Cationic surfactants carry charges opposite to those of anionic surfactants, so they are often referred to as “inverted soaps”. Chemically, they contain 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 petrochemicals; hence, fatty amines serve as important raw materials for cationic surfactants. Cationic surfactants exhibit limited detergency, yet they feature prominent antibacterial properties and high affinity for adsorption onto hard surfaces. In cosmetics, they are commonly used as hair conditioners, bactericides, bacteriostatic agents, softeners and anti‑caries additives.
Fatty amines are organic bases in essence. They are uncharged in neutral solutions and show no cationic surface activity under such conditions, and become lipophilic at pH values above 7. Tertiary amine salts obtained through neutralization with inorganic or organic acids possess sufficient solubility and are fully recognized as effective cationic surfactants. Organic salts are generally more water‑soluble than inorganic salts. They are rarely incorporated into detergent and cleaner formulations.
Non‑quaternary cationic species are highly sensitive to pH, multivalent ions and electrolytes.
Ethoxylation of fatty amines yields ethoxylated amines. After pH adjustment, such surfactants are compatible with cationic surfactants and deliver satisfactory detergency.
Neutralization of fatty amines with salicylic acid or α‑chlorobenzoic acid can enhance their antifungal performance.
Quaternary ammonium compounds derived from fatty amines represent the most widely used class of cationic surfactants.
Quaternary ammonium salts display good stability in acidic or alkaline media (below 100 °C). Their solubility is correlated with alkyl‑chain length: longer alkyl chains correspond to lower water solubility. Monoalkyltrimethyl quaternary ammonium salts with C16‑C18 chains are sparingly soluble in water, soluble in polar solvents, and insoluble in non‑polar solvents. Dialkyldimethyl quaternary ammonium salts dissolve in non‑polar solvents but are insoluble in water.
The distinctive functions of quaternary ammonium salts stem from their adsorption onto negatively‑charged surfaces as well as their bactericidal and disinfectant effects. It should be emphasized that combination with anionic surfactants, oxides, peroxides, silicates, silver nitrate, sodium citrate, sodium tartrate, borax, kaolin, proteins and certain polymers most readily causes reduced bactericidal activity or turbidity.
The solubility of alkyl quaternary ammonium salts depends on their hydrophilic groups; more hydrophilic groups bring better water solubility. Typical preparations include 5 wt% isopropanol solutions or 10 wt% aqueous solutions with a pH range of 6‑9. They have excellent chemical stability, being resistant to light, heat, strong acids and strong alkalis, and demonstrate penetration, antistatic properties, bactericidal activity (optimal for C12‑C16 homologues) and outstanding corrosion‑inhibiting performance. Alkyldimethyl quaternary ammonium salts contain two long‑chain alkyl hydrophobic moieties. They possess favourable softening and antistatic properties together with moderate bactericidal capacity, alongside good wetting and emulsifying behaviours. They are less irritating than alkyltrimethyl quaternary ammonium salts. They exhibit cationic character under weakly acidic conditions and form non‑ionic species under neutral and alkaline conditions.
Alkyl Imidazoline Salts
Alkyl imidazolines are synthesized by reacting fatty acids with substituted ethylenediamines. Upon heating (typically 220‑240 °C), the resulting amidoethylamine converts into alkyl imidazoline.
Alkyl imidazoline is an organic monocyclic tertiary amine and acts as a moderately strong base. As a typical cationic surfactant, it adsorbs firmly onto negatively‑charged surfaces such as hair, skin, teeth, glass, paper, fibres, metals and silicon‑containing materials. Salts formed with water‑soluble acids tend to form gels at high concentrations. Salts of acetic acid, nicotinic acid, phosphoric acid and sulphuric acid are water‑soluble, whereas their long‑chain fatty‑acid salts are oil‑soluble. They show good storage stability under moisture‑proof conditions, yet crystal precipitation may occur after long‑term storage or at low temperatures; homogeneity can be restored by warming and stirring. Exposure to water or moisture triggers gradual hydrolysis that alters their functionality. Prolonged heating even up to 165 °C does not impair stability, although discoloration may take place. It is noteworthy that alkyl imidazoline, as a moderately strong base, exerts substantial irritation to skin and eyes, while alkyl imidazoline salts have markedly lower irritancy.
Post time: Aug-25-2026