Steartrimonium Chloride - Cationic Surfactant CAS 112-03-8

Steartrimonium Chloride (Octadecyltrimethylammonium Chloride) - Properties, Applications, and Safety

Steartrimonium chloride (CAS 112-03-8), also known as octadecyltrimethylammonium chloride, trimethylstearylammonium chloride, or STAC, is a quaternary ammonium compound belonging to the family of cationic surfactants. It features a long hydrophobic C18 alkyl chain (stearyl group) attached to a positively charged quaternary ammonium head group, which provides strong substantivity to negatively charged surfaces such as hair keratin, skin, and textile fibers. This long-chain structure enhances water repellency, softening properties, and conditioning performance compared to shorter-chain quaternary ammonium compounds.

Chemical and Physical Properties

Steartrimonium chloride has the molecular formula C₂₁H₄₆ClN and a molecular weight of approximately 348.06 g/mol. Key properties include:

• Appearance: White to off-white powder, flakes, or paste

• Density: Approximately 0.91 g/mL at 20°C

• Boiling Point: Approximately 249°C at 101,325 Pa

• Flash Point: Approximately 180°C (open cup)

• HLB Value: 15.7 (hydrophilic-lipophilic balance)

• Surface Tension (0.1% solution): 34 × 10⁻³ N/m

• pH (0.05% aqueous solution): 5.5–8.5 at 20°C

• Solubility: Soluble in hot water (above approximately 50°C), alcohols, and organic solvents; limited solubility in cold water

• CMC (Critical Micelle Concentration): Approximately 5.4 × 10⁻⁵ mol/L at 25°C

Mechanism of Action

As a cationic surfactant, steartrimonium chloride carries a positive charge that enables strong electrostatic adsorption to negatively charged surfaces. When applied to hair, it forms a protective hydrophobic film that coats the hair shaft, reduces friction between fibers, neutralizes static electricity, and improves wet and dry combability. On skin and fabrics, it imparts a soft, smooth feel by reducing surface friction. The long C18 alkyl chain provides enhanced substantivity and superior anti-static performance compared to shorter-chain analogues.

Applications

Hair Care: Steartrimonium chloride is widely used in conditioners, hair masks, leave-in treatments, and rinse-off products as a conditioning agent, anti-static agent, and glazing agent. It improves softness, shine, and manageability, reduces frizz and static, and protects against mechanical damage. It is also used in shampoos to provide conditioning benefits.

Skin Care: Functions as an emulsifier and mild conditioning agent in creams, lotions, and moisturizers.

Textile and Leather Processing: Used as a fabric softener, anti-static agent, and finishing agent in textile and leather processing.

Coatings and Polishes: Applied as an emulsifier and surface modifier in polishes, coatings, and industrial formulations.

Preservative: At low concentrations, steartrimonium chloride may function as a preservative in cosmetic formulations.

Safety and Toxicology

The Cosmetic Ingredient Review (CIR) Expert Panel has evaluated the safety of steartrimonium chloride and concluded that it is safe for use in rinse-off products and safe for use in leave-on products at concentrations up to 0.25%. When used primarily as a preservative, the maximum concentration is limited to 0.1%. At higher concentrations, it may cause mild skin and eye irritation. The compound is not considered genotoxic and has low sensitization potential. It is incompatible with anionic surfactants, with which it may form precipitates.

Environmental Profile

Steartrimonium chloride is classified as very toxic to aquatic organisms and may cause long-term adverse effects in the aquatic environment. It is not readily biodegradable and should be handled with care to prevent environmental release.

Storage and Handling

Steartrimonium chloride should be stored in tightly sealed containers in a cool, dry place, protected from moisture and strong oxidizing agents. It should be kept away from heat and ignition sources. Use appropriate personal protective equipment (gloves, safety goggles, protective clothing) when handling. In case of skin or eye contact, rinse thoroughly with water.

Conclusion

Steartrimonium chloride (CAS 112-03-8) is a versatile and effective cationic surfactant with a long C18 alkyl chain that provides superior conditioning, antistatic, and softening properties. It is widely used in hair care, textile processing, and coatings, and is considered safe for use in cosmetic products at recommended concentrations. Its stability and compatibility with cationic and nonionic systems make it a valuable ingredient in modern formulations. Ongoing research and development continue to optimize its applications while addressing environmental considerations.