Proposed structure of an imidazolium-based ionic liquid.
An ionic liquid (IL) 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 100 °C (212 °F). While ordinary liquids such as water and gasoline are predominantly made of electrically neutralmolecules, ionic liquids are largely made of ions. These substances are variously called liquid electrolytes, ionic melts, ionic fluids, fused salts, liquid salts, or ionic glasses.
Any salt that melts without decomposing or vaporizing usually yields an ionic liquid. Sodium chloride (NaCl), for example, melts at 801 °C (1,474 °F) into a liquid that consists largely of sodium cations (Na+ ) and chloride anions (Cl− ). Conversely, when an ionic liquid is cooled, it often forms an ionic solid—which may be either crystalline or glassy.
The ionic bond is usually stronger than the Van der Waals forces between the molecules of ordinary liquids. Because of these strong interactions, salts tend to have high lattice energies, manifested in high melting points. Some salts, especially those with organic cations, have low lattice energies and thus are liquid at or below room temperature. Examples include compounds based on the 1-ethyl-3-methylimidazolium (EMIM) cation and include: EMIM:Cl, EMIMAc (acetate anion), EMIM dicyanamide, (C 2H 5)(CH 3)C 3H 3N+ 2·N(CN)− 2, that melts at −21 °C (−6 °F); and 1-butyl-3,5-dimethylpyridinium bromide which becomes a glass below −24 °C (−11 °F).
Low-temperature ionic liquids can be compared to ionic solutions, liquids that contain both ions and neutral molecules, and in particular to the so-called deep eutectic solvents, 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.
When Tawny crazy ants (Nylanderia fulva) combat Fire ants (Solenopsis invicta), the latter spray them with a toxic, lipophilic, alkaloid-based venom. The Tawny crazy ant then exudes its own venom, formic acid, and self-grooms with it, an action which de-toxifies the Fire ant venom. The mixed venoms chemically react with one another to form an ionic liquid, the first naturally occurring IL to be described.
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 room temperature ionic liquids was ethylammonium nitrate (C 2H 5)NH+ 3·NO− 3 (m.p. 12 °C), reported in 1914 by Paul Walden. In the 1970s and 1980s, ionic liquids based on alkyl-substituted imidazolium and pyridinium cations, with halide or tetrahalogenoaluminate anions, were developed as potential electrolytes in batteries.
Although many classical ILs are hexafluorophosphate and tetrafluoroborate salts, bistriflimides[(CF 3SO 2) 2N]− are also popular.
Ionic liquids are often moderate to poor conductors of electricity, non-ionizing, highly viscous and frequently exhibit low vapor pressure. Their other properties are diverse: many have low combustibility, are thermally stable, with wide liquid regions, and favorable solvating properties for a range of polar and non-polar compounds. Many classes of chemical reactions, such as Diels-Alder reactions and Friedel-Crafts reactions, can be performed using ionic liquids as solvents. ILs can serve as solvents for biocatalysis. 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 acids, bases, or ligands, and are precursors salts in the preparation of stable carbenes. They have been found to hydrolyse. Because of their distinctive properties, ionic liquids have been investigated for many applications.
Cations commonly found in ionic liquids
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 depend on the liquid's components. The thermal stability of ionic liquids is <225 °C.
The solubility properties of ILs are diverse. Saturated aliphatic compounds are generally only sparingly soluble in ionic liquids, whereas alkenes show somewhat greater solubility, and aldehydes can be completely miscible. Solubility differences can be exploited in biphasic catalysis, such as hydrogenation and hydrocarbonylation processes, allowing for relatively easy separation of products and/or unreacted substrate(s). Gas solubility follows the same trend, with carbon dioxide gas showing good solubility in many ionic liquids. Carbon monoxide 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 common ionic liquids.
Some ILs do not freeze down to very low temperatures (even −150 °C), The glass transition temperature was detected below −100 °C in the case of ionic liquids with N-methyl-N-alkylpyrrolidinium cations and fluorosulfonyl-trifluoromethanesulfonylimide (FTFSI).
Water is a common impurity in ionic liquids, as it can be absorbed from the atmosphere and influences the transport properties of RTILs, even at relatively low concentrations.
Table salt NaCl and ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide at 27 °С
Room-temperature ionic liquids (RTILs) consist of salts derived from 1-methylimidazole, i.e., 1-alkyl-3-methylimidazolium. Examples include 1-ethyl-3-methyl- (EMIM), 1-butyl-3-methyl- (BMIM), 1-octyl-3 methyl (OMIM), 1-decyl-3-methyl-(DMIM), 1-dodecyl-3-methyl- docecylMIM). Other imidazolium cations are 1-butyl-2,3-dimethylimidazolium (DBMIM), 1,3-di(N,N-dimethylaminoethyl)-2-methylimidazolium (DAMI), and 1-butyl-2,3-dimethylimidazolium (BMMIM). Other N-heterocyclic cations are derived from pyridine: 4-methyl-N-butyl-pyridinium (MBPy) and N-octylpyridinium (C8Py). Conventional quaternary ammonium cations also form ILs, e.g. tetraethylammonium (TEA) and tetrabutylammonium (TBA).
Low-temperature ionic liquids (below 130 K) have been proposed as the fluid base for an extremely large diameter spinning liquid-mirror telescope to be based on the Moon. Low temperature is advantageous in imaging long-wave infrared light, which is the form of light (extremely red-shifted) 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 in lunar vacuum conditions to prevent evaporation.
Protic ionic liquids
Protic ionic liquids are formed via a proton transfer from an acid to a base. In contrast to other ionic liquids, which generally are formed through a sequence of synthesis steps, protic ionic liquids can be created more easily by simply mixing the acid and base.
Polymerized ionic liquids, poly(ionic liquid)s or polymeric ionic liquids, all abbreviated as PIL is the polymeric form of ionic liquids. They have half of the ionicity of ionic liquids since one ion is fixed as the polymer moiety to form a polymeric chain. PILs have a similar range of applications, comparable with those of ionic liquids but the polymer architecture provides a better chance for controlling the ionic conductivity. They have extended the applications of ionic liquids for designing smart materials or solid electrolytes.
An IL based on tetraalkylphosphonium iodide is a solvent for tributyltin iodide, which functions as a catalyst to rearrange the monoepoxide of butadiene. This process was commercialized as a route to 2,5-dihydrofuran, but later discontinued.
ILs improve the catalytic performance of palladium nanoparticles. Furthermore, ionic liquids can be used pre-catalysts for chemical transformations. In this regard dialkylimidazoliums such as [EMIM]Ac have been used in the combination with a base to generate N-heterocyclic carbenes (NHCs). These imidazolium based NHCs are known to catalyse a number transformations such as the Benzoin condensation and the OTHO reaction.
Recognizing that approximately 50% of commercial pharmaceuticals are 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.
The dissolution of cellulose by ILs has attracted interest. A patent application from 1930 showed that 1-alkylpyridinium chlorides dissolve cellulose. Following in the footsteps of the lyocell process, which uses hydrated N-Methylmorpholine N-oxide, as a non-aqueous solvent for the dissolution of the pulp and paper. The dissolution of cellulose–based materials like tissue paper 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 from this cellulose matrix was studied. The "valorization" of cellulose, i.e. its conversion to more valuable chemicals, has been achieved by the use of ionic liquids. Representative products are glucose esters, sorbitol, and alkylgycosides. IL 1-butyl-3-methylimidazolium chloride dissolves freeze driedbanana pulp and with an additional 15% dimethyl sulfoxide, lends itself to Carbon-13 NMR analysis. In this way the entire complex of starch, sucrose, glucose, and fructose can be monitored as a function of banana ripening.
Beyond cellulose, ILs have also shown potential in the dissolution, extraction, purification, processing and modification of other biopolymers such as chitin/chitosan, starch, alginate, collagen, gelatin, keratin, and fibroin. For example, ILs allow for the preparation of biopolymer materials in different forms (e.g. sponges, films, microparticles, nanoparticles, and aerogels) and better biopolymer chemical reactions, leading to biopolymer-based drug/gene-delivery carriers. Moreover, ILs enable the synthesis of chemically modified starches with high efficiency and degrees of substitution (DS) and the development of various starch-based materials such as thermoplastic starch, composite films, solid polymer electrolytes, nanoparticles and drug carriers.
Nuclear fuel reprocessing
The IL 1-butyl-3-methylimidazolium chloride has been investigated for the recovery of uranium and other metals from spent nuclear fuel 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 are potential heat transfer and storage media in solar thermal energy systems. Concentrating solar thermal facilities such as parabolic troughs and solar power towers 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 Cmim 4[BF 4] have more favorable liquid-phase temperature ranges (-75 to 459 °C) and could therefore be excellent liquid thermal storage media and heat transfer fluids.
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 polymers 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.
ILs can replace water as the electrolyte in metal-air batteries. ILs are attractive because of their low vapor pressure. Furthermore, ILs have an electrochemical window 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.
Some ionic liquids have been shown to reduce friction and wear in basic tribological testing, and their polar nature makes them candidate lubricants for tribotronic applications. While the comparatively high cost of ionic liquids currently prevents their use as neat lubricants, adding ionic liquids in concentrations as low as 0.5 wt% may significantly alter the lubricating performance of conventional base oils. Thus, the current focus of research is on using ionic liquids as additives to lubricating oils, often with the motivation to replace widely used, ecologically harmful lubricant additives. However, the claimed ecological advantage of ionic liquids has been questioned repeatedly and is yet to be demonstrated from a lifecycle perspective.
Ionic liquids' low volatility effectively eliminates a major pathway for environmental release and contamination.
Ionic liquids' aquatic toxicity is as severe as or more so than many current solvents.
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Ionic Liquids Biological Effects Database, free database on toxicology and ecotoxicology of ionic liquids