Correlation between the standard Gibbs energies of an anion transfer from water to highly hydrophobic ionic liquids and to 1,2-dichloroethane

Abstract

Cyclic voltammetry is used to investigate the transfer of several semihydrophobic and hydrophilic anions (F−, Cl−, Br−, I−, NO3-, NO2-, SCN−, BF4-, ClO4-, PF6-) across the polarized interface between an aqueous electrolyte solution and a highly hydrophobic ionic liquid (IL) membrane. Three ILs are examined being composed of the trioctadecylmethylammonium (TOMA+), tridodecylmethylammonium (TDMA+) or tetradodecylammonium (TDA+) cation and the tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (TFPB−) anion. The standard Gibbs energies of the anion transfer from water to IL, ΔGtr,i0,w→IL, are evaluated from the voltammetric measurements by applying the classical tetraphenylarsonium-tetraphenylborate hypothesis. Comparison of data for various ILs points to a small systematic effect of the cationic IL component, which is manifested by somewhat lower values of ΔGtr,i0,w→IL for most anions in the presence of TDMA+ or TDA+, indicating their stronger association with the anions. The capillary electrophoresis measurements suggest that the degree of interaction of anions with the IL cations in water could follow the order TDMA+ >TOMA+ >TDA+. It is shown that a linear correlation with the nearly unity slope between ΔGtr,i0,w→IL and the standard Gibbs energy of anion transfer from water to 1,2-dichloroethane (DCE), ΔGtr,i0,w→DCE, can be established for all three ILs studied. An extended correlation including both the present and the literature values of ΔGtr,i0,w→IL for the ion transfer from water to TDMATFPB reveals the consistency of data for the semihydrophobic cations and anions, and the presence of a weak solvation effect favoring the transfer of hydrophilic anions (F−, Cl−, Br−, I−) and disfavoring the transfer of hydrophilic cations (H+, Li+, Na+, K+, Rb+, Cs+).