Foam (aka suds, lather) occurs where air is entrapped in a surfactant solution – for example as the result of shaking. The surfactant molecules arrange themselves at the liquid/air interface, thus creating the envelopes we know as soap bubbles. During cleaning processes, this formation of foam is often desirable because it ensures, for example, that water is able to penetrate the structure of a fabric more effectively. Heavy foaming can, however, also be disruptive – in which case defoamers are used, which are also based on surfactants.
Used as wetting agents, surfactants reduce the interfacial tension between a solid and a liquid. That enables water, for example, to become more evenly distributed over a hard surface rather than coagulating into droplets. The solid is then said to have been wetted.
Surfactants can make generally mutually repellent liquids, such as water and oil, miscible in the form of an emulsion. Because of the “amphiphilic” – that is to say simultaneously “water-attracting” and “water-repelling” – nature of surfactants, they are able to penetrate the oil with their fat-dissolving portion. The oil droplets caused e.g. by stirring, remain finely distributed through the action of the hydrophilic component of the molecule.
A further action mode of surfactants lies in their ability to promote the detachment of small solid particles from solid surfaces, as in the removal of dirt from textiles. Surfactants support the formation and retention of a suspension by becoming deposited around the solid particles, thus inhibiting their agglomeration and preventing them from sinking. Further deposition on solid surfaces which are covered by a surfactant layer is also avoided.