Ionic Liquids


Ionic liquids (ILs) are substances consisting of ions only, but this definition is different from the classic definition of a molten salt.
The latter is a high-melting, highly viscous and highly corrosive liquid, while an ionic liquid due to the lower symmetry and larger size of the ions is liquid at a much lower temperature (< 100°C) and has a lower viscosity.

Left: Crystal structure of sodium chloride (yellow: sodium cations, red: chloride anions); right: Crystal structure of [EMIM][NTf2] (yellow: sulphur, red, oxygen, blue: nitrogen, green: fluor, grey: carbon).

Ionic liquids are considered as environmentally friendly substitutes for volatile organic solvents, not only because of their low vapor pressures but more importantly because of their ability to act as catalysts.
Moreover, ionic liquids posses several other attractive properties, including chemical and thermal stability, non flammability, high ionic conductivity and a wide electrochemical potential window.

Cations (top) and anions (bottom) of the most commonly used ionic liquids.

The development of ionic liquids goes back to the beginning of the 20th century when Hurley and Wier developed 1948 the first ionic liquids with chloroaluminate ions as bath solutions for electroplating aluminum.
In the early 1980s chloroaluminate melts as non aqueous, polar solvents for the investigation of transition metal complexes were used by the groups of Seddon and Hussey.
At the end of the 1980s first publications with ionic liquids described as new reaction media and catalysts for organic synthesis appeared.

SciFinder search result for the keywords "ionic liquid" and "ionic liquids" up to Dec 2007.

Ever since, the number of publications on ionic liquids in various fields of chemistry has grown significantly, exceeding 2400 in 2006.

References

  • Ionic Liquids in Synthesis (Eds.: Peter Wasserscheid, Tom Welton), Wiley-VCH, 2007
  • Ionic Liquids as Green Solvents. Progress and Prospects (ACS Symposium) (Eds.: Robin R. Rogers, Kenneth R. Seddon), Oxford University Press, 2003.
  • Ionic Liquids. Industrial Applications for Green Chemistry (ACS Symposium) (Eds.: Robin R. Rogers, Kenneth R. Seddon), Oxford University Press, 2002.

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