The charge of cationic surfactants is opposite to that of anionic surfactants, so cationic surfactants are often called “inverse soaps”. In terms of their chemical structure, they have at least one long-chain hydrophobic group and one positively charged hydrophilic group. The long-chain hydrophobic groups are generally derived from fatty acids or petroleum chemicals, so fatty amines are important raw materials for cationic surfactants. Cationic surfactants have limited detergency, but their antibacterial properties and affinity for adsorption on hard surfaces are quite prominent. In cosmetics, they are generally used as hair conditioners, fungicides, bacteriostats, softeners, and anti-caries additives, etc.
1.Fatty amine salt
Fatty amines are essentially organic bases. They are uncharged in neutral solutions, lacking cationic surface activity at this point, and are lipophilic when the pH is greater than 7. Tertiary amine salts obtained by neutralization with inorganic or organic acids have sufficient solubility and are fully recognized as effective cationic surfactants. Organic salts are generally more soluble in water than inorganic salts. They are rarely used in detergent and cleanser formulations.
Non-quaternary ammonium salt cations are very sensitive to pH, multivalent ions, and electrolytes. Ethoxylated amines, produced by ethoxylation of fatty amines, are surfactants that can be compatible with cationic surfactants after pH adjustment and have good detergency. When fatty amines are neutralized with salicylic acid or α-chlorobenzoic acid, their antifungal function can be enhanced. Quaternary ammonium compounds among fatty amines are the most widely used type of cationic surfactants.
Quaternary ammonium salts have good stability in acidic or alkaline media (below 100°C). The solubility of quaternary ammonium salts is related to the length of the alkyl chain; the longer the chain, the lower the solubility in water. C16-C18 monoalkyl trimethyl quaternary ammonium salts are poorly soluble in water, soluble in polar solvents, and insoluble in non-polar solvents. Dialkyl dimethyl quaternary ammonium salts are soluble in non-polar solvents and insoluble in water.
The special functions of quaternary ammonium salts lie in their adsorption on negatively charged surfaces and their bactericidal and disinfectant effects. It is extremely important to note that compounding with anionic surfactants, oxides, peroxides, silicates, silver nitrate, sodium citrate, sodium tartrate, borax, kaolin, proteins, and some polymers is most likely to reduce their bactericidal power or cause turbidity.
The solubility of alkyl quaternary ammonium salts is related to hydrophilic groups; the more hydrophilic groups there are, the better the water solubility. A 5% mass fraction of isopropanol or a 10% mass fraction aqueous solution has a pH of 6-9. They have good chemical stability, resistance to light, heat, strong acids, and strong alkalis, and possess permeability, antistatic properties, bactericidal effects (with C12-C16 being the most effective), and excellent corrosion inhibition. Alkyl dimethyl quaternary ammonium salts have two long-chain alkyl groups as hydrophobic groups, exhibiting good softness, antistatic properties, certain bactericidal ability, as well as good wetting and emulsifying effects. They are less irritating than alkyl trimethyl quaternary ammonium salts, show cationic properties under weak acids, and form non-ionic compounds under neutral and alkaline conditions.
2.Alkyl imidazoline salt
Alkyl imidazoline is a product formed by the reaction of fatty acids and substituted ethylenediamines. The amide ethylamine produced by heating (usually at 220~240℃) is converted into alkyl imidazoline.
Alkyl imidazoline is an organic monocyclic tertiary amine and is a medium-strong base. It is a typical cationic surfactant that can be firmly adsorbed on negatively charged surfaces, such as hair, skin, teeth, glass, paper, fibers, metals, and silicon-containing materials. Salts formed with water-soluble acids tend to form gels at high concentrations. Acetates, nicotinates, phosphates, and sulfates are water-soluble, while their long-chain fatty acid salts are oil-soluble. It has good storage stability under moisture-proof conditions, but crystals may precipitate after long-term storage or storage in low-temperature environments, and it can return to a uniform state when warmed and stirred. Contact with water or moisture will gradually cause hydrolysis, changing its functions. Long-term heating, even when the temperature rises to 165℃, will not affect its stability, but the color will change. It is worth noting that since alkyl imidazoline is a medium-strong base, it is highly irritating to the skin and eyes, while alkyl imidazoline salts have significantly reduced irritation.
Post time: Feb-25-2026