Introduction
Measurement of surface and interfacial tension can be performed with a force tensiometer. These instruments are based on measuring the forces exerted on a probe that is positioned at the liquid-gas or liquid-liquid interface.
Surface tension acts to contract and minimize any liquid surface. If a surface is being enlarged by the formation of a liquid lamella, as a test body, such as a Wilhelmy plate or Du Noüy ring, is in contact with it, the surface tension exerts a tensile force that can be measured. Within a tensiometer this force is measured by a high precision weighing system and, hence, the surface tension of the liquid can be calculated.
Using a tensiometer, also the interfacial tension between two liquids can be measured with the help of a test body. Since tensiometry is based on force measurements it is – in contrast to the optical analysis of pendant drops – not necessary that one of the liquids is transparent or that the refractive index differs between the two liquids. Therefore, tensiometry is in many cases a handy alternative to the optical measurement of surface and interfacial tensions.
Wetting phenomena
With tensiometry, it is also possible to investigate the wetting of and dynamic contact angles on solid samples that are used as the test body. For this purpose, the sample is dipped into a liquid with known surface tension and pulled back out again. In this case, the weighing system detects both the buoyancy of the body and the lamella weight. During the evaluation, the buoyancy contribution is removed from the data so that the advancing angle can be determined for the immersion and the receding angle for the withdrawal based on the Wilhelmy equation. In addition, the advancing contact angle of powders and fiber bundles can be determined by the Washburn method.
Surfactants and CMC
Interfacially active compounds like surfactants consist of a hydrophilic and a hydrophobic part, hence they adsorb preferably at interfaces. There they reduce the surface or interfacial tension, respectively, which is essential for good washing performance or the stability of foams. The amount of surfactant molecules that can be accommodated on a certain surface area is limited. The only possibility for ‘excess’ surfactant molecules to shield their hydrophobic parts from water is by forming micelles.
The characteristic surfactant concentration that, when reached, initiates the formation of micelles is called “critical micelle concentration” (CMC). It can easily be determined with a measurement series that varies the surfactant concentration: below the CMC the surface tension decreases with increasing concentration because more and more surfactant molecules adsorb at the surface. Above the CMC, further added surfactant serves only to form micelles and the surface tension stays constant.
Wide field of application
Other tensiometric methods, like the determination of liquid or solid density, utilize the measured buoyancy. Specialized sample holder sets for this purpose. Other specific test bodies and accessory modules can be used in order to investigate sedimentation and penetration properties, adhesion, or surface pressure.
Related products
DataPhysics Dynamic Contact Angle Measuring Devices and Tensiometer (DCAT)
The universal measuring devices for the force-based measurement of interfacial parameters and phenomena
