Surface Active Agents

2.14.1 Cationic Surfactants. Cationic surfactants are quaternary ammonium or pyri-dinium salts that are ionic in water and have surface-active properties. These properties are associated with the cationic head, which has high affinity for water, and a long hydrocarbon tail, which has high affinity for lipids. Cationic surfactants show potent activity against Gram-positive bacteria and lower activity against Gram-negative bacteria (71). Pseudomonas species and Mycobacterium tuberculosis are resistant.

The mechanism of action of cationic surfactants is association with cell wall protein followed by penetration and disruption of the cell membrane. The resistance of Gram-negative bacteria is attributed to difficulty in penetrating the outer membrane (72). There is no activity against bacterial spores. Desirable features of cationic surfactants include water solubility, low toxicity, relatively good tissue penetration, and freedom from stains and corrosion. Disadvantages include inactivation by anionic surfactants (all traces of soap must be removed from skin), reduced effectiveness in the presence of blood serum and pus, strong adsorption on fibrous material such as cotton, occasional allergic responses on prolonged use, and resistant organisms.

The important surfactants are benzalko-nium chloride, benzethonium chloride, methybenzethonium chloride, and cetylpyri-dinium chloride. Benzalkonium chloride is a mixture of alkylbenzyldimethylammonium chlorides having the general formula [C6H5CH2N(CH3)2R]+ CI", where R is a mixture of alkyl groups of which C12H25, C14H29, and C16H33 are the main components. It is used as an antiseptic for skin and mucosa in concentrations of 1:75 to 1:20,000. Other uses include irrigation and disinfection of surgical instruments. Benzethonium chloride (39) is also is used as a skin disinfectant and irrigant of mucous membranes. Methylbenzethonium chloride (Diaperene, 40) is used specifically for control of diaper rash in infants caused by Bacterium ammoniagenes, a species that liberates ammonia.

Cetylpyridinium chloride (1-hexadecylpyri-dinium chloride, 41) has its positively charged nitrogen as part of a pyridine ring. It finds use as a general anesthetic, irrigant for mucous membranes, and as a component of mouth-washes and lozenges.

Structure-activity relationships have been

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