where g SL represents the interfacial tension between the solid The different methods available in the literature to calculate the and the liquid, g S and g L are respectively the surface tension surface tension of a solid from contact angle measurements are of the solid and the surface tension of the liquid in equilibdiscussed and compared. The discussion is based on the contact rium with their own vapors, and u is the contact angle. angles of water, glycerol, and diiodomethane measured at 20ЊC Although Young did not prove this equation, several justifion the surface of a side-chain liquid crystalline polymer. Some cations were given later, the most correct one based on the discrepancies exist among the results obtained with the different minimization of the Helmholtz energy of the system (2). It methods, mainly between the values yielded by Neumann's equacomes out from this equation that the difference g S 0 g SL tion and those obtained with approaches that postulate the decomcan be obtained from the experimental values of g L and u. position of the surface tension into several terms associated with different types of molecular interactions (methods of Owens and On the other hand, the work of adhesion (the negative of Wendt and of Good and van Oss). The physicochemical basis of the Gibbs energy of adhesion) for a liquid and a solid in these various treatments is discussed.