Acid-base surface free energies of solids and the definition of scales in the Good-van Oss-Chaudhury theory

The overwhelming basicity of all analysed surfaces strongly dependent on th e choice of liquid triplet used for contact angle measurements and the nega tive values sometimes obtained for the square roots of the acid-base parame ters can be summarized as the main problems arising from the application of the Good-van Oss-Chaudhury (GvOC) theory to the calculation of Lewis acid- base properties of polymer surfaces from contact angle data. This paper tri es to account for these problems, namely: (1) the Lewis base, or electron d onor component, is much greater than the Lewis acid or electron-acceptor co mponent because of the reference values for water chosen in the original Gv OC theory. A direct comparison of the acidic component with the basic one o f the same materials has no meaning. A new reference scale for water which is able to overcome this problem is suggested. For the calculation of acid- base components, a best-fit approach is proposed which does not require any starting information about the liquids or polymers and can yield estimates of the acid-base parameters for both the liquids and the polymers involved ; (2) the strong dependence of the value of the acid-base components on the three liquids employed is due to ill-conditioning of the related set of eq uations, an intrinsic and purely mathematical feature which cannot be compl etely cured by any realistic improvement in experimental accuracy. To reduc e or eliminate the effect, one only needs a proper set of liquids, represen tative of all kinds of different solvents: (3) the negative coefficients ap pear as a simple consequence of measurement uncertainty, combined with the possible ill-conditioning of the equation set. We cannot exclude, however, that in some cases they could have a different origin.