Solid surface tension, contact angle approaches

The advancing contact angles, measured on smooth silicon surfaces coated with the polymer brushes or the silane, were used to calculate the solid surface tension ysv according to the equation of state approach (EQS) [39] ... 

The measurement of contact angles on solids in powder or particle form is a challenging task, and numerous methods have been proposed 7. In this section, three successful indirect approaches are discussed capillary penetration into columns of powders, sedimentation volume of particles, and solidification fronts of particles. These methods are indirect because they provide the solid surface tension rather than the contact angle. For the sake of completeness, various direct approaches are also summarized briefly. 

In view of the close chemical similarity between paraffin and polyethylene, the large difference in the estiniates of the solid surface tensions is rather unexpected and deserves comment. This difference is, by virtue of the assumptions and related estimates used in the present approach, reflected in the rather different contact angles observed for water on these two solids. This contact angle difference has been confirmed and commented on by Adam and Elliott [1]. They attribute the difference from the expected behavior for polyethylene to the presence of traces of polar impurities. 

This approach to the determination of s is generally useful, but suffers from observed non-linearities when polar liquids are used. More thorough understanding of the relations between measured contact angles and solid surface tensions depends on the combine-... 

As noted above, PMTFPS exhibits somewhat surprising surface properties. According to our measurements, its liquid surface tension is higher than that of PDMS, its critical surface tension using n-alkane contact angle test liquids is similar to the PDMS value, while its solid surface tension judged by the Owens and Wendt [10] approach with water and diiodomethane liquids has a value considerably less than that of PDMS. 

In combination with the Dupre equation (equation (7.15)) and Young s equation (equation (7.7)), this geometric mean combining rule is the basis for different approaches to calculate the solid surface tension from contact angle data (see Section 2.4 below). 

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). 

However, the most important difference between the two approaches lies on the way the surface tension components are estimated. In the van Oss-Good approach, the surface tension components for liquids and for solids are estimated from a wide range of experimental data (liquid-liquid interfacial tensions, contact angles, etc.) often regressed simultaneously for various solids and liquids. As we discussed, there are no predictive or estimation methods proposed by van Oss-Good for calculating these surface tension components. 

Available methods for the measurement of contact angles on polymers and approaches for the estimation of solid surface tension values from contact angle data are discussed in a recent review (84). 

Approaches to Determine Solid Surface Tension by Contact Angle... 

In summary, there are three basic approaches to use contact angle data to determine the surface tensions of solid surfaces. These approaches are the Zisman method, the surface tension component methods, and the equation of state. Within these three approaches, there are many variants. It is reasonable to wonder the merit, accuracy, and limitation of some of the methods. The Zisman method is an empirical approach based on the correlation between the cosines of the contact angles on a solid surface versus the surface tensions of the test liquids. With alkanes, linear plots are usually obtained, and the critical solid surface tension (yc) is determined by extrapolating... 

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