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 H₂O 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_tG⁰, of H⁺ from water into the mixture and thence of Δ_tG⁰(i) for other simple ions. The variations in Δ_tG⁰(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_tG⁰(OH⁻), comparable with Δ_tG⁰(OH⁻) for the latter two co-solvents and in contrast to Δ_tG⁰(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.