Equilibria involving protons in mixtures of water with propane-1,2-diol Gibbs energies of transfer of protons and other ions from water into the mixture

Abstract

The variation of the concentration of 4-nitroaniline in mixtures of water and propane-1,2-diol containing protons conforms to that expected from an equilibrium between two solvated species of the proton with one containing the diol. Values for the equilibrium quotient, K _c , for the exchange of H ₂ O by a propane-1,2-diol molecule in the solvation sphere of the proton have been calculated and used in the estimation of the Gibbs energy of transfer, Δ _t G ⁰ , of H ⁺ from water into the mixture and thence of Δ _t G ⁰ (i) for other simple ions. The variations in Δ _t G ⁰ (i) for simple positive and negative ions, except OH ⁻ , with solvent composition compare well with those found with other added hydroxy-co-solvents. However, propane-1,2-diol, which enhances structure in the mixtures, like alcoholic co-solvents, but unlike ethane-1,2-diol and glycerol, has negative Δ _t G ⁰ (OH ⁻ ), comparable with Δ _t G ⁰ (OH ⁻ ) for the latter two co-solvents and in contrast to Δ _t G ⁰ (OH ⁻ ) for added alcohols. The enhancement of the stability of OH ⁻ in the mixtures over that in water is not just a simple function of the effect of neighbouring electron-withdrawing OH groups in the co-solvent molecule or of their effect on the solvent structure.