Deep eutectic solvent
Encyclopedia
A deep eutectic solvent or DES is a type of ion
ic 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 salt
s with hydrogen donors such as amine
s and carboxylic acid
s. 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 :
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 compound
s 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 liquid
s 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.
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
ic solvent
Solvent
A solvent is a liquid, solid, or gas that dissolves another solid, liquid, or gaseous solute, resulting in a solution that is soluble in a certain volume of solvent at a specified temperature...
with special properties composed of a mixture which forms a eutectic with a melting point
Melting point
The melting point of a solid is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depends on pressure and is usually specified at standard atmospheric pressure...
much lower than either of the individual components. The first generation eutectic solvents were based on mixtures of quaternary ammonium salt
Salt
In chemistry, salts are ionic compounds that result from the neutralization reaction of an acid and a base. They are composed of cations and anions so that the product is electrically neutral...
s with hydrogen donors such as amine
Amine
Amines are organic compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are derivatives of ammonia, wherein one or more hydrogen atoms have been replaced by a substituent such as an alkyl or aryl group. Important amines include amino acids, biogenic amines,...
s and carboxylic acid
Carboxylic acid
Carboxylic acids are organic acids characterized by the presence of at least one carboxyl group. The general formula of a carboxylic acid is R-COOH, where R is some monovalent functional group...
s. The deep eutectic phenomenon was first described in 2003 for a mixture of choline chloride
Choline chloride
Choline chloride is an organic compound and a quaternary ammonium salt. It has a choline cation with chloride anion. Alternative names are hepacholine, biocolina and lipotril.-Synthesis:...
(2-hydroxyethyl-trimethylammonium chloride) and urea
Urea
Urea or carbamide is an organic compound with the chemical formula CO2. The molecule has two —NH2 groups joined by a carbonyl functional group....
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 :
| 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
Metal
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...
salts like lithium chloride
Lithium chloride
Lithium chloride is a chemical compound with the formula LiCl. The salt is a typical ionic compound, although the small size of the Li+ ion gives rise to properties not seen for other alkali metal chlorides, such as extraordinary solubility in polar solvents and its hygroscopic...
(solubility
Solubility
Solubility is the property of a solid, liquid, or gaseous chemical substance called solute to dissolve in a solid, liquid, or gaseous solvent to form a homogeneous solution of the solute in the solvent. The solubility of a substance fundamentally depends on the used solvent as well as on...
2.5 mol
Mole (unit)
The mole is a unit of measurement used in chemistry to express amounts of a chemical substance, defined as an amount of a substance that contains as many elementary entities as there are atoms in 12 grams of pure carbon-12 , the isotope of carbon with atomic weight 12. This corresponds to a value...
/L) and copper(II) oxide
Copper(II) oxide
Copper oxide or cupric oxide is the higher oxide of copper. As a mineral, it is known as tenorite.-Chemistry:It is a black solid with an ionic structure which melts above 1200 °C with some loss of oxygen...
(solubility 0.12 mol/L). In this capacity these solvents are used for metal cleaning prior to electroplating
Electroplating
Electroplating is a plating process in which metal ions in a solution are moved by an electric field to coat an electrode. The process uses electrical current to reduce cations of a desired material from a solution and coat a conductive object with a thin layer of the material, such as a metal...
. Because the solvent is conductive it also has a potential application in electropolishing
Electropolishing
Electropolishing, also known as electrochemical polishing or electrolytic polishing , is an electrochemical process that removes material from a metallic workpiece. It is used to polish, passivate, and deburr metal parts. It is often described as the reverse of electroplating...
. Organic compound
Organic compound
An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...
s such as benzoic acid
Benzoic acid
Benzoic acid , C7H6O2 , is a colorless crystalline solid and the simplest aromatic carboxylic acid. The name derived from gum benzoin, which was for a long time the only source for benzoic acid. Its salts are used as a food preservative and benzoic acid is an important precursor for the synthesis...
(solubility 0.82 mol/L) also have great solubility and this even includes cellulose
Cellulose
Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to over ten thousand β linked D-glucose units....
. Compared to ordinary solvents, eutectic solvents also have a very low VOC
Volatile organic compound
Volatile organic compounds are organic chemicals that have a high vapor pressure at ordinary, room-temperature conditions. Their high vapor pressure results from a low boiling point, which causes large numbers of molecules to evaporate or sublimate from the liquid or solid form of the compound and...
and are non-flammable.
Other deep eutectic solvents of choline chloride are formed with malonic acid
Malonic acid
Malonic acid is a dicarboxylic acid with structure CH22. The ionised form of malonic acid, as well as its esters and salts, are known as malonates. For example, diethyl malonate is malonic acid's ethyl ester...
at 0 °C, phenol
Phenol
Phenol, also known as carbolic acid, phenic acid, is an organic compound with the chemical formula C6H5OH. It is a white crystalline solid. The molecule consists of a phenyl , bonded to a hydroxyl group. It is produced on a large scale as a precursor to many materials and useful compounds...
at -40 °C and glycerol
Glycerol
Glycerol is a simple polyol compound. It is a colorless, odorless, viscous liquid that is widely used in pharmaceutical formulations. Glycerol has three hydroxyl groups that are responsible for its solubility in water and its hygroscopic nature. The glycerol backbone is central to all lipids...
at -35 °C.
Compared to ionic liquid
Ionic liquid
An ionic liquid is a salt in the liquid state. In some contexts, the term has been restricted to salts whose melting point is below some arbitrary temperature, such as . While ordinary liquids such as water and gasoline are predominantly made of electrically neutral molecules, ILs are largely made...
s 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
Biodegradation
Biodegradation or biotic degradation or biotic decomposition is the chemical dissolution of materials by bacteria or other biological means...
.
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.
External links
- Deep eutectics at www.rsc.org Website