The two theories in brief

While most (if not all) textbooks on colloids and interfaces limit their discussion about interfacial theories to Girifalco-Good and Fowkes/Owens-Wendt, the discussion is hardly complete without presenting the two most modem, possibly most widely used and certainly most controversial, theories, the acid-base theory of Carel van Oss, Manoj Chaudhury and Robert Good (from now on called here van Oss-Good) and the equation of state approach of A. Wilhelm Neumann. These theories, already presented in Chapter 3 (Equations 3.18 and 3.25 and 3.26), have resulted to extensive discussions — not the least between their developers, often with rather direct and not always entirely poUtc statements about the capabilities and limitations. Numerous articles have been published about these two theories both by their developers and by others. Thus, the pertinent hterature is enormous but we attempt a short review here. 

We discuss first the background of the two approaches. Each one presents a different school in theories for the interfacial tension. The van Oss-Good is the latest well-known version of the surface component theories , where the geometric mean rule 

Actually, a similar approach to Equation 15.11 was proposed earlier by Small (1953) who also stated that polarity is often far less crucial than the dispersion and hydrogen bonding forces. He also mentioned that the geometric mean mle breaks down if size/shape of the involved compounds differ too much. 

The van Oss-Good equation can result in either positive or negative interfacial tensions, the latter simply meaning miscible liquids. Thus, it is possible for the van Oss-Good theory to predict repulsive van der Waals forces which can be present in certain systems (van Oss et al, 1988, 1989). Because van Oss-Good can also predict negative interfacial tensions, it has been shown to predict well the solubility in aqueous polymer solutions (van Oss and Good, 1992) where Owens-Wendt fails. It has also been applied with success to biopolymers (van Oss et al, 

Naturally, apphcation of Equation 15.11 to sohd interfaces using the contact angle equation requires data for at least three liquids on the same solid. We return later on the choice of Uquids. 

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