Thermodynamics of association of cholesterol in an aprotic solvent

Abstract

The colligative properties of benzene solutions of cholesterol up to 0.25 mol kg–1 in the temperature range 298–333 K have been determined by vapour pressure osmometry. The relations for the concentration dependence of the osmotic coefficient, activity coefficient and excess thermodynamic functions are given in analytical form, in which the regression coefficients are correlated with the virial coefficients, which, according to the McMillan–Mayer theory, characterize the contributions arising from the formation of pairs, triplets etc. of solute particles in a definite medium. On the basis of Prigogine's theory for associated solutions it may be concluded that the solvent–solute interactions are weak. In addition, the non-ideal behaviour of the system investigated was discussed on the basis of an association model, including an extended series of multimers, the formation of which is described in terms of two independent parameters: the dimerization constant β2 and constant K. The values of the dimerization constant β2 and constant K were determined by the curve-fitting method, whereas the concentration of monomer was obtained via Bjerrum's integral. On the basis of van't Hoff's relation for the temperature dependence of the equilibrium constants, the thermodynamic functions of association processes were determined and are discussed with respect to the oligomeric species formed.