.^{15–19,21,23–25,28,29} For instance, it is well known that [C_nmim][PF₆] with a hydrophobic anion and [C_nmim][BF₄] with a hydrophilic anion exhibit differing HP phase transitions: the former tends to crystallize under high pressure^15,16,21 and the latter tends to form a HP glassy state.^23,30 The difference in these phase transitions is due to the cation–anion affinity (such as the position of the anion for the [C_nmim] cation) and cation–anion interaction (hydrophobicity and hydrophilicity). In addition to these transitions, the previous Raman results have revealed that the single conformer (trans or gauche conformer) for the NCCC angles of the [C₄mim] cation of [C₄mim][PF₆] is predominant in compression crystallization.^15,16,21,31 A predominance of the single conformer of [C₄mim][PF₆] in compression crystallization has also been observed in many MLs and can be explained by the molecular packing order effect.^1,5–7 In contrast, the HP glassy state of [C₄mim][BF₄] induced a higher gauche conformer population than the liquid state.³⁰

Here it is intriguing as to why the higher gauche conformer population of [C₄mim][BF₄] is needed to form a glassy state under high pressure. Although it has been noted that the conformational change of the alkyl chain is strongly related to phase transition,^{15–19,21,23–25,28,29} the preferential role (increase of the gauche conformer) of the alkyl-chain length of [C₄mim][BF₄] in HP glassy formation is still unclear. Elucidation of the preferential role of the alkyl-chain length under high pressure is related to the origin of the complicated high pressure phase transition of ILs such as the decompression crystallization mechanism.^18,19,23

In this study, to deepen the previous study,²³ we investigated the pressure effect on the conformational equilibrium of [C_nmim][BF₄] (n = 3–8) using Raman spectroscopy to elucidate the preferential role of the alkyl-chain length in glass formation under high pressure. Fig. 1 shows (a) the chemical structures of [C_nmim][BF₄] and (b) the representative trans and gauche conformers of the [C_nmim] cation. The present finding is that the pressure-induced conformational stability of the [C_nmim] cation is related to competition between the coulombic interaction and the alkyl-chain packing. In combination with the conformational preference of the [C_nmim] cation under a high-pressure glassy state, the alkyl-chain length of the [C_nmim] cation in [C_nmim][BF₄] contributes to the maintenance of the liquid structure, and its adjustment (increase of the gauche conformer) induced glass formation under high pressure, thereby avoiding compression crystallization.

Fig. 1 (a) Chemical structures of 1-alkyl-3-methylimidazolium tetrafluoroborate ([C_nmim][BF₄]) and (b) the representative trans and gauche conformers for the NCCC and CCCC angles of the alkyl-chain length.

Experiments

1-Alkyl-3-methylimidazolium tetrafluoroborate ([C_nmim][BF₄], n = 3–10) was investigated in this study (Kanto Chemical Co. for n = 4; Ionic Liquids Technologies Inc. for n = 3, 5, 6, and 7; and Wako Pure Chemical Co. for n = 8, 9, and 10). Raman spectra were measured using a JASCO NR-1800 Raman spectrophotometer equipped with a single monochromator and a liquid nitrogen-cooled charge-coupled device detector. The spectra were excited by the 514.5 nm line of an Ar⁺ laser (Lexel) with a power of 350 mW. The pressure-loading device was a screw-type diamond anvil cell (DAC, SR-DAC-KYO3-3d, Kyowa) with anvil cutlet 0.6 mm in size. We used a SUS301 gasket with a 0.3 mm hole. The pressure was determined from the spectral shift of the R₁ fluorescence line of several ruby balls (0.015 mm diameter) placed in the sample chamber of the DAC.³² The error of the estimated pressure was ±0.05 GPa. The sample was prepared in a glove box filled with argon gas to mitigate contamination from water in air. All Raman spectral measurements of [C_nmim][BF₄] were performed in the liquid (298 K and 0.1 MPa) and glassy (77 K and 0.1 MPa or 298 K and high pressure) states. High-pressure experiments for [C₉mim][BF₄] and [C₁₀mim][BF₄] were not performed because both ILs were crystallized at low pressure (ca. 0.4–0.7 GPa). Each sample was normally compressed in steps of approximately 1 GPa h⁻¹, including the duration time. To improve the data quality, all spectral measurements were repeated at least three times. All of the spectral lines were fitted with Gaussian–Lorentzian mixing functions provided by GRAMS/386 software (Galactic Industries Co.).

Results and discussion

Conformational stability of [C_nmim][BF₄] under high pressure

Fig. 2(a) shows the Raman spectra of [C_nmim][BF₄] in the region from 560 to 680 cm⁻¹ at several alkyl-chain lengths at 298 K and 0.1 MPa. The vibrational assignments of the Raman band in this region of [C_nmim][BF₄] were well established by Hamaguchi et al.³³ Two peaks at ∼600 cm⁻¹ and ∼625 cm⁻¹, assigned to the CH₂ rocking bands, correspond to the whole gauche and trans conformers of the alkyl chain in the [C_nmim] cation, respectively. The Raman intensity at ∼600 cm⁻¹ (the gauche conformer) decreases, and that at ∼625 cm⁻¹ (the trans conformer) increases, with increasing alkyl-chain length. To further investigate the effect of the alkyl-chain length on the conformational equilibrium of the [C_nmim] cation, Fig. 2(b) shows the intensity ratio (r = I_gauche/I_trans) between the conformers as a function of alkyl-chain length (n). The r-value using the same vibrational mode corresponds to the conformational free energy change (ΔG^{trans→gauche} = −RT ln(I_gauche/I_trans), where R and T are the gas constant and temperature).^34,35 A drastic decrease in the r-value was observed in the range from n = 5 to 6, and this value is almost constant above n = 6. This indicates that the conformational stability of the [C_nmim] cation below n = 5 is different from that above n = 5.

Fig. 2 (a) Raman spectral changes in the CH₂ rocking region of [C_nmim][BF₄] at several alkyl-chain lengths (n). (b) Changes in the intensity ratio (r) between the conformers of the [C_nmim] cation as a function of n. The inset figure in (b) shows changes in p_g of [C_nmim][BF₄] as a function of n. Data were re-plotted from ref. 23.

Remarkably, the r-value shows similar behavior to the change in the glass transition pressure (p_g) of [C_nmim][BF₄] in relation to n, as reported by Yoshimura et al.²³ (the inset in Fig. 2(b)), although the r-value shifts to higher n rather than p_g. The similar behavior of r-values and p_g-values indicates that the conformational stability of the [C_nmim] cation at 0.1 MPa and 298 K is related to the glass transition behavior of [C_nmim][BF₄]. Related to these results, Yamamuro et al.³⁶ used X-ray and neutron scattering methods to show that the structure of low-temperature (LT) glassy state of [C_nmim]-based ILs is similar to that in the liquid state. Moreover, Endo et al.³⁷ used Raman spectroscopy to demonstrate that the gauche conformer of [C₄mim][PF₆] increases when [C₄mim][PF₆] forms the LT glassy state. Considering their results, we suggest that the investigation of local structural stability, such as the conformational change of the [C_nmim] cation in the liquid state, is an important aspect of the HP glass formation mechanism of [C_nmim][BF₄]. In view of the pressure effect on the conformational equilibrium of the cation, we discuss changes in the conformational stability of the [C_nmim] cation.

As a representative result, Fig. 3(a) shows the Raman spectra in the CH₂ rocking region of [C₅mim][BF₄] at several pressures. The peak at 600 cm⁻¹ (gauche conformer) increases with increasing pressure, whereas that at 625 cm⁻¹ (trans conformer) decreases. The increase of the gauche conformer under high pressure was observed in other [C_nmim][BF₄]. Similarly, previous Raman studies showed that the gauche conformer of MLs such as alkanes and haloalkanes increases with increasing pressure until crystallization occurs.^1,5–7,38 Pressure-induced conformational preferences of the [C_nmim] cation prior to phase transition are consistent with those of alkanes and haloalkanes.

Fig. 3 (a) Raman spectral changes in the CH₂ rocking region of [C₅mim][BF₄] at several pressures. (b) Changes in the logarithmic intensity ratio between the conformers of the [C_nmim] cation versus pressure (●: [C₃mim][BF₄]; ○: [C₅mim][BF₄]; ▲: [C₆mim][BF₄]; △: [C₇mim][BF₄]; ■:[C₈mim][BF₄]). The straight lines represent the results of the least-squares fit.

To further investigate the pressure effect on the conformational equilibrium of the [C_nmim] cation, we determined the difference in the partial molar volume (PMV) (ΔV^{trans→gauche}) between the whole trans and gauche conformers on the basis of the pressure dependence of the relative Raman intensities, as shown in Fig. 3(b). The relationship between the Raman intensities for the conformers and pressure has been well established.^38–41 Assuming that the ratio of the Raman scattering cross sections between the conformers is independent of pressure, ΔV^{trans→gauche} is given by

where p is the pressure. I_gauche and I_trans indicate the relative Raman intensity of CH₂ rocking bands due to the whole gauche and trans conformers, respectively. Using the slopes in Fig. 3(b), ΔV^{trans→gauche} is determined to be the range from −0.73 to −0.03 cm³ mol⁻¹ throughout the studied n range. Although the order of the ΔV^{trans→gauche} values of [C_nmim][BF₄] is consistent with those of MLs such as alkanes and haloalkane (0 to −1.5 cm³ mol⁻¹),^{8,9,38,40,42–45} the absolute values are slightly smaller than those of MLs.

Fig. 4 shows the changes in the ΔV^{trans→gauche} value of [C_nmim][BF₄] versus n; the data are combined with the previous results for alkanes.^8,9,44,45 Remarkably, the negative values of ΔV^{trans→gauche} became larger when n = 3–5 and became smaller above n = 5. Whereas ΔV^{trans→gauche} of alkanes became larger above n = 5. Although the conformational preference of [C_nmim][BF₄] until high pressure phase transition is similar to that of alkanes, for ILs that construct cations and anions, the absolute value and inversion of the slope in ΔV^{trans→gauche}versus n are quite different from those of alkanes that construct only neutral alkyl chains. We suppose that the abrupt change in the conformational stability at n = 5 is related to the pressure effect on the nanostructure of [C_nmim][BF₄], because it is well known that the formation of a nanostructure of [C_nmim]-based IL with a shorter alkyl-chain length (n < 5) is weak, and the increase of the alkyl-chain length of the cation (n > 5) induced enhancement of the nanostructure.¹² In fact, our recent studies demonstrated that an abrupt conformational change occurs in the [C₈mim] cation of [C₈mim][BF₄] having the enhanced nanostructure after the disruption of the nanostructure by the applied pressure (as we discuss below).²³ Based on these results, as the formation of a nanostructure of [C_nmim][BF₄] at n < 5 is relatively weak, the conformational stability of the [C_nmim] cation might be largely affected by the pressure. On the other hand, the enhanced nanostructure of [C_nmim][BF₄] at n > 5 is disrupted by the pressure before the conformational change of the [C_nmim] cation occurs.

Fig. 4 Volume changes (ΔV^{trans→gauche}) between the conformers of the [C_nmim] cation of [C_nmim][BF₄] as a function of n (combined with the results for [C₄mim][BF₄]⁴¹ and alkanes^8,9,45,46). Closed and open circles represent [C_nmim][BF₄] and alkanes, respectively.

Here, we discuss the origin of the inversion of slope in ΔV^{trans→gauche} for [C_nmim][BF₄]. Considering the coulombic interaction and the alkyl-chain packing effect in [C_nmim][BF₄], the crystallization of [C_nmim][BF₄] with a shorter alkyl-chain length is plausible to be caused by the larger contribution of the coulombic interaction rather than by the alkyl-chain packing effect, whereas that with a longer alkyl-chain length is due to the larger contribution of the alkyl-chain packing effect rather than that of the coulombic interaction. If the competition between the coulombic interaction and the alkyl-chain packing effect is reflected in the inversion of slope in ΔV^{trans→gauche} for [C_nmim][BF₄], the corresponding pressure-induced frequency shifts for the BF₄ stretching (ν_BF4) band (observed at ∼750 cm⁻¹) suggesting the coulombic interaction and the CH₂ rocking (ν_CH2) band (which indicates the alkyl-chain packing effect) should occur in response to n. Thus, we checked the pressure-induced frequency shifts of ν_BF4 and ν_CH2 of [C_nmim][BF₄]. ν_BF4 and two ν_CH2 bands (the gauche and trans conformers) shifted to higher frequencies with increasing pressure. These higher frequency shifts in response to pressure show the repulsion effect of the cation/anion species, and they are consistent with the general trend of the pressure-induced frequency shift of MLs.^1,40,50

Fig. 5 shows the changes in the values for the pressure dependency of ν_BF4 ((∂ν_BF4/∂p)_T) and ν_CH2 ((∂ν_CH2/∂p)_T) of [C_nmim][BF₄] as a function of n. The values of (∂ν_BF4/∂p)_T (▲) are almost constant throughout the studied n range. On the other hand, the values of (∂ν_CH2/∂p)_T show different behavior from (∂ν_BF4/∂p)_T. On the whole, the (∂ν_CH2/∂p)_T value (●) of the gauche conformer with the smaller PMV is larger than that for the trans conformer (○) throughout the studied n range. Remarkably, the values of (∂ν_CH2/∂p)_T for both conformers increase up to n = 5 and become constant above n = 5. Related to this, as stated above, the increase in the alkyl-chain length induced the enhancement of the alkyl-chain packing effect of [C_nmim]-based ILs (nanoheterogeneity enhancement),^46–49 although the coulombic interaction arising from the imidazolium ring and the anion is independent of n. These indicate that the pressure-induced environmental change around the gauche conformer is larger than that around the trans conformer throughout the studied n range. In addition, changes in (∂ν_CH2/∂p)_T of both conformers versus n are in good agreement with the inversion of slope in ΔV^{trans→gauche}.

Fig. 5 Pressure dependency of the frequency shifts of BF₄ stretching (triangles) and CH₂ rocking (circles) bands of [C_nmim][BF₄] as a function of n. Closed and open circles represent the gauche and trans conformers, respectively. The lines represent the results of the least-square and sigmoidal fits.

Based on these results, and because the (∂ν_BF4/∂p)_T values are almost constant in relation to n and the (∂ν_CH2/∂p)_T values of both conformers increase up to n = 5, the alkyl-chain packing effect (cation–cation interaction) of [C_nmim][BF₄] at n = 3–5 is shielded by the coulombic interaction (cation–anion interaction). In contrast, with a further increase of the alkyl-chain length above n = 5, the alkyl-chain packing effect of [C_nmim][BF₄] above n = 5 might be stronger than the coulombic interaction. According to the recent MD simulations of [C₈mim][BF₄] by Russina et al.,²⁵ the coulombic correlation (cation–anion interaction) in the pair distribution functions is relatively unchanged up to 1 GPa. They noted that the coulombic interaction of [C₈mim][BF₄] is independent of pressure; pressure changes will largely affect the conformation of the alkyl chain. Their results are consistent with the present results of the frequency shifts versus n.

Based on these results, the inversion of slope in ΔV^{trans→gauche} for [C_nmim][BF₄] might be related to the competition between the coulombic interaction and the alkyl-chain packing effect of the conformers together with BF₄ anions. Here, ΔV^{trans→gauche} was simply decomposed into two contributions:

ΔV^{trans→gauche} = ΔV_C + ΔV_P (2)

where ΔV_C and ΔV_P are the coulombic interaction volume difference and the difference in the volumes due to alkyl-chain packing between the conformers together with BF₄ anions, respectively. From these contributions, we can suggest that in [C_nmim][BF₄] with a short alkyl chain (n < 5), conformational stability is affected by the coulombic interaction, and the contribution of ΔV_C is larger than that of ΔV_P. On the other hand, in [C_nmim][BF₄] with a long alkyl chain (n > 5), since the alkyl-chain packing effect becomes larger with increasing n, the contribution of ΔV_P is larger than that of ΔV_C. Accordingly, the inversion of slope in ΔV^{trans→gauche} for [C_nmim][BF₄] versus n corresponds to a change in the dominant factor (from ΔV_C to ΔV_P).

Consequently, we can conclude that the inversion of slope in ΔV^{trans→gauche} for [C_nmim][BF₄] is due to the competition between ΔV_C and ΔV_P: the former is predominant for ΔV^{trans→gauche} below n = 5, and the latter is predominant above n = 5. These results indicate that this competition is reflected in the r-values, which correspond to ΔG^{trans→gauche}versus n.

Preferential role of the alkyl-chain length in glassy formation of [C_nmim][BF₄] under high pressure

As seen in the previous section, the conformational stability of [C_nmim][BF₄] versus n under high pressure has been discussed, and changes in the conformational stability of the [C_nmim] cation are due to the competition between the coulombic interaction and the alkyl-chain packing effect. In this section, we discuss the preferential role of the alkyl-chain length in conjunction with the glass formation of [C_nmim][BF₄] under high pressure.

Fig. 6(a) shows the Raman spectra in the CH₂ rocking region of [C_nmim][BF₄] in the HP glassy state at several n values. The spectra showed similar spectral shapes throughout the studied n range. To further investigate the conformational preference of the [C_nmim] cation, we determined the fraction of the gauche conformer (f_gauche); in combination with the liquid state and retrieved results, this is shown in Fig. 6(b). In the liquid state, a drastic decrease in f_gauche was observed from n = 5 to 6. The f_gauche values (f_gauche = 0.65–0.68) in the HP glassy state are higher than those in the liquid state (f_gauche = 0.53–0.65) and linearly decrease with increasing n. Moreover, the retrieved f_gauche values are almost consistent with the f_gauche values before compression.

Fig. 6 (a) Raman spectra in the CH₂ rocking region of [C_nmim][BF₄] (n = 3, 5, and 6) in the HP glassy state. (b) Changes in the fraction (f_gauche) of the gauche conformer of the [C_nmim] cation of [C_nmim][BF₄] as a function of n at 298 K and 0.1 MPa (○), HP glassy state at 298 K (●), retrieved results (▲), and supercooled glassy state (△). Here the f_gauche values are determined by the equation f_gauche = [I_gauche/(I_gauche + I_trans)] (I is the integrated Raman intensity). The Raman spectra of [C₈mim][BF₄] in the supercooled glassy state was recorded at a cooling rate of 2 K min in the temperature region from 298 K to 180 K. The lines represent the results of the least-square or sigmoidal fits.

Related to these results, our recent high-pressure small-angle X-ray scattering (HP-SAXS) results demonstrated that the intensity of a low-Q peak (∼2.8 Å⁻¹) that shows the nanostructure in the SAXS profiles of [C₈mim][BF₄] drastically decreases up to 2.0 GPa.²³ In response to the HP-SAXS results, Russia et al.²⁵ used MD simulations to demonstrate that the simulated low-Q peak (∼2.8 Å⁻¹) of [C₈mim][BF₄] vanished below 1 GPa. Similar MD simulation results were also obtained in the case of ammonium-based ILs.⁵¹ These results indicate that the nanostructure of ILs is disrupted with increasing pressure. Moreover, our Raman results showed that [C₈mim][BF₄] forms the glassy state above 2.2 GPa, and that the gauche conformer of the [C₈mim] cation increases above 1 GPa.²³ Combined with these results, after the nanostructure of [C_nmim]-based ILs is disrupted under high pressure, further compression will induce an increase in the gauche conformer of the [C_nmim] cation; [C_nmim][BF₄] then formed the glassy state. On the other hand, the pressure release caused the reversal of the conformational preference in the liquid state. It is evident that an increase in the disordered conformer (gauche conformer) of the glassy state showing a metastable phase is necessary to avoid crystallization of the highly ordered conformer (trans conformer). Thus, it is intriguing which alkyl-chain part in the [C_nmim] cation contributes the HP glassy state.

A remarkable result is the difference in the f_gauche between the ambient pressure and the HP glassy state depending n, when n is classified into two regions: (1) n = 3–5 and (2) n = 6–8. Since the second region is consistent with the region of ΔV^{trans→gauche} inversion, the increase in f_gauche from 0.1 MPa to HP in the former region is due to the coulombic interaction and that in the latter region is due to the alkyl-chain packing effect. Recent MD simulations demonstrated the difference in the conformational preference of the alkyl-chain length of ILs under high pressure.^25,52 Russina et al.²⁵ showed that the increase in the gauche conformer occurred for the cation-tail portion above C = 5 (C is the carbon number of the alkyl chain) in [C₈mim][BF₄] rather than for the cation-head portion below C = 5. On the other hand, Sharma et al.⁵² reported that the conformational preference of the cation in 1-alkyl-methylpyrrolidinium bis(trifluoromethylsulfonyl) imide ([Pyrr_1,n][NTf₂], n = 8 and 10) under high pressure is opposite to that of [C₈mim][BF₄]; the increase in the gauche-kink conformer of [Pyrr_1,n] cations occurred below C = 3–5 (the cation-head portion) under high pressure. They reported that the different pressure dependency of the cation-head and cation-tail portions of these ILs is due to the difference in the conformational flexibility between the imidazolium ring-BF₄ and the pyrrolidinium ring-NTf₂. These results indicate that the conformational stability of the cation-head portion under high pressure is different from that of the cation-tail portion.

Based on all the results, we propose the preferential role of the alkyl-chain length in glass formation of [C_nmim][BF₄] under high pressure. As mentioned in the Introduction, high pressure causes glass formation of [C_nmim][BF₄] in the range n = 3–8 and the gauche conformer increases. Moreover, the inversion of slope in ΔV^{trans→gauche} occurred at n = 5 due to competition between the coulombic interaction and the alkyl-chain packing effect. Considering these results, it is apparent that the alkyl-chain length plays an important role in glass formation; i.e., if the alkyl chain carbon number is longer than 5, in addition to the cation-head portion curling, the cation-tail curling is necessary to inhibit the crystallization due to the alkyl-chain packing effect (Fig. 7). This adjustment is a likely consequence, because in the case of [C_nmim][BF₄] the BF₄ anion is positioned around the top of the imidazolium-ring.⁵⁴ Since such a conformational role was not observed for alkanes under crystallization at high pressure, the role of the alkyl chain of ILs is quite different from that of alkanes in the absence of the coulombic interaction. Moreover, this cation-tail (conformational) adjustment effect can explain the decompression crystallization, as seen in [C₂mim][BF₄] and [C₈mim][BF₄] at around 1 GPa.^18,23 The decompression crystallization is related to the relaxation of the restricted terminal alkyl-chain group under high pressure, i.e., the cation-tail portion via the cation-tail adjustment effect. Very recently, Faria et al.⁵³ reported that the oligomerization of 1-allyl-3-methylimidazolium dicyanamide ([allylC₁im][N(CN)₂]) occurred above 8.0 GPa. They mentioned that the terminal double bond of the allyl-group directly involves the HP oligomerization reaction. The terminal double bond in their study corresponds to the cation-tail part of the [C_nmim] cation in our study. This result ensures that the cation-tail portion of imidazolium-based ILs is an important key of the HP phase transition of ILs after glass transition (>3.0 GPa).

Fig. 7 Schematic representation for the preferential role of the alkyl-chain length of (a) [C₃mim][BF₄] and (b) [C₈mim][BF₄].

On the other hand, in the case of supercooled glass, the situation is quite different. There is no need to make a conformational adjustment under no compression. The f_gauche value is comparable to that at the liquid state at 0.1 MPa, as seen in Fig. 6(b).

It is difficult to mention more about the relationship between the cation-head and cation-tail portions under high pressure since the direct conformational probability for each dihedral angle of the [C_nmim] cation is still unclear. Further theoretical investigations, such as MD simulations under high pressure, e.g., the systematic conformational probability of the cation-head and cation-tail portions and the pressure dependence of the cation–anion configuration, have provided us with further knowledge for understanding the glass formation and crystallization mechanism of [C_nmim]-based ILs under high pressure.

Conclusions

The preferential role of the alkyl-chain in glass formation under high pressure was provided in view of the conformational stability of 1-alkyl-3-methylimidazolium tetrafluoroborate ([C_nmim][BF₄], n = 3–8). To evaluate the results, we determined the intensity ratio (r) and the differences in the partial molar volume (ΔV^{trans→gauche}) between the conformers of the [C_nmim] cation using Raman intensities, and both values were classified into two alkyl-chain length groups at the border of n = 5. The conformational stability of the [C_nmim] cation is due to the competition between the coulombic interaction and the alkyl-chain packing effect of the conformers together with BF₄ anions. In combination with the conformational preference under HP glassy states, the alkyl-chain part plays an important role in glass formation; i.e., the cation-tail portion (above C = 5) adjusts to inhibit the crystallization (the conformational adjustment effect).

However, the present findings on the preferential role of the alkyl-chain part is only the case for [C_nmim][BF₄]; it is unclear whether the cation-tail adjustment effect can adapt other ILs. A recent MD simulation study of the ester functionalized imidazolium-based ILs with various anions indicated that the nanoscale organization of ILs is affected by the cation–anion interaction, the cation–anion configuration, and the alkyl-side chain terminal of cations.⁵⁵ In fact, the increase in the gauche conformer of the cation-tail or cation-head portion is largely dependent on the cationic and anionic species (as in the case of other imidazolium-based and pyrrolidinium-based ILs with NTf₂).⁵⁶ Related to this, the conformational preference (trans > gauche) of [C₄mim][I] at 0.1 MPa is different from that of [C₄mim][BF₄] (trans < gauche) by the difference in the cation–anion configuration.⁵⁴ Based on these results, studies of the counter anion effect on the conformational stability of ILs under high pressure are clearly important for further elucidation of the role of alkyl-chain flexibility in the phase transition behavior. We are currently investigating the topics which will be published elsewhere.

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