Benzalkonium Chloride: Chemical Properties, pH, and Uses

Benzalkonium chloride (BAC) is a mixture of alkylbenzyldimethylammonium chlorides with varying alkyl chain lengths (C₈–C₁₈). It is a quaternary ammonium compound (QAC) and a cationic surfactant.

Acid-Base Properties

• The quaternary nitrogen bears a permanent positive charge; it has no proton to donate (no N–H). Therefore, BAC is not a Brønsted acid.
• It can act as a very weak Lewis base (electron-pair donor) via the nitrogen lone pair, but steric hindrance from four alkyl groups makes this negligible.
• Reported pK_a ~12 refers to the conjugate acid of the tertiary amine precursor, not BAC itself. In aqueous solution, BAC is a neutral salt (cation + Cl⁻); its solutions are typically pH 6–8.

Key Properties

• Antimicrobial: Disrupts microbial cell membranes (bacteria, enveloped viruses, fungi). Used as disinfectant, preservative, antiseptic.
• Surfactant: Lowers surface tension; wetting, emulsifying, foaming agent.
• Stability: Stable over wide pH and temperature ranges; inactivated by anionic surfactants (soaps), organic matter, hard water.

Common Uses

• Hand sanitizers, surface disinfectants (0.1–0.2%).
• Ophthalmic/nasal preservatives (0.004–0.01%).
• Cosmetics, personal care products (preservative).
• Industrial biocides, cooling water treatment.

Safety Note

Concentrated solutions are corrosive. Prolonged exposure may cause contact dermatitis. Avoid mixing with anionic cleaners (neutralizes both).