Ionic Liquid - History

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. One of the earliest truly room temperature ionic liquids was ethylammonium nitrate (C₂H₅)NH+
3·NO−
3 (m.p. 12 °C), synthesized 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 initially developed for use as electrolytes in battery applications.

An important property of the imidazolium halogenoaluminate salts is that 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. Two major drawbacks for some applications were moisture sensitivity and acidity/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. Recently a new class of air– and moisture–stable, neutral ionic liquids became available. Research has also been moving away from hexafluorophosphate and tetrafluoroborate towards less toxic alternatives such as bistriflimide − or away from halogenated compounds completely. Moves towards less toxic cations have also been growing, with compounds like ammonium salts (such as choline) proving to be as flexible a scaffold as imidazolium.