## 1 Deep Eutectic Solvents (DESs)  
During recent decades, important efforts have been made to find new solvents that are safe, cheap, and safe for the environment and health, and that also have good properties for industrial applications. The principal features of solvents are related to solubility, recyclability, thermal and chemical stability, selectivity, flammability, corrosivity, viscosity, melting point, (eco)toxicity, physicochemical properties, availability, and cost [1, 2]. Several groups of solvents have been studied in order to find competitive, safer, and greener substitutes for traditional solvents, including ionic liquids (ILs), supercritical solvents, and solvents from biomass [3, 4]. ILs present interesting properties, such as low vapor pressure, high thermal, chemical, and electrochemical stability, being non-flammable [5, 6, 7], and the ability to dissolve or extract several chemicals (organics and inorganics; polar and nonpolar) [8], and can be used in separations [9] or as catalysts, among other important properties [10, 11, 12]. It should be noted that these characteristics are not common to all ionic liquids, and properties depend on the ions. Thus, it is possible to find different families of ILs based on the cation structure (pyridine ring, imidazolium, tetraalkylammonium), which can be linked to different anions (halides, tetrafluoroborate, dicyanada, triflate, etc.). However, the main problem of these chemicals is their toxicity. There are some studies about their toxicity, biodegradability, and ecotoxicity, but there is not yet enough evidence, and more studies are needed in order to ensure the safety of ILs for the environment and health [13, 14, 15, 16, 17]. Supercritical fluids present the following characteristics: they can increase the transfer of mass and heat due to their high diffusion coefficient and low viscosity; their properties can be adjusted by changing pressure and temperature; and they can be easily removed and recycled by reversing conditions of temperature and pressure. Some supercritical fluids include supercritical carbon dioxide (scCO\({}_{2}\)), supercritical water (scH\({}_{2}\)O), and supercritical ammonia (scNH\({}_{3}\)). Most of their uses are related to organic synthesis, extractions, and solubilization, among others [18; 19; 20; 21; 22]. However, although scCO\({}_{2}\) might be considered a good alternative, it is a nonpolar solvent, and it cannot be used in many applications. In the case of scH\({}_{2}\)O, it is important to note that the critical pressure and temperature are too high, so its uses are limited. Supercritical water behaves as a nonpolar solvent; therefore, it is not good for dissolving inorganic compounds. Finally, scNH\({}_{3}\) presents accessible critical pressure and temperature, but it is toxic and highly reactive and, therefore, it is dangerous to use in high-scale applications. Essentially, two problems have been found with this type of solvent: on the one hand, it is difficult and expensive to perform the installation of the equipment and, on the other hand, the polarity of these solvents is limited [23]. Solvents from biomass have been also widely studied, because their properties are based on green chemistry precepts [24]. These are compounds obtained via fermentation, enzymatic, or esterification processes. Agricultural and some industrial activities can produce large amounts of raw materials, used to produce commodity and fine chemicals, with lignocellulosic biomass being the production material with the largest potential volume and lowest cost. Some of these chemicals include fatty acid esters, furfural, levulinic acid, terpen, glycols and their esters, low-molecular-weight alcohols, and lactates [25]. A new type of solvent--the so-called deep eutectic solvents (DESs)--was defined by [PERSON] et al. in 2003. DESs are liquid compounds that present a diminution of the melting point above 100 \({}^{\circ}\)C when it is compared to the forming substances [26; 27].