8 High Pressure Phase Behavior of Two Imidazolium - Based Ionic Liquids ,

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