Recent Advances on Ionic Liquid Uses in Separation Techniques

Abstract

Ionic liquids (ILs) are non-molecular solvents; specifically, molten salts with low melting points, often below 100 °C and even below room temperature, thus allowing these solvents to remain liquid [1,2,3]. Chemically, they are formed by a bulky organic cation and a variety of usually smaller inorganic/organic anions to achieve electrical neutrality. Therefore, the number of combinations of cations and anions with substituted head-groups is unlimited, whereas the hydrophobicity range is guaranteed by the alkyl chain length of the organic moieties [4]. The physico-chemical properties of ILs include volatility, flammability, thermal stability, viscosity, solvation ability, an electrochemical window and relatively low toxicity, among others [1,2]. All of them can be tuned and modulated by the appropriate selection of both IL cation and anion—but what makes ILs so attractive? The main reason for their popularity is that they are considered a greener alternative to pollutant organic solvents, traditionally used in many technical and scientific fields, to decrease the environmental impact and overall risk of chemical exposure. Consequently, the scientific interest in ILs has been extended to different fields of chemistry with a high impact. In analytical chemistry, ILs have attracted special attention in separation techniques, where new ILs with specific properties, such as amphiphilic, magnetic and chiral properties, among others, are continuously being developed for new applications [3]. All this research has resulted in a rapid growth of the literature on the development and use of ILs, with an almost exponential increase, which usually involves books and Special Issues, such as the one we presented in Separations in 2019–2020. The Special Issue (SI), entitled “Recent Advances on Ionic Liquid Uses in Separation Techniques”, was devoted to applications of ILs in chromatography, extraction for sample preparation and related techniques. The scope was extended to deep eutectic solvents (DES), which are a promising class of solvents with similar properties to ILs [5]. The SI gathered the collaborations of different authors with wide experience in the IL or DES fields that discussed the state-of-the-art preparation and application of IL-based composites and hydrophobic DES in analytical sample preparation, or presented new results in different HPLC modes for the analysis of basic compounds.