In a broad sense, "amphoteric surfactant" is a collective term encompassing both the narrower definitions of "amphoteric surfactant" and "amphoteric ionic surfactant," referring to surfactants whose molecular structure simultaneously possesses two or more hydrophilic groups: anionic, cationic, and nonionic. Amphoteric surfactants specifically refer to surfactants whose molecular structure contains one or more positive and negative charge centers (or dipole centers) connected by bridging chains (hydrocarbon chains, fluorocarbon chains, etc.).
It is a mild surfactant. Unlike single anionic or cationic surfactants, amphoteric surfactant molecules possess both acidic and basic groups at one end of the molecule. The acidic group is mostly a carboxyl, sulfonic acid, or phosphate group, while the basic group is an amino or quaternary ammonium group. They can be miscible with anionic and nonionic surfactants and are resistant to acids, alkalis, salts, and alkaline earth metal salts.
Lecithin in egg yolks is a natural amphoteric surfactant. Currently commonly used synthetic amphoteric surfactants mostly have a carboxylic acid group as their anionic moiety, with a few having a sulfonic acid group. Most of these surfactants have cationic moieties that are amine or quaternary ammonium salts. Those with amine salts forming the cationic moieties are called amino acid type; those with quaternary ammonium salts forming the cationic moieties are called betaine type.
All three types of surfactants are readily degraded under aerobic conditions, while amide betaine and amphoteric imidazolines are still readily degraded under anaerobic conditions. Toxicity studies on Cyclocarya paliurus and Daphnia magna showed that the half-maximal inhibitory concentrations (EC50) of all three types of surfactants were higher than 5 mg/L. The EC50 of amphoteric imidazoline surfactants was in the range of 20 mg/L to 200 mg/L, indicating lower toxicity to aquatic organisms.
