Additive and nonadditive surface tension components and the interpretation of contact angles

TL;DR: In this paper, it was shown that the relation between contact angles and the three controlling parameters ybw, yg, and 7s of a polar solid, S, cannot be established by means of one single equation.

Abstract: It is demonstrated that, while the Lifshitz-van der Waals (rLW) and the polar, Lewis acid-base (rAB) surface tension components are additive, the Lewis acid-base electron-acceptor (7') and electron-donor (y-) surface tension parameters (which on account of the intrinsic asymmetry of their interactions are connected in a more complex manner) are not additive. Contact angle data cannot, in general, be correlated with a single surface tension property. The relation between contact angles and the three controlling parameters ybw, yg, and 7s of a polar solid, S, cannot be established by means of one single equation. These three unknowns can, however, be determined by means of contact angle measurements with three different liquids L (of which two must be polar and H-bonding), which are completely characterized as to their respective rkw, yL+, and y i parameters by using a modified Young-Good-GirifalcoFowkes equation 3 times. In view of these considerations. recent claims to have proven the applicability of a single "equation of state" are shown to be spurious.