Sample Treatments Based on Ionic Liquids

Abstract

The term ionic liquid (IL) is nowadays used to define a broad class of semi-organic salts or salt mixtures composed entirely by ions which are liquid in the range 180-600 K. They are hydrogen bonded substances with strong interionic interactions which condition their crystallographic structure. The presence of at least one constituent ion with a delocalized charge prevents the formation of a stable crystal lattice (Palacio & Bhushan, 2008). Although ILs are known since 1914, their expansion arrives with the availability of air and moisture stable ones. This second generation of ILs combines the unique properties of the former with the ease of handling of the latter. Their most relevant attributes or properties are as follows: negligible vapor pressure, good thermal stability, tunable viscosity and miscibility with water, inorganic and organic substances, a wide electrochemical window, high conductivity, high heat capacity and suitability to control reactions. The electronic and steric features of the cation and anion condition their physical-chemical properties. Their synthetic flexibility permits the modulation or adaptation of certain properties of the IL for a particular objective. Density, viscosity and solubility (crucial in any extraction procedure) can be cited among the more tuneable features. Moreover, the nature of cation and anion structures influences the acid/basic and solvent properties and raises the differences with conventional molecular solvents. Finally, the potential introduction of functional groups capable of having complementary interactions with other solvents or molecules opens up new variability on ILs properties and behavior. A huge number of potential cation-anion combinations are possible, which gives rise to an unusual large number of different compounds. The most popular high weight organic cations are imidazolium, pyridinium, pyrrolidinium quaternary ammonium and tetraalkylphosphonium. The most used by far are the 1,3-dialkylimidazolium salts as the likely result of their easily tailored properties. Regarding the most preferred organic anions, perfluorated (e.g. trifluoromethanesulphonate) and halogen–free (e.g. n-alkylsulphates) are among the most reported in the literature. The presence of impurities is a very remarkable aspect in the use of any IL, as its properties can be dramatically altered by the presence of trace amounts of compounds usually arising from unreacted starting material or water. This fact has led to irreproducibility between reactions and procedures. The overcoming of such limitation is crucial for the implementation of ILs in analytical science and their consolidation as an alternative to traditional molecular organic solvents.