Origin of the correlation between the standard Gibbs energy of ion transfer and the solubility of water in organic solvents

Abstract

Standard Gibbs energies ΔwoGi0 of the Cs+ and Li+ ion transfer from water to 1,2-dichlorobenzene, and to an n-octanol + α,α,α-trifluorotoluene mixture were obtained from the steady-state voltammetric measurements. The equilibrium concentrations of water in these solvents were determined by the Karl Fischer method. Analogous data have previously been reported for the transfer of these ions to α,α,α-trifluorotoluene, 1.6-dichlorohexane, 1,4-dichlorobutane, 1,2-dichloroethane, o-nitrophenyl octyl ether, nitrobenzene, and n-octanol. Based on this collection of data, a correlation was established between the experimental values of ΔwoGi0 for the Cs+ or Li+ ion transfer and the reported content of water in the organic solvent characterized by the ratio cW/cS of the molar concentrations of water (cW) and the organic solvent molecules (cS) in the organic solvent S. This correlation exhibits a rapid decay of ΔwoGi0 in the range of the small values of the ratio cW/cS= 0.0025 - 0.02, while towards higher values of cW/cSthe standard Gibbs energy of ion transfer tends to attain a constant value. The relationship was successfully simulated using the equations derived for a Born-type electrostatic model of solvation of the ion, which was combined with a thermodynamic model of the preferential solvation in the mixed solvents. The mechanism of an ion transfer assumes the partial replacement of the water molecules in the first hydration layer of the transferred ion by molecules of the organic solvent. Spectroscopic evidence is presented pointing to the absence of water clusters in pure 1,2-dichloroethane saturated with water, which therefore should not play a role in the ion transfer mechanism.