What is a cationic surfactant?

Cationic surfactants are mainly nitrogen-containing organic amine derivatives, because the nitrogen atom in its molecule contains a lone pair of electrons, so it can be combined with the hydrogen in the acid molecule by hydrogen bonding, so that the amino group is positively charged. Therefore, they have good surface activity only in acidic medium; in alkaline medium, it is easy to precipitate and lose surface activity. In addition to nitrogen-containing cationic surfactants, there are a small number of cationic surfactants containing sulfur, phosphorus, arsenic and other elements.

I. Development:

As early as 1896, F. Kraft and others found that the hydrochloride of cetylamine has the property of soap foaming, but it was not until after 1928 that cationic surfactants were adopted in the industry, such as used as fibre softeners, static preventive agents, water-repellent agents, dyeing auxiliaries, mineral flotation and bactericidal agents, etc. However, its consumption is lower than that of anionic and nonionic surfactants. However, its consumption is lower than that of anionic surfactants and nonionic surfactants, and it is mainly used in special applications such as friction reduction and bactericidal effects. Cationic surfactants dissociate in water to form surface-active cations, which have the opposite charge to anionic surfactants, and are often referred to as ‘inverse soaps’. In terms of its chemical structure, it contains at least one long-chain hydrophobic group and one positively charged hydrophilic group. The long chain hydrophobic group is usually derived from fatty acids or petrochemicals. The positive charge of surface-active cations is generally carried by nitrogen atoms, but can also be carried by sulphur and phosphorus atoms, but the vast majority of the hundreds of cationic surfactants currently commercially available contain positively charged nitrogen atoms. Fatty amines are therefore important raw materials for cationic surfactants.

Cationic surfactants, like other types of surfactants, adsorb at interfaces or on surfaces, and form micelles in solution when they reach a certain concentration (critical micelle concentration, or CMC), thus, lowering the surface tension of the solvent and exhibiting surface activity. It has emulsifying, solubilising, wetting, washing and dispersing effects. The detergent effect of cationic surfactants is limited, and their bacteriostatic properties and affinity for hard surface adsorption are more prominent. Generally, hard surfaces are negatively charged, and positively charged cationic surfactants have very obvious activity on them. Cationic surfactants are easily adsorbed by human skin, hair and teeth. For example, they are mainly used in cosmetics as bactericidal and antibacterial agents, hair conditioning agents, skin softeners and anti-caries additives.

The molecular structure of cationic surfactants contains positively charged groups, such as amine groups and quaternary ammonium groups. Cationic surfactants can form ion pairs and micellar structures in aqueous solution, and have a series of special chemical and physical properties.

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II. Common cationic surfactants:

1. Quaternary ammonium salts: such as cetyltrimethylammonium bromide (CTAB), cetyltrimethylammonium chloride (CTAC). These cationic surfactants have good solubility and emulsification properties, and are often used as wetting agents, emulsifiers and bactericides. 

2. amine-based surfactants: such as cetylamine (C16-amine), cetyl dimethylamine (C16-dimethylamine), etc. They have good wetting properties. They have good wetting properties, and are often used as wetting agents and dispersants in textile, paper and dye industries.

3. Amino acetaldehyde surfactants: such as cetyl acetaldehyde amine (C16-aminoaldehyde), etc. They have good emulsifying and dispersing properties. They have good emulsifying and dispersing properties and are commonly used in dyestuffs, coatings and paints industries.

III. Main functions:

- Surface activity: cationic surfactants can reduce the surface tension of liquids, making it easier to wet solid surfaces, and improve the dispersion and wettability of liquids on solids.

- Emulsifier: Cationic surfactants can form emulsified liquids between water and oil, making oil-water mixtures more stable and easier to disperse and mix.

- Sterilants: Some cationic surfactants have good bactericidal and antimicrobial properties and are commonly used in disinfecting and cleaning products.

- Softeners: Cationic surfactants can soften fibres and increase the feel and comfort of fabrics.

- Dispersants: Cationic surfactants can disperse solid particles in water and prevent them from regrouping.

It should be noted that due to the positive charge characteristics of cationic surfactants, their interaction with anionic substances (e.g., soaps, anionic dyes, etc.) will result in precipitation or loss of surface activity, and therefore their simultaneous presence with these substances needs to be avoided when using them. In addition, cationic surfactants have a greater impact on the environment and biological toxicity, and should be used with attention to environmental protection and safe use.

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