THE THERMODYNAMIC PROPERTIES OF TETRAHYDROFURAN-WATER ASSOCIATION AND ITS INFLUENCE ON RETENTION IN LIQUID CHROMATOGRAPHY

Abstract

Employing density data reported for a range of tetrahydrofuran-water mixtures, measured at different temperatures, the tetrahydrofuran-water association constant was calculated at each temperature, employing previously established procedures. 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., 1.642 Kcal/mol, and the standard free entropy of distribution was 3.00 cal/mol. These values indicate strong association and the relative low standard free entropy shows a comparatively small change in molecular freedom after association. This suggests that the solvents are also 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. At 60°C the association constant was reduced to about half that at 25°C