Owens-Wendt theory

The Owens-Wendt theory (Good and Girifalco, 1960 Owens and Wendt, 1969) accounts for local interactions and separates the surface tension into dispersive and polar components (o and O ) for the solid-vapor, and liquid-vapor interfaces (Osv and Olv) (Parry et al, 2010 Wood et al., 2010b) ... 

The success of the Fowkes equation, but also the need for better theories, has been illustrated in Examples 3.3 and 3.4. Extensions of the Fowkes equation have been proposed, which account explicitly for polar and hydrogen bonding effects in the expression for the interfacial tension using geometric-mean rules for all terms. Two such well-known theories are the Owens-Wendt theory, which is often used for polymer surfaces, and the Hansen/Skaarup (van Krevelen and Hoftyzer, 1972 Hansen, 2000) model. Both models are presented below. The Hansen/Skaarup... 

Derive an expression for the work of adhesion as a function of the surface tensions of the solid and the liquid. Then derive the expression for the solid-liquid work of adhesion from the Owens-Wendt theory. Compare the results from the two theories. Which one is expected to perform best ... 

Similarly for the Owens-Wendt theory (see Chapter 3), the work of adhesion is the cross term again without the negative sign ... 

The Owens-Wendt theory often performs better than Fowkes equation for polar systems, but problems have been often reported, e.g. for systems like ethanol-water and acetone-water, which are erroneously predicted to be as immiscible as benzene-water. Nonetheless, Owens-Wendt is an old theory, well-established in certain fields, with extensive parameter tables and many successful appKcations, particularly for polymers. Table 6.2 shows some parameters of the Owens-Wendt equation for liquids and some polymers. 

Table 6.2 Owens-Wendt theory parameters for certain liquids and some polymers. In mN m ...

We have a solid substrate - a flame-treated polypropylene (PP) film coated with a polymer - that is a vinyhdene chloride/methyl acrylate co-polymer. The surface of two sohds, coating-PP, is immersed in a solution of a surfactant, SDS (this is the weU-known sodium dodecyl sulfate). Is there a dangCT of separation of the two solids due to the surfactant Use the data available in the table below for the three mateials and assume the vahdity of the Owens-Wendt theory (Chapter 3). 

Using the Owens-Wendt theory, we have (the specific surface tensions of the two solids and the liquid are simply calculated as the difference between the total and the dispersion parts of the surface tension). AU values are in mN... 

Problem 6.2 Characterization of a PVC surface with the Owens-Wendt theory The contact angles of water and methylene iodide have been measured on a PVC (poly(vinyl chloride)) surface to be equal to 87° and 36°, respectively. The surface tension of water is at 25 °C equal to 72.8 with a dispersion part equal to 21.8. For methylene iodide, the surface tension is 50.8, and the dispersion part is 49.5. All surface tension values are in mN m. Assuming the validity of the Owens-W endt theory, calculate the surface tensions (total, dispersion and specific) of the sohd PVC surface and comment briefly on the results. 

Table 15.2 Parameters of the van Oss-Good and Fowkes/Owens-Wendt theories for some liquids (In mN m ) at room temperature...

Erbil (1997) writes that most researchers in the field today have abandoned the Owens-Wendt theory (which also predicts measurable interfacial tensions for certain mixtures which are known to be miscible). 

Osterhold and Armbmster (1998) used the van Oss-Good theory in the study of adhesion changes due to surface modification (see Figure 6.14). Adhesion phenomena and their relation to acid-base interactions and other theories are also summarized by Clint (2001a,b). He showed that adhesion calculated from theories can be linked to experimental values. He presented results with both the van Oss-Good and Owens-Wendt theories but recommends the former for polar systems. 

Calculate the specific contributions to the solid surface tension based on the Owens-Wendt theory and then compute the contact angle of water on these polymers using the OW theory. Compare the results to the experimental data. 

How accurately do the van Oss-Good and Owens-Wendt theories perform for such interfaces Are these theories able to identify when we get miscible and we get immiscible liquid mixtures Compare their performance with each other and against the experimental observations. 

In a large part of the (current) literature the Lifshitz-van der Waals component (o, is simply termed dispersion component and the Lewis acid-base interactions (o ) are interpreted as polar interactions even though the material s dipole moments may be zero or the interactions originating from permanent dipoles are very small and can be easily associated with the dispersion part [6]. The misleading denominations go back to a historical misidentification of the acid-base interactions as polar interactions in the Owens-Wendt-Rabel-Kaelble [7-9] approach to calculate the IFT [6] (OWRK model). However, as an impact on the SFE calculation by this misinterpretation of this old theory occurs only when a monopolar base interacts with a monopolar acid, this nomenclature is still widely used. And here in this work we will also use the terms dispersion and po/ar interactions to differentiate the two major contributions to SFE, ST, and IFT. For a detailed discussion of the use of contact angles in determining SFE of solids and other methods of determining SFE, see Etzler [10]. 

It can be seen that these theories resemble the Fowkes equation but one or two additional cross terms are added to account for the specific interactions (a combined specific terms is used for Owens-Wendt while both polar and hydrogen bonding terms are used in the Hansen equation). The relevant equations for the surface and interfacial tensions for these two theories are given in Equations 3.22-3.24 ... 

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 can see that the Owens-Wendt contains two terms from the contribution of dispersion and specific forces, while in the first theory the work of adhesion depends only on the values of the solid and liquid surface tensions. The two theories discussed here belong to the two different theory families (component and direct)... 

To do so, we must perform contact angle measurements for different liquids on the same solid. The minimum number of required data depends on the theory. In the case of Owens-Wendt we need data for two liquids on the same solid, while the most advanced theories (Hansen/Beerbower and van Oss-Good) require data for at least three liquids on the same solid. Graphical methods can also be used, e.g. the Owens-Wendt equation can be written in the form ... 

All interfacial theories such as Fowkes, Owens-Wendt, Hansen and van Oss-Good can in principle be applied to all types of various interfaces (liquid-liquid, solid-liquid and even solid-solid ones). 

However, all theories have advantages and shortcomings (see, for example, Balkenende et al.. 1998), but based on numerous investigations the van Oss-Good is possibly the best tool to-date, even though many companies have working experience with other theories as well, especially the Owens-Wendt (particularly for applications related to polymers). 

Adhesion can be either wanted (paints, glues, etc.) or not (biofouling, cleaning, etc.). In either case, studies should ensure at first good wetting and also estimation of the theoretical work of adhesion via use of an interfacial theory, e.g. Owens-Wendt or van Oss-Good. 

Analyse the results using both the Owens-Wendt and the van Oss-Good theories for interfacial tensions. Compare and discuss the results. Can we conclude that the new binders have simQar/higher surface tensions than the existing ones and could thus be considered useful alternatives to the existing products ... 

In the Fowkes and many subsequent equations, it is assumed that the surface tension can be divided into imaginary contributions from the various intermo-lecular forces, e.g. dispersion and remaining specific forces in the Fowkes and subsequently in the Owens-Wendt equation (OW) (Equations 3.19 and 3.22a). These theories have since been known as surface component or simply component theories . It is a very useful concept in the sense that, if these surface components could be calculated reliably, we could obtain information about the contribution of the various intermolecular forces on the... 

Similar corrections should apply to Girifalco-Good, Owens-Wendt and all the other theories. But... 

Table 15.3 Components of surface tension for various solids (polymers) from the Owens-Wendt (OW) theory and the Zisman plot (critical surface tension). All values are in mN m . When two values are mentioned it is because they are based on different experimental values for the contact angle...

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. 

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,... 

We can conclude that all theories have positive and negative features, advantages and problems or, in other words, areas/systems where they can be reliably used and others where they cannot. Even the classical theories of Fowkes and Owens-Wendt are still much in use. Actually, the latter, despite its criticism by,... 

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... 

Which of the two classical theories, of Fowkes and Owens-Wendt, will perform best in describing or predicting the interfacial tensions for these three mixtures ... 

Compare the Fowkes, Owens-Wendt and van Oss-Good theories for these non-aqueous liquid-liquid interfacial tensions. Which model performs best ... 

Testing the theories for interfacial tension, either for liquids or solids, is not an easy task. In the case of liquids, small impurities may affect the values of interfacial tensions, as Owens (1970) and others have stated. On the other hand, if accurate data are available for obtaining the surface components and if there is confidence on a specific theory we can get useful information about the solids, e.g. a semiquantitative measure of surface composition (Owens and Wendt,... 

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