Nonaqueous Microemulsions Containing Ionic Liquids – Properties and Applications

There is a still growing interest in ionic liquids (ILs) in general and room temperature ionic liquids (RTILs) in particular resulting from their fascinating and outstanding properties and wide range of potential applications. The research field of ILs was almost entirely related to imidazolium, pyridinium and pyrrolidinium based substances in the last decade (Welton 1999; Earle & Seddon 2000; Wasserscheid & Keim 2000). Beside these classical aprotic ionic liquids, attention has been paid to protic (Greaves & Drummond 2007; Greaves et al., 2008 b) and to the development of less toxic ILs (Tao et al., 2006; Fukaya et al., 2007; Pernak et al., 2007; Zech et al., 2009 a). ILs are often considered as future solvents for catalysis (Welton 1999; Wasserscheid & Keim 2000; Pârvulescu & Hardacre 2007; van Rantwijk & Sheldon 2007), chemical reactions (Haumann & Riisager 2008; Martins et al., 2008), extractions (Blanchard et al., 1999), and electrochemical purposes (Hapiot & Lagrost 2008). Apart from these applications, ILs also stimulated research in classical colloid and surface chemistry. The formation of amphiphilic association structures in and with ionic liquids, such as micelles, vesicles, microemulsions and liquid crystalline phases has been reviewed three times between 2007 (Hao & Zemb 2007) and 2008 (Qiu & Texter 2008; Greaves & Drummond 2008 a), reflecting the growing interest and progress in this field. In this review we focus on ILs in nonaqueous microemulsions, because significant new work has been reported in this field since these earlier reviews have been published. Microemulsions are thermodynamically stable, isotropic transparent mixtures of at least a hydrophilic, a hydrophobic and an amphiphilic component. The first microemulsion structures termed at that time “oleophatic hydro-micelle” were discovered in 1943 by Hoar and Schulman (Hoar & Schulman 1943). The term microemulsion was introduced by Schulman and coworkers in 1959 describing optically isotropic transparent solutions consisting of water, oil, surfactant and alcohol (Schulman et al., 1959). A more recent definition was given by Danielsson and Lindman: “A microemulsion is a system of water, oil and an amphiphile which is a single optically isotropic and thermodynamically stable liquid solution” (Danielsson & Lindman 1981). Herein, “water” corresponds to a polar phase that is classically an aqueous solution that can contain electrolytes and other