Recent progress in the determination of solid surface tensions from contact angles

Advancing contact angles of different liquids measured on the same solid surface fall very close to a smooth curve when plotted as a function of liquid surface tension, i.e., γlvcosθ versus γlv. Changing the solid surface, and hence γsv, shifts the curve in a regular manner. These patterns suggest that γlvcosθ depends only on γlv and γsv. Thus, an “equation of state for the interfacial tensions” was developed to facilitate the determination of solid surface tensions from contact angles in conjunction with Youngʹs equation. However, a close examination of the smooth curves showed that contact angles typically show a scatter of 1–3° around the curves. The existence of the deviations introduces an element of uncertainty in the determination of solid surface tensions. ishing that (i) contact angles are exclusively a material property of the coating polymer and do not depend on experimental procedures and that (ii) contact angle measurements with a sophisticated methodology, axisymmetric drop shape analysis (ADSA), are highly reproducible guarantees that the deviations are not experimental errors and must have physical causes. The contact angles of a large number of liquids on the films of four different fluoropolymers were studied to identify the causes of the deviations. ic molecular interactions at solid–vapor and/or solid–liquid interfaces account for the minor contact angle deviations. Such interactions take place in different ways. Adsorption of vapor of the test liquid onto the solid surface is apparently the only process that influences the solid–vapor interfacial tension (γsv). The molecular interactions taking place at the solid–liquid interface are more diverse and complicated. Parallel alignment of liquid molecules at the solid surface, reorganization of liquid molecules at the solid–liquid interface, change in the configuration of polymer chains due to contact with certain probe liquids, and intermolecular interactions between solid and liquid molecules cause the solid–liquid interfacial (γsl) tension to be different from that predicted by the equation of state, i.e., γsl is not a precise function of γlv and γsv. In other words, the experimental contact angles deviate from the “ideal” contact angle pattern. ic criteria are proposed to identify probe liquids which eliminate specific molecular interactions. Octamethylcyclotetrasiloxane (OMCTS) and decamethylcyclopentasiloxane (DMCPS) are shown to meet those criteria, and therefore are the most suitable liquids to characterize surface tensions of low energy fluoropolymer films with an accuracy of ± 0.2 mJ/m2.