Dynamic surface tension

The dynamic surface tension (SFT) or interfacial tension (IFT) is the value of the SFT or IFT referred to a particular surface age or interface age. In the case of liquids with surface-active substances (surfactants), this can differ from the equilibrium value.

Background

As a result of their adsorption at the surface or interface, surfactants bring about a reduction in the SFT or IFT. Immediately after the surface is produced, the SFT or IFT has the same value as the pure liquid(s). The value then reduces until an equilibrium value is reached. The time required for this depends on the diffusion rate and the adsorption rate of the surfactant.

There is a difference between the dynamic SFT/IFT and the equilibrium value on the one hand and dynamic and static methods on the other. With a dynamic tensiometer (e.g. drop volume tensiometer or bubble pressure tensiometer), the size of the interface changes during the measurement. In spite of this, if the change is sufficiently slow, an equilibrium value can often be measured as well as the time-dependent values. In return, the establishment of the equilibrium value with respect to time can also be pursued using static methods in which the size of the interface does not change (e.g. Wilhelmy plate method).

Evaluation of high-speed processes and quality control of surfactant-containing baths

Interfaces are produced extremely quickly in processes such as spraying, foaming, cleaning, printing, emulsifying or coating. In such processes it is not just the equilibrium value of the surface tension that is the decisive influence, but also the kinetics of the interface formation. The molecular mobility of the surfactants used becomes an important factor in the formation of the SFT or IFT.

Measurements of the dynamic surface tension with a bubble pressure tensiometer are also used for an indirect determination of the concentration of cleaning or wetting agents in industrial baths, where the surfactant concentration is usually above the CMC. In a certain surface age range, the dynamic SFT strongly correlates with the surfactant concentration and can thus be used to determine the content of surface-active substances.

Measuring methods

• Bubble pressure method: The maximum internal pressure of a gas bubble which is formed in a liquid by means of a capillary is measured.
• Drop volume method: The volume of a drop of liquid produced at a vertical capillary is measured at the moment of its detachment. This method is mainly used for measuring the interfacial tension.
• Interfacial rheology: Measurement of the progression of the SFT or IFT on a drop, the surface area of which changes in sinusoidal cycles. The measurement reveals the viscoelastic behavior of the surface or interface of a surfactant solution, which often correlates to the stability of foams or emulsions. The analysis is carried out using drop shape analysis on a pendant drop or by measuring the Laplace pressure on a spherical drop.