Ions in Aqueous Acetic Acid Mixtures: Solvent Reorganization around Protons and Gibbs Energies of Transfer from Water

The photo-absorbing, basic sensor, 4-nitroaniline, has been used to determine theequilibrium constant for solvent reorganization around the proton in mixtures ofvarying composition of water with acetic acid. In all the mixtures used, theself-ionization of the acetic acid was suppressed. In contrast to mixtures of waterwith the related ethanol or acetone, this equilibrium is shifted more toward thewater-solvated species as the mole fraction x₂ of the cosolvent increases. TheGibbs energy of transfer of protons from water into the mixture G^o_t(H⁺) can bederived with the aid of this equilibrium constant for the solvent reorganization.Using G^o_t(H⁺), G^o_t(i) for i denoting anions and other cations can be evaluated.In comparison the G^o_t(i) for cations have lower negative values than when eitherethanol or acetone is added to water. Correspondingly, for halide anions, thepositive G^o_t(i) with added acetic acid are rather less than is found with eitherethanol or acetone added. The influence on the ion-solvent interaction of bothelectron withdrawing hydroxy and carbonyl groups in acetic acid may beresponsible for this. Although G^o_t(i) for C10^–₄ and Re0^–₄ are also positive, both picrateions and OH^– give negative values with acetic acid added to water. With picrateions, the hydrophobic effect of the carbon ring produces stabilization in themixture relative to water. With OH^–, complete conversion to acetate anionsoccurs. As is found with other cosolvents, the contribution of the charge onacetate anion to G^o_t(CH₃COO^–) is found to increase as x₂ rises. The aciddissociation constant K_a for acetic acid is found to decrease slowly as x₂ rises to0.5, followed by a rapid decrease for x₂ greater than 0.7 where dimerization ofacetic acid occurs.