Ionic solvation in water + co-solvent mixtures. Part 4.—Free energies of transfer of single ions from water into water + t-butyl alcohol mixtures

Abstract

The method used to calculate the free energy of transfer of the proton, ΔG^°_t(H⁺), from water into mixtures of water with various co-solvents has now been applied to water + t-butyl alcohol mixtures: the extra-thermodynamic assumptions involved in these calculations are detailed in Part 1. These values for ΔG^°_t(H⁺) are then used to separate ΔG^°_t for the hydrogen halides to give values for ΔG^°_t for Cl^–, Br^– and I^–; and the values for ΔG^°_t(Cl^–) are used in turn to separate ΔG^°_t for salts MCl, where M⁺= Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺ and Ag⁺. Values for ΔG^°_t for the hydrogen halides and for the salts are obtained from data available on the appropriate electrode potentials. Values for K_c=[(Bu^tOH,H⁺)_solv]/[H⁺_aq][Bu^tOH_solv] have been determined experimentally using the previously described spectrophotometric method with p-nitroaniline. The variations of ΔG^°_t with solvent composition for the single ions are compared with the variations for ΔG^°_t in other water + co-solvent mixtures and they are related to solvent structure in the mixtures: a maximum is found in ΔG^°_t(halide) and –ΔG^°_t(M⁺) at mole fractions of t-butyl alcohol ∼0.1.