Amphoteric surfactant
An amphoteric surfactant is a surfactant whose head group can carry a positive charge, a negative charge or both, depending on the pH of the solution. Betaines such as cocamidopropyl betaine are common examples, prized for mildness and foam boosting.
What is an amphoteric surfactant?
An amphoteric surfactant has both acidic and basic groups in its head. Depending on pH, it can behave like a cationic surfactant, carrying a positive charge in acidic solutions, or like an anionic one in alkaline solutions. When the head carries both a positive and a negative charge at once, the molecule is described as zwitterionic, and the term is often used interchangeably with amphoteric.
Common examples
- Betaines, such as cocamidopropyl betaine, one of the most widely used amphoterics.
- Sultaines, related to betaines.
- Amphoacetates.
- Amine oxides, which behave as cationic at low pH and as nonionic at neutral and higher pH, and are often grouped with amphoterics.
Properties and uses
Amphoteric surfactants are valued less as primary cleaners than as co-surfactants:
- Mildness. Added to anionic surfactants, they reduce skin and eye irritation, which is why they are common in shampoos and body washes.
- Foam boosting and stabilization.
- Viscosity building in liquid formulations.
- Compatibility with anionic, nonionic and, in many cases, cationic surfactants.
- Performance across a wide pH range, useful in acidic and alkaline cleaners.
Frequently asked questions
Zwitterionic describes a molecule carrying positive and negative charges at the same time. Amphoteric describes a surfactant whose charge changes with pH. Many amphoteric surfactants are zwitterionic over a range of pH, so the terms are often used interchangeably.
It boosts and stabilizes foam, builds viscosity, and makes formulations based on anionic surfactants milder.
Sources
- Surfactants and Interfacial Phenomena, 4th edition, M. J. Rosen and J. T. Kunjappu, Wiley, 2012
- surfactant, IUPAC Compendium of Chemical Terminology (Gold Book)