ABSTRACT
Although the determination of the surface free energy of solids, as well as that of liquids, is still an open problem, the approach of van Oss, Good, and Chaudhury (vGC) [1-3] is commonly accepted at present. The main problem is with the choice of an absolute scale of the acid-base parameters for the liquids used for the determination of contact angles. Up to now, it has been based on the assumption
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of van Oss and co-workers that for water the electron donor parameter, y ^ , is equal to the electron acceptor one, yt+. As the experimental value of the acid-base component for water equals 51.0 mJ/m2 (at 20°C) [4], both parameters have to be equal to 25.5 mJ/m2, if one accepts the combining rule for the electron donor-acceptor interactions to be appropriate. The rule is applied for the dispersion interaction (or Lifshitz-van der Waals interaction, }/|LW, in the vGC approach). A consequence of this equality is a particular set of these parameters for other liquids used as probes for contact angle or thin-layer wicking methods. However, very recently, different values of y, and \ \ ' for water have been suggested [5, 6], On the basis of solvatochromic parameters, Lee [5] suggests that for water at room temperature the ratio of the electron donor and electron acceptor parameters equals 1.8. As a result, the acidic parameter for water y,+ = 34.2 mJ/m2 and the basic one y, = 19 mJ/m2. Moreover, Della Volpe and Siboni [6], while discussing the mathematical problem involved in solving the equations with three unknowns to determine the acid-base parameters, offer another set of these parameters for water, namely y,+ = 65 mJ/m2 and y,~ — 10 mJ/m2. As can be seen, in the latter two approaches a higher acidic character (proton-donating) rather than basic is preferred for water. Nevertheless, irrespective of which set of parameters for the interfacial interactions is accepted as the correct one, which at present cannot be judged, the relative changes in the acid and base parameters of a given surface can still give some insight into the interfacial process studied.
