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Dispersive & polar parts of the surface energy and surface tension

The cohesion between the atoms and molecules which causes the surface energy/tension of a substance can be explained by different types of interaction. In particular, one can differentiate between dispersive and polar interactions. Interactions caused by temporary fluctuations of the charge distribution in the atoms/molecules are called dispersive interactions (van der Waals interaction). Polar interactions comprise Coulomb interactions between permanent dipoles and between permanent and induced dipoles (e.g. hydrogen bonds). The surface energy/tension σi of component i is additively made up of dispersive σid and polar parts σip according to:

Dispersive and polar part

Because van der Waals interactions occur between all atoms and molecules there is no substance with a surface energy/tension that solely consists of a polar part. On the other hand there are substances which don't have any polar groups, such as alkanes that consist of hydrocarbon chains. Therefore their surface energy/tension is purely dispersive.

Comparing the ratio between the dispersive and the polar part of the surface energy/tension for two phases allows for a prediction of the adhesion between these two phases. The closer the ratios match the more interactions are possible between the phases and the higher the adhesion which is to be expected (see figure 1). A high potential for interaction between two phases also leads to a small interfacial energy/tension.

Figure 1: Illustration of the interactions between two phases with equal/similar (top) or different (bottom) dispersive and polar parts of the surface energy/tension

There are different models for the calculation of the interfacial energy/tension which are based on the dispersive and polar parts of the surface energies/tensions of the involved phases. These models are used to determine the polar and dispersive parts of the surface tension of a liquid. For this purpose the surface tension of the liquid and the interfacial tension between the liquid and a liquid with purely dispersive surface tension (which is known or was measured beforehand) is being measured. Additionally, the models for interfacial energy/tension can be used to determine the surface energy of a solid via contact angle measurements with different test liquids.

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