As a result of their adsorption at the surface or interface, surfactants bring about a reduction in the SFT or IFT. After a certain time, which depends on the system, thermodynamic equilibrium is established between the surface concentration (surface excess concentration) of the surfactant and the concentration in the volume phase. The static surface tension is the value measured in this state.
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. In 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).
- Plate method according to Wilhelmy: The force acting on an optimally wettable plate which is immersed vertically in the liquid is measured in this method.
- Rod method: As the plate method, wherein a cylindrical rod with a smaller wetted length is used for measurement with a smaller liquid volume.
- Ring method according to Du Noüy: The force acting on an optimally wettable ring as a result of the tension of the withdrawn liquid lamella when removing the ring is measured in this method. The method is only quasi-static, as an equilibrium value is only measured if the lamella is stretched sufficiently slowly.
- Spinning drop method: A horizontal capillary filled with a bulk phase and a drop phase is set in rotation. The diameter of the drop which is elongated by centrifugal force correlates with the interfacial tension.
- Pendant Drop method: The shape of a drop suspended from a needle is determined from the surface tension and the weight of the drop. The surface tension can be ascertained from the image of the drop using drop shape analysis.