Surfactants are so-called surface-active substances, as they reduce the surface tension of liquids or the interfacial tension of two-phase systems due to adsorption at the surface or interface respectively.
A common structural feature of most surfactants is the presence of at least one (non-polar) hydrophobic region and at least one (polar) hydrophilic region. Due to the different chain lengths in many surfactants, reference is often made to a hydrophilic "head" and a hydrophobic "tail".
When adsorption takes place at the interface, the region with less affinity to the bulk phase protrudes from the interface, while the region with greater affinity aligns itself towards the volume phase. Surfactants are most frequently used as the bulk phase in water so that the hydrophobic group is the one pointing outwards.
Surfactants are categorized according to the nature of the hydrophilic group:
- Anionic surfactants: Bonds with an anionic group (e.g. carboxylates or sulphonates), often in the form of alkaline metal salt.
- Cationic surfactants: Bonds with a cationic group, e.g. quaternary amines.
- Ampholytic surfactants: Bonds with an anionic and a cationic group; frequently carboxylate and quaternary amine group.
- Non-ionic surfactants: Bonds with non-ionic, polar groups such as alcohol, ether or ethoxylate.
Surface tension comes about, because, from an energy point of view, the presence of liquid molecules is preferred in the volume phase rather than at a surface or interface. In contrast, due to the structure of surfactants, presence of the liquid molecules at the interface is more beneficial, and therefore the work required to form the surface (= surface tension) is reduced when surfactants are present.
When the surface is fully overlaid with surfactant molecules, aggregates, so-called micelles, form in the volume phase. Micelles can incorporate substances which are not soluble in the bulk liquid (e.g. oil in water). The action of surfactants is based on this, e.g. for washing.
The transition concentration at which micelles start to form is described as the critical micelle concentration (CMC). The CMC is an important characteristic for surfactants.
Surfactants are of tremendous importance in industry. They are used wherever the contact or mixing between different phases is to be improved: in cleaning, wetting and coating, in emulsification, dispersion and flooding. Surfactants are also responsible for the formation of foam.
The interfacial and surface-active effect of surfactants and the CMC are measured with the help of tensiometers. The effects of the addition of surfactants on the wetting of solid surfaces are investigated by measuring the contact angle.