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 Kc=[(ButOH,H+)solv]/[H+aq][ButOHsolv] 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.