Van Oss-Good theory

A major observation from the van Oss-Good theory is that, unlike other theories, the interfacial tension may be positive or negative. Note that the LW term is given by a geometric-mean Fowkes equation, but this is not the case for the Lewis AB asymmetric term. 

Following the success of the van Oss-Good theory and other investigations, many leading scientists have... 

Table 3.4 Parameters of the van Oss-Good theory for some classical test liquids (in mN m ). Notice the high values of the base contribution to the surface tension in all cases...

Even the van Oss-Good theory has been criticized, e.g. due to the very high basic values, but despite that it has found wide applicability in describing interfacial phenomena (interactions) involving polymers, paints, proteins and other complex systems (like polymer surface characterization, CMC determination of surfactants, protein adsorption, cell adhesion, enzyme-substrate interactions). 

Water is typically used as one of the three fluids used and is considered to have equal LA-LB (acid-base) components (=25.5 mN m ) and a LW (van der Waals) value of 21.8 mN m (recall the dispersion value obtained from Fowkes equation. Chapter 3, Example 3.3). Water is thus often considered as a reference fluid in this type of calculation and all other AB values are relative to those of water . Other reference values for the van Oss-Good theory are discussed in Chapters 3 and 15. 

Table 6.3 Summary of the results for the van Oss-Good theory s acidic (+) and basic (-) parameters of the two polymeric surfaces, as calculated by various methods...

As evident from the above, the van Oss-Good theory parameters for solids depend greatly on the liquids chosen and the exact calculation method as well as the values of the experimental contact angle data. Nevertheless, trends and average values are presented for several solids in the literature. Table 6.4 presents values for some solids together with the critical surface tension, when available. There is good correspondence between the solid surface tension and the critical surface tension in some cases, but there are differences for some solids, as might have been anticipated. 

Table 6.4 Parameters of the van Oss-Good theory for some polymers (in mN m ). Notice the high values of the base contribution to the surface tension in all cases, exactly as was the case for the liquids based on which these surface tensions are estimated (Table 3.4). Where available, the Zisman critical surface tensions are also shown (also expressed in mN m )...

In the next example, we will use the van Oss-Good theory to explain the selective plating of laser machines. This example is based on the experiments and modelling approach presented by Zhang, Kontogeorgis and Hansen (2011). First, the background of the problem and the procedure are briefly presented. 

Example 6.3. Explanation of selective plating of laser machines surfaces using the van Oss-Good theory (based on Zhang, Kontogeorgis and Hansen, 2011). 

In a recent case study (see Svendsen et al, 2007 and also Problem 6.1), in collaboration with a paint company, the adhesion of six different epoxies-silicon systems has been studied. These paints are used in marine coating systems. Some epoxies showed adhesion problems in practice while others did not. The purpose of the study was to understand the origin of these problems and whether adhesion could be described/ correlated to surface characteristics, e.g. surface tensions. An extensive experimental study has been carried out including both surface analysis (contact angle measurements on the six epoxies, surface tension of silicon at various temperatures, atomic force microscopy (AFM) studies of the epoxies), as well as measurements of bulk properties (pull-off adhesion tests and modulus of elasticity). Theoretical analysis included both estimation of Zisman s critical surface tensions and surface characterization using the van Oss-Good theory. 

When using the van Oss-Good theory for interfacial tensions, the parameters of the solid surface are typically estimated from ... 

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 ... 

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,... 

Good and co-workers have established several selected liquids for which they have estimated LW, acid/base components (Table 3.4). These should be used in the analysis of solid surfaces together with contact angle data. However, it is not a priori known which liquids should be used for performing contact angle experiments. Van Oss et al. (1988) have established that the optimum approach is to always use water (essentially aU van Oss-Good theory parameters are relative to water), one non-polar liquid (often methylene iodide) and another polar liquid. 

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. 

As mentioned, both theories have been extensively used by other researchers, with the van Oss-Good theory being more popular in independent studies. 

Some applications of the van Oss-Good theory are presented in Chapters 3 and 6. McCafferty (2002) characterized PVC and PMMA surfaces and based on the obtained values he presented a discussion of adhesion phenomena in various systems which are... 

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. 

We have also successfully used the van Oss-Good theory in various applications in the paint industry (see Case study Paints, Chapter 6, and Problem 6.1 Svendsen et ah, 2007), characterization of polymers (Problem 6.11) and laser-based surface treatment studies (Zhang, Kontogeorgis and Hansen, 2011 Example 6.3). In all cases, we find that the van Oss-Good basic components are systematically higher than the acidic ones but, even with this limitation, the method is useful for a qualitative assessment... 

Della Volpe and Siboni recommend that the acid-base values in the van Oss-Good theory should not be compared too much within the same solid but comparatively between different solids and it is more correct, for example, to compare acid values and base values between different solids rather than compare acid to base values. They also point out that the liquids to be used in the van Oss-Good theory should be chosen with care, according to the guidelines, e.g. from the proposers. 

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. 

Lee also proposes to use the LSER (linear solvation energy relationship) concept in its full context and he identifies that the polarity parameter of LSER is related to the spreading pressure. The remaining parameters of Kamlet-Taft LSER (acid, base and solubility parameter) have already their analogue in the van Oss-Good theory (acid, base and LW contributions to the surface tension). Thus, Lee believes that the spreading pressure should be included in new developments and in a proper evaluation of the van Oss-Good approach. 

Despite the many successful applications, several researchers have expressed serious objections about the foundations of the surface tension component theories like the one proposed by van Oss et al. Douillard (1997) states that it is based on an oversimplified thermodynamic basis because the additivity of the various force contributions can be claimed for the enthalpy but not for the free energy and thus not for the surface tension either. What Douillard (1997) really claims is that the van Oss-Good (and other similar theories) essentially do not account for the entro-pic effects which can be significant in some cases or imply that enthalpic and entropic effects are proportional and that the effect of temperature is not appropriately accounted for. He suggests that a surface enthalpy components theory is possibly the way ahead but he admits that this may be more complex and - to our knowledge - this has not been as yet pursued. Despite these limitations, Douillard explains why several of these assumptions can be less important for several systems and thus the van Oss-Good theory often works quite well. 

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. 

Panayiotou has recently (2012a, 2014) proposed a new theory for the interfacial tension that - at least at a first level - bears some similarities to the van Oss-Good theory. The surface tension is divided into a van der Waals (essentially a dispersion) part d, a polarity contribution pz and an asymmetric hydrogen bonding contribution hb ... 

Indeed, Equations 15.15 and 15.16 resemble very much the corresponding equations of the van Oss-Good theory, e.g. as compared to Equations 15.11 and 6.3.2. 

The acid-base (+/-) contributions in the van Oss-Good theory are now called o/Z> this is largely a notation difference. A more important difference is that two terms are used now for the van der Waals forces... 

During the last decade or so, most new developments focused on improving the van Oss-Good theory, especially by introducing new parameters for the reference liquids used. These parameters improve the performance and result in relatively more correct values for the acid and base components of solids when the theory is used together with contact angle data for surface analysis. 

Nevertheless, as also discussed in this chapter, there are several problems even with the van Oss-Good theory and, while usefiil in many practical applications, it should be used with care and by experienced users who arc familiar with the interpretations of the results. Due to these problems, it is most likely that we have not as yet seen the last in the development of theories for the interfacial tension. The recent Panayiotou theory based on the partial solvation parameters may prove to be a promising tool for predictive calculations of interfacial tensions. 

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

According to the van Oss-Good theory, zero or negative interfadal tensions indicate miscible liquid mixtures while of course positive interfacial tensions are obtained for immiscible liquids having a distinct interface. 

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 ... 

Calculate from these data the LW contribution of the van Oss-Good theory for these four liquids and compare the results to reported values in literature from other methods, e.g. data from liquid-liquid interfaces or solid-liquid interfaces with other solids (50.8 for diiodomethane, 29-33.6 for EG, 33.5-39 for formamide and 32.8-44.01 for 1-bromonaphthalene). 

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