Oss-Good and Neumann theories

J,5 What do we believe about the van Oss-Good and Neumann theories ... 

The discussion of the van Oss-Good and Neumann theories is compheated by many factors, e.g. the validity range of the Young equation, the role of spreading pressure, the accuracy of experimental contact angle data and others. 

We have already mentioned that theories for interfacial tension constimte a controversial topic and the advent of the van Oss-Good and Neumann methods has not made the topic less controversial. Actually, these two are possibly the most widely discussed theories today and for this reason, and to pay justice to all opinions, we will divide the discussion into different sections the opinions of the developers on their own and on each other s theories and some of the independent studies carried out by others before expressing our own views. 

Many theories for estimating the interfacial tensions have been presented in Sections 3.5.1-3.5.3. The equations for the surface and interfacial tensions as well as for the work of adhesion are summarized in Table 3.6. Notice that the work of adhesion corresponds to the cross term of the interfacial tension expression (under the square roots), which reflects different contributions of intermolecular forces, according to the various theories (either the total surface tensions in Girifalco—Good and Neumann, only those contributions due to dispersion forces in Fowkes, due to both dispersion and specific forces in Owens-Wendt, separately dispersion, polar and hydrogen bonding ones in Hansen/Beerbower, or the van der Waals and as5mimetric acid/base effects in van Oss et ai). 

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. 

Good and co-workers have frequently pointed out (e.g. van Oss et al., 1987) that the inability of Neumann theory to predict negative liquid-liquid interfacial tensions results in often poor agreement for liquid-liquid interfaces and for predicting misci-bUity (miscibUity is equivalent to zero or negative interfacial tensions). 

In addition, Kwok (Kwok and Neumann, 1996 Kwok et ai, 1998, 1999) showed that the van Oss-Good theory predicts problematic results for several liquid-liquid interfaces for which experimental data are available. While for many aqueous systems the interfacial tensions are predicted rather satisfactorily, the performance of van Oss-Good for several non-aqueous mixtures is not good. In some cases finite values for the interfacial tension are predicted for mixtures which are known to be miscible such as bromonaphthalene with alkanes and squalene-diiodo-methane. The results or at least the main conclusions appear to be independent of the source of reference liquids and parameters used for the van Oss-Good method (original ones or those from Lee, see Section 15.3.4). Kwok and Neumann (1996) state, on basis of these calculations, that it is surprising to see an approach published in well reputed journals when it can be shown to be false by anybody who possesses a simple calculator and a few drops of the liquids used in the approach . Statements like this can be found in several of the articles published by Neumann and coworkers. 

Delia Volpe and Siboni are, despite the problems and limitations which they themselves point out, much more confident about the van Oss-Good theory than for the Neumann theory. They have considered the latter theory in several publications and their conclusion is, in agreement with others, that the Neumann theory is of semi-empirical nature and should be applied only to apolar surfaces. 

It appears, among several researchers, to be a consensus that the Neumann theory is best apphed to non-polar liquids and solids (Correia et al., 1989 Drelich and Miller, 1994) and that, all things being equal, the van Oss-Good theory performs best overall for the analysis of sohd surfaces. 

Overall, we beheve that the van Oss-Good method is, despite its well-documented limitations, the best among the two recent theories and moreover it can be used for surface analysis as well. The Neumann... 

The Fowkes, Owens-Wendt and the Girifalco-Good theories are those most often presented in textbooks on colloids and interfaces but they would not be the ones we recommend using today. The more modem theories of Neumann and the acid-base by van Oss-Good are better choices. Strangely they are not presented in textbooks but they are the ones most widely accepted in engineering practice. The consensus today is that the Neumann theory has a rather weak theoretical justification but can be reliably used to estimate surface tensions of rather non-polar or slightly... 

Compare also your results to the analysis carried out by the opponents of the van Oss-Good theory, the group of Neumann and co-workers (see, for example, Kwok and Neumann, 1996 Kwok et ah, 1998, Kwok, 1999). What do you observe ... 

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