Introduction to Ionic Liquids

Currently, ionic liquids (ILs) is a topic of interest to physical, organic inorganic chemists as well as biologists due to their unusual physical, chemical, and biological properties. The fine-tuning of physical, chemical, and biological properties with the help of their cations, anions as well as side chains attached to them makes these suitable for various applications. Unlike inorganic salts, these salts generally have low melting points (sometimes below room temperature), and remain liquid over a wide range of temperatures including room temperature, therefore termed as room temperature ionic liquids (RTILs). A variety of cations and anions can serve the purpose of the synthesis of ionic liquids. Cations and anions are chosen in such a way that their structures are asymmetric in nature and hence their packing in the lattice is not closed one and hence their melting points are not as pronounced as in the case of inorganic salts having symmetric cations and anions. Ionic liquids solely consist of ions only and are liquid at room temperature; therefore, these salts can serve the purpose of a unique solvent that is ionic in nature, unlike molecular solvents, which are molecular in nature. The other important characteristic features such as large electrochemical window, high thermal and chemical stability, and low vapor pressure make these salts suitable for various applications such as electro-analysis, synthesis, catalysis, separation, extraction, mass spectrometry, dye aggregation, excited state proton transfer reactions, sensing, CO2 capture, and energy-related applications. Due to their ionic nature, these salts possess quite interesting and unusual solvent properties and many research groups have reported unusual solvation processes within these solvent media. It has been reported that the ions of ionic liquids not only interact with each other but also interact with solute species. The quantification of the interactions between ionic liquid ions and solute species is a topic of interest to many researchers. The present chapter provides an overview of various salient features associated with ionic liquids.