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Introduction to Ionic Liquids

What Are Ionic Liquids?

Ionic Liquids are a new class of purely ionic, salt-like materials that are liquid at unusually low temperatures. Currently, it’s “official” definition uses the boiling point of water as a point of reference: “These are ionic compounds which are liquids below 100 °C.” More commonly, These have melting points below room temperature; some of them even have melting points below 0 °C. These new materials are liquid over a wide temperature range (300–400 °C) from the melting point to the decomposition temperature of these compounds.

Ionic Liquids

If we compare a typical ionic liquid, e.g., 1-ethyl-3-methylimidazolium ethylsulfate (m.p. <-20 °C), with a typical inorganic salt, e.g., table salt (NaCl, m.p. 801 °C), it becomes obvious why there is a difference. The ionic liquid has a significantly lower symmetry! Furthermore, the charge of the cation as well as the charge of the anion is distributed over a larger volume of the molecule by resonance. As a consequence, the solidification of the ionic liquid will take place at lower temperatures. In some cases, especially if long aliphatic side chains are involved, a glass transition is observed instead of a melting point.

Anion and cation, large and small

Figure 1.The charge of the cation as well as the charge of the anion is distributed over a larger volume of the molecule by resonance.

The strong ionic (Coulomb-) interaction within these substances results in a negligible vapor pressure (unless decomposition occurs), a non-flammable substance, and in a high thermally, mechanically as well as electrochemically stable product. In addition to this very interesting combination of properties, they offer other favorable properties: for example, very appealing solvent properties and immiscibility with water or organic solvents that result in biphasic systems.

The choice of the cation has a strong impact on the properties of the ionic liquid and will often define the stability. The chemistry and functionality of the ionic liquid is, in general, controlled by the choice of the anion. The combination of a broad variety of cations and anions leads to a theoretically possible number of 1018 Ionic Liquids. However, a realistic number will be magnitudes lower. Today about 1000 of these compounds are described in the literature, and approximately 300 are commercially available. Typical structures combine organic cations with inorganic or organic anions:

Common Ionic Liquids and Their Structures

Figure 2.Common Ionic Liquids and Their Structures

The growing academic and industrial interest in ionic liquid technology is represented by the yearly increase in the number of publications (source Sci-Finder™): starting from far below 100 in 1990 to more than 1500 papers published in 2004.

Ionic Liquid Papers from 1990-2004

Figure 3.Ionic Liquid Papers from 1990-2004

Our portfolio includes 130+ ionic liquids, ranging from well-known imidazolium and pyridinium derivatives to ammonium, pyrrolidinium, phosphonium, and sulfonium derivatives.

Use and Applications

The reason for the increase in number of publications can be attributed to the unique properties of these new materials. Ultimately, the possible combinations of organic cations and anions places chemists in the position to design and fine-tune physical and chemical properties by introducing or combining structural motifs and, thereby, making tailor-made materials and solutions possible. The following chart summarizes their important properties potential and current applications: 1-29

Important Properties of Ionic Liquids and Their Potential and Current Applications

Figure 4.Important Properties of Ionic Liquids and Their Potential and Current Applications

Ionic Liquids as Process and Performance Chemicals

The corresponding applications of these ionic fluids can be divided in their use as process chemicals and performance chemicals. In 1948, this class of electrically conducting materials (1-butylpyridinium chloride/AlCl3) first drew attention for their use as electrolytes for electrodeposition of aluminium. 30-32 This field is still the subject of current research efforts. Other important applications in this context are their use as electrolytes for batteries and fuel cells.

The major breakthrough came with the advent of less corrosive, air-stable materials in 1992. Wilkes and Zaworotko introduced 1-ethyl-3-methylimidazolium tetrafluoroborate and focused on solvent applications. 33 Although 1-butyl-3-methylimidazolium hexafluorophosphate and -tetrafluoroborate still dominate the current literature, there are better choices with respect to performance and handling. It was demonstrated in aqueous media the hexafluorophosphate anion and the tetrafluoroborate anion will degrade, resulting in the formation of HF, a noxious and aggressive acid.

Developing Applications

Other interesting applications have been suggested, derived from the unique combination of physical properties. For example, the use of ionic liquids as thermal fluids combines their heat capacity with thermal stability and a negligible vapor pressure. It is quite feasible to expect that many more applications will be brought forward by potential users and some of them will be realized in the next couple of years. In order to enable potential users to make their use, it will be necessary to give technological support and to make a range of these liquids available for testing. This support also includes the development of toxicological data and government required notification as well as the implementation of recycling concepts for the ionic liquid.

References

1.
Wang H, Lu Q, Ye C, Liu W, Cui Z. 2004. Friction and wear behaviors of ionic liquid of alkylimidazolium hexafluorophosphates as lubricants for steel/steel contact. Wear. 256(1-2):44-48. http://dx.doi.org/10.1016/s0043-1648(03)00255-2
2.
Artificial Muscles: Potential application, personal note..
3.
Armstrong DW, Zhang L, He L, Gross ML. 2001. Ionic Liquids as Matrixes for Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry. Anal. Chem.. 73(15):3679-3686. http://dx.doi.org/10.1021/ac010259f
4.
Andre M, Loidl J, Laus G, Schottenberger H, Bentivoglio G, Wurst K, Ongania K. 2005. Ionic Liquids as Advantageous Solvents for Headspace Gas Chromatography of Compounds with Low Vapor Pressure. Anal. Chem.. 77(2):702-705. http://dx.doi.org/10.1021/ac048737k
5.
Protein-Crystallization: Ionic Liquids Technologies, Patent pending.
6.
Moriguchi T, Yanagi T, Kunimori M, Wada T, Sekine M. 2000. Synthesis and Properties of Aminoacylamido-AMP:  Chemical Optimization for the Construction of anN-Acyl Phosphoramidate Linkage. J. Org. Chem.. 65(24):8229-8238. http://dx.doi.org/10.1021/jo0008338
7.
Hayakawa Y, Kawai R, Hirata A, Sugimoto J, Kataoka M, Sakakura A, Hirose M, Noyori R. 2001. Acid/Azole Complexes as Highly Effective Promoters in the Synthesis of DNA and RNA Oligomers via the Phosphoramidite Method. J. Am. Chem. Soc.. 123(34):8165-8176. http://dx.doi.org/10.1021/ja010078v
8.
Eckstein M, Villela Filho M, Liese A, Kragl U. 2004. Use of an ionic liquid in a two-phase system to improve an alcohol dehydrogenase catalysed reductionElectronic supplementary information (ESI) available: experimental section. See http://www.rsc.org/suppdata/cc/b4/b401065e/. Chem. Commun..(9):1084. http://dx.doi.org/10.1039/b401065e
9.
Liu Y, Wang M, Li J, Li Z, He P, Liu H, Li J. Highly active horseradish peroxidase immobilized in 1-butyl-3-methylimidazolium tetrafluoroborate room-temperature ionic liquid based sol?gel host materials. Chem. Commun..(13):1778-1780. http://dx.doi.org/10.1039/b417680d
10.
Wasserscheid, P.; Welton, T.. Organic Synthesis & Catalysis: For a general overview see: Ionic Liquids in Synthsis; Wiley-VCH: Weinheim, 2003.
11.
Bösmann, A.; Datsevich, L.; Jess, A.; Lauter, A.; Schmitz, C.; Wasserscheid, P. . 2001. Deep Desulfuration: Chem. Comm..2494–2495.
12.
Nakashima K, Kubota F, Maruyama T, Goto M. 2003. Chem. Mater..(15):1825–1829.
13.
Zhou Y, Antonietti M. 2003. Synthesis of Very Small TiO2Nanocrystals in a Room-Temperature Ionic Liquid and Their Self-Assembly toward Mesoporous Spherical Aggregates. J. Am. Chem. Soc.. 125(49):14960-14961. http://dx.doi.org/10.1021/ja0380998
14.
Zhou Y, Antonietti M. 2004. A Series of Highly Ordered, Super-Microporous, Lamellar Silicas Prepared by Nanocasting with Ionic Liquids. Chem. Mater.. 16(3):544-550. http://dx.doi.org/10.1021/cm034442w
15.
Holbrey JD, Seddon KR. 1999. The phase behaviour of 1-alkyl-3-methylimidazolium tetrafluoroborates; ionic liquids and ionic liquid crystals. J. Chem. Soc., Dalton Trans..(13):2133-2140. http://dx.doi.org/10.1039/a902818h
16.
Haristoy D, Tsiourvas D. 2003. Novel Ionic Liquid-Crystalline Compounds Bearing Oxadiazole and Pyridinium Moieties as Prospective Materials for Optoelectronic Applications. Chem. Mater.. 15(10):2079-2083. http://dx.doi.org/10.1021/cm021365g
17.
Wu, B.; Reddy, R. G.; Rogers, R. D. . Thermal Fluids: Proceedings of Solar Forum, 2001.
18.
See also: Ionic Liquids Technologies GmbH & Co. KG. http://www.iolitec.de
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Yanes EG, Gratz SR, Baldwin MJ, Robison SE, Stalcup AM. 2001. Capillary Electrophoretic Application of 1-Alkyl-3-methylimidazolium-Based Ionic Liquids. Anal. Chem.. 73(16):3838-3844. http://dx.doi.org/10.1021/ac010263r
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Sato T, Masuda G, Takagi K. 2004. Electrochemical properties of novel ionic liquids for electric double layer capacitor applications. Electrochimica Acta. 49(21):3603-3611. http://dx.doi.org/10.1016/j.electacta.2004.03.030

22.
Zell CA, Freyland W. 2003. In Situ STM and STS Study of Co and Co?Al Alloy Electrodeposition from an Ionic Liquid. Langmuir. 19(18):7445-7450. http://dx.doi.org/10.1021/la030031i
23.
Scheeren CW, Machado G, Dupont J, Fichtner PFP, Texeira SR. 2003. Nanoscale Pt(0) Particles Prepared in Imidazolium Room Temperature Ionic Liquids:  Synthesis from an Organometallic Precursor, Characterization, and Catalytic Properties in Hydrogenation Reactions. Inorg. Chem.. 42(15):4738-4742. http://dx.doi.org/10.1021/ic034453r
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Fortunato R, Afonso CA, Reis M, Crespo JG. 2004. Supported liquid membranes using ionic liquids: study of stability and transport mechanisms. Journal of Membrane Science. 242(1-2):197-209. http://dx.doi.org/10.1016/j.memsci.2003.07.028
30.
Hurley, F. H. . U.S. Patent 2,446,331, 1948.
31.
Wier, T. P.; Hurley, F. S. . U.S. Patent 2,446,349, 1948.
32.
Wier, T. P.. U.S. Patent 2,446,350, 1948.
33.
Wilkes JS, Zaworotko MJ. 1992. Air and water stable 1-ethyl-3-methylimidazolium based ionic liquids. J. Chem. Soc., Chem. Commun..(13):965. http://dx.doi.org/10.1039/c39920000965