Ionic Liquids Research Paper

2.1 Introduction to Ionic Liquids

Ionic liquids (ILs) are liquids composed entirely of organic cations and inorganic or organic anions. Those systems that are liquids at high temperatures for example NaCl with melting point at ≈ 800 0C is normally termed as molten salts, on the other hand room-temperature ILs are liquids below 100˚C that have received considerable attention as substitutes for volatile organic solvents. Owing to their fascinating properties, such as negligible vapour pressure, large liquidous range, high thermal stability, good ionic conductivity, high electrochemical stability, they are considered favourable solvents for chemical syntheses. ILs are also called as designer solvents because their physical and chemical properties

Since many ILs has negligible vapour pressures they generally do not evaporate or boil at high temperature, they slowly form glasses at low temperatures. For example, 1-alkyl-3-metylimidazolium salts typically have Tg in the range –70 to –90 0C and Td ranging from 250 to over 450 0C. Therefore these Ils have liquid ranges of 300 0C which is larger in comparision to water having liquid range from 0 to 100 0C at atmospheric pressure. Ethanol has a melting point of -114.1 0C and boiling point of 78.5 0C and , therefore, a liquid range of 192.6 0C. Thus because of hight Td i. e. high thermal stabilities, experiments involving these ILs can be carried out at hidh temperature without any sovent

Furthermore, it has also been evidenced that the slopes of the relationships versus do not reflect the size of each species, suggesting that in these media the diffusion of an ion may depend on its counter-ion. In agreement with this hypothesis, the ratios between the molar conductivity, determined by impedance measurements, and the conductivity calculated from the NMR diffusion coefficients, applying the Nernst–Einstein equation without considering ion association, are smaller than unity. In particular, they range from 0.6 to 0.8 for and and from 0.3 to 0.5 for and . This behaviour has been considered as evidence that in salts most of the individual ions contribute to the ionic conduction, whereas in the case of and the presence of ion pairs and neutral ion aggregates has been proposed [38]. The transport properties in a family of dialkylimidazolium ILs ([emim][Br], [emim][I], [emim] [TfO], [emim][Tf2N], [mmim][I], [pmim][I], [bmim][I], [hmim][I], [hpmim][I]) have been examined[41] The four [emim]þ salts bearing different anions presented very similar conductivities, despite the different meltingpoints. The diffusion coefficients, however, fell into two groups, with the halide salts showing considerably lower coefficients than the triflyl salts. In this case, the difference has been ascribed to differences in