The thermodynamic properties of methanol–water association and its effect on solute retention in liquid chromatography

Abstract

Employing density data reported for a range of methanol–water mixtures measured at different temperatures, the methanol–water association constant was calculated at each temperature. The standard free enthalpy and entropy of association were then determined from the linear relationship that exists between the logarithm of the association constant and the reciprocal of the absolute temperature. It is shown that the standard free enthalpy is very high for simple association, viz., 1.916 kcal mol⁻¹, and the standard free entropy of distribution was 2.05 cal K⁻¹ mol⁻¹. These values indicate strong association and the relatively low standard free entropy shows a comparatively small change in molecular freedom after association. This indicates that the solvents are probably strongly associated with themselves when not associating with each other. Using the thermodynamic parameters and molar volumes derived from the calculation, the volume fractions of unassociated methanol, unassociated water and methanol–water associate were calculated for a range of initial methanol–water mixtures at different temperatures. The association decreased fairly rapidly with increased temperature as would be expected. From the calculated composition of the ternary mixtures, together with published retention data for the distribution coefficients of a series of aromatic hydrocarbons and aliphatic acids between methanol–water mixtures and hexadecane, it is shown that dispersive interactions are almost exclusively controlled by the volume fraction of free unassociated methanol. It is also shown that the other components (the methanol–water associate and the free water) exert significant control only over polar interactions.