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Ionic liquid

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Ionic liquid
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Ionic liquid

An ionic liquid is a liquid that contains essentially only ions. Some ionic liquids, such as ethylammonium nitrate, are in a dynamic equilibrium where at any time more than 99.99% of the liquid is made up of ionic rather than molecular species. In the broad sense, the term includes all molten salts, for instance, sodium chloride at temperatures higher than 800 °C. Today, however, the term "ionic liquid" is commonly used for salts whose melting point is relatively low (below 100 °C). In particular, the salts that are liquid at room temperature are called room-temperature ionic liquids, or RTILs.

1-butyl-3-methylimidazolium salts or bmim
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1-butyl-3-methylimidazolium salts or bmim

Contents

History

Whereas the date of discovery, as well as discoverer, of the "first" ionic liquid is disputed, one of the earlier known ionic liquids was [EtNH3][NO3] (m.p. 12 °C), the synthesis of which was published in 1914.[#endnote_Walden] Much later, series of ionic liquids based on mixtures of 1,3-dialkylimidazolium or 1-alkylpyridinium halides and trihalogenoaluminates, initially developed for use as electrolytes, were to follow.[#endnote_Chum],[#endnote_Levisky] An important property of the imidazolium halogenoaluminate salts was that they were tuneable – viscosity, melting point and the acidity of the melt could be adjusted by changing the alkyl substituents and the ratio of imidazolium or pyridinium halide to halogenoaluminate.[#endnote_Gale]

A major drawback was their moisture sensitivity and, though to a somewhat lesser extent, their acidity/basicity, the latter which can sometimes be used to an advantage. In 1992, Wilkes and Zawarotko reported the preparation of ionic liquids with alternative, 'neutral' anions such as [PF6]- and [BF4]-, allowing a much wider range of applications for ionic liquids.[#endnote_Wilkes] It was not until recently that a class of new, air- and moisture stable, neutral ionic liquids, was available that the field attracted significant interest from the wider scientific community.

More recently, people have been moving away from [PF6]- and [BF4]- since they are highly toxic, and towards new anions such as bistriflimide or even away from halogenated compounds completely. Moves towards less toxic cations have also been growing, with compounds like ammonium salts (such as choline) being just as flexible a scaffold as imidazole.

The early influence of the group of Kenneth Seddon at Queen's University, Belfast, which has now turned into the first research centre dedicated to ionic liquids, QUILL, should also not be underestimated.[#endnote_Seddon]

Characteristics

The notable characteristics of ionic liquids are their non-measurable vapor pressure (recent papers have actually shown that there is a small but non negligible vapour pressure [#endnote_Earle], which will not be a surprise to anybody who has worked with them, since many have a noticeable smell), non-flammability, thermal stability, wide liquid range, and solvating properties for diverse kinds of materials. It is known that, as in conventional organic solvents, many kinds of chemical reactions such as Diels-Alder reactions and Friedel-Crafts reactions occur in ionic liquids. More recently, there has been a lot of work showing ionic liquids to be very suitable solvents for biocatalysis [#endnote_Walker]. Another important feature of ionic liquids is their designability: miscibility with water or organic solvents can be tuned through sidechain lengths on the cation and choice of anion. Furthermore, they can be functionalized to act as acids, bases or ligands. Because of their properties, ionic liquids attract great attention in many fields, including organic chemistry, electrochemistry, catalysis, physical chemistry, and engineering; see for instance magnetic ionic liquid.

It should, however, be borne in mind that there are indications that under certain conditions some ionic liquids can be distilled, that some ionic liquids generate flammable gases on thermal decomposition (such as 1-butyl-3-methylimidazolium nitrate), and that the thermal stability as well as melting point depend on the components of the ionic liquid and thus affect the liquid window to some extent. These exceptions illustrate the nature of ionic liquids as a diverse class of fluids, rather than a small group of individual examples.

The solubility of different species in imidazolium ionic liquids tends to be dependent mainly on polarity and hydrogen bonding ability. Simple aliphatic compounds are generally only sparingly soluble in ionic liquids, whereas olefins show somewhat greater solubility, and aldehydes can be completely miscible. This 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 exceptional solubility in many ionic liquids, carbon monoxide being less soluble in ionic liquids than in many popular organic solvents, and hydrogen being only slightly soluble, similar to the solubility in water, and probably varying relatively little between the more popular ionic liquids. Different groups have, however, employed different analytical techniques, which in turn have yielded somewhat different absolute solubility values.

Room temperature ionic liquids

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 ions. A wide range of anions is employed, from simple halides, which generally inflect high melting points, to inorganic anions such as tetrafluoroborate and hexafluorophosphate and to large organic anions like bis-trifluorsulfonimide, triflate or tosylate. There are also many interesting examples of uses of ionic liquids with simple non-halogenated organic anions such as formate or glycolate. As an example, the melting point of 1-butyl-3-methylimidazolium tetrafluoroborate or [bmim][BF4] with an imidazole skeleton is about -80 °C, and it is a colorless liquid with high viscosity at room temperature.

Food science

The application range of ionic liquid also extends to food science. [bmim]Cl for instance is able to completely dissolve freeze dried banana pulp and the solution with an additional 15% DMSO lends itself to Carbon-13 NMR analysis. In this way the entire banana compositional makeup of starch, sucrose, glucose, and fructose can be monitored as a function of banana riping [#endnote_Fort].

Safety

Due to their non-volatility, ionic liquids are generally considered as having a low impact on the environment and human health, and thus recognized as solvents for green chemistry. However, it remains to be seen how 'environmentally-friendly' ILs will be regarded once widely used by industry. Research into IL aquatic toxicity has shown them to be as toxic or more so than many current solvents already in use [#endnote_Pretti]. Available research also shows that mortality isn't necessarily the most important metric for measuring their impacts in aquatic environments, as sub-lethal concentrations have been shown to change organisms' life histories in meaningful ways. According to these researchers balancing between zero VOC emissions, and avoiding spills into waterways (via waste ponds/streams, etc.) should become a top priority. However, with the enormous diversity of substituents available to make useful ILs, it should be possible to design them with useful physical properties and less toxic chemical properties.

Despite their low vapor pressure many ionic liquids have also found to be combustable and therefore require carefull handling [#endnote_Smiglak]. Brief exposure (5 to 7 seconds) to a flame torch will ignite these IL's and some of them are even completely consumed by combustion.

See also

External links

Major academic groups
Other
  • [Ionic Liquids @ organic-chemistry.org]
  • [Toxicity article on rcs.org]

    • References

    1. ↑  P. Walden, Bull. Acad. Sci. St. Petersburg 1914, 405-422
    2. ↑  Electrochemical scrutiny of organometallic iron complexes and hexamethylbenzene in a room temperature molten salt H. L. Chum, V. R. Koch, L. L. Miller, R. A. Osteryoung Journal of the American Chemical Society 1975, 97, 3264.[DOI]
    3. ↑  Dialkylimidazolium chloroaluminate melts: a new class of room-temperature ionic liquids for electrochemistry, spectroscopy and synthesis J. S. Wilkes, J. A. Levisky, R. A. Wilson, C. L. Hussey Inorganic Chemistry 1982, 21, 1263-1264. [DOI]
    4. ↑  Potentiometric investigation of dialuminum heptachloride formation in aluminum chloride-1-butylpyridinium chloride mixtures R. J. Gale, R. A. Osteryoung Inorganic Chemistry 1979, 18, 1603.[DOI]
    5. ↑  J. S. Wilkes, M. J. Zaworotko Chemical Communications 1992, 965-967
    6. ↑ ''ESI Special Topics - Ionic liquids. Interview with Kenneth Seddon [link]
    7. ↑  The distillation and volatility of ionic liquids Martyn J. Earle, José M.S.S. Esperança, Manuela A. Gilea, José N. Canongia Lopes, Luís P.N. Rebelo, Joseph W. Magee, Kenneth R. Seddon and Jason A. Widegren Nature, 2006, 439, 831 [link]
    8. ↑  Cofactor-dependent enzyme catalysis in functionalized ionic solvents Adam J. Walker and Neil C. Bruce Chemical Communications, 2004, 22, 2570 [DOI abstract]
    9. ↑  Use of ionic liquids in the study of fruit ripening by high-resolution 13C NMR spectroscopy: green solvents meet green bananas Diego A. Fort, Richard P. Swatloski, Patrick Moyna, Robin D. Rogers and Guillermo Moyna Chemical Communications, 2006, 714 - 716 [DOI abstract]
    10. ↑  Acute toxicity of ionic liquids to the zebrafish (Danio rerio) C Pretti, C Chiappe, D Pieraccini, M Gregori, F Abramo, G Monni and L Intorre, Green Chem., 2005 [DOI abstract]
    11. ↑  Combustible ionic liquids by design: is laboratory safety another ionic liquid myth? Marcin Smiglak et al. Chemical Communications, 2006, (Advance Article) DOI:[10.1039/b602086k]
    12. ↑  Air and water stable ionic liquids in physical chemistry F. Endres, S. Zein El Abedin, Phys.Chem.Chem.Phys., 8 (2006) 2101.[link]

     

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