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The recent interest in ionic liquids has stemmed from their immeasurably low vapor pressure. Original publications called it zero, but Earle et al. have recently shown that some ionic liquids can be distilled, albeit at very low pressures. Thus their use as solvents for chemical reactions, replacing volatile organic solvents (VOCs) with their environmental problems, was an early area of investigation. Indeed most work published (and publications on ionic liquids continue to proliferate at a greater than exponential rate) has concentrated on their use as solvents. A search of the Chemical Abstracts Database suggests two reviews are published each week on some aspect of chemistry involving ionic liquids. Thus a review such as this can never be current!The composition of ionic liquids can be varied by changing either the cation or the anion to give different properties. In many cases studied, the cation consists of a heterocyclic system (usually nitrogen-based) with a relatively simple anion. Other cations studied include ammonium salts, phosphonium salts and cholinium derivatives, yielding a wide range of options. Making structural changes means that, at least in principle, ionic liquids can be designed for a particular application, giving rise to the term “designer solvents.” A generally accepted estimate suggests that some 106 simple ionic liquids are possible and 1018 ternary ionic liquids could exist. Properties such as viscosity, density, melting point, polarity, etc., can be specified and, in some cases, predicted. The variation of measured and predicted melting points with alkyl chain length in a series of alkyl methyl imidazolium hexafluorophosphates is shown in F-2. This shows that, at least in principle, models can be developed that allow one to predict such properties and thus design (rather than screen for) ionic liquids with the desired properties. The National Institute of Standards and Technology has a database of reviewed ionic liquid properties. Early work often did not record the levels of water or halides to which many of the measured properties are very sensitive and therefore should be used with caution.
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