Unique Characteristics of Ionic Liquids Comprised of Long-Chain Cations and Anions: A New Physical Insight Chiranjib Banerjee, Sarthak Mandal, Surajit Ghosh, Jagannath Kuchlyan, Niloy Kundu, and Nilmoni Sarkar* Department of Chemistry, Indian Institute of Technology, Kharagpur 721302, WB, India * S Supporting Information ABSTRACT: We have designed a unique class of surface active ionic liquids (SAILs) and utilized them to prepare IL-in- oil microemulsions as well as large unilamellar vesicles (LUVs). The IL-in-oil microemulsions were characterized by a phase behavior study, regular swelling behavior, and also by spectral shift of coumarin-480 probe molecules. The LUVs were characterized by dynamic light scattering and transmission electron microscope measurements. Our work opens up the possibility of creating a huge number of IL-in-oil micro- emulsions as well as LUVs simply by replacing the cation of NaAOT with a long chain cation. 1. INTRODUCTION Room temperature ionic liquids (RTILs) have attracted much attention as a class of solvents because of their special physical and chemical properties (such as nonflammability, low volatility, and high thermal stability) and a wide range of potential applications in chemical synthesis, industrial process- ing, and energy storage. 1 Their unique properties and the rising necessity of sustainable, “green” chemistry have also led to an unparalleled increase in interest in such salts. RTILs are organic salts composed entirely of ions, and unlike the common organic salts, which melt at high temperatures, these salts melt at considerable low temperature, primarily due to the presence of sterically mismatched ions. 2-4 The most popular cationic components of the RTILs are substituted imidazolium ions, while [BF 4 ] - , [PF 6 ] - , and [(CF 3 SO 2 ) 2 N] - are most frequently used as anionic components. Because the properties of RTILs are very much dependent on their constituent ions, it is possible to obtain a RTIL of a desired property by tuning the cationic and anionic constituents; such liquids are called “designer solvents”. 5,6 Microemulsions are spatially ordered and thermodynamically stable macromolecular assembly formed by two or more immiscible liquids, which are stabilized by surfactants. These microheterogeneous systems can solubilize both polar and nonpolar substances and have been applied to many fields: for chemical reaction, nanomaterial synthesis, and several organic transformations. 7-13 The idea behind microemulsion is simple and straightforward; long-chain amphiphilic molecules (whose one end is polar and other end is nonpolar), generally called the surfactant molecules, are dissolved in a nonpolar solvent within a certain concentration range. The polar end of the amphiphilic surfactant tries to shade them from the unfavorable nonpolar solvent interaction. As a result, they can form an aggregated structure where all the polar ends directed toward the core of the aggregates. In the last few decades a lot of studies have been performed to reveal the structure as well as the biological importance of the confined water in the pool of microemulsion. Nowadays, room-temperature ionic liquids (RTILs) are also being used as a polar solvent because they constitute “green” substituents to classic (volatile) organic solvents. The structure of microemulsions is a field of current interest. To study ionic liquid microemulsions, it is necessary to investigate the structure of the microemulsion. In particular, the research into IL-containing nonaqueous microemulsions was motivated by the fact that, in spite of the useful properties of ILs, poor solubility of nonpolar solutes in neat ILs was a major hindrance in the path of their potential applications. This can be overcome using the hydrocarbon domains provided by IL-in- oil microemulsions. 14,15 The thermal stability of such a system over aqueous microemulsions is another important factor. 16 Gao et al. 17 first reported the formation of IL-in-oil micro- emulsions in [C 4 mim][BF 4 ]/TX-100/cyclohexane system. The size and shape of these microemulsions were verified from SANS studies by Eastoe et al., 18 where regular swelling behavior with the addition of the IL was also observed, implying that the volume of dispersed nanodomains was proportional to the amount of IL added. Following this, several other reports on similar types of systems are available in the literature. 19-24 From these it is evident that cyclohexane and benzene are commonly used nonpolar solvents. But the problems associated Received: February 12, 2013 Revised: March 6, 2013 Published: March 8, 2013 Article pubs.acs.org/JPCB © 2013 American Chemical Society 3927 dx.doi.org/10.1021/jp4015405 | J. Phys. Chem. B 2013, 117, 3927-3934