"Amphoteric surfactant" refers to a surfactant with both anionic and cationic properties. From its structure, there are both cations and anions connected to the hydrophobic group. Its structure can be expressed as follows: See attachment
It is a mild surfactant. The amphoteric surfactant molecule is different from a single anionic or cationic type. There are both acidic and basic groups at one end of the molecule. The acidic groups are mostly carboxyl, sulfonic or phosphoric acid groups, and the basic groups are amine or quaternary ammonium groups, which can be mixed with anionic and nonionic surfactants, and are resistant to acids, alkalis, salts and alkaline earth metal salts.
Lecithin in egg yolk is a natural amphoteric surfactant. Nowadays, the commonly used synthetic amphoteric surfactants have mostly carboxylic acid groups and a few sulfonic acid groups. The cationic part is mostly amine salt or quaternary amine salt. The cationic part composed of amine salt is called amino acid type; the cationic part composed of quaternary amine salt is called betaine type.
The aqueous solution of amino acid type amphoteric surfactant is alkaline. If you add hydrochloric acid slowly while stirring, there will be no change when it becomes neutral. When slightly acidic, a precipitate is formed. If hydrochloric acid is added to become strongly acidic, the precipitate will dissolve again. This means that when it is alkaline, it behaves as an anionic surfactant, and when it is acidic, it behaves as a cationic surfactant. However, when the cationic and anionic properties are just at the equilibrium isoelectric point, the hydrophilicity becomes smaller, and precipitation is formed.
Betaine type amphoteric surfactant, the biggest feature is that it can be dissolved in acidic, neutral or alkaline aqueous solutions. There is no precipitation even at the isoelectric point. In addition, penetration, detergency and antistatic properties are also good. Therefore, it is a better emulsifier and softener.
*The isoelectric point refers to the state when the amphoteric electrolyte is ionized in a solution and the degree of ionization of acid and alkali is equal.