Ionic Liquids and Green Futures (Progress in Science: Current Commentary) (Report‪)‬

Ionic liquids (ILs) offer manifold prospects as solvents (1) for the extraction and processing of materials while avoiding or minimising the handling, inadvertent release and ultimate disposal of more common organic solvents. As the name implies, ILs are frequently liquid at room temperature and consist entirely of ionic species, in contrast to more usual pure liquids like water and acetone, which contain electrically neutral molecules. An IL is a salt in which either or both of the ions are large, and the cation possesses a low degree of symmetry. These aspects tend to diminish the lattice energy of the crystalline state of the salt and therefore reduce the melting point. There are in general two principal kinds of IL, those that are simple salts consisting of a single anion and a single cation and others known as binary ionic liquids which are salts involving an equilibrium. As an example, [[C.sub.2][H.sub.5]N[H.sup.+.sub.3]][N[O.sup.-.sub.3]] is a simple salt that exhibits simple melting behaviour, whereas the binary ionic liquid systems furnished by mixtures of aluminium(III) chloride and 1,3-dialkylimidazolium chlorides contain a number of different ionic species with melting points that vary according to the quantity of each component. In consequence of their negligible vapour pressure, high thermal stability, profound lack of flammability, and tailorable solubility for particular compounds, ILs have many "green"/environmental applications of which the range of examples chosen below merely gives an indication. One of the earliest truly room temperature ionic liquids was ethylammonium nitrate [[C.sub.2][H.sub.5]N[H.sup.+.sub.3]][N[O.sup.-.sub.3] (m.p. 12[degrees]C), reported by Walden in 1914 (2). In the 1970s and 1980s, ionic liquids based on alkyl-substituted imidazolium and pyridinium cations, with halide or trihalogenoaluminate anions, were initially developed for use as electrolytes in battery applications (3,4). An important feature of the imidazolium/pyridinium halogenoaluminate salts is that their physical properties, e.g. viscosity, melting point, and acidity, may be fine-tuned according to the nature of the alkyl substituents and the ratios of imidazolium/pyridinium and halide/halogenoaluminate ions (5). Sensitivity to moisture/water and acidity/basicity presented an obstacle for some applications of ILs which was circumvented by the creation of air- and water-stable ILs with 'neutral' weakly coordinating anions such as hexafluorophosphate (P[F.sub.6.sup.-]) and tetrafluoroborate [(B[F.sub.4.sup.-]).sup.1]. ILs containing less toxic anions such as bistriflimide (6) [[[(C[F.sub.3]S[O.sub.2]).sub.2]N].sup.-] are being developed along with others completely devoid of halogens. ILs with less toxic cations have also been devised, with ammonium salts (e.g. based on choline (7)), appearing to offer an almost equally adaptable scaffold as imidazolium does.