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^[1] :

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.

Some have suggested the possibility that carboxylic acids in conjunction with compounds such as sugar alcohols show promise and potential as a DES. A small group of undergraduate researchers at a small college in middle Tennessee have reportedly formulated a solvent that shows potential for having practical applications in green chemistry and could possibly have commercial applications in varying fields. They have not released their formulations or ratios, however it has been rumored the solvent was derived from some combination of carboxylic acid and an unknown sugar alcohol.

Due to the increase in research activities on DESs and their applications, it was necessary to characterize them (measure their physical properties and establish a database). So far, one research only published by American Chemical Society/Journal of Chemical and Engineering Data, had dealt with the task of the characterization of these solvents. That research took DESs based on Phosphonium salts as a subject to its study, because this type of salts was yet to be studied.

The applicability of DESs in industry is still subject to a wide study. A search on the internet for "Deep Eutectic Solvents" can return lot of results, but a specific search with proper keywords can lead to the exact research groups that are researching on DESs. Few papers are already published on DESs' applications, and it is expected that this will change dramatically in the near future.

References

1. ^ Andrew Abbott; John Barron, Karl Ryder and David Wilson (2007). "Eutectic-Based Ionic Liquids with Metal-Containing Anions and Cations". Chem. Eur. J. 13: 6495 – 6501. doi:10.1002/chem.200601738. 

1. Andrew P. Abbott, Glen Capper, David L. Davies, Raymond K. Rasheed, Vasuki Tambyrajah (2003). "Novel solvent properties of choline chloride/urea mixtures". Chemical Communications (1): 70–71. doi:10.1039/b210714g. 
2. Abbott, A. P.; Boothby, D.; Capper, G.; Davies, D. L.; Rasheed, R. K. (2004). "Deep Eutectic Solvents Formed between Choline Chloride and Carboxylic Acids: Versatile Alternatives to Ionic Liquids". J. Am. Chem. Soc. 126 (29): 9142–9147. doi:10.1021/ja048266j. PMID 15264850. 
3. Mukhtar A. Kareem; Farouq S. Mjalli; Mohd Ali Hashim; Inas M. AlNashef (2010). "Phosphonium-Based Ionic Liquids Analogues and Their Physical Properties". J. Chem. & Eng. Data 55 (11): 4632–4637. doi:10.1021/je100104v. 

External links

• Deep eutectics at www.rsc.org Website