Deep eutectic solvent
A deep eutectic solvent or DES is a type of ionic solvent with special properties composed of a mixture which forms a eutectic with a melting point much lower than either of the individual components. The first generation eutectic solvents were based on mixtures of quaternary ammonium salts with hydrogen donors such as amines and carboxylic acids. The deep eutectic phenomenon was first described in 2003 for a mixture of choline chloride (2-hydroxyethyl-trimethylammonium chloride) and urea in a 1:2 mole ratio, respectively. Choline chloride has a melting point of 302 °C and that of urea is 133 °C. The eutectic mixture melts as low as 12 °C. There are four types of eutectic solvents:
|Type I Eutectic||metal salt + organic salt (e.g. ZnCl2 + choline chloride)|
|Type II Eutectic||metal salt hydrate + organic salt (e.g. CoCl2*6H2O + choline chloride)|
|Type III Eutectic||organic salt + hydrogen bond donor (e.g. choline chloride + urea)|
|Type IV Eutectic||metal salt (hydrate) + hydrogen bond donor (e.g. ZnCl2 + urea)|
The DESs have been studied for their applicability in industry at lab level, and the DES described above was found to be able to dissolve many metal salts like lithium chloride (solubility 2.5 mol/L) and copper(II) oxide (solubility 0.12 mol/L). In this capacity these solvents are used for metal cleaning prior to electroplating. Because the solvent is conductive it also has a potential application in electropolishing. Organic compounds such as benzoic acid (solubility 0.82 mol/L) also have great solubility and this even includes cellulose. Compared to ordinary solvents, eutectic solvents also have a very low VOC and are non-flammable. Other deep eutectic solvents of choline chloride are formed with malonic acid at 0 °C, phenol at -40 °C and glycerol at -35 °C. Compared to ionic liquids which share many charactistics but are ionic compounds and not ionic mixtures, deep eutectic solvents are cheaper to make, much less toxic and sometimes biodegradable.
Hayyan et al. (2013) first questioned whether DESs are benign or toxic. They used two gram-positive and two gram-negative bacteria to examine the toxicity of ChCl-G, ChCl-EG, ChCl-triethylene glycol, and ChCl-U. In their study, the DESs tested had no toxic effect on any of the bacteria studied, but the cytotoxicity of the tested DESs was much higher than that of their individual components, indicating that their toxicological behavior is different. They suggested that the toxicity and cytotoxicity of DESs varied according to the structure of the components and highlighted the need for careful use of the terms non-toxicity and biodegradability of DESs.
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