All of the cationic surfactants are quaternary ammonium compounds (Table 6.3). For that reason, they are always ionized in water and exhibit surface-active properties. The compounds, with a polar head group and nonpolar hydrocarbon chain, form micelles by concentrating at the interface of immiscible solvents. The surface activity of these compounds, exemplified by lauryl triethylammonium sulfate, results from two structural moieties: (a) a cationic head group, which has a high affinity for water and (b) a long hydrocarbon tail, which has an affinity for lipids and nonpolar solvents.
At the right concentration (the critical micelle concentration), the molecules concentrate at the interface between immiscible solvents, such as water and lipid, and water-in-oil or oil-in-water emulsions may be formed with the ammonium head group in the water layer and the nonpolar hydrocarbon chain associated with the oil phase. The synthesis and antimicrobial actions of the members of this class of com
TABLE 6.3 Analogs of Dimethylbenzylammonium Chloride
pounds were first reported in 1908, but it was not until the pioneering work of Gerhard Domagk in 193513 that attention was directed to their usefulness as antiseptics, disinfectants, and preservatives.
The cationic surfactants exert a bactericidal action against a broad spectrum of Gram-positive and Gram-negative bacteria. They are also active against several pathogenic species of fungi and protozoa. All spores resist these agents. The mechanism of action probably involves dissolution of the surfactant into the microbial cell membrane, destabilization, and subsequent lysis. The surfactants may also interfere with enzymes associated with the cell membrane.
The cationic surfactants possess several other properties. In addition to their broad-spectrum antimicrobial activity, they are useful as germicides. They are highly water soluble, relatively nontoxic, stable in solution, nonstaining, and noncorrosive. The surface activity causes a keratolytic action in the stratum corneum and, hence, provides good tissue penetration. In spite of these advantages, the cationic surfactants present several difficulties. Soaps and other anionic detergents inactivate them. All traces of soap must be removed from skin and other surfaces before they are applied. Tissue debris, blood, serum, and pus reduce the effectiveness of the surfactants. Cationic surfactants are also adsorbed on glass, talc, and kaolin to reduce or prevent their action. The bactericidal action of cationic surfactants is slower than that of iodine. Solutions of cationic surfactants intended for disinfecting surgical instruments, gloves, etc. should never be reused because they can harbor infectious microorganisms, especially Pseudomonas and Enterobacter spp.
Alkylbenzyldimethylammonium chloride (Zephiran) is a mixture of alkylbenzyldimethylammonium chlorides of the general formula [C6H5CH2N(CH3)2R]+Cl~, where R represents a mixture of alkyl chains beginning with C8H17 and extending to higher homologues with C12H25, C14H29, and C16H33. The higher-molecular-weight homologues compose the major fractions. Although variations in the physical and antimicrobial properties exist between individual members of the mixture, they are of little importance in the chemistry of the overall product. Benzalkonium chloride occurs as a white gel that is soluble in water, alcohol, and organic solvents. Aqueous solutions are colorless, slightly alkaline, and very foamy.
Benzalkonium chloride is a detergent, an emulsifier, and a wetting agent. It is used as an antiseptic for skin and mucous membranes in concentrations of 1:750 to 1:20,000. For irrigation, 1:20,000 to 1:40,000 concentrations are used. For storage of surgical instruments, 1:750 to 1:5,000 concentrations are used, with 0.5% NaNO3 added as a preservative.
Benzyldimethyl[2-[2-[[4-(1,1,3,3-tetramethylbutyl) tolyl]oxy]ethoxy]ethyl]ammonium chloride (Diaparene) is a mixture of methylated derivatives of methylbenzethonium chloride. It is used specifically for the treatment of diaper rash in infants, caused by the yeast Candida albicans, which produces ammonia. The agent is also used as a general antiseptic. Its properties are virtually identical to those of ben-zethonium chloride.
Benzyldimethyl[2-[2-[^-(1,1,3,3-tetramethylbutyl)phe-noxy]ethoxy]ethyl]ammonium chloride (Phemerol chloride) is a colorless crystalline powder that is soluble in water, alcohol, and most organic solvents. The actions and uses of this agent are similar to those of benzalkonium chloride. It is used at a 1:750 concentration for skin antisepsis. For the irrigation of mucous membranes, a 1:5,000 solution is used. A 1:500 tincture is also available.
1-Hexadecylpyridinium chloride is a white powder that is very soluble in water and alcohol. In this compound, the quaternary nitrogen atom is a member of an aromatic pyridine ring.
The cetyl derivative is the most active of a series of alkylpyridinium compounds. It is used as a general antiseptic in concentrations of 1:100 to 1:1,000 for intact skin, 1:1,000 for minor lacerations, and 1:2,000 to 1:10,000 for the irrigation of mucous membranes. Cetylpyridinium chloride is also available in the form of throat lozenges and a mouthwash at a 1:20,000 dilution.
1,6-Di(4'-chlorophenyldiguanido)hexane gluconate (Hibiclens) is the most effective of a series of antibacterial biguanides originally developed in Great Britain.14
The antimicrobial properties of the biguanides were discovered as a result of earlier testing of these compounds as possible antimalarial agents (Chapter 7). Although the biguanides are technically not bisquaternary ammonium compounds and, therefore, should probably be classified separately, they share many physical, chemical, and antimicrobial properties with the cationic surfactants. The biguanides are strongly basic, and they exist as dications at physiological pH. In chlorhexidine, the positive charges are counterbalanced by gluconate anions (not shown). Like cationic surfactants, these undergo inactivation when mixed with anionic detergents and complex anions such as phosphate, carbonate, and silicate.
Chlorhexidine has broad-spectrum antibacterial activity but is not active against acid-fast bacteria, spores, or viruses. It has been used for such topical uses as preoperative skin disinfection, wound irrigation, mouthwashes, and general sanitization. Chlorhexidine is not absorbed through skin or mucous membranes and does not cause systemic toxicity.
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