Determination of Solid Surface Tension by Contact Angle

Abstract

In this chapter, approaches to determine solid surface tension by contact angle are briefly reviewed and assessed. These approaches include the Zisman method, various versions of the surface tension component methods, and the equation of state methods. The Zisman method is an empirical approach based on the relationship between the cosine of the contact angle and the surface tensions of the test liquids. The approach allows the determination of the critical surface tension of the solid. However, it is limited to low surface energy surfaces as data points from high surface tension liquids deviate from linearity due to polar and H-bonding interactions. The surface tension component approach is pioneered by Fowkes who assumed that (1) surface tension can be partitioned into individual independent components and (2) the work of adhesion can be expressed as the geometric means of the surface tension components. The original Fowkes method only considered dispersion interaction, and the methodology has been extended to include polar and H-bonding interactions in the extended Fowkes method or electron donor and acceptor interactions in the vOCG method. The equation of state assumes that the interfacial liquid–solid surface tension depends on the surface tension of the liquid and solid only. The interface surface tension was obtained by curve fitting with contact angle data and adjustable parameters. While the equation of state approach has been improved and three different versions have been developed, the basic thermodynamic assumption and the methodology were seriously challenged by many researchers in the field. It is important to note that both surface tension component methods and equation of state methods are semiempirical and that there are many assumptions in each methodology. Both approaches inherit a reversible work-of-adhesion assumption from Dupre. Specifically, for two immiscible liquids, the free energy change at the interface is equated to the interfacial tension of the newly formed interface subtracted by the surface tensions of the precursor liquids. The validity of this assumption is always questionable when one of the components is solid as the surface molecules or segments in solid have no mobility during any interfacial interaction. In view of this questionable assumption and the semiempirical nature of the contact angle approach, we propose a simpler and more direct approach to move forward. Since the motivation of determining surface tension is to be able to predict surface wettability and adhesion, we suggest measuring the advancing and receding angle of the solid surface instead. They have recently been shown to correlate to wettability and adhesion, respectively, by force measurements.