Cationic Surfactants

Cationic surfactants are primarily nitrogen-containing organic amine derivatives. Because the nitrogen atoms in their molecules contain lone pairs of electrons, they can bond with hydrogen atoms in acid molecules via hydrogen bonds, giving the amino group a positive charge. Therefore, they exhibit good surface activity in acidic media; however, they easily precipitate and lose their surface activity in alkaline media. Besides nitrogen-containing cationic surfactants, there are also a small number of cationic surfactants containing elements such as sulfur, phosphorus, and arsenic.

1. Development Status

Cationic surfactants have a relatively short history of widespread industrial use, and their demand has been growing rapidly year by year. However, due to their main applications as bactericides, fiber softeners, and antistatic agents, their usage is relatively lower compared to anionic and nonionic surfactants.

The research and development and application of cationic surfactants in my country started relatively late, but the pace of development has been rapid. In 1981, there were 18 varieties of industrial cationic surfactants, accounting for 13.5% of the total number of industrial surfactant varieties. By 1990, this number had risen to 45, accounting for 15.5%, with a total of 105 varieties including those for civilian use. However, due to the narrow application range and small usage of cationic surfactants, the output was extremely low, remaining at only a few thousand tons annually until 2002, less than 1% of the total surfactant production.

Cationic surfactants generally possess excellent emulsifying, wetting, detergency, bactericidal, softening, antistatic, and anticorrosive properties. Due to their unique properties and applications, they have great development potential. As the scope of industrial and civilian applications continues to expand, their variety and demand will continue to increase.

2 categories

Most commercially valuable cationic surfactants are derivatives of organic nitrogen compounds, with their positive ionic charge carried by nitrogen atoms. Some novel cationic surfactants also have their positive ionic charge carried by atoms such as phosphorus, sulfur, iodine, and arsenic. Based on their chemical structure, cationic surfactants can be mainly classified into four types: amine salts, quaternary ammonium salts, heterocyclic salts, and cyclohexane salts.

Amino salt type

Amine salt-type cationic surfactants are a general term for primary, secondary, and tertiary amine salt surfactants. They have very similar properties, and many products are mixtures of primary and secondary amines. These surfactants are mainly salts formed from aliphatic amines and inorganic acids, and are only soluble in acidic solutions. Under alkaline conditions, amine salts readily react with alkalis to form free amines, reducing their solubility and thus limiting their application.

Quaternary ammonium salt type

Quaternary ammonium salt cationic surfactants are the most important type of cationic surfactant, and their properties and preparation methods differ from those of amine salts. These surfactants are soluble in both acidic and alkaline solutions, possess a range of excellent properties, and exhibit good compatibility with other types of surfactants, thus enjoying a wide range of applications.

Heterocyclic

The heterocycles contained in cationic surfactant molecules are mainly nitrogen-containing morpholine rings, pyridine rings, imidazole rings, piperazine rings, and quinoline rings.

Rosal type

Cationic surfactants can be classified according to the different heteroatoms they contain, such as N, P, As, S, and I, but this classification method is rarely used. Based on the different atoms carrying positive charges, cationic surfactants also include sulfonium salts, iodosulfonium salts, and iodosulfonium and iodosulfonium salt compounds.

3 Synthesis

The main reaction for synthesizing cationic surfactants is the N-alkylation reaction, in which the reaction of tertiary amines with alkylating agents to form quaternary ammonium salts is also called the quaternization reaction.

Alkyl quaternary ammonium salts

Alkyl quaternary ammonium salts are an important type of quaternary ammonium cationic surfactant, widely used as bactericides, fiber softeners, mineral flotation agents, and emulsifiers. Their structural characteristic is the presence of four alkyl groups attached to the nitrogen atom; that is, all four hydrogen atoms of the ammonium ion ⁽ NH₄⁺ ₎ are replaced by alkyl groups. Typically, only one or two of these alkyl groups are long-chain hydrocarbon alkyl groups, while the remaining alkyl groups have one or two carbon atoms. Based on their structural characteristics, there are three main methods for synthesizing alkyl quaternary ammonium salts: reaction of higher alkyl halides with lower tertiary amines, reaction of higher alkylamines with lower alkyl halides, and the formaldehyde-formic acid method.

Quaternary ammonium salts containing heteroatoms

The quaternary ammonium salts containing heteroatoms mentioned here generally refer to quaternary ammonium salts whose hydrophobic hydrocarbon chains contain heteroatoms such as O, N, and S, which are surfactants whose lipophilic groups contain amide bonds, ether bonds, ester bonds, or thioether bonds. Because the hydrophilic quaternary ammonium cation and the alkyl hydrophobic group are linked through amide, ester, ether, or thioether groups, rather than being directly connected, these quaternary ammonium salts are also called indirectly linked cationic surfactants.

Quaternary ammonium salts containing benzene rings

These surfactants are mainly used as bactericides, foaming agents, wetting agents, and dye fixing agents. In the synthesis process, the main method for introducing aromatic rings is to react benzyl chloride as an alkylating agent with a tertiary amine. Benzyl chloride is prepared by the side-chain chlorination of toluene. To avoid chlorination of the benzene ring, this reaction must be carried out in an enamel-lined kettle or a glass-lined tower reactor.

heterocyclic quaternary ammonium salts

The heterocycles contained in quaternary ammonium salt molecules are mainly morpholine rings, piperazine rings, pyridine rings, quinoline rings, and imidazole rings.

Amino salt type

Amino salt cationic surfactants are mainly classified into three categories: long-chain alkyl primary amine salts, secondary amine salts, and tertiary amine salts.

4 Applications

Cationic surfactants possess excellent bactericidal, softening, antistatic, and anticorrosive properties, as well as certain emulsifying and wetting properties, and are often used as phase transfer catalysts. However, these surfactants are rarely used alone as detergents because many substrates typically carry a negative charge on their surfaces in aqueous solutions, especially alkaline solutions. During application, the positively charged surfactant will form an arrangement of hydrophilic groups facing inward and hydrophobic groups facing outward on the substrate surface, making the substrate surface hydrophobic and hindering washing, or even producing negative effects. Furthermore, unlike other surfactants, the primary application of these surfactants is not to reduce surface tension, but rather to utilize their structural characteristics for other specific purposes.