**4.1. Ionic liquids**

ILs are substances composed entirely of ions that are liquids at room temperature (Endes et al., 2008). While common salts, such as table salt (i.e. sodium chloride), have melting points around 800 °C, the upper limit for the melting point so that they can be classified as ILs is usually set at 100 °C. Therefore, they can be considered as room-temperature molten salts (Keskin et al., 2007). Such behaviour is obviously related to their chemical composition: ILs are usually composed of bulky and asymmetric organic cations and smaller inorganic or organic anions. This feature makes their lattice energy quite low and, therefore, so does their melting point (Keskin et al., 2007). There are many of them depending on their chemical composition, but almost every IL can be classified, according to its cationic structure, into one of the seven families reported by Torimoto et al. (Torimoto et al., 2010).

Connected to their composition, one of the most interesting properties concerning ILs is their tunability. As a matter of fact, by changing the nature of the constituent ions it is possible to obtain ILs with very different physical and chemical properties (Endes et al., 2008). Theoretically, the combination of cations and anions would lead to a number of ILs as high as 1018 (Chiappe et al., 2005). In practice, however, this number would be much smaller, but it gives an idea about the broad range of physicochemical properties that can be obtained. For instance, ILs with the same imidazolium cation, [BuMeIm]1, can be either hydrophilic (combined with [BF4] or [AlCl4]) or hydrophobic (if the anion is [PF6] or [Tf2N]) (Torimoto et al., 2010). Here it is worth mentioning the fact that some ILs with magnetic properties have also been prepared by using [FeCl4] anions. Tunability is especially interesting in the case of MFs, since it would allow synthesizing tailor-made MFs specifically designed for every single application.

Nevertheless, there are two common features for many of them that are rather interesting from the technological viewpoint. These are their negligible vapour pressure and flammability, at least when compared to those of conventional solvents such as volatile organic compounds (VOCs). Both characteristics ensure, first of all, thermal stability and vacuum resistance and, what is more, an insignificant contribution to air pollution. As a matter of fact, they have been labelled as "green solvents" due to their negligible impact on atmospheric pollution (Keskin et al., 2007; Torimoto et al., 2010). Finally, it is possible to recycle them, which would contribute to reduce their release to the aquatic environment and lead to more efficient and economic industrial processes (Haerens et al., 2010; Wu et al., 2009).
