Amphoteric surfactants carry both a positive and a negative charge on the same head group. Which charge dominates depends on the pH of the solution, so the same molecule behaves cationically in acid, anionically in alkali, and as a neutral zwitterion in between. That flexibility, combined with an unusually low irritation profile, makes them the standard co-surfactant in personal care and mild household cleaners.
- Head group charge
- Both, pH dependent (zwitterionic)
- Typical chemistries
- Betaines, amphoacetates, sultaines, amine oxides
- Key metric
- Isoelectric point
- Primary role
- Mildness, foam boosting, viscosity building
How amphoteric surfactants work
The head group of an amphoteric surfactant contains an anionic function (usually a carboxylate or sulfonate) and a cationic function (usually a quaternary nitrogen). At low pH the acidic group is protonated and the molecule reads as cationic. At high pH it is deprotonated and the molecule reads as anionic. At the isoelectric point both charges are fully expressed and cancel, leaving a net-neutral zwitterion.
Because the molecule can present either face, amphoterics are compatible with anionic, cationic and non-ionic partners alike. They are one of the few surfactants that can sit in the same formulation as a cationic without precipitating an anionic partner, which is why they appear in almost every 2-in-1 cleansing product.
- Universal compatibility across all surfactant classes.
- Marked reduction of the irritation potential of anionic systems.
- Foam boosting and foam stabilisation at low use levels.
- Viscosity building in salt-thickened surfactant systems.
The main amphoteric chemistries
Most commercial amphoterics derive from fatty acids or fatty amines coupled to a small charged fragment.
| Alkyl betaines | Cocamidopropyl betaine (CAPB) is the dominant grade. Foam booster, thickener, irritation reducer. |
|---|---|
| Sultaines | Hydroxysultaines. Retain performance at low pH where betaines lose it. |
| Amphoacetates | Sodium cocoamphoacetate and diacetate. The mildest common class, used in baby and sensitive-skin cleansers. |
| Amine oxides | Non-ionic at neutral pH, cationic below roughly pH 3. Often grouped with amphoterics for their pH-switching behaviour. |
| Amino acid derivatives | Glycinates, alaninates, sarcosinates. Mild, biodegradable, higher cost. |
Where amphoterics fit in a formulation
Amphoterics are rarely the primary surfactant. Their economics and detergency do not support it. They are used at 20 % to 40 % of the total actives to do three jobs at once: cut the irritation of the anionic base, lift and stabilise the foam, and provide the salt-response that lets the product be thickened without a polymer.
The design decision is the isoelectric point relative to the product pH. Formulating too close to it can cause haze or reduced solubility, so a betaine that performs perfectly at pH 5.5 may need to be swapped for a sultaine in an acidic system.
Feedstock and the bio-based angle
The fatty portion of most amphoterics comes from coconut or palm-kernel oil, so the class carries the same feedstock questions as palm-derived non-ionics: certification, land-use pressure and supply concentration outside Europe.
PureSurf approaches this from the other direction, building surfactants on the PureSynth platform from renewable European side streams rather than tropical oils. NEXOVANT, the bio-based amine-oxide grade, occupies the same functional slot as an amine-oxide co-surfactant: it boosts foam, builds viscosity and reduces the anionic load, with a critical micelle concentration measured by BASF SE at 33 mg/L against 73 mg/L for the C12 to C14 amine oxide benchmark.
Frequently asked questions
What is an amphoteric surfactant?
A surfactant whose head group carries both a positive and a negative charge. Which charge dominates depends on pH: cationic in acid, anionic in alkali, and net-neutral zwitterionic at the isoelectric point.
Which surfactants are amphoteric?
Cocamidopropyl betaine (CAPB), hydroxysultaines, sodium cocoamphoacetate and cocoamphodiacetate, amino acid derivatives such as sarcosinates and glycinates, and amine oxides through their pH-switching behaviour.
What is the difference between amphoteric and zwitterionic?
Zwitterionic strictly describes a molecule carrying both charges simultaneously at a given pH. Amphoteric describes a molecule that can act as either an acid or a base depending on pH. In surfactant practice the two terms are used interchangeably.
Why are amphoteric surfactants milder?
They form mixed micelles with the anionic surfactant in the formulation. Those larger, less mobile micelles bind less strongly to skin proteins, which lowers the irritation potential of the whole system.
