On the Selection of Test Liquids for the Evaluation of Acid-Base Properties of Solid Surfaces by Contact Angle Goniometry

The determination of the surface free energy (SFE) components from contact angle data using the acid-base (AB) model is often not consistent when various sets of test liquids are utilized. Using contact angle data from the literature, mathematical instabilities of the model are discussed as one of the origins of these inconsistencies. In order to obtain reliable results, the difference of the ratios of the basic and acidic parts of the interfacial free energy of the test liquids should be large. If this condition is not satisfied, small deviations of the measured contact angles (within the range of experimental errors) can produce major differences in the AB components of the SFE. An optimization of the results of several pairs of test liquids can help to obtain reliable results.