Solute-solvent interactions and acid-base dissociation in mixed solvent systems

Abstract

Data for the pK of cationic weak acids BH+ in binary mixed solvents have been collected and critically analyzed. This dissociation reaction is isoelectric, and electrostatic charging effects should have a minimal influence on the change of pK with solvent composition. Thus conditions are favorable for an examination of the effects of solute-solvent interactions on the dissociation reaction with little interference from electrostatic contributions. Standard Gibbs transfer energies derived from the change in pK were xamind, with the aid of solubility data, in terms of the contributions of the individual species. Comparisons were made with the corresponding quantities for the alkali chlorides and the results correlated with the aid of the activities of the two components of the solvent mixture. In solvents of which water was one component, it was possible to relate the transfer energy for hydrochloric acid to the activity of water alone, to a good approximation. At both extremes of the composition range, there are indications that preferential solvation can account for the “observed” differences in transfer energies of the ions BH+ and H+. The results therefore confirm the earlier conclusion that solute-solvent interactions, rather than electrostatic effects associated with the change in dielectric constant, are largely responsible for the changes of pK with composition of the solvent mixtures.