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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 1 to 2 by mole mixture of choline chloride (2-hydroxyethyl-trimethylammonium chloride) and urea. Choline chloride has a melting point of 302 °C and that of urea is 133 °C. The eutectic mixture however melts as low as 12 °C.

Additional recommended knowledge

This DES is 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 applied in metal cleaning for 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 (filtration paper!). 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.

References

^ Novel solvent properties of choline chloride/urea mixtures Andrew P. Abbott, Glen Capper, David L. Davies, Raymond K. Rasheed, Vasuki Tambyrajah Chemical Communications, 2003, (1), 70 - 71 Graphic abstract
^ Deep Eutectic Solvents Formed between Choline Chloride and Carboxylic Acids: Versatile Alternatives to Ionic Liquids Abbott, A. P.; Boothby, D.; Capper, G.; Davies, D. L.; Rasheed, R. K.; J. Am. Chem. Soc.; 2004; 126(29); 9142-9147. Abstract