In order to be able to calculate the surface free energy from the contact angle, the second unknown variable σsl must be determined.
The interfacial tension σsl is calculated based on the two surface tensions σs and σl and the interactions between the phases. These interactions are interpreted as the geometric mean of a disperse part σD and the corresponding acid parts σ+ and base parts σ- of the surface tension or surface free energy.
According to the Lewis acid-base theory, polar interactions take place when an electron acceptor (+) impinges on an electron donor (-). The geometric mean is accordingly formed from the respectively opposing parts.
At least three liquids are required to determine the surface free energy of the solid: a purely dispersive liquid and two liquids with known acid and base parts. As the neutral point in the Lewis scale, water should be used in all cases.
- R. J. Good; C. J. van Oss, The Modern Theory of Contact Angles and the Hydrogen bond Components of Surface Energies. In: G. I. Loeb; M. E. Schrader (Hrg.): Modern approaches to wettability. 1992, P. 1-27.
- R. J. Good, Contact Angle, Wetting and Adhesion: a Critical Review. In: K. L. Mittal (Hrg.): Contact Angle, Wettability and Adhesion. Festschrift in Honor of Professor Robert J. Good. Utrecht 1993, P. 3-36.
- C. J. van Oss; M. K. Chaudhury; R. J. Good, Interfacial Lifschitz-van der Waals and Polar Interactions in Macroscopic Systems. In: J. Chem. Rev. 88 (1988), P. 927-941.