Subsequent redox transitions as a tool to understand solvation in ionic liquids

Ionic liquid (IL) solvent effect on the redox potential values of three anionic reactants (one quinone derivative and two polytungstates) is investigated by means of cyclic voltammetry. Redox potential dependencies on the solvent nature are analyzed in two series of ILs (comprising the same cation and the same anion), and compared with literature data for molecular solvents. Very positive redox potential values for selected reactants in ILs can be explained assuming strong association between anionic forms of reactants and cations of IL, which was found to be more pronounced than that in tetraalkylammonium electrolyte solutions in low- and moderate-permittivity molecular solvents. It is shown that ion–ion interactions are primarily responsible for solvation energetics in ILs, and the reduction of multicharged anions is accompanied by association of the product with cation. The importance of the account for ionic association when considering kinetics and equilibria in ILs is stressed.