The polar part is that part of the surface free energy of a solid or the surface tension of a liquid which is due to polar interactions. When the polar part and the dispersive part of the surface tension of liquids are known, the surface free energy of a solid can be calculated by measuring the contact angle using different models.
Non-covariant, polar interactions occur in molecules with a dipole moment. These are molecules with a permanent inequality of the electron density due to different electronegativities of the bonding partners while at the same time the molecule is asymmetrical (e.g. water). Molecules with a dipole moment can form polar interactions with one another.
The surface tension ;σ is based on cohesive interactions (work of cohesion) within a phase, which, according to Owens, Wendt, Rabel and Kaelble and also Wu, are dispersive ;(σD) or polar ;(σP). In each case the sum of the parts makes up the total surface tension.
When there is contact with a second phase, the extent of adhesive interactions (work of adhesion ;WA) depends on whether similar interactions can be formed with the adjacent phase. This can be seen from the equation according to Owens, Wendt, Rabel und Kaelble for example:
In the extended Fowkes method for determining the surface free energy, discrimination is also made between hydrogen bonds and other polar interactions. Oss & Good describe polar interactions in accordance with the Lewis acid-base model.
In the models mentioned, the combination of polar and dispersive parts does not contribute to the adhesive interactions.