Anionic surfactants


The defining feature of the anionic surfactant is, of course, that it is an anion (i.e. a negatively charged ion). All of the soaps (the fatty acid salts) are anionic surfactants (see the section on soaps for more information).

Some common soaps: sodium oleate, sodium palmitate, sodium myristate and sodium stearate.

One of the first steps in the development of surfactants that were insensitive to metal lines was the development of the alkyl sulfate surfactants. Like the soaps, these are anionic surfactants. In fact, probably the most studied surfactant over the years is one of these alkyl sulfates: sodium dodecyl sulfate (SDS). Unlike soaps, alkyl sulfates will not precipitate in low pH solutions.

Sodium dodecyl sulfate, one of the most common of the alkyl sulfate style of anionic surfactant.

Teacher Note: Unfortunately SDS is also sensitive to hard water. A sample of SDS to which calcium chloride is added tends to form a precipitate just like soaps. Since SDS is commonly used in wool-washing blends, dishwashing detergents and in washing powder, this has interesting consequences.

Other commonly used anionic surfactants are the alkyl benzenesulfonates, alkyl sulfonates and the alkyl phosphates.

Two alkyl phosphates, an alkyl sulfonate, and an alkyl benzene sulfonate

Sulfosuccinates are similar to the alkyl sulfonates and were developed in 1939.

The sulfosuccinate surfactant sodium di(2-ethylhexyl) sulfosuccinate (sold under the name Aerosol-OT or AOT).

Uses of Anionic Surfactants

Anionic surfactants are used all over the place. They make up around 49% of all surfactants made. They are used in shampoos, in dishwashing detergents and in washing powders. In many industrial and commercial applications, anionic surfactants are no longer used on their own. Typically, they are used in conjunction with nonionic surfactants to provide even greater stability.

Long-term exposure to anionic surfactants has been linked to swelling of the skin in a conditioned allergic reaction. This swelling is temporary, although it tends to increase the susceptibility of the skin to permeation by other substances. Anionic surfactants are generally avoided in cosmetic products, but their use in shampoos and other products can still lead to irritation (that's why some people suggest changing shampoos every month or so).