Methanol-water association and its effect on solute retention in liquid chromatography

Previous work on the association of methanol with water in methanol-water mixtures has been extended. With the use of published data on the volume change on mixing, density, and refractive index, the mean value for the association constant for methanol and water is found to be 5.22 /times/ 10/sup -3/ at 23/degrees/C. Curves relating the associated water concentration, associated methanol concentration, and the concentration of the methanol-water associate against the original volume composition of the solvent mixture are given. The predicted composition of the ternary solvent mixtures thus formed is used to explain the magnitude of the distribution coefficient of a number of different solutes between n-hexadecane and a range of methanol-water mixtures. It is shown that for solutes distributed largely in the aqueous phase, the distribution of the solute is controlled almost exclusively by the concentration of the methanol unassociated with water. For those solutes distributed primarily in the hydrocarbon phase, the distribution coefficient has a second-order dependence on the methanol unassociated with water, probably due to the need for two methanol molecules to associate with the solute to render it soluble in the aqueous mixture.