Deep eutectic solvent
Deep eutectic solvents are systems formed from a eutectic mixture of Lewis or Brønsted acids and bases which can contain a variety of anionic and/or cationic species. They are classified as types of ionic solvents with special properties. They incorporate one or more compound in a mixture form, to give a eutectic with a melting point much lower than either of the individual components. One of the most significant deep eutectic phenomenon was observed for a mixture of choline chloride and urea in a 1:2 mole ratio. The resulting mixture has a melting point of 12 °C (far less than the melting point of choline chloride, 302 °C and urea, 133 °C), which makes it liquid at room temperature.
|Type I||Quaternary ammonium salt + metal chloride|
|Type II||Quaternary ammonium salt + metal chloride hydrate|
|Type III||Quaternary ammonium salt + hydrogen bond donor|
|Type IV||Metal chloride hydrate + hydrogen bond donor|
Type I Eutectics therefore also include the wide range of chlorometallate ionic liquids widely studied in the 1980s, such as the ever-popular imidazolium chloroaluminates which are based on mixtures of AlCl3 + 1-Ethyl-3-methylimidazolium chloride. In addition to ionic liquids with discrete anions, the electrodeposition of a range of metals has been previously carried out in deep eutectic solvents (DESs). These are quaternary ammonium salts (e.g. choline chloride, ChCl), metal salts or metal salt hydrates and hydrogen bond donors (e.g. urea) and are commonly divided into four groups (Table 1), have been particularly successful on a large scale for metal polishing and immersion silver deposition. While most ionic liquids and DESs include a quaternary ammonium ion as the cationic component, it has recently been shown that eutectics can also be formed between a metal salt (hydrate) and a simple amide or alcohol to form a metalliferous solution composed of cations and anions via disproportionation processes e.g.
2AlCl3 + urea ↔ [AlCl2•urea]+ + [AlCl4]−
These so-called Type 4 eutectics are useful as they produce cationic metal complexes, ensuring that the double layer close to the electrode surface has a high metal ion concentration.
In contrast with ordinary solvents, such as Volatile Organic Compounds (VOC), DESs have a very low vapour pressure, and thus are non-flammable. The same reference mentions that DESs have a relatively high viscosities which might hinder their industrial applications as they might not flow easily in process streams. DESs possess favorably low densities and can be liquid at a wide range of temperatures, going to around -50 °C for some DESs.
Compared to modern ionic liquids based on discrete anions, such as bistriflimide, which share many characteristics but are ionic compounds and not ionic mixtures, DESs are cheaper to make, are less toxic and sometimes biodegradable. Therefore, DESs can be used as safe, efficient, simple, and low–cost solvents. To date, there are numerous applications that had been studied for DESs. By varying the components of the DES and their molar ratios, new DESs can be produced. For this reason, many new applications are presented in the literature every year. Some of the earliest applications of DESs were the electrofinishing of metals using DESs as electrolytes. Organic compounds such as benzoic acid (solubility 0.82 mol/L) have great solubility in DESs, and this even includes cellulose.  For this reason, DESs were applied as extraction solvents for such material from their complex matrices. They were also studied for their applicability in the production and purification of biodiesel, and their ability to extract metals for analysis. Incorporating microwave heating with deep eutectic solvent can efficiently increase the solubility power of DES and reduce the time required for complete dissolution of biological samples at atmospheric pressure. It is noteworthy that proton-conducting DESs (e.g. the mixture of imidazolium methanesulfonate and 1H-1,2,4-triazole in a 1:3 mole ratio or the mixture of 1,2,4-triazolium methanesulfonate and 1H-1,2,4-triazole in a 1:3 mole ratio, wherein the Brønsted base may act as the hydrogen bond donor) have also found applications as proton conductors for fuel cells .
Owing to their unique composition, DES are promising solvating environments, affecting the structure and self-assembly of solutes. For example, the self-assembly of sodium dodecyl sulfate (SDS) in DES has recently been studied, implying DES can form microemulsions different from those in water. In another case, the solvation of the polymer polyvinylpyrrolidone (PVP) in DES is distinct from water, whereby the DES appear to be a better solvent for the polymer.
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