Components of surface free energy of cholesterol in the presence of bile salts

Abstract

Measurements of the contact angle for water and diiodomethane on cholesterol discs covered with films of the bile salts sodium cholate, sodium deoxycholate and sodium chenodeoxycholate, were made. Using these values of contact angles, the total surface free energy of a cholesterol/bile salt film and its components resulting from different kinds of intermolecular interactions were calculated. For this purpose both the geometric mean approach and the approach of van Oss et al. to solid—liquid interfacial free energy were used. Next, using the Lifshitz—van der Waals component, and the electron-acceptor and electron-donor parameters of the surface free energy of cholesterol covered with bile salt films, the free energy of interaction in the system cholesterol/bile salt film—water—cholesterol/bile salt film was calculated. On the basis of the contact angle measurements and calculations with the two theoretical approaches, it was found that the cholesterol surface free energy strongly changed when the concentration of bile salt in solution reached a fairly narrow concentration range, defining an operational critical micellar concentration of the bile salt. At this concentration, a change of sign of the free energy of interaction between two particles of cholesterol in water from negative to positive occurs, which indicates the existence of repulsive forces under these conditions.