Importance of surface layers in solid surface free energy determination

The problem of the solid surface free energy determination by using direct, theoretical and indirect methods was considered. Among the indirect methods, measurements based on the contact angle in the solid–liquid drop–air, solid–hydrocarbon drop–water, solid–water drop–hydrocarbon and solid–air bubble–water systems were discussed. In the studies, the importance of surface layers in determination of the surface free energy of solids from the Young’s equation was taken into account. As follows, there are considerable differences between the surface free energy values of a given solid obtained by direct or theoretical methods and those from the contact angle measurements in proper three-phase systems. It is suggested that the difference between these values can result from the presence of the surface layer at the solid–air and/or solid–liquid interfaces, which is not commonly taken into account.