High Pressure Phase Behavior of Two Imidazolium-Based Ionic Liquids, [bmim][BF4] and [bmim][PF6]

Room temperature ionic liquids (RTILs) consisting of organic cations and inorganic anions remain in the liquid state at room temperature (Welton, 1999). Due to their dual nature as both salts and fluids, vast interests in investigating the properties and applications of RTILs have been conducted. Because of the low melting point, almost zero vapor pressure, wide electrochemical window, and high recyclibility, RTILs are very attractive for a wide range of applications such as catalysis (Welton, 1999), synthesis (Itoh et al., 2004), and electrochemical devices (Galinski et al, 2006; Sato et al., 2004). Generally, RTILs are classified into several groups depending on the cationic series, imidazolium, pyridinium, and alphatic quaternary ammonium series. Most of studies so far concern with the results at atmospheric pressure, but recently an increasing number of works using high pressure is reported (Chang et al., 2007, 2008a, 2008b, 2008c; Su et al., 2009, 2010; Umebayashi et al., 2009a, 2009b). A merit of the use of high pressure as a varialbe is summarized as follows. We can change, in a cotrolled way, the intermolecular interactions without encountering major pertubations, but an alteration in temperature of a chemical system at atmospheric pressure produces a simultaneous change in the thermal energy and the volume. To separate the thermal and volume effects, we need to execute high pressure experiments. RTILs typically exhibit different bonding interactions, but unlike in other liquids, they are characterized by the presence of Coulomb interactions among the constituent ions. Thus, many RTILs are easily supercooled, and may introduce a rich phenomenology in the dynamics and also the phase transition. A bulky, asymmetric organic cation prevents ions from packing and the solidification. Therefore, it is most likely that the liquid structure of RTILs is determined by a balance between long-range electrostatic forces and local geometric factors. Application of high pressure is the ideal tool to tune and/or alter the liquid structure along with the bonding properties of the materials. If the RTIL system is compressed, the reaction moves to favor the components with smaller volume. In this


Introduction
Room temperature ionic liquids (RTILs) consisting of organic cations and inorganic anions remain in the liquid state at room temperature (Welton, 1999). Due to their dual nature as both salts and fluids, vast interests in investigating the properties and applications of RTILs have been conducted. Because of the low melting point, almost zero vapor pressure, wide electrochemical window, and high recyclibility, RTILs are very attractive for a wide range of applications such as catalysis (Welton, 1999), synthesis (Itoh et al., 2004), and electrochemical devices (Galinski et al, 2006;Sato et al., 2004). Generally, RTILs are classified into several groups depending on the cationic series, imidazolium, pyridinium, and alphatic quaternary ammonium series. Most of studies so far concern with the results at atmospheric pressure, but recently an increasing number of works using high pressure is reported (Chang et al., 2007(Chang et al., , 2008a(Chang et al., , 2008b(Chang et al., , 2008cSu et al., 2009Su et al., , 2010Umebayashi et al., 2009aUmebayashi et al., , 2009b. A merit of the use of high pressure as a varialbe is summarized as follows. We can change, in a cotrolled way, the intermolecular interactions without encountering major pertubations, but an alteration in temperature of a chemical system at atmospheric pressure produces a simultaneous change in the thermal energy and the volume. To separate the thermal and volume effects, we need to execute high pressure experiments. RTILs typically exhibit different bonding interactions, but unlike in other liquids, they are characterized by the presence of Coulomb interactions among the constituent ions. Thus, many RTILs are easily supercooled, and may introduce a rich phenomenology in the dynamics and also the phase transition. A bulky, asymmetric organic cation prevents ions from packing and the solidification. Therefore, it is most likely that the liquid structure of RTILs is determined by a balance between long-range electrostatic forces and local geometric factors. Application of high pressure is the ideal tool to tune and/or alter the liquid structure along with the bonding properties of the materials. If the RTIL system is compressed, the reaction moves to favor the components with smaller volume. In this situation, an intriguing question is raised: if we compress RTILs using pressure as an external factor, what happens to the phase behavior; the crystallization occurs or the liquid state holds up to a certain pressure? The aim of the present manuscript is to show the phase stabilities of imidazolium-based (i.e., known as prototype) RTILs at high pressures over 1 GPa. From the results, we may shed light on the relative importance of the hydrophilic vs. hydrophobic part of molecules in the collapse and aggregation (crystallization) processes. Additionally, it is now well established that the 1-butyl-3-methylimidazolium ([bmim]) cation, has a trans-trans (tt)-gauche-trans (gt) equilibrium of the NCCC and CCCC angles of the butyl group as shown in Figure 1 (Hamaguchi & Ozawa, 2005). We wonder that the conformational analysis of [bmim] cation under high pressure is necessary to obtain the structural information of the high pressure phase behavior of the RTIL in relation to the packing efficiency. Actually, the solution structures including the conformational behavior o f R T I L s h a v e b e e n s t u d i e d t o r e v e a l t h e physical and chemical properties such as conductivity, viscosity, and melting point (Chang et al., 2007(Chang et al., , 2008cNishikawa et al., 2008Nishikawa et al., , 2009Rivera-Calzada et al., 2008;Seki et al., 2009). The remainder of the paper is organized as follows. In section 2, we summarize the experimental methods.

High pressure measurements
For high pressure experiments, we used a diamond anvil cell (DAC, Kyowa Co. Ltd., SR-DAC-KYO3-3d) as shown in Figure 3. The sample solution was placed together with a small amount of powdered ruby chip for a pressure marker in a SUS301 gasket mounted in the DAC ( Figure 4). The spectral shift of the R 1 fluorescence line of ruby chip was used for a pressure calibrant (Mao et al., 1978). The error of the pressure was ±0.05 GPa. The DAC was mounted under the microscope of a JASCO NR-1800 spectrophotometer ( Figure 5).

Raman measurements
Raman spectroscopy is often used to explore bonding structures of liquids, because it provides information on the local structure in the liquid state. The observation of the C-H stretching vibration is one of the keys to characterize the imidazolium-based cations and can serve as a useful probe to reflect the phase transition behavior. Raman spectra were measured by a JASCO NR-1800 Raman spectrophotometer equipped with a single monochromator and a charge coupled device (CCD) detector. The 514.5 nm line from a Lexel Ar + laser was typically used as an excitation source with a power 250 mW. For complimentary measurements of temperature-induced Raman spectral changes at a normal pressure, we controlled the sample temperature by LINKAM THMS-600 (Japan hitech Co.). The cooling rate was 5 K/min. For the experiments on conformational variation with temperature, temperature was decreased from 298 K to 113 K in 10 K decrements. The obtained Raman spectra were fitted with Gaussian-Lorentzian mixing function using the GRAMS/386 software (Galactic Ind. Co. Ltd.).

Geometric volume calculations
Density functional theory (DFT) calculations have been used as an investigation methodology for the conformational analysis (Holomb et al., 2008;Umebayashi et al., 2005). We performed the DFT calculations of geometries of the tt ([bmim])-anion and the gt ([bmim])-anion complexes. All DFT calculations were carried out using GAUSSIAN03 program (Frish et al., 2003). For the present calculation, we used the Becke's three-parameter (B3) exchange function (Becke, 1998). The B3 exchange function was combined with Lee-Yang-Peer correlation function (B3LYP) (Lee et al., 1998). The 6-311+G(d) basis set was used. Geometric volume calculations have been conducted as an analysis methodology for obtaining the information on a relationship between the partial molar volume (PMV) and the structure of a molecule (Edelsbrunner et al., 1995;Imai et al., 2005;Ling et al., 1998;Takekiyo et al., 2006aTakekiyo et al., , 2006b. Geometric component of [bmim] cation in the RTILs enables the calculation of the solvent accessibility of the molecular surface by assuming that RTILs are hard-sphere probes. The details of geometric volume calculations were described in the previous papers (Edelsbrunner et al., 1995;Ling et al., 1998;Takekiyo et al., 2006aTakekiyo et al., , 2006b. We have calculated the van der Waals (V w ) and the molecular volumes (V M ). V M is composed of the V w and the void volume (V v ). V v is structural void within the solvent inaccessible core of the solute molecule. Hence, the V v can be obtained by the subtraction of Figure 6 shows the definition of respective volume contributions schematically. For the radius of the probe, the average radius of each optimized [bmim] conformer-anion using B3LYP/6-311+G(d) level as a hard-sphere probe was used. The radius of the solvent probe was estimated to be 5.93 Å for [bmim] [PF 6 ].

Phase stabilities of 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF 4 ]) and hexafluorophosphate ([bmim][PF 6 ]) at high pressures
Firstly we provide the results of the Raman spectral changes of [bmim] [BF 4 ] as a function of pressure in Figure 7. We follow the detailed peak assignments of the CH stretching mode (ν CH ) reported previously (Heimer et al., 2006). The wavenumber region from 2750 to 3075 cm -1 is due from the ν CH of the alkyl chain of [bmim] cation and that from 3075 to 3200 cm -1 is from the ν CH mode of the imidazolium ring, respectively. We can see that the ν CH band shifts to a higher frequency with accompanying a broadening of the band upon compression. This frequency shift might arise from the contraction of C-H bonds and the overlap repulsion effect by applied pressure (Chang et al., 2008a). We note that at 1.4 GPa a larger change in the spectrum of [bmim] [BF 4 ] was obtained. The spectral resolution of each peak in the ν CH of the alkyl chain becomes unclear; the peak intensities at 2873 cm -1 and 2910 cm -1 due from the symmetric ν CH (ν CH ss ) of propyl (C6, C7 and C8) group and from the ν CH ass of butyl (C6, C7, C8 and C9) group apparently decrease along with a pronounced peak broadening, whereas the peak at 3168 cm -1 assigned to the ring ν CH ss of C(4)H and C(5)H groups can be still clearly observable. Importantly, spectral changes due from the alkyl chain are larger than those from the imidazolium ring, indicating that the environment around the butyl chain is larger perturbed than that around the imidazolium ring with applied pressure. This pressure-enhanced change in the ν CH mode might be a key to provide the stability of the material under high pressure conditions. Specifically, the wavenumber shift of the most intense peak at around 2965 cm -1 assigned to the C(9)H asymmetric stretching mode (ν C(9)H ass ) of [bmim] cation as a function of pressure is shown in Figure 8. The ν C(9)H ass linearly shifts to a higher frequency with increasing pressure without any discontinuous jump under 1.4 GPa. A distinct phase change such as crystallization could not be recognized and [bmim] [BF 4 ] is still stable at this pressure range. Additionally, unlike common molecular liquids, RTILs show a structural feature on the nanoscopic level, which is one of the characteristic features appear in RTILs. Based on the simulation (Raabe & Koehler, 2008) and experimental (Xiao et al., 2007) studies, imidazolium-based RTILs show the nanoscale spatial heterogeneity (nano-phase separation) where an anion and a positively charged imidazolium ring pair up, and the pairs aggregate and form polar domains. Alkyl side chains of imidazolium cations also aggregate and form nonpolar domains. This domain structure due from the separation of each polar/nonpolar domain is a cause of the heterogeneity. The nano-phase separations exist when an imidazolium cation has over 4 (i.e., longer than butyl chain) on the alkyl side chain (Lopes & Padua, 2006;Wang & Voth, 2005). The "nano-heterogeneity" may have an influence on the non-equilibrium phase behavior. On the other hand, Figure 9 shows the pressure-induced Raman ν CH spectral changes of [bmim] [PF 6 ] having the hydrophobic anion, as compared to the BF 4¯ anion. We can see that the fine structures are appeared in the Raman ν CH spectrum at 0.2 GPa. Previous high pressure-differential thermal analysis (HP-DTA) study of [bmim] [PF 6 ] showed that the phase transition occurs at ～0.1 GPa (Su et al., 2009). Therefore, this change corresponds to the (liquid-crystalline) phase transition. Further compression above 0.2 GPa, all the observed Raman ν CH frequencies shift to higher frequencies as shown in Figure 10. Interestingly, the Raman ν CH band at around 2916 cm -1 due from the butyl chain clearly separates into two peaks (▲: 2923 cm -1 and △: 2935 cm -1 ) at 1.1 GPa. Additionally, a new peak at 2985 cm -1 (◇) appeared at above 1.7 GPa. These spectral changes might imply the existence of other high pressure solid-solid phase transitions at around 1.1 and 1.7 GPa other than reported one at 0.2 GPa. In previous differential scanning calorimetry (DSC) and Raman studies in the heating process (scanning rate =5.0 mK/s), Endo et al. (Endo et al., 2010)    As mentioned above, imidazolium [C n -mim] cations having longer alkyl chains of n ≥ 4 tend to aggregate within the polar or non-polar regions and the nanoscale spatial heterogeneity occurs (Lopes & Padua, 2006;Wang & Voth, 2005). We suppose that there may be a shortening of the size of the micelle-like domain structure in [bmim][BF 4 ] upon compression. As another important feature of [bmim] cation in the liquid state, we point out that two stable conformers, the tt ( trans-trans) form and the gt ( gauche-trans) form (Hamaguchi & Ozawa, 2005;Ozawa et al., 2003) due to the rotational isomerism of butyl group exist, which will be described in detail below. These may be a cause for the difference in the phase behavior in response to the external functions (pressure and temperature). As stated briefly above, Raman spectral changes in the low temperature phases of [bmim][PF 6 ] were already reported by Endo et al. (Endo et al., 2010). We will discuss the relation to the conformational changes of [bmim] cation in the next section.

Conformational preferences of [bmim][BF 4 ] and [bmim][PF 6, ] with the relation to the phase stabilities
Thermodynamic study concerning the conformational equilibrium of RTILs is useful to clarify the relationship between the conformation and complicated phase transition www.intechopen.com behavior of RTILs (Endo et al., 2010;Hamaguchi & Ozawa, 2005;Imanari et al., 2008;Katayanagi et al., 2004 Now it is well established that [bmim] cation in the liquid state has a tt-gt equilibrium of the NCCC and CCCC angles of the butyl group as already shown in Fig. 1 (Hamaguchi & Ozawa, 2005;Ozawa et al., 2003). Recently, Endo et al. (Endo et al., 2010) Figure 12. Two peaks at 580 and 620 cm -1 assigned to the CH 2 rocking mode of NCCC moiety are due to the gt and tt conformers of [bmim] cation, respectively (Hamaguchi & Ozawa, 2005;Ozawa et al., 2003). We found that the Raman intensity due to the gt conformer drastically increases at 0.2 GPa, and the intensity of the tt conformer decreases.  (Chang et al., 2007). To discuss the observed results further, the pressure dependence of the intensity ratio (I gt /I tt ) between the conformers of [bmim] cation is shown in Figure 13. The value of I gt /I tt drastically increases up to 0.2 GPa, but after high pressure phase transition (crystallization) occurs, the I gt /I tt shows a constant value up to 1.0 GPa. From these results, we can conclude that the high pressure phase of [bmim] [PF 6 ] prefers the gt conformer. We note that further compression up to 2.0 GPa induces two more changes in the I gt /I tt ratio, implying the existence of other high-pressure crystalline phase transitions in agreement with the results as shown in section 3.1. On the other hand, as to the [bmim] [BF 4 ], the value of I gt /I tt slightly increases with applied pressure as shown in Figure 13, i.e., the gt conformer is also a dominant specie and stabilized under high pressure (Takekiyo et al., 2011). Therefore, the predominance of the gt conformer under high pressure seems to be less dependent on the anionic species such as PF 6¯, BF 4¯ (Takekiyo et al., 2011), andCl¯ (Change et al., 2007). In relation to this, previous Raman and IR studies of simple organic liquids such as dihaloethane (Taniguchi et al., 1981), halopropane (Kato & Taniguchi, 1990), and n-alkane in neat solution (Kato & Taniguchi, 1991;Schoen et al., 1979) show that the gauche conformer is more stable than the trans conformer under high pressure. Thus, the higher stability of gt conformer of [bmim] [PF 6 ] under high pressure is agreement with the pressure-induced conformational change of the simple organic liquids (Kato & Taniguchi, 1990, 1991Schoen et al., 1979;Taniguchi et al., 1981).

The origin of the phase behavior: Interpretation from geometric volume calculations
In this section, we discuss why the gt conformer of [bmim] cation has a higher stability at high pressure. Here, to clarify the main factor of the conformational stability of [bmim] cation under high pressure, we calculated the optimized structures of the tt ([bmim])-counter anion (e.g., PF 6 -) and the gt ([bmim])-anion complexes using B3LYP/6-311+G(d) level. We expected that the pressure shifts the conformational equilibrium to the conformation with the smaller partial molar volume (PMV). From the pressure dependence of the I gt /I tt value in Figure 13, we can predict the PMV difference between the conformers for [bmim] cations in [bmim] [PF 6 ] and [bmim] [BF 4 ]. We found that the PMV of the gt conformer of [bmim] cation is smaller than that of the tt conformer (ΔV tt→gt <0). To discuss the main factor of ΔV tt→gt <0 of [bmim] cation, we additionally performed the geometric molecular volume calculation of [bmim] cation by simple molecular volume calculation method (Imai et al., 2005;Takekiyo et al., 2006aTakekiyo et al., , 2006b. Here, we assume that the molecular volume (V M ) composes of the van der Waals volume (V W ) and the structural void volume (V v ). Figure 14 shows the change of V M of [bmim] cation as a function of the NCCC angle. Conformational change from the trans to the gauche conformers accompanies with the decrease of V M value (ΔV M tt→gt = -1.55 cm 3 /mol). In this calculation the value of -0.13 cm 3 /mol for ΔV W tt→gt and -1.42 cm 3 /mol for ΔV v tt→gt are obtained. We found that ΔV v tt→gt is the predominant factor in ΔV M tt→gt . This calculation results well explain the experimental results (ΔV tt→gt <0). Therefore, the negative volume change of [bmim] cation is mainly ascribed to the contribution from the ΔV v tt→gt . We conclude that the preference of the gt conformer having the smaller PMV in the high-pressure phase of [bmim] [PF 6 ] is largely due to the void volume contribution, which relates to the difference in the molecular packings between the conformers of [bmim] cation.  The results indicate that a contribution of the anion is significant to the phase stability at high pressures. These clearly contrast with the results in the low-temperature phase behaviors of both RTILs. Another important fact deduced from the present study is conformational change in [bmim] cation occurs concordant with the phase transition. In an effort to elucidate the origin of the phase behavior, interpretation from theoretical calculations were done. We showed the conclusion that a key to understand the conformational preference of gt conformer in the high-pressure [bmim][PF 6 ] phase is the void volume contribution of [bmim] cation . RTILs have received significant attention for their use in multidisciplinary chemistry areas (Itoh et al., 2004;Galinski et al., 2006;Sato et al., 2004;Welton, 1999). Investigations on the high pressure behavior of the materials have important significance regardless of their basic knowledge or understanding of the potential application. As Sui et al. (Su et al., 2009) pointed out, one direct impact of the understanding of the high pressure phase behavior of RTILs can be recycling or purification of RTILs. High pressure crystallization may be an effective way to recycle RTILs at the room temperature, as crystallization temperature increases as the pressure rises. The green nature of RTILs as solvent is difficult to evaporate, which prevents the pollution on environment. In another point of view, however, exactly because of the merit in RTILs, distillation through evaporation, which is an important approach to recycle and purity common organic solvents, cannot be applied to RTILs (Chang et al., 2008b;Su et al., 2010). Moreover, RTILs are very hazardous to the environment if they are released. For instance, a hydrolysis of BF 4¯ anion in the presence of water is sometimes known to occur (Freire et al., 2010). On the other hand, as present results shown, pressure plays a critical role in regulating the structures and properties of materials. High pressure is unique tool to explore "new" chemistry by creating novel substances and altering the properties of existing compounds. Pressure may use as a way to explore the concealed properties of fluids and solids. One example is that the high-pressure development of functional materials with potential industrial applications, such as high-energy density materials, superhard materials and hydrogen storage materials, has become a vibrant area of high pressure research.