Interfacial tension

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The interfacial tension describes mechanical tensions and thus forces that occur at the boundary between two different phases that are in contact with each other. The two phases form a common interface that is under interfacial tension. Under various phases is understood to be phases that do not mix, such. B. water and oil or glass and water.

The phases can be liquid, solid or gaseous. Interfacial tension between liquids and gas phases is usually referred to as surface tension . For interfacial tension in solids, see elastic interfacial tension .

Phenomena

The interfacial tension describes the reasons why ...

  • falling water or water on panes of glass disintegrates into drops ;
  • the surface of a liquid in a test tube can show a curve (→  meniscus );
  • Liquids in a glass tube can rise a little if one end of the tube is immersed vertically in the liquid (→  capillarity );
  • some insects can walk over the water (→  water strider );
  • the water can run off a light, thin rain jacket (→  wetting ).

Interfacial tension is "interfacial energy" or "interfacial work"

The interfacial tension is a mechanical tension in the interface with forces that can cause a reduction in the interface. Deviating from the other definition of mechanical tension, it is given as force per length , e.g. B. in the SI units  N / m (see surface tension #Physical background ).

At the same time, as interface work or energy, it describes the energy that has to be converted in order to enlarge the interface by 1 m 2 under isothermal conditions. Here too, the definition of work per area results in the unit N / m or kg / s 2 .

Interfacial tension means that work has to be done to enlarge the interface and that energy is released as the area decreases. For these energetic reasons, a system like water / air strives for the smallest possible interface: the water does not “voluntarily” take on the shape of a plate, but forms droplets.

Phase and phase boundary

Fig. 1: Directions of intermolecular forces in the phase and on the surface .

Within a phase, the forces act in all spatial directions between the particles that form the phase ( cohesion ). The particles can be molecules, metal atoms or the ions of a salt. In the interior of the phase, the forces cancel each other out. This is not the case at the phase boundary. There are no neighbors here who belong to their own phase.

In a liquid phase of water (Fig. 1), dipole-dipole moments act on the molecules in all spatial directions. This is not the case at the phase boundary (edge ​​of a droplet). A water molecule on the edge has far fewer neighbors. A water molecule that moves from inside the phase to the phase boundary must have the energy to overcome part of the dipole-dipole moments. If it moves in the other direction, corresponding energy is released. In order to enlarge the area of ​​the phase boundary, energy has to be expended, since more particles are now required to form the larger area. Therefore, water strives for a minimal surface and thus forms droplets.

Direction of interfacial tension

Fig. 2: Soap bubble
Fig. 3: Foam bubbles between two glass plates

In the case of a soap bubble (Fig. 2), the "bubble skin" phase is delimited on both sides by a gas phase. The two interfaces of the "bubble skin" tighten the inner gas phase. The forces act in the direction of the expansion of the bladder skin. This direction becomes clearer by looking at Fig. 3, which shows a cross-section through foam . Due to the direction of the interfacial tension, the “bladder membranes” have the shortest connecting lines (straight lines).

Liquid-gas-wall interface

Fig. 4: Wetting a wall

If a gas-liquid interface touches a solid wall, a certain angle is established between the wall and the surface of the liquid. Fig. 4 shows this contact angle for a case of a vertical wall that can be easily wetted. The stronger the wetting , the smaller the angle and the higher the upper edge of the liquid rises. This behavior in narrow tubes is called the capillary effect. The degree of wetting depends on the type of liquid, the material of the surface and its nature, e.g. B. their roughness .

Influenceability

The size of the interfacial tension can be influenced by:

Thermoplastic polymers are immiscible with one another in the molten state. Compatibilizers lower the interfacial tension between the phases in copolymers and reduce the phase separation and agglomeration of the different basic material molecules.

Individual evidence

  1. ↑ a b Ralph-Dieter Maier, Michael Schiller: Handbuch Kunststoff Additive. ISBN 978-3-446-43291-8 , p. 21 ( limited preview in Google book search).