An emulsion is a mixture of substances comprising liquids which are not soluble in one another and in which no phase boundary is visible. At the same time, finally distributed droplets of one phase are present in another phase, wherein one of the phases is usually hydrophobic and the other hydrophilic.
Surfactants, as so-called emulsifiers, contribute to the production and stabilisation of emulsions. These are either of natural origin (e.g. in the case of milk) or are added to the mixture (e.g. in cosmetics). Emulsifiability describes the tendency of mixtures to form an emulsion or the effort required to achieve this.
When forming the fine droplets which are present in an emulsion, a large interface is produced between the available volume of the emulsified phase and the bulk phase. The work necessary to achieve this - in practice agitating or shaking - corresponds to the interfacial tension. Surfactants reduce the interfacial tension and give rise to smaller droplets and a reduced tendency of the droplets to flow together (coalescence) when mixing.
Microemulsions, which form spontaneously under certain conditions (surfactant content, salt content, temperature) and are stable when the conditions are maintained, are a special case.
Measurements of the interfacial tension are carried out in order to investigate the emulsifiability. Use is made of force sensor tensiometers with the Du Noüy ring method or the Wilhelmy plate method depending on the measuring range. The conditions for the formation of a microemulsion are investigated with a spinning drop tensiometer.