Dynamic Heterogeneity in Room-Temperature Ionic Liquids

Room temperature ionic liquids (RTILs) are comprised of bulky organic cations and anions (Holbrey & Seddon, 1999; Weingärtner, 2008). Because of the intricate interplay of various interand intramolecular interactions, RTILs have rich dynamical properties and have found diverse applications. In particular, negligible vapor pressure of RTILs makes them a green alternative of conventional organic solvent. For understanding of RTIL’s dynamical behavior at the molecular level, solvation and rotational dynamics have been studied experimentally (Arzhantsev et al., 2007; 2006; Cang et al., 2003; Funston et al., 2007; Ingram et al., 2003; Jin et al., 2007; Karmakar & Samanta, 2002a;b; Lang et al., 2006) and theoretically (Jeong et al., 2007; Kobrak, 2006; 2007; Kobrak & Znamenskiy, 2004; Shim et al., 2006; 2007; Shim & Kim, 2009). It is found that the ultrafast relaxation in a subpicosecond time regime contributes substantially to solvation dynamics, disproving the diffusion-controlled solvation in RTILs. On the other hand, in the long time regime, solvation and rotational dynamics of RTILs show slow nonexponential relaxations, which is a characteristic of glassy liquids (Jeong et al., 2008; Shim et al., 2005b). In this chapter, we particularly focus on the dynamical properties of RTILs as a viscous liquid (Rodriguez & Brennecke, 2006). High viscosity of RTILs has two different aspects in nanoscale applications such as solar cells (Noda et al., 2003; Wang et al., 2003) and capacitors (Tsuda & Hussey, 2007): it is advantageous to preventing the leakage of electrolyte, but disadvantageous to enhancing transport properties. In order to realize the potential applications of RTILs, it is necessary to scrutinize the transport process and relaxation dynamics in RTILs microscopically via molecular dynamics (MD) simulations (Bhargava & Balasubramanian, 2005; Klähn et al., 2008; Popolo & Voth, 2004; Zhao et al., 2009). One notable feature observed in simulation studies of RTILs is so called dynamic heterogeneity (Habasaki