Cationic surfactants are mainly nitrogen-containing organic amine derivatives. Because the nitrogen atom in the molecule contains a lone pair of electrons, it can combine with the hydrogen in the acid molecule through hydrogen bonds to make the amino group positively charged. It has good surface activity in acidic medium; it is easy to precipitate and lose surface activity in alkaline medium. In addition to nitrogen-containing cationic surfactants, there are also a small number of cationic surfactants containing elements such as sulfur, phosphorus, and arsenic.
Commercially valuable cationic surfactants are mostly derivatives of organic nitrogen compounds, and their positive ion charges are carried by nitrogen atoms. There are also some new cationic surfactants whose positive ion charges are carried by atoms such as phosphorus, sulfur, iodine, and arsenic. According to the chemical structure of cationic surfactants, they can be mainly divided into four types: amine salt type, quaternary ammonium salt type, heterocyclic type and laminium salt type.
Amine salt type natural cationic surfactant is a general term for primary amine salt, secondary amine salt and tertiary amine salt surfactant. Their properties are very similar, and many products are mixtures of primary and secondary amines. These surfactants are mainly salts of fatty amines and inorganic acids, which are only soluble in acidic solutions. However, under alkaline conditions, amine salts are prone to react with alkalis to generate free amines and reduce their solubility, so the scope of use is limited.
Quaternary ammonium salt type cationic surfactant is the most important type of cationic surfactant, and its properties and preparation methods are different from those of amine salt type. This type of surfactant is soluble in both acidic and alkaline solutions, has a series of excellent properties, and has good compatibility with other types of surfactants, so it has a wide range of applications.
The heterocycles contained in cationic surfactant molecules are mainly nitrogen-containing morpholine rings, pyridine rings, imidazole rings, piperazine rings and quinoline rings.
Cationic surfactants can be divided into surfactants containing N, P, As, S, l and other elements according to the difference of heteroatoms, but this classification method is rarely used. According to the different atoms carrying the positive charge, cationic surfactants also include sulfonium salts, sulfonium salts, iodine sulfonium salts, and sulfonium salt compounds.