The cohesion of atoms and molecules is determined by various interactions. This also applies to atoms and molecules at the interface. The interfacial energy of an interface can be described as consisting of polar and dispersion force interactions. The ratio of polar and dispersion components influences the wetting and adhesion of two phases.

Dispersion and polar interactions between molecules

The cohesion of atoms and molecules, which determines the interfacial energy of a substance, is due to different types of interactions. A distinction can be made between dispersion and polar interactions.

The interactions due to temporal fluctuations in the charge distribution of the atoms or molecules are called dispersion force or Van-der-Waals-interactions. Polar interactions include Coulomb-interactions between permanent dipoles, and between permanent and induced dipoles. An example of polar interactions are hydrogen bonds, such as those formed by water molecules.

How is the interfacial energy composed of polar and dispersion components?

The interfacial energy σ can be understood as being composed of a dispersion component σ^d and a polar component σ^p:

Dispersion force interactions occur in all atoms and molecules. However, there are substances that have no polar groups. Their interfacial energy is therefore purely made of dispersion components. Examples for such substances, whose interfacial energy consists of only dispersion force interactions, are alkanes composed of hydrocarbon chains.

Influence of the polar and dispersion components on adhesion

Predictions about the adhesion of two phases can be made if the ratio of dispersion to polar components are compared. The more similar the ratio of the dispersion and polar components, the higher the interactions between the phases (see Figure 2). In this case, a stronger adhesion between the two phases can be expected.

How surface activation influences the polar component of the surface energy

Polymers have a very low surface energy, as the polar part of the surface energy is low or non-existent. Furthermore, impurities on the surface can lower the surface energy. As a result, plastics are difficult to print, wet, or bond.

Therefore, various methods are used in industry to increase the surface energy of polymers, or more precisely: the polar part of the surface energy. Such methods are often referred to as surface pre-treatment or surface activation. Examples are flame treatment, coating, or plasma treatment of the surface.