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 neutral molecules, ionic liquids are largely made of ions and short-lived ion pairs. These substances are variously called liquid electrolytes, ionic melts, ionic fluids, fused salts, liquid salts, or ionic glasses. [1][2][3]

The chemical structure of 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM]PF6), a common ionic liquid.
Proposed structure of an imidazolium-based ionic liquid.

Ionic liquids have many potential applications. They are powerful solvents and can be used as electrolytes. Salts that are liquid at near-ambient temperature are important for electric battery applications, and have been considered as sealants due to their very low vapor pressure.

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. 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 compounds based on the 1-Ethyl-3-methylimidazolium (EMIM) cation and include: EMIM:Cl, EMIM dicyanamide, (C
2H
5)(CH
3)C
3H
3N+
2·N(CN)−
2, that melts at −21 °C (−6 °F);[4] and 1-butyl-3,5-dimethylpyridinium bromide which becomes a glass below −24 °C (−11 °F).[5]

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.[6]

The term "ionic liquid" in the general sense was used as early as 1943.[7]

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.[8]

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.[9] One of the earliest truly room temperature ionic liquids was ethylammonium nitrate (C
2H
5)NH+
3·NO−
3 (m.p. 12 °C), reported in 1914 by Paul Walden.[10] 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.[11][12]

For the imidazolium halogenoaluminate salts, their physical properties—such as viscosity, melting point, and acidity—could be adjusted by changing the alkyl substituents and the imidazolium/pyridinium and halide/halogenoaluminate ratios.[13] Two major drawbacks for some applications were moisture sensitivity and acidity or basicity. In 1992, Wilkes and Zawarotko obtained ionic liquids with 'neutral' weakly coordinating anions such as hexafluorophosphate (PF−
6) and tetrafluoroborate (BF−
4), allowing a much wider range of applications.[14]

Although many classical ILs are hexafluorophosphate and tetrafluoroborate salts, bistriflimide [(CF
3SO
2)
2N]−
 are also popular.

Characteristics

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.[15] 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.[16] 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.[17] In the original work by Martyn Earle, et al., the authors wrongly concluded that the vapor was made up of individual, separated ions,[18] but was later proven that the vapors formed consisted of ion pairs.[19] 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.[20] The thermal stability of a task-specific ionic liquid, protonated betaine bis(trifluoromethanesulfonyl)imide is of about 534 K (502 °F) and N-butyl-N-methyl pyrrolidinium bis(trifluoromethanesulfonyl)imide was thermally stable up to 640 K.[21] The upper limits of thermal stability of ionic liquids reported in the literature are usually based upon fast (about 10 K/min) TGA 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.[22]

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.

Room-temperature varieties
Table salt NaCl and ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide at 27 °С

Room-temperature ionic liquids (RTILs) consist of bulky and asymmetric organic cations such as 1-alkyl-3-methylimidazolium, 1-alkylpyridinium, N-methyl-N-alkylpyrrolidinium and ammonium ions. Phosphonium cations are less common, but offer some advantageous properties.[23][24] A wide range of anions are employed, ranging from simple halides, which generally suffer high melting points, to inorganic anions such as tetrafluoroborate and hexafluorophosphate, and to large organic anions like bistriflimide, triflate or tosylate. There are also many potential uses of ionic liquids with simple non-halogenated organic anions such as formate, alkylsulfate, alkylphosphate or glycolate. The melting point of 1-butyl-3-methylimidazolium tetrafluoroborate is about −80 °C (−112 °F) and it is a colorless liquid with high viscosity at room temperature. If a highly asymmetric cation is combined with a highly asymmetric anion, formed ionic liquid may not freeze down to very low temperatures (down to −150 °C), and 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).[25] 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.

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,[26] for example, with nitrile groups on either the cation or anion (CN-IL). In various C-C coupling reactions catalyzed by a palladium 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.[27]

Low-temperature varieties

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.[28] 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.

Poly(ionic liquid)s

Polymerized ionic liquids, poly(ionic liquid)s or polymeric ionic liquids, all abbreviated as PIL is the polymeric form of ionic liquids.[29] 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.[30][31]

Magnetic ionic liquids

Magnetic ionic liquids can be synthesized by incorporating paramagnetic elements into ionic liquid molecules. One example is 1-butyl-3-methylimidazolium tetrachloroferrate.

Commercial applications

Many applications have been considered, some commercialized briefly, and others remain under development.[32]

Organic synthesis

A liquid 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.[33]

Pharmaceuticals

Recognizing 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.[34][35]

ILs can extract specific compounds from plants for pharmaceutical, nutritional and cosmetic applications, such as the antimalarial drug artemisinin from the plant Artemisia annua.[36]

Cellulose processing

The dissolution of cellulose by ILs has attracted interest.[37] A patent application from 1930 showed that 1-alkylpyridinium chlorides dissolve cellulose.[38] 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.[39] 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.[40] IL 1-butyl-3-methylimidazolium chloride dissolves freeze dried banana 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.[41][42]

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.[43][44][45] Protonated betaine bis(trifluoromethanesulfonyl) imide has been investigated as a solvent for uranium oxides.[46] 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.[47][48]

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.[49]

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 polymers in plastic waste streams. This has been achieved using lower temperature extraction processes than current approaches[50] and could help avoid incinerating plastics or dumping them in landfill.

Batteries

ILs can replace water as the electrolyte in metal-air batteries. ILs are attractive because of their low vapor pressure, increasing battery life by drying more slowly. Furthermore, ILs have an electrochemical window of up to six volts[51] (versus 1.23 for water) supporting more energy-dense metals. Energy densities from 900-1600 watt-hours per kilogram appear possible.[52]

Dispersing agent

ILs can act as dispersing agents in paints to enhance finish, appearance and drying properties.[53] ILs are used for dispersing nanomaterials at IOLITEC.

Carbon capture

ILs and amines have been investigated for capturing carbon dioxide CO
2 and purifying natural gas.[54][55][56]

Tribology

Some ionic liquids have been shown to reduce friction and wear in basic tribological testing,[57][58][59][60] 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.[61]

Safety

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.[62][63][64] 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 reductions against waterway spills (via waste ponds/streams, etc.) requires further research. Ionic liquids' substituent diversity simplifies the process of identifying compounds that meet safety requirements.

Ultrasound can degrade solutions of imidazolium-based ionic liquids with hydrogen peroxide and acetic acid to relatively innocuous compounds.[65]

Despite low vapor pressure many ionic liquids are combustible and therefore require careful handling.[66] Brief exposure (5 to 7 seconds) to a flame torch can ignite some Ionic liquids. Complete combustion is possible for some Ionic liquids.[67]

See also
  • MDynaMix software for ionic liquids simulations
  • 1-Butyl-3-methylimidazolium hexafluorophosphate (BMIM-PF6) for an often encountered ionic liquid
  • Trioctylmethylammonium bis(trifluoromethyl-sulfonyl)imide
  • Aza-Baylis–Hillman reaction for the use of a chiral ionic liquid in asymmetric synthesis.
  • Ionic liquids in carbon capture
  • NanoFlowcell which uses ionic liquid in its car batteries
  • Ioliomics, or studies of ions in liquids

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