Methods for the calculation of surface free energy of solids

Purpose: The main purpose of this paper is the analysis of the most common methods for the calculation of the surface free energy (SFE) of solids, utilising the results of the contact angle measurements. The calculation deals also with the SFE at the interface, especially that at the surface of polymers and polymeric materials. The survey has been meant to ease the understanding of physical processes occurring at the solid-liquid interface and to help to find proper measuring methods with respect to various physical systems. Design/methodology/approach: The presented analysis has been based on the papers of the fundamental nature as well as on the specialised literature reports. The results of the experimental and theoretical studies of the author of this article are also considered. Findings: Different assumptions have been made in the individual methods for calculating the SFE of polymeric materials. Thus, the SFE values for a given material, obtained by various methods and with use of different measuring liquids, are not consistent. The method for the calculation of the SFE of porous or granulated materials, powders, and fibres, in which the Washburn equation is utilised, is very useful in practice. Currently, there is no appropriate alternative to this method. Research limitations/implications: The method for calculating the SFE with use of the equation of state requires further investigation, both experimental and theoretical. Further studies on the phenomena associated with penetration of the measuring liquids into the bulk of an examined material and on the relevant changes concerning this material, including the changes in its SFE, have also to be carried out. Practical implications: The presented results of the investigations may be applied in optimisation of the current and derivation of the new methods for calculating the SFE of solids and liquids, including mainly the SFE of polymers in the solid state. Originality/value: Because of differences in the assumptions made in most of the methods for the SFE calculation and of differences in the interactions between the individual measuring liquids and the examined material, the SFE values for various polymer materials may be compared with one another only when the contact angle measurements have been made using the same measuring liquids and when the SFE calculations have been performed by the same method.