Contact angle acid-base theory

Interfadal tension between two fluid phases is a definite and accurately measurable property depending on the properties of both phases. Also, the contact angle, depending now on the properties of the three phases, is an accurately measurable property. Experimental approaches are described, e.g., in Refs. 8,60, and 63 and in Ref. 62, where especially detailed discussion of the Wilhehny technique is presented. Theories such as harmonic mean theory, geometric mean theory, and acid base theory (reviewed, e.g., in Refs. 8, 20, and 64) allow calculation of the soHd surface energy (because it is difficult to directly measure) from the contact angle measurements with selected test liquids with known surface tension values. These theories require introduction of polar and dispersive components of the surface free energy. 

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]. 

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. 

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. 

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