Deep eutectic solvent

In contrast with ordinary solvents, such as Volatile Organic Compounds (VOC), DESs have a very low vapour pressure, and thus are non-flammable.[7] 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.[8]

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 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.[9] Organic compounds such as benzoic acid (solubility 0.82 mol/L) have great solubility in DESs, and this even includes cellulose.[10] 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,[11][12] and their ability to extract metals for analysis.[13] 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.[14] 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[15] .[16]

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.[17] 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.[18] It has been also shown that depending on state of matter of the solute homogeneous or heterogeneous mixtures are formed.[19]

DES have also been studied for their potential use as more environmentally sustainable solvents for extracting gold and other precious metals from ore.[20] Some solvent extraction work has been done using DES solvents, Mark Foreman at Chalmers has in recent years published several papers on this topic. He wrote about the use of the solvents for battery recycling from an applied point of view[21] and he also published what may be the first ever serious study of solvent extraction of metals from DES [22]. Foreman has also published two pure research papers on the activity issues in DES, in the first[23] he pointed out that activity coefficients in DES do appear to deviate wildly away from their values in sodium chloride solution while in his later paper[24] he provides a mathematical model for the activity coefficients in DES using the SIT equation.