Surface free energy of cholesterol and bile salts from contact angles

Abstract

Measurements of contact angles for water, ethylene glycol, diiodomethane, and bromoform on cholesterol, and water, diiodomethane, and bromoform on bile salts pellets were conducted. Using the values of the contact angles, total surface free energy of cholesterol and bile salts and components of the energy resulting from different kinds of intermolecular interactions were calculated. For this purpose, various approaches to the solid—liquid interfacial free energy have been considered, though some older ones were criticized. Thus, it was found that the surface free energy of cholesterol originates mainly from the Lifshitz—van der Waals interactions, while that of bile salts is due to apolar Lifshitz—van der Waals, and to a great extent to the polar (Lewis acid—base) electron-donor and electron-acceptor interactions. It was found that all the tested approaches to the solid—liquid interfacial energies may be useful; however, the one describing the surface free energy of a solid and a liquid in terms of apolar Lifshitz—van der Waals and polar acid—base interactions gives more information about properties of the cholesterol and bile salts surfaces. Using values of the Lifshitz—van der Waals, electron-donor, and electron-acceptor components of the surface free energy of cholesterol, bile salts, and water, the free energy of interaction between two particles of cholesterol in water, and of cholesterol—bile salt and of bile salt—bile salt immersed in water was calculated. From these calculations it appears that bile salts adsorbed on cholesterol surfaces cause the free energy of interaction between the cholesterol particles immersed in water to be positive. As a result, the cholesterol dispersions should be stable (unflocculated). However, no electrostatic interactions due to the surface electric charge were taken into account.