Introduction: Ionic Liquids

Ionic liquids (ILs) are normally defined as compounds completely composed of ions with melting point below 100 °C. The first IL (ethylammonium nitrate) was reported by Paul Walden in 1914, who at that time never realized that ILs would become a major scientific area after almost one century. Actually, ILs as innovative fluids have received wide attention only during the past two decades. The number of SCI papers published on ILs has exponentially increased from a few in 1996 to >5000 in 2016, exceeding the annual growth rates of other popular scientific areas. This indicates that more and more researchers are engaged in studying this exciting area, with the outcomes being plentiful. A multidisciplinary study on ILs is emerging, including chemistry, materials science, chemical engineering, and environmental science. More specifically, some important fundamental viewpoints are now different from the original concepts, as insights into the nature of ILs become deeper. For example, the physicochemical properties of ILs are now recognized as ranging broadly from the oft quoted “nonvolatile, non-flammable, and air and water stable” to those that are distinctly volatile, flammable, and unstable. This is attributed to numerous combinations of cations and anions that meet the definition of ILs, leading to a diverse suite of behaviors. Regardless, ILs remain more desirable than conventional volatile solvents and/or catalysts in many physical and chemical processes, often exhibiting “green” and “designer” properties to a useful degree.