The interfacial energy describes the potential energy at an interface, whereas the interfacial tension describes the force per length at the interface. Using a model experiment, it can be shown that the interfacial tension of liquid-gas and liquid-liquid interfaces is equal to the interfacial energy.

What is the difference between interfacial tension and interfacial energy?

The interfacial energy ε is the potential energy stored in an interface between two phases. It indicates how much energy is needed to increase the size of the interface:

The higher the interfacial energy, the more energy or work is required to enlarge the interface. This has an effect, for example, on phenomena such as wetting.

Another concept to describe the interface is the interfacial tension σ. The interfacial tension considers the - tangentially acting - tensile force that counteracts an increase of the size of the interface. This tensile stress is defined as the force per length, acting along a line in the interface. The interfacial tension is given in the unit Newton per metre:

With the help of a model experiment, it can be shown that for liquid-liquid or liquid-gas interfaces, the interfacial energy ε is equal to the interfacial tension σ. In the literature, therefore, the same formula symbol, σ, is often used for interfacial tensions and interfacial energies.

Model experiment: interfacial energy and interfacial tension are equivalent for liquids

At the interfaces between a liquid and a gas or between two liquids, the two concepts - interfacial energy and interfacial tension - can be equated. This can be ascertained with the help of a model experiment.

For this purpose, a thin film of liquid, for example soap suds, is stretched in a U-shaped wire, to the open side of which a movable stirrup is attached (see Figure 2). An equilibrium is established between the surface forces of the liquid film and the weight of the stirrup. In order to move the stirrup by Δs and thus increase the surface area by ΔA = 2bΔs (double the rectangular area, since the film has a front and a back), the force F must be applied.

Through the liquid surface, a tensile stress acts on the yoke, which counteracts the surface enlargement. This tensile stress is also called surface tension. Against this tensile stress, the work ΔW_Stir is performed on the stirrup to increase the surface area:

For liquids, the work ΔW_Stir performed on the stirrup is equal to the work ΔW_Sur needed to increase the surface area:

This means that for liquid-gaseous interfaces, the surface tension σ and the surface energy ε are equal. The same considerations apply to a liquid-liquid interface.

Why does the equivalence of interfacial tension and interfacial energy not apply to solids?

The equivalence between interfacial energy and interfacial tension only applies to liquids, because in a liquid, the atoms or molecules can be displaced within the phase without additional work. If one were to carry out the model experiment with a solid, one would also have to take into account the work required to displace the atoms or molecules within the phase. Therefore, the equation of surface energy and surface tension is not possible for solids.