What do van Oss-Good and Neumann say about their own theories

2 What do van Oss-Good and Neumann say about their own theories  

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. 

Van Oss-Good (and others) have recognized that, even with the recommended approach mentioned above, the basic components are almost always higher than the acidic ones, which is physically not correct. All results of the theory should be interpreted having this model deficiency in mind. Still, neither this nor the fact that different liquid triplets yield different results is a serious issue, according to the developers. When the peculiarities of the van Oss-Good method mentioned above are taken into consideration, solid surfaces can still be characterized successfully and useful information can be obtained also for practical phenomena such as wetting and adhesion. 

However, they too have observed several problems with their theory for some solid-liquid combinations, i.e. deviations from the general plots yi cos =f yi), e.g. see Tavana and Neumann (2007) and Tavana et al. (2004, 2005). 

They believe that these deviations are not due to experimental error or problems of the theory but aU have a clear explanation. In most cases, they attribute the problems to either interaction between the solid and the liquid and/or the presence of a non-zero spreading pressure due to vapour adsorption from the liquid. They improve the smoothness of their plots, in these cases, by eliminating several (in some cases many of the) liquids, sometimes even alkanes, used in the analysis in order to maintain the maximum possible inertness of the test liquids used to estimate the solid surface tension using the Neumann method. They mention that with this approach, i.e. careful selection of test liquids (bulky molecules are often useful), some of the experimental contact angle data, even with the goniometer method used in the extensive studies of Zisman, can be used in the context of the Neumann method (Kwok and Nemnann, 2000b). 

---