Anionic surfactant
An anionic surfactant is a surfactant whose water-attracting head group carries a negative charge in water. Sulfates, sulfonates and soaps are common examples, and anionic surfactants are the most widely used class in cleaning products.
What is an anionic surfactant?
Surfactants are classified by the charge on their head group. In an anionic surfactant, the head group is negatively charged when the molecule is dissolved in water, balanced by a positive counterion such as sodium.
Common examples
- Sulfates, such as sodium lauryl sulfate and sodium laureth sulfate; see sulfate-free surfactant.
- Sulfonates, such as linear alkylbenzene sulfonate, one of the highest-volume synthetic surfactants, used widely in laundry detergents.
- Carboxylates, including soaps, which are salts of fatty acids.
- Phosphate esters.
Properties and uses
Anionic surfactants are generally strong cleaners and good foamers, and they are relatively inexpensive, which is why they are the backbone of laundry, dishwashing and personal cleansing products. They adsorb readily on positively charged surfaces and lift particulate soil.
Their main weakness is sensitivity to water hardness: calcium and magnesium ions can bind to the negative head groups, reducing performance and, with soaps, forming insoluble scum. Formulators counter this with builders and chelating agents, or by blending with nonionic and amphoteric surfactants. Anionic surfactants generally should not be combined with cationic surfactants, because the opposite charges can make them precipitate.
Frequently asked questions
Yes. Soap is the sodium or potassium salt of a fatty acid, and its carboxylate head group is negatively charged.
Their charged head groups repel each other at the bubble surface, which helps stabilize thin liquid films, so many anionic surfactants produce abundant, stable foam.
Sources
- Surfactants and Interfacial Phenomena, 4th edition, M. J. Rosen and J. T. Kunjappu, Wiley, 2012
- surfactant, IUPAC Compendium of Chemical Terminology (Gold Book)