Interfacial tension Neumann theory

Neumann and co-workers have used the term engulfrnent to describe what can happen when a foreign particle is overtaken by an advancing interface such as that between a freezing solid and its melt. This effect arises in floatation processes described in Section Xni-4A. Experiments studying engulfrnent have been useful to test semiempirical theories for interfacial tensions [25-27] and have been used to estimate the surface tension of cells [28] and the interfacial tension between ice and water [29]. 

Figure 3.16 Validation of the Neumann theory for interfacial tensions. Reprinted from Kwok and Neumann (1999), with permission from Elsevier...

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). 

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. 

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. 

Colloid Chemistry (1997 and subsequent editions) edited by K.S. Birdi. Many methods for interfacial tension arc discussed in Chapters 2 and 9 of the CRC Handbook, both written by H.Y. ErbU (Erbil, 1997). He considers that the Neumann approach is controversial in many respects, especially since it ignores chemical effects and thus its range of apphea-bility should be apolar systems. Erbil does not object to the empirical practical value of the Neumann approach but he believes that his theory is of rather limited applicability range. 

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