Reduction of the surface and interfacial tension is one of the most commonly measured properties of surfactants in solution. Measurements of the surface or interfacial tension of liquid systems is accomplished readily by a number of methods. The static or quasi-static methods allow reaching the equilibrium state of a liquid interface and hence are able to measure the equilibrium tension.
With the tensiometer STA-1 one can measure the static surface tension of liquids. Pure liquids, aqueous surfactant solutions, or liquids which contain soluble substances are suitable for such measurements. The ring tensiometry (Ring tensiometry after du Noüy) is the most frequently used technique to measure the static surface tension of pure liquids and solutions.
A thin platinum wire ring is inserted below the interface and held horizontal. Then the ring is pulled up through the surface and the force is measured continuously by a balance. During the pull out of the ring the measured force increases, run through a maximum and decreases. The force maximum corresponds with the surface tension σ of the liquid under study.