Surfactants lower the surface tension of an aqueous solution and are used as wetting agents, detergents, emulsifiers, antiseptics, and disinfectants. As antimicrobials, they alter the energy relationship at interfaces. Based on the position of the hydrophobic moiety in the molecule, surfactants are classified as anionic or cationic.
Soaps are dipolar anionic detergents with the general formula RCOONa/K, which dissociate in water into hydrophilic K+ or Na+ ions and lipophilic fatty acid ions. Because NaOH and KOH are strong bases (whereas most fatty acids are weak acids), most soap solutions are alkaline (pH 8–10) and may irritate sensitive skin and mucous membranes. Soaps emulsify lipoidal secretions of the skin and remove, along with most of the accompanying dirt, desquamated epithelium and bacteria, which are then rinsed away with the lather. The antibacterial potency of soaps is often enhanced by inclusion of certain antiseptics, eg, hexachlorophene, phenols, carbanilides, or potassium iodide. They are incompatible with cationic surfactants.
Cationic detergents are a group of alkyl- or aryl-substituted quaternary ammonium compounds (eg, benzalkonium chloride, benzathonium chloride, cetylpyridinium chloride) with an ionizable halogen, such as bromide, iodide, or chloride. The major site of action of these compounds appears to be the cell membrane, where they become adsorbed and cause changes in permeability. The activity of older quaternary ammonium compounds is reduced by hard water and by porous or fibrous materials (eg, fabrics, cellulose sponges) that adsorb them. They are also inactivated by anionic substances (eg, soaps, proteins, fatty acids, phosphates). Therefore, they are of limited value in the presence of blood and tissue debris. However, newer dialkyl quaternary ammonium compounds (fourth generation, including dodecyl dimethyl ammonium bromide, dioctyl dimethyl ammonium bromide, etc) purportedly remain active in hard water and are tolerant of anionic residues. Fifth-generation quaternaries are mixtures of the fourth generation with the second generation and demonstrate greater biocidal activity under conditions of high soil load, making them useful disinfectants in barns and footbaths. Quaternary ammonium compounds are effective against most bacteria, enveloped viruses, some fungi (including yeasts), and protozoa but not against nonenveloped viruses, mycobacteria, and spores. Aqueous solutions of 1:1,000 to 1:5,000 have good antimicrobial activity, especially at slightly alkaline pH, and are commonly used for disinfection of noncritical instruments and hard surface cleaning. When applied to skin, they may form a film under which microorganisms can survive, which limits their reliability as antiseptics. Concentrations >1% are injurious to mucous membranes.
Octenidine dihydrochloride is a cationic surfactant used increasingly in Europe as an alternative to quaternaries, chlorhexidine, and iodophores for skin, mucous membrane, and wound antisepsis.