Polar part

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.

Background

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.