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, Delta G(tr,i)(0,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 Delta G(tr,i)(0,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 Delta G(tr,i)(0,w-IL) and the standard Gibbs energy of anion transfer from water to 1,2-dichloroethane (DCE), Delta G(tr,i)(0,w-DCE), can be established for all three ILs studied.
An extended correlation including both the present and the literature values of Delta G(tr,i)(0,w-IL) 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+, Rb+, CS+).