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General Introduction
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1.1. General Context
Ionic Liquids (ILs) are ionic compounds that belong to the molten salts group, often
denominated by RTILs (Room Temperature Ionic Liquids). They are usually formed by
large organic cations and organic or inorganic anions, allowing them to remain liquid at or
near room temperature and at atmospheric pressure,
since they don’t have an ordered crystalline structure.
Unlike molecular liquids, the ionic nature of these
liquids results in a unique combination of properties
for most ILs, namely their high thermal stability,
large liquidus range, high ionic conductivity, negligible vapor pressure, non-flammability
and a high solvating capacity for organic, inorganic and organometallic compounds.
Indeed, these properties have determined their high potential to be exploited as “green
solvents” in the past several years 1, 2 . In addition, the huge number of possible
combinations between cations and anions allows the possibility of tuning ILs, which can be
designed for a particular application or to show a specific set of intrinsic properties, thus
they are also commonly described as “designer solvents” 3 .
Besides the existence of over six hundred of common solvents used nowadays in industry,
ILs are an innovative proposal. In the environmental field and with the intent of
diminishing the air pollution, ILs show low vapor pressures and can substitute the typical
and commonly used volatile organic compounds (VOCs). This is the main reason by which
ILs are designated as “green solvents”. However, the fact of showing negligible vapor
pressure is not enough to assure that these compounds are in fact “green”. Properties such
as toxicity and biodegrability must be accessed before such assumptions.
In the past years it was believed that ILs did not evaporate, but recently Earle et al. 1
showed that with high temperature and low pressure it is possible to distillate ILs and to
separate mixtures composed by different ILs using fractional distillation. Nevertheless,
although distillable, this group of researchers 1 concluded that the vapor pressure of the ILs
stays negligible at environmental conditions of pressure and temperature.
ILs were first discovered, almost occasionally, in 1914, during the 1 st World War, by Paul
Walden when testing new explosives for the substitution of nitroglycerin 4 . By the
neutralization of ethylamine with concentrated nitric acid it was possible to synthesize
+
-
Figure 1. Example of a common
IL, 1-butyl-3-methylimidazolium
bis(trifluormethylsulfonyl)imide.
3