Deep eutectic solvents or DESs are solutions of Lewis or Brønsted acids and bases which form a eutectic mixture.[1] Deep eutectic solvents are highly tunable through varying the structure or relative ratio of parent components and thus have a wide variety of potential applications including catalytic, separation, and electrochemical processes.[1][2] The parent components of deep eutectic solvents engage in a complex hydrogen bonding network which results in significant freezing point depression as compared to the parent compounds.[3] The extent of freezing point depression observed in DESs is well illustrated by a mixture of choline chloride and urea in a 1:2 mole ratio. Choline chloride and urea are both solids at room temperature with melting points of 302 °C (decomposition point) and 133 °C respectively, yet the combination of the two in a 1:2 molar ratio forms a liquid with a freezing point of 12 °C.[4] DESs share similar properties to ionic liquids such as tunability and lack of flammability yet are distinct in that ionic liquids are neat salts composed exclusively of discrete ions.[1] In contrast to ordinary solvents, such as Volatile Organic Compounds (VOC), DESs are non-flammable, and possess low vapour pressures and toxicity.[5]
The first generation eutectic solvents were based on mixtures of quaternary ammonium salts with hydrogen bond donors such as amines and carboxylic acids. DESs are classified into four types based on composition:[6]
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 include a wide range of chlorometallate ionic solvents which were widely studied in the 1980s, such as imidazolium chloroaluminates which are based on mixtures of AlCl3 + 1-Ethyl-3-methylimidazolium chloride.[7] Type II eutectics are identical to Type I eutectic in composition yet include the hydrated form of the metal halide. Type III eutectics consist of hydrogen bond acceptors such as quaternary ammonium salts (e.g. choline chloride) and hydrogen bond donors (e.g urea, ethylene glycol) and include the class of metal-free deep eutectic solvents.[2][8] Type III eutectics have been successfully used in metal processing applications such as electrodeposition, electropolishing, and metal extraction. Type IV eutectics are similar to type III yet replace the quaternary ammonium salt hydrogen bond acceptor with a metal halide hydrogen bond acceptor while still using an organic hydrogen bond donor such as urea. Type IV eutectics are of interest for electrodeposition as they produce cationic metal complexes, ensuring that the double layer close to the electrode surface has a high metal ion concentration.[8]
Wide spread practical use of DESs in industrial process or devices has thus far been hindered by relatively high viscosities and low ionic conductivities. Additionally, lack of understanding of the relationship between parent compound structure and solvent function has prevented development of general design rules. Work to understand structure-function relation is on-going.
^ abcSmith, Emma L.; Abbott, Andrew P.; Ryder, Karl S. (12 November 2014). "Deep Eutectic Solvents (DESs) and Their Applications". Chemical Reviews. 114 (21): 11060–11082. doi:10.1021/cr300162p. hdl:2381/37428. PMID 25300631.
^ abGurkan, Burcu; Squire, Henry; Pentzer, Emily (19 December 2019). "Metal-Free Deep Eutectic Solvents: Preparation, Physical Properties, and Significance". The Journal of Physical Chemistry Letters. 10 (24): 7956–7964. doi:10.1021/acs.jpclett.9b01980. OSTI 1608304. PMID 31804088. S2CID 208643425.
^"Deep Eutectic Solvents" (PDF). kuleuven.be. University of Leicester. Retrieved 17 June 2014.
^Abbott, Andrew P.; Capper, Glen; Davies, David L.; Rasheed, Raymond K.; Tambyrajah, Vasuki (19 December 2003). "Novel solvent properties of choline chloride/urea mixtures". Chemical Communications (1): 70–71. doi:10.1039/b210714g. PMID 12610970.
^García, Gregorio; Aparicio, Santiago; Ullah, Ruh; Atilhan, Mert (16 April 2015). "Deep Eutectic Solvents: Physicochemical Properties and Gas Separation Applications". Energy & Fuels. 29 (4): 2616–2644. doi:10.1021/ef5028873.
^Abbott, Andrew P.; Barron, John C.; Ryder, Karl S.; Wilson, David (27 July 2007). "Eutectic-Based Ionic Liquids with Metal-Containing Anions and Cations". Chemistry: A European Journal. 13 (22): 6495–6501. doi:10.1002/chem.200601738. PMID 17477454.
^Wilkes, John S.; Levisky, Joseph A.; Wilson, Robert A.; Hussey, Charles L. (March 1982). "Dialkylimidazolium chloroaluminate melts: a new class of room-temperature ionic liquids for electrochemistry, spectroscopy and synthesis". Inorganic Chemistry. 21 (3): 1263–1264. doi:10.1021/ic00133a078.
^ abAbbott, Andrew P.; Al-Barzinjy, Azeez A.; Abbott, Paul D.; Frisch, Gero; Harris, Robert C.; Hartley, Jennifer; Ryder, Karl S. (2014). "Speciation, physical and electrolytic properties of eutectic mixtures based on CrCl3·6H2O and urea". Physical Chemistry Chemical Physics. 16 (19): 9047–9055. Bibcode:2014PCCP...16.9047A. doi:10.1039/c4cp00057a. hdl:2381/37718. PMID 24695874.
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