Deep eutectic solvents or DESs are solutions of Lewis or Brønsted acids and bases which form a

eutectic mixture A eutectic system or eutectic mixture ( ) is a type of a homogeneous mixture that has a melting point lower than those of the constituents. The lowest possible melting point over all of the mixing ratios of the constituents is called the ''eutec ...

. 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. 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. The extent of freezing point depression observed in DESs is well illustrated by a mixture of choline chloride and

urea Urea, also called carbamide (because it is a diamide of carbonic acid), is an organic compound with chemical formula . This amide has two Amine, amino groups (–) joined by a carbonyl functional group (–C(=O)–). It is thus the simplest am ...

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. 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. In contrast to ordinary solvents, such as

volatile organic compound Volatile organic compounds (VOCs) are organic compounds that have a high vapor pressure at room temperature. They are common and exist in a variety of settings and products, not limited to Indoor mold, house mold, Upholstery, upholstered furnitur ...

s, DESs are non-flammable, and possess low vapour pressures and toxicity. Traditional eutectic solvents are mixtures of

quaternary ammonium In organic chemistry, quaternary ammonium cations, also known as quats, are positively-charged polyatomic ions of the structure , where R is an alkyl group, an aryl group or organyl group. Unlike the ammonium ion () and the primary, secondary, o ...

salt In common usage, salt is a mineral composed primarily of sodium chloride (NaCl). When used in food, especially in granulated form, it is more formally called table salt. In the form of a natural crystalline mineral, salt is also known as r ...

s with

hydrogen bond In chemistry, a hydrogen bond (H-bond) is a specific type of molecular interaction that exhibits partial covalent character and cannot be described as a purely electrostatic force. It occurs when a hydrogen (H) atom, Covalent bond, covalently b ...

donors such as

amine In chemistry, amines (, ) are organic compounds that contain carbon-nitrogen bonds. Amines are formed when one or more hydrogen atoms in ammonia are replaced by alkyl or aryl groups. The nitrogen atom in an amine possesses a lone pair of elec ...

s and

carboxylic acid In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group () attached to an Substituent, R-group. The general formula of a carboxylic acid is often written as or , sometimes as with R referring to an organyl ...

s. Classic examples are choline and various ureas. DESs can be classified on the basis of their composition:
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. 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. 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. 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.

Natural deep eutectic solvents

Natural deep eutectic solvents (NADES) are bio-based deep eutectic solvents which are composed of two or more compounds that are generally plant based

primary metabolite A primary metabolite is a kind of metabolite that is directly involved in normal growth, development, and reproduction. It usually performs a physiological function in the organism (i.e. an intrinsic function). A primary metabolite is typically pre ...

s, i.e.

organic acid An organic acid is an organic compound with acidic properties. The most common organic acids are the carboxylic acids, whose acidity is associated with their carboxyl group –COOH. Sulfonic acids, containing the group –SO2OH, are re ...

sugar Sugar is the generic name for sweet-tasting, soluble carbohydrates, many of which are used in food. Simple sugars, also called monosaccharides, include glucose Glucose is a sugar with the Chemical formula#Molecular formula, molecul ...

alcohols In chemistry, an alcohol (), is a type of organic compound that carries at least one hydroxyl () functional group bound to a Saturated and unsaturated compounds, saturated carbon atom. Alcohols range from the simple, like methanol and ethanol ...

s and

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although over 500 amino acids exist in nature, by far the most important are the 22 α-amino acids incorporated into proteins. Only these 22 a ...

s. Work done by Choi, Spronsen ''et al''. showed that water can be present as part of the

solvent A solvent (from the Latin language, Latin ''wikt:solvo#Latin, solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a Solution (chemistry), solution. A solvent is usually a liquid but can also be a solid, a gas ...

, being strongly retained in the liquid and which cannot be evaporated.

Research

Compared to modern

ionic liquid An ionic liquid (IL) is a salt (chemistry), salt in the liquid state at ambient conditions. In some contexts, the term has been restricted to salts whose melting point is below a specific temperature, such as . While ordinary liquids such as wate ...

s based on discrete anions, such as

bistriflimide Bistriflimide, also known variously as bis(trifluoromethane)sulfonimide, bis(trifluoromethanesulfonyl)imide, bis(trifluoromethanesulfonyl)imidate (and variations thereof), informally and somewhat inaccurately as triflimide or triflimidate'','' or ...

, which share many characteristics but are ionic compounds and not ionic mixtures, DES are cheaper to make and sometimes

biodegradable Biodegradation is the breakdown of organic matter by microorganisms, such as bacteria and fungi. It is generally assumed to be a natural process, which differentiates it from composting. Composting is a human-driven process in which biodegrada ...

. Therefore, DES can be used as safe, efficient, simple, and low–cost solvents. To date, there are numerous applications that have been studied for DES. By varying the components of the DES and their molar ratios, new DES can be produced. For this reason, many new applications are presented in the literature every year. Some of the earliest applications of DES were the electrofinishing of metals using DES as electrolytes.

Organic compound Some chemical authorities define an organic compound as a chemical compound that contains a carbon–hydrogen or carbon–carbon bond; others consider an organic compound to be any chemical compound that contains carbon. For example, carbon-co ...

s such as

benzoic acid Benzoic acid () is a white (or colorless) solid organic compound with the formula , whose structure consists of a benzene ring () with a carboxyl () substituent. The benzoyl group is often abbreviated "Bz" (not to be confused with "Bn," which ...

(solubility 0.82 mol/L) have great solubility in DES, and this even includes

cellulose Cellulose is an organic compound with the chemical formula, formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of glycosidic bond, β(1→4) linked glucose, D-glucose units. Cellulose is an important s ...

For this reason, DES were applied as extraction solvents for such material from their complex matrices. DES as extraction solvents in the separation of aromatic hydrocarbons from naphtha was also studied and promising results were published in 2012 and 2013. They were also studied for their applicability in the production and purification of biodiesel, and their ability to extract metals for analysis. 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. It is noteworthy that proton-conducting DES (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. 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. In another case, the solvation of the polymer

polyvinylpyrrolidone Polyvinylpyrrolidone (PVP), also commonly called povidone, is a water-soluble polymer compound made from the monomer ''N''-vinylpyrrolidone. PVP is available in a range of molecular weights and related viscosities, and can be selected according ...

(PVP) in DES is distinct from water, whereby the DES appear to be a better solvent for the polymer. It has been also shown that depending on

state of matter In physics, a state of matter is one of the distinct forms in which matter can exist. Four states of matter are observable in everyday life: solid, liquid, gas, and Plasma (physics), plasma. Different states are distinguished by the ways the ...

of the solute

homogeneous Homogeneity and heterogeneity are concepts relating to the uniformity of a substance, process or image. A homogeneous feature is uniform in composition or character (i.e., color, shape, size, weight, height, distribution, texture, language, i ...

heterogeneous Homogeneity and heterogeneity are concepts relating to the uniformity of a substance, process or image. A homogeneous feature is uniform in composition or character (i.e., color, shape, size, weight, height, distribution, texture, language, i ...

mixtures are formed. DES have also been studied for their potential use as more environmentally sustainable solvents for extracting gold and other precious metals from

ore Ore is natural rock or sediment that contains one or more valuable minerals, typically including metals, concentrated above background levels, and that is economically viable to mine and process. The grade of ore refers to the concentration ...

. 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 and he also published what may be the first ever serious study of solvent extraction of metals from DES. Foreman has also published two pure research papers on the activity issues in DES, in the first he pointed out that activity coefficients in DES do appear to deviate wildly away from their values in

sodium chloride Sodium chloride , commonly known as Salt#Edible salt, edible salt, is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. It is transparent or translucent, brittle, hygroscopic, and occurs a ...

solution while in his later paper he provides a mathematical model for the activity coefficients in DES using the SIT equation. Lastly, DES involvement in thermoelectrical field was researched through the incorporation of DES in thermoelectric polymer for the synthesis of improved thermoelectric polymer films.{{cite journal , last1=Rahman , first1=Shahriar Mufid , last2=Mohd Said , first2=Suhana Binti , last3=Subramanian , first3=Balamurugan , last4=Long , first4=Bui Duc , last5=Kareem , first5=Mukhtar A. , last6=Soin , first6=Norhayati , title=Synthesis and Characterization of Polymer Electrolyte Using Deep Eutectic Solvents and Electrospun Poly(vinyl alcohol) Membrane , journal=Industrial & Engineering Chemistry Research , date=3 August 2016 , volume=55 , issue=30 , pages=8341–8348 , doi=10.1021/acs.iecr.6b01754

References

Solvents Ionic liquids