Effect of water solubility in organic solvents on the standard Gibbs energy of ion transfer across a water/organic solvent interface

Abstract

Ion transfer voltammetry is used to study the transfer of alkali metal and proton cations and the Cl− anion from the aqueous solution of MCl (M+ = H+, Li+, Na+, K+, Rb+, Cs+) to the solution of bis(triphenylphosphoranylidene)ammonium tetrakis(pentafluorophenyl)borate in α,α.α-trifluorotoluene (TFT) or 1,2-dichloroethane (DCE). The scale of the applied potentials is converted to the scale of the Galvani potential differences on the basis of the voltammetric measurements of the standard ion transfer potential for the tetraethylammonium (TEA+) ion used as a reference ion in situ. The interfacial tension measurements at the water/TFT interface yield the zero-charge potential difference, which is close to the expected zero value. The standard Gibbs energies of ion transfer ΔwoGi0 are evaluated from the voltammetric data on considering both the effect of the association between the transferred ion and the counter-ion of the organic solvent phase, and the effect of the ion migration in the aqueous phase. The values of ΔwoGi0 for the ion transfer to TFT are found to be considerably higher than those obtained for the ion transfer to DCE. This difference is proposed to be related to the significantly lower solubility of water in TFT, possibly requiring the removal of the ion hydration shell in the course of the ion transfer from water to TFT. Such conclusion is supported by the calculations of ΔwoGi0 by using an advanced Born-type model of ion solvation.