Ionic liquid
Overview
 
An ionic liquid is a salt in the liquid
Liquid
Liquid is one of the three classical states of matter . Like a gas, a liquid is able to flow and take the shape of a container. Some liquids resist compression, while others can be compressed. Unlike a gas, a liquid does not disperse to fill every space of a container, and maintains a fairly...

 state. In some contexts, the term has been restricted to salts whose 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...

 is below some arbitrary temperature, such as 100 °C (212 °F). While ordinary liquids such as water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

 and gasoline
Gasoline
Gasoline , or petrol , is a toxic, translucent, petroleum-derived liquid that is primarily used as a fuel in internal combustion engines. It consists mostly of organic compounds obtained by the fractional distillation of petroleum, enhanced with a variety of additives. Some gasolines also contain...

 are predominantly made of electrically neutral
Electric charge
Electric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter. Electric charge comes in two types, called positive and negative. Two positively charged substances, or objects, experience a mutual repulsive force, as do two...

 molecules, ILs are largely made of ion
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...

s and short-lived ion pairs. These substances are variously called liquid electrolytes, ionic melts, ionic fluids, fused salts, liquid salts, or ionic glasses.

ILs have many applications, such as powerful 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...

s and electrically conducting fluids (electrolyte
Electrolyte
In chemistry, an electrolyte is any substance containing free ions that make the substance electrically conductive. The most typical electrolyte is an ionic solution, but molten electrolytes and solid electrolytes are also possible....

s).
Encyclopedia
An ionic liquid is a salt in the liquid
Liquid
Liquid is one of the three classical states of matter . Like a gas, a liquid is able to flow and take the shape of a container. Some liquids resist compression, while others can be compressed. Unlike a gas, a liquid does not disperse to fill every space of a container, and maintains a fairly...

 state. In some contexts, the term has been restricted to salts whose 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...

 is below some arbitrary temperature, such as 100 °C (212 °F). While ordinary liquids such as water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

 and gasoline
Gasoline
Gasoline , or petrol , is a toxic, translucent, petroleum-derived liquid that is primarily used as a fuel in internal combustion engines. It consists mostly of organic compounds obtained by the fractional distillation of petroleum, enhanced with a variety of additives. Some gasolines also contain...

 are predominantly made of electrically neutral
Electric charge
Electric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter. Electric charge comes in two types, called positive and negative. Two positively charged substances, or objects, experience a mutual repulsive force, as do two...

 molecules, ILs are largely made of ion
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...

s and short-lived ion pairs. These substances are variously called liquid electrolytes, ionic melts, ionic fluids, fused salts, liquid salts, or ionic glasses.

ILs have many applications, such as powerful 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...

s and electrically conducting fluids (electrolyte
Electrolyte
In chemistry, an electrolyte is any substance containing free ions that make the substance electrically conductive. The most typical electrolyte is an ionic solution, but molten electrolytes and solid electrolytes are also possible....

s). Salts that are liquid at near-ambient temperature are important for electric battery applications, and have been used as sealants due to their very low vapor pressure
Vapor pressure
Vapor pressure or equilibrium vapor pressure is the pressure of a vapor in thermodynamic equilibrium with its condensed phases in a closed system. All liquids have a tendency to evaporate, and some solids can sublimate into a gaseous form...

.

Any salt that melts without decomposing
Pyrolysis
Pyrolysis is a thermochemical decomposition of organic material at elevated temperatures without the participation of oxygen. It involves the simultaneous change of chemical composition and physical phase, and is irreversible...

 or vaporizing usually yields an IL. Sodium chloride
Sodium chloride
Sodium chloride, also known as salt, common salt, table salt or halite, is an inorganic compound with the formula NaCl. Sodium chloride is the salt most responsible for the salinity of the ocean and of the extracellular fluid of many multicellular organisms...

 (NaCl), for example, melts at 801 °C (1,473.8 °F) into a liquid that consists largely of sodium
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...

 cations and chloride anions . Conversely, when an IL is cooled, it often forms an ionic solid—which may be either crystal
Crystal
A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...

line or glass
Glass
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...

y.

The ionic bond
Ionic bond
An ionic bond is a type of chemical bond formed through an electrostatic attraction between two oppositely charged ions. Ionic bonds are formed between a cation, which is usually a metal, and an anion, which is usually a nonmetal. Pure ionic bonding cannot exist: all ionic compounds have some...

 is usually stronger than the Van der Waals force
Van der Waals force
In physical chemistry, the van der Waals force , named after Dutch scientist Johannes Diderik van der Waals, is the sum of the attractive or repulsive forces between molecules other than those due to covalent bonds or to the electrostatic interaction of ions with one another or with neutral...

s between the molecules of ordinary liquids. For that reason, common salts tend to melt at higher temperatures than other solid molecules. Some salts are liquid at or below room temperature. Examples include 1-ethyl-3-methylimidazolium dicyanamide, ·, that melts at −21 C; and 1-butyl-3,5-dimethylpyridinium bromide which becomes a glass below −24 C.

Low-temperature IL can be compared to ionic solutions, liquids that contain both ions and neutral molecules, and in particular to the so-called deep eutectic solvent
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...

s, mixtures of ionic and non-ionic solid substances which have much lower melting points than the pure compounds. Certain mixtures of nitrate salts can have melting points below 100 °C.

The term "ionic liquid" in the general sense was used as early as 1943.

History

The discovery date of the "first" ionic liquid is disputed, along with the identity of its discoverer. Ethanolammonium nitrate (m.p. 52–55 °C) was reported in 1888 by S. Gabriel and J. Weiner. One of the earliest truly room temperature ionic liquids was ethylammonium nitrate · (m.p. 12 °C), synthesized in 1914 by Paul Walden
Paul Walden
Paul Walden was a Latvian-German chemist known for his work in stereochemistry and history of chemistry. In particular he invented the stereochemical reaction known as Walden inversion and synthesized the first room-temperature ionic liquid, ethylammonium nitrate.-Early years:Walden was born in...

. In the 1970s and 1980s ionic liquids based on alkyl-substituted imidazolium and pyridinium
Pyridinium
Pyridinium refers to the cationic form of pyridine. This can either be due to protonation of the ring nitrogen or because of addition of a substituent to the ring nitrogen, typically via alkylation. The lone pair of electrons on the nitrogen atom of pyridine is not delocalized, and thus pyridine...

 cations, with halide
Halide
A halide is a binary compound, of which one part is a halogen atom and the other part is an element or radical that is less electronegative than the halogen, to make a fluoride, chloride, bromide, iodide, or astatide compound. Many salts are halides...

 or trihalogenoaluminate anions, were initially developed for use as electrolytes in battery applications.

An important property of the imidazolium halogenoaluminate salts is that their physical properties—such as viscosity
Viscosity
Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...

, 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...

, and acidity—could be adjusted by changing the alkyl substituent
Substituent
In organic chemistry and biochemistry, a substituent is an atom or group of atoms substituted in place of a hydrogen atom on the parent chain of a hydrocarbon...

s and the imidazolium/pyridinium and halide/halogenoaluminate ratios. Two major drawbacks for some applications were moisture sensitivity and acidity/basicity. In 1992, Wilkes and Zawarotko obtained ionic liquids with 'neutral' weakly coordinating anions such as hexafluorophosphate
Hexafluorophosphate
Hexafluorophosphate is an anion with chemical formula of . This octahedral species is isoelectronic with sulfur hexafluoride, SF6, and is valence isoelectronic with the highly stable superacid anion fluoroantimonate . As a non-coordinating anion, it is a poor nucleophile...

  and tetrafluoroborate
Tetrafluoroborate
Tetrafluoroborate is the anion BF4−. This tetrahedral species is isoelectronic with tetrafluoromethane, CF4 and tetrafluoroammonium NF4+, and is valence isoelectronic with many stable and important species including the closely related anion perchlorate, ClO4−...

 , allowing a much wider range of applications. Recently a new class of air– and moisture–stable, neutral ionic liquids became available. Research has also been moving away from hexafluorophosphate and tetrafluoroborate towards less toxic alternatives such as bistriflimide
Bistriflimide
Bistriflimide, systematically known as bissulfonimide and colloquially known as TFSI is a non-coordinating anion with the chemical formula of [2N]-. The anion is widely used in ionic liquids, since it is less toxic and more stable than more "traditional" counterions such as tetrafluoroborate...

  or away from halogenated compounds completely. Moves towards less toxic cations have also been growing, with compounds like ammonium
Ammonium
The ammonium cation is a positively charged polyatomic cation with the chemical formula NH. It is formed by the protonation of ammonia...

 salts (such as choline
Choline
Choline is a water-soluble essential nutrient. It is usually grouped within the B-complex vitamins. Choline generally refers to the various quaternary ammonium salts containing the N,N,N-trimethylethanolammonium cation....

) proving to be as flexible a scaffold as imidazolium.

Characteristics

Ionic liquids are often moderate to poor conductors of electricity, non-ionizing (e.g. non-polar), highly viscous and frequently exhibit low vapor pressure
Vapor pressure
Vapor pressure or equilibrium vapor pressure is the pressure of a vapor in thermodynamic equilibrium with its condensed phases in a closed system. All liquids have a tendency to evaporate, and some solids can sublimate into a gaseous form...

. Their other properties are diverse: many have low combustibility, excellent thermal stability, wide liquid regions, and favorable solvating properties
Solvation
Solvation, also sometimes called dissolution, is the process of attraction and association of molecules of a solvent with molecules or ions of a solute...

 for a range of polar and non-polar compounds. Many classes of chemical reaction
Chemical reaction
A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...

s, such as Diels-Alder reaction
Diels-Alder reaction
The Diels–Alder reaction is an organic chemical reaction between a conjugated diene and a substituted alkene, commonly termed the dienophile, to form a substituted cyclohexene system. The reaction can proceed even if some of the atoms in the newly formed ring are not carbon...

s and Friedel-Crafts reaction
Friedel-Crafts reaction
The Friedel–Crafts reactions are a set of reactions developed by Charles Friedel and James Crafts in 1877. There are two main types of Friedel–Crafts reactions: alkylation reactions and acylation reactions. This reaction type is a form of electrophilic aromatic substitution...

s, can be performed using ionic liquids as solvents. Recent work has shown that ionic liquids can serve as solvents for biocatalysis
Biocatalysis
Biocatalysis is the use of natural catalysts, such as protein enzymes, to perform chemical transformations on organic compounds. Both enzymes that have been more or less isolated and enzymes still residing inside living cells are employed for this task....

. The miscibility of ionic liquids with water or organic solvents varies with side chain lengths on the cation and with choice of anion. They can be functionalized to act as acid
Acid
An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...

s, base
Base (chemistry)
For the term in genetics, see base A base in chemistry is a substance that can accept hydrogen ions or more generally, donate electron pairs. A soluble base is referred to as an alkali if it contains and releases hydroxide ions quantitatively...

s or ligand
Ligand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...

s, and have been used as precursor salts in the preparation of stable carbenes. Because of their distinctive properties, ionic liquids are attracting increasing attention in many fields, including organic chemistry
Organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation of carbon-based compounds, hydrocarbons, and their derivatives...

, electrochemistry
Electrochemistry
Electrochemistry is a branch of chemistry that studies chemical reactions which take place in a solution at the interface of an electron conductor and an ionic conductor , and which involve electron transfer between the electrode and the electrolyte or species in solution.If a chemical reaction is...

, catalysis
Catalysis
Catalysis is the change in rate of a chemical reaction due to the participation of a substance called a catalyst. Unlike other reagents that participate in the chemical reaction, a catalyst is not consumed by the reaction itself. A catalyst may participate in multiple chemical transformations....

, physical chemistry
Physical chemistry
Physical chemistry is the study of macroscopic, atomic, subatomic, and particulate phenomena in chemical systems in terms of physical laws and concepts...

, and engineering
Engineering
Engineering is the discipline, art, skill and profession of acquiring and applying scientific, mathematical, economic, social, and practical knowledge, in order to design and build structures, machines, devices, systems, materials and processes that safely realize improvements to the lives of...

; see for instance magnetic ionic liquid
Magnetic ionic liquid
A magnetic ionic liquid was identified by Satoshi Hayashi and Hiro-o Hamaguchi of the University of Tokyo in 2004 as an ionic liquid based on the imidazole 1-butyl-3-methylimidazolium chloride and ferric chloride....

.
Despite their extremely low vapor pressures, some ionic liquids can be distilled under vacuum conditions at temperatures near 300 °C. In the original work by Martyn Earle, et al., the authors wrongly concluded that the vapor was made up of individual, separated ions, but was later proven that the vapors formed consisted of ion-pairs. Some ionic liquids (such as 1-butyl-3-methylimidazolium nitrate) generate flammable gases on thermal decomposition. Thermal stability and 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...

 depend on the liquid's components. Thermal stability of various RTILs (Room Temperature Ionic Liquid) are available. The thermal stability of a task-specific ionic liquid, protonated betaine bis(trifluoromethanesulfonyl)imide is of about 534 kelvins (501.5 °F) and N-Butyl-N-Methyl pyrrolidinium bis(trifluoromethanesulfonyl)imide was thermally stable up to 640 K. The upper limits of thermal stability of ionic liquids reported in the literature are usually based upon fast (about 10 K/min) TGA
Thermogravimetric analysis
Thermogravimetric analysis or thermal gravimetric analysis is a type of testing performed on samples that determines changes in weight in relation to change in temperature. Such analysis relies on a high degree of precision in three measurements: weight, temperature, and temperature change...

 scans, and they do not imply long-term (several hours) thermal stability of ionic liquids, which is limited to less than 500 K for most ionic liquids.

The solubility of different species in imidazolium ionic liquids depends mainly on polarity and hydrogen bonding ability. Saturated aliphatic compounds are generally only sparingly soluble in ionic liquids, whereas olefins show somewhat greater solubility, and aldehyde
Aldehyde
An aldehyde is an organic compound containing a formyl group. This functional group, with the structure R-CHO, consists of a carbonyl center bonded to hydrogen and an R group....

s can be completely miscible. This can be exploited in biphasic catalysis, such as hydrogenation
Hydrogenation
Hydrogenation, to treat with hydrogen, also a form of chemical reduction, is a chemical reaction between molecular hydrogen and another compound or element, usually in the presence of a catalyst. The process is commonly employed to reduce or saturate organic compounds. Hydrogenation typically...

 and hydrocarbonylation processes, allowing for relatively easy separation of products and/or unreacted substrate(s). Gas solubility follows the same trend, with carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

 gas showing exceptional solubility in many ionic liquids. Carbon monoxide
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...

 is less soluble in ionic liquids than in many popular organic solvents, and hydrogen is only slightly soluble (similar to the solubility in water) and may vary relatively little between the more commonly used ionic liquids. Different analytical techniques have yielded somewhat different absolute solubility values.

Room temperature varieties

Room temperature ionic liquids consist of bulky and asymmetric organic cations such as 1-alkyl-3-methylimidazolium, 1-alkylpyridinium, N-methyl-N-alkylpyrrolidinium and ammonium
Ammonium
The ammonium cation is a positively charged polyatomic cation with the chemical formula NH. It is formed by the protonation of ammonia...

 ions. Less common, but offering some advantageous properties are phosphonium cations. A wide range of anions are employed, from simple halide
Halide
A halide is a binary compound, of which one part is a halogen atom and the other part is an element or radical that is less electronegative than the halogen, to make a fluoride, chloride, bromide, iodide, or astatide compound. Many salts are halides...

s, which generally inflect high melting points, to inorganic anions such as tetrafluoroborate
Tetrafluoroborate
Tetrafluoroborate is the anion BF4−. This tetrahedral species is isoelectronic with tetrafluoromethane, CF4 and tetrafluoroammonium NF4+, and is valence isoelectronic with many stable and important species including the closely related anion perchlorate, ClO4−...

 and hexafluorophosphate
Hexafluorophosphate
Hexafluorophosphate is an anion with chemical formula of . This octahedral species is isoelectronic with sulfur hexafluoride, SF6, and is valence isoelectronic with the highly stable superacid anion fluoroantimonate . As a non-coordinating anion, it is a poor nucleophile...

 and to large organic anions like bistriflimide
Bistriflimide
Bistriflimide, systematically known as bissulfonimide and colloquially known as TFSI is a non-coordinating anion with the chemical formula of [2N]-. The anion is widely used in ionic liquids, since it is less toxic and more stable than more "traditional" counterions such as tetrafluoroborate...

, triflate
Triflate
Trifluoromethanesulfonate, also known by the trivial name triflate, is a functional group with the formula CF3SO3-. The triflate group is often represented by -OTf, as opposed to -Tf...

 or tosylate. There are also many interesting uses of ionic liquids with simple non-halogenated organic anions such as formate
Formate
Formate or methanoate is the ion CHOO− or HCOO− . It is the simplest carboxylate anion. It is produced in large amounts in the hepatic mitochondria of embryonic cells and in cancer cells by the folate cycle Formate or methanoate is the ion CHOO− or HCOO− (formic acid minus one hydrogen ion). It...

, alkylsulfate, alkylphosphate or glycolate. The melting point of 1-butyl-3-methylimidazolium tetrafluoroborate
Tetrafluoroborate
Tetrafluoroborate is the anion BF4−. This tetrahedral species is isoelectronic with tetrafluoromethane, CF4 and tetrafluoroammonium NF4+, and is valence isoelectronic with many stable and important species including the closely related anion perchlorate, ClO4−...

 with an imidazole
Imidazole
Imidazole is an organic compound with the formula C3H4N2. This aromatic heterocyclic is a diazole and is classified as an alkaloid. Imidazole refers to the parent compound, whereas imidazoles are a class of heterocycles with similar ring structure, but varying substituents...

 skeleton is about −80 C and it is a colorless liquid with high viscosity
Viscosity
Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...

 at room temperature.

In many synthetic processes using transition metal catalysts, metal nanoparticles play an important role as the actual catalyst or as a catalyst reservoir. ILs are an appealing medium for the formation and stabilization of catalytically active transition metal nanoparticles. More importantly, ILs can be made that incorporate coordinating groups, for example, with nitrile
Nitrile
A nitrile is any organic compound that has a -C≡N functional group. The prefix cyano- is used interchangeably with the term nitrile in industrial literature. Nitriles are found in many useful compounds, one example being super glue .Inorganic compounds containing the -C≡N group are not called...

 groups on either the cation or anion (CN-IL). In various C-C coupling reactions catalyzed by a palladium
Palladium
Palladium is a chemical element with the chemical symbol Pd and an atomic number of 46. It is a rare and lustrous silvery-white metal discovered in 1803 by William Hyde Wollaston. He named it after the asteroid Pallas, which was itself named after the epithet of the Greek goddess Athena, acquired...

 catalyst, it has been found that palladium nanoparticles are better stabilized in CN-IL compared to non-functionalized ionic liquids; thus enhanced catalytic activity and recyclability are realized.

Low temperature varieties

Low temperature ionic liquids (below 130 K
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...

) have been proposed as the fluid base for an extremely large diameter spinning liquid mirror
Liquid mirror
Liquid mirror telescopes are telescopes made with a reflective liquid. The most common liquid used is mercury, but other liquids will work as well . The container for the liquid is rotating so that the liquid assumes a paraboloidal shape. A paraboloidal shape is precisely the shape needed for the...

 telescope to be based on the Earth's moon. Low temperature is advantageous in imaging long wave infrared light which is the form of light (extremely red-shifted
Redshift
In physics , redshift happens when light seen coming from an object is proportionally increased in wavelength, or shifted to the red end of the spectrum...

) that arrives from the most distant parts of the visible universe. Such a liquid base would be covered by a thin metallic film that forms the reflective surface. Low volatility is important for use in lunar vacuum conditions.

Applications

ILS find a variety of industrial applications. A few industrial applications are described below; more detailed information can be found in a recent review article.

Chemical industry

The first major industrial IL application was the BASIL (Biphasic Acid Scavenging utilizing Ionic Liquids) process by BASF
BASF
BASF SE is the largest chemical company in the world and is headquartered in Germany. BASF originally stood for Badische Anilin- und Soda-Fabrik . Today, the four letters are a registered trademark and the company is listed on the Frankfurt Stock Exchange, London Stock Exchange, and Zurich Stock...

, in which 1-alkylimidazole scavenged the acid from an existing process. This then results in the formation of an IL which can easily be removed from the reaction mixture. This increased the space/time yield of the reaction by a factor of 80,000.

Eastman operated an IL-based plant for the synthesis of 2,5-dihydrofuran from 1996 to 2004.

The dimersol process is a traditional way to dimerize short chain alkene
Alkene
In organic chemistry, an alkene, olefin, or olefine is an unsaturated chemical compound containing at least one carbon-to-carbon double bond...

s into branched alkenes of higher molecular weight. Y. Chauvin
Yves Chauvin
Yves Chauvin is a French chemist and Nobel Prize laureate. He is honorary research director at the Institut français du pétrole and a member of the French Academy of Science. Chauvin received his degree from the Lyon School of Chemistry, Physics and Electronics in 1954.He was awarded the 2005...

 and H. Olivier-Bourbigou developed an IL-based add-on to this process called the difasol process.

Ionikylation is an IL–based process developed by Petrochina
PetroChina
PetroChina Company Limited is a Chinese oil company and is the listed arm of state-owned China National Petroleum Corporation , headquartered in Dongcheng District, Beijing. It is China's biggest oil producer, and was the world's most valuable company by market value as of September 28th 2010...

 for the alkylation of four-carbon olefins with isobutane
Isobutane
Isobutane, also known as methylpropane, is an isomer of butane. It is the simplest alkane with a tertiary carbon. Concerns with depletion of the ozone layer by freon gases have led to increased use of isobutane as a gas for refrigeration systems, especially in domestic refrigerators and freezers,...

. Their 65,000 tonne per year plant is claimed to be the biggest industrial application of ionic liquids to date.

Pharmaceuticals

Recognising that approximately 50% of commercial pharmaceuticals are organic salts, ionic liquid forms of a number of pharmaceuticals have been investigated. Combining a pharmaceutically active cation with a pharmaceutically active anion leads to a Dual Active ionic liquid in which the actions of two drugs are combined.

Cellulose processing

At a volume of some 700 billion tons, 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....

 is the earth’s most widespread natural organic chemical and, thus, highly important as a renewable resource. But even out of nature's annual 40 billion tons output, only approx. 5% is used as feedstock for further processing. More intensive exploitation of cellulose, as a renewable feedstock, is aided by the development of suitable solvents for mechanical and chemical processing. ILs have been shown to be highly effective at solvating
Solvation
Solvation, also sometimes called dissolution, is the process of attraction and association of molecules of a solvent with molecules or ions of a solute...

 cellulose to technically useful concentrations. This may allow for development of new processes and intensification of existing ones substituting existing cellulose solvents for ILs. The use of molten salts in cellulose processing originally dates back to a patent application from 1930 where mixtures of molten 1-alkylpyridinium chlorides were used to dissolve cellulose, followed by subsequent chemical and mechanical processing. Further advances have generally led to lowering of melting points and viscosities to a point where there are now room temperature ionic liquids available that dissolve cellulose.

Making viscose
Viscose
Viscose is a viscous organic liquid used to make rayon and cellophane. Viscose is becoming synonymous with rayon, a soft material commonly used in shirts, shorts, coats, jackets, and other outer wear.-Manufacture:...

-based fibres from dissolving pulp currently involves the use, and subsequent disposal, of great volumes of various chemical auxiliaries, esp. carbon disulfide . Major volumes of wastewater
Wastewater
Wastewater is any water that has been adversely affected in quality by anthropogenic influence. It comprises liquid waste discharged by domestic residences, commercial properties, industry, and/or agriculture and can encompass a wide range of potential contaminants and concentrations...

 need to be disposed of. Following in the footsteps of the lyocell
Lyocell
Lyocell is a regenerated cellulose fiber made from dissolving pulp . It was first manufactured in 1987 by Courtaulds Fibres UK at their pilot plant S25...

 process, which uses hydrated N-Methylmorpholine N-oxide
N-Methylmorpholine N-oxide
N-Methylmorpholine-N-oxide, NMO or NMMO is an organic compound. This heterocyclic amine oxide and morpholine derivative is used in organic chemistry as a co-oxidant and sacrificial catalyst in oxidation reactions for instance in osmium tetroxide oxidations and the Sharpless asymmetric...

, as a novel non-aqueous solvent for the dissolution of the pulp, it has been suggested that IL can greatly simplify these processes, serving as solvents that are potentially recyclable. The “Institut für Textilchemie und Chemiefasern” (ITCF) in Denkendorf and BASF are jointly investigating the properties of fibers spun from solutions of cellulose dissolved in IL in a pilot plant.
The dissolution of cellulose–based materials like tissue paper
Tissue paper
Tissue paper is a lightweight paper or, light crêpe paper. Tissue can be made both from virgin and recycled paper pulp.-Properties:Key properties are: strength, absorbency, basis weight, thickness , brightness, stretch, appearance and comfort....

 waste, generated in chemical industries and at research laboratories, in room temperature IL 1-butyl-3-methylimidazolium chloride, bmimCl and the recovery of valuable compounds by electrodeposition
Electrodeposition
Electrodeposition may refer to:*Electroplating*Electrophoretic deposition*Underpotential deposition...

 from this cellulose matrix was studied.

Dispersants

ILs can act as dispersing agents in paint
Paint
Paint is any liquid, liquefiable, or mastic composition which after application to a substrate in a thin layer is converted to an opaque solid film. One may also consider the digital mimicry thereof...

s to enhance finish, appearance and drying properties.

ILs are used for dispersing nanomaterials at IOLITEC.

Gas handling

ILs have several properties that make them useful in gas storage and handling applications, including low vapor pressure, stability at high temperatures, and solvation for a wide variety of compounds and gases. They also have weakly coordinating anions and cations which are able to stabilize polar transition states. Many ionic liquids can be reused with minimal loss of activity.

The company Air Products uses ILs instead of pressurized cylinders as a transport medium for reactive gases such as trifluoroborane, phosphine
Phosphine
Phosphine is the compound with the chemical formula PH3. It is a colorless, flammable, toxic gas. Pure phosphine is odourless, but technical grade samples have a highly unpleasant odor like garlic or rotting fish, due to the presence of substituted phosphine and diphosphine...

 and arsine
Arsine
Arsine is the chemical compound with the formula AsH3. This flammable, pyrophoric, and highly toxic gas is one of the simplest compounds of arsenic...

. The gases are dissolved in the liquids at or below atmospheric pressure and are easily withdrawn from the containers by applying a vacuum.

Gas manufacturer Linde exploits the low solubility of hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

 in ILs to compress the gas up to 450 bar in filling stations
Hydrogen station
A hydrogen station is a storage or filling station for hydrogen, usually located along a road or hydrogen highway, or at home as part of the distributed generation resources concept. The stations are usually intended to power vehicles, but can also be used to power small devices. Vehicles use...

 by using an ionic liquid piston compressor
Ionic liquid piston compressor
An ionic liquid piston compressor, ionic compressor or ionic liquid piston pump is a hydrogen compressor based on an ionic liquid piston instead of a metal piston as in a piston-metal diaphragm compressor.-Principle:...

, which has only 8 moving parts (down from about 500 in a conventional piston pump
Reciprocating compressor
A reciprocating compressor or piston compressor is a positive-displacement compressor that uses pistons driven by a crankshaft to deliver gases at high pressure....

).

IL 1-butyl-3-methylimidazolium chloride has been used for separating hydrogen from ammonia borane
Ammonia borane
Ammonia borane is the chemical compound with the formula H3NBH3. The colourless solid is the simplest molecular boron-nitrogen-hydride compound and has attracted attention as a source of hydrogen fuel.-Synthesis:...

.

ILs and 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 can be used to capture carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

  and purify natural gas.

Gas treatment

ION Engineering is commercializing ILs and amines for capture and natural gas sweetening.

Nuclear fuel reprocessing

IL 1-butyl-3-methylimidazolium chloride has been investigated as a non-aqueous electrolyte media for the recovery of uranium
Uranium
Uranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...

 and other metals from spent nuclear fuel
Nuclear fuel
Nuclear fuel is a material that can be 'consumed' by fission or fusion to derive nuclear energy. Nuclear fuels are the most dense sources of energy available...

 and other sources. Protonated betaine bis(trifluoromethanesulfonyl) imide has been investigated as a solvent for uranium oxides.
Ionic liquids, N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide and N-methyl-N-propylpiperidinium bis(trifluoromethylsulfonyl)imide, have been investigated for the electrodeposition of Europium and Uranium metals respectively.

Solar thermal energy

ILs have potential as a heat transfer and storage medium in solar thermal energy
Solar thermal energy
Solar thermal energy is a technology for harnessing solar energy for thermal energy . Solar thermal collectors are classified by the United States Energy Information Administration as low-, medium-, or high-temperature collectors. Low-temperature collectors are flat plates generally used to heat...

 systems. Concentrating solar thermal facilities such as parabolic trough
Parabolic trough
A parabolic trough is a type of solar thermal energy collector. It is constructed as a long parabolic mirror with a Dewar tube running its length at the focal point. Sunlight is reflected by the mirror and concentrated on the Dewar tube...

s and solar power tower
Solar power tower
The solar power tower is a type of solar furnace using a tower to receive the focused sunlight. It uses an array of flat, movable mirrors to focus the sun's rays upon a collector tower...

s focus the sun's energy onto a receiver which can generate temperatures of around 600 °C (1,112 °F). This heat can then generate electricity in a steam or other cycle. For buffering during cloudy periods or to enable generation overnight, energy can be stored by heating an intermediate fluid. Although nitrate salts have been the medium of choice since the early 1980s, they freeze at 220 °C (428 °F) and thus require heating to prevent solidification. Ionic liquids such as [][] have more favorable liquid-phase temperature ranges (-75 to 459 °C) and could therefore be excellent liquid thermal storage media and heat transfer fluids.

Food and bioproducts

IL 1-butyl-3-methylimidazolium chloride completely dissolves freeze dried banana
Banana
Banana is the common name for herbaceous plants of the genus Musa and for the fruit they produce. Bananas come in a variety of sizes and colors when ripe, including yellow, purple, and red....

 pulp and with an additional 15% DMSO
Dimethyl sulfoxide
Dimethyl sulfoxide is an organosulfur compound with the formula 2SO. This colorless liquid is an important polar aprotic solvent that dissolves both polar and nonpolar compounds and is miscible in a wide range of organic solvents as well as water...

, lends itself to Carbon-13 NMR
Carbon-13 NMR
Carbon-13 NMR is the application of nuclear magnetic resonance spectroscopy to carbon. It is analogous to proton NMR and allows the identification of carbon atoms in an organic molecule just as proton NMR identifies hydrogen atoms...

 analysis. In this way the entire banana complex of starch
Starch
Starch or amylum is a carbohydrate consisting of a large number of glucose units joined together by glycosidic bonds. This polysaccharide is produced by all green plants as an energy store...

, sucrose
Sucrose
Sucrose is the organic compound commonly known as table sugar and sometimes called saccharose. A white, odorless, crystalline powder with a sweet taste, it is best known for its role in human nutrition. The molecule is a disaccharide composed of glucose and fructose with the molecular formula...

, glucose
Glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...

, and fructose
Fructose
Fructose, or fruit sugar, is a simple monosaccharide found in many plants. It is one of the three dietary monosaccharides, along with glucose and galactose, that are absorbed directly into the bloodstream during digestion. Fructose was discovered by French chemist Augustin-Pierre Dubrunfaut in 1847...

 can be monitored as a function of banana ripening.

ILs can extract specific compounds from plants for pharmaceutical, nutritional and cosmetic applications, such as the antimalarial drug artemisinin
Artemisinin
Artemisinin , also known as Qinghaosu , and its derivatives are a group of drugs that possess the most rapid action of all current drugs against falciparum malaria. Treatments containing an artemisinin derivative are now standard treatment worldwide for falciparum malaria...

 from the plant Artemisia annua
Artemisia annua
Artemisia annua, also known as Sweet Wormwood, Sweet Annie, Sweet Sagewort or Annual Wormwood , is a common type of wormwood that is native to temperate Asia, but naturalized throughout the world.-Characteristics:...

.

Waste recycling

ILs can aid the recycling of synthetic goods, plastics and metals. They offer the specificity required to separate similar compounds from each other, such as separating polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...

s in plastic waste streams. This has been achieved using lower temperature extraction processes than current approaches and could help avoid incinerating plastics or dumping them in landfill.

Batteries

Researchers have identified ILs that can replace water as the electrolyte in metal-air batteries. ILs have great appeal because they evaporate at much lower rates than water, increasing battery life by drying slower. Further, ILs have an electrochemical window
Electrochemical window
The electrochemical window of a substance is the voltage range between which the substance doesn't get oxidized nor reduced.This range is important for the efficiency of an electrode, out of this range, water gets electrolysed, spoiling the electrical energy that is intended for another...

 of up to six volts (versus 1.23 for water) supporting more energy-dense metals. Energy densities from 900-1600 watt-hours per kilogram appear possible.

A Metal-air battery draws oxygen through a porous ambient "air" electrode (-cathode) and produces water, hydrogen peroxide
Hydrogen peroxide
Hydrogen peroxide is the simplest peroxide and an oxidizer. Hydrogen peroxide is a clear liquid, slightly more viscous than water. In dilute solution, it appears colorless. With its oxidizing properties, hydrogen peroxide is often used as a bleach or cleaning agent...

, or hydroxide anions depending on the nature oxygen reduction catalyst and electrolyte. These compounds store the electrons released by the oxidation of the anode.

Safety

ILs' non-volatility effectively eliminates a major pathway for environmental release and contamination. However, this property is distinct from toxicity. ILs' aquatic toxicity is as severe as or more so than many current solvents. Mortality isn't necessarily the most important metric for measuring impacts in aquatic environments, as sub-lethal concentrations change organisms' life histories in meaningful ways. Balancing 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...

 reductions against waterway spills (via waste ponds/streams, etc.) requires further research. ILs' substituent diversity simplify the process of identifying compounds that meet safety requirements.

Ultrasound
Ultrasound
Ultrasound is cyclic sound pressure with a frequency greater than the upper limit of human hearing. Ultrasound is thus not separated from "normal" sound based on differences in physical properties, only the fact that humans cannot hear it. Although this limit varies from person to person, it is...

 can degrade solutions of imidazolium-based ionic liquids with hydrogen peroxide
Hydrogen peroxide
Hydrogen peroxide is the simplest peroxide and an oxidizer. Hydrogen peroxide is a clear liquid, slightly more viscous than water. In dilute solution, it appears colorless. With its oxidizing properties, hydrogen peroxide is often used as a bleach or cleaning agent...

 and acetic acid
Acetic acid
Acetic acid is an organic compound with the chemical formula CH3CO2H . It is a colourless liquid that when undiluted is also called glacial acetic acid. Acetic acid is the main component of vinegar , and has a distinctive sour taste and pungent smell...

 to relatively innocuous compounds.

Despite low vapor pressure
Vapor pressure
Vapor pressure or equilibrium vapor pressure is the pressure of a vapor in thermodynamic equilibrium with its condensed phases in a closed system. All liquids have a tendency to evaporate, and some solids can sublimate into a gaseous form...

 many ionic liquids are combustible
Combustion
Combustion or burning is the sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat and conversion of chemical species. The release of heat can result in the production of light in the form of either glowing or a flame...

 and therefore require careful handling. Brief exposure (5 to 7 seconds) to a flame torch can ignite some ILs. Complete combustion is possible for some ILs.

See also

  • MDynaMix
    MDynaMix
    MDynaMixis a general purpose molecular dynamics software package for simulations mixtures of molecules,interacting by AMBER/CHARMM like force fields in a periodic boundary conditions....

     software for ionic liquids simulations
  • Ionic Liquids Biological Effects Database, free database on toxicology and ecotoxicology of ionic liquids
  • 1-Butyl-3-methylimidazolium hexafluorophosphate
    1-Butyl-3-methylimidazolium hexafluorophosphate
    1-Butyl-3-methylimidazolium hexafluorophosphate, also known as BMIM-PF6, is a viscous, colourless, hydrophobic and non-water soluble ionic liquid. Together with 1-butyl-3-methylimidazolium tetrafluoroborate, BMIM-BF4, it is one of the most widely studied ionic liquids and is commercially available...

     (BMIM-PF6) for an often encountered ionic liquid
  • Trioctylmethylammonium bis(trifluoromethyl-sulfonyl)imide
    Trioctylmethylammonium bis(trifluoromethyl-sulfonyl)imide
    Trioctylmethylammonium bisimide is an ionic liquid that is produced by Solvent Innovation....

  • Aza-Baylis–Hillman reaction for the use of a chiral ionic liquid in asymmetric synthesis.
  • Deep eutectic solvent
    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...

  • 1st international Ionic Liquids web portal, All Around Ionic Liquids
  • Ionic Liquid Property Database with analytical and synthesis data, from delph-IL

Further reading

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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