Surface-Active Agents
John J. Merianos
Gerald McDonnell
Surface-active agents (surfactants) are amphiphilic compounds, which means one portion of the molecule is hydrophilic, or “water-loving,” and the other portion of the molecule is lipophilic, or “oil-loving.” The origin of the term amphiphile comes from the Greek word amphi, meaning that all surfactant molecules consist of at least two parts: one hydrophilic and one lipophilic. Chemically, the hydrophilic moiety of the molecule may be a carboxylate, sulfate, sulfonate, phosphate, or some other polar group. The lipophilic portion of the molecule is a nonpolar group, usually of a hydrocarbon nature. A schematic presentation of surfactant molecules is given in Figure 21.1, which illustrates the hydrophilic head group with a hydrophobic tail. The different surfactant molecules are typically classified based on the overall charge at their hydrophilic (polar) head groups: nonionic having no overall charge, cationic with a positive charge, anionic a negative charge, and amphoteric (or zwitterionic) with two oppositely charged groups. Figure 21.1 also shows how surfactants form micelles (or aggregates of surfactant molecule) and can be used to solubilize drugs or other insoluble molecules in various formulations as well as to aid in the removal of materials (eg, lipids on surfaces) in liquids such as water. By far, surfactants’ most valuable application to the pharmaceutical and agricultural industries is their ability to solubilize drugs as a result of their amphiphilic nature, but they are also widely used as cleaning and disinfection agents.
PHYSICAL PROPERTIESThe fundamental property of surfactants is their tendency to accumulate at interfaces, such as solid-liquid (suspension), liquid-liquid (emulsion), or liquid-vapor (foam).1 Surface-active agents possess both water-soluble and oil-soluble characteristics. The dual solubility of the surfactants makes them able to exhibit unique properties. If a compound is completely water soluble or completely oil soluble, it will not collect at the interface; instead, it will dissolve in the medium in which it is soluble. It is the nonpolar group that allows the compound to be partially oil soluble and the polar group that allows the compound to be partially water soluble.
Another important characteristic of surfactants is their capacity to lower surface or interfacial tension. Surface tension of a liquid is the force that opposes the expansion of the surface area of that liquid. Interfacial tension is similar to surface tension; however, one important difference is the location at which the tension occurs. Interfacial tension takes place at the interface of the two immiscible liquids, whereas surface tension takes place between the liquid surface and the air. Electrostatic or molecular forces are responsible for surface and interfacial tension. These forces are the reason for the mutual attraction of molecules for one another. Many molecules, although electrically neutral, have an uneven distribution of electrical charge, thus giving them polarity, or a negative or positive center of electricity. These negative and positive centers of electricity create the electrostatic forces within molecules.
The molecules in a liquid possess electrostatic forces, giving rise to intermolecular attraction. This attraction between “like” molecules is caused by cohesive forces. Adhesional forces occur because of the molecular attraction between the liquid’s surface and the liquid molecules. The net result of the adhesional forces and the cohesive forces at the interface of two immiscible liquids is known as interfacial tension. If the cohesive forces, which tend to hold the liquid molecules together, are stronger than the adhesional forces, which tend to pull the surface of the molecules apart, the interfacial tension will be high and the two liquids will not mix. To lower the interfacial tension between two immiscible liquids, the interface between the two liquids must be altered by some means. This may be accomplished by adding a substance that is
capable of orienting itself between the liquid layers. On collection of a surface-active substance at the interface, there is a reduction of interfacial tension. These surface-active compounds are called emulsified agents, which help to hold together two immiscible liquids, forming a water-in-oil or an oil-in-water emulsion.
capable of orienting itself between the liquid layers. On collection of a surface-active substance at the interface, there is a reduction of interfacial tension. These surface-active compounds are called emulsified agents, which help to hold together two immiscible liquids, forming a water-in-oil or an oil-in-water emulsion.
 FIGURE 21.1 A representation of the structure of surfactant monomers and associated classification based on their overall charge (nonionic, anionic, cationic, and amphoteric). Also shown (right) are examples of micelles or aggregates of surfactants and the locations of solubilizates in spherical micelles. A, Ionic surfactant (solubilized molecules in this case have no hydrophilic groups). B, Nonionic surfactant (with polar solubilizate). Based on Zografi et al1 and Harvey.2 |
Stearic acid is an example of a surface-active compound. When stearic acid is placed in water, the molecules will collect at the surface. The molecules cannot go beyond the surface because the hydrophobic (or lipophilic groups) are insoluble in the water. Because of the dual-solubility characteristics of stearic acid molecules, they are adsorbed on the surface of the liquid and then are able to lower the surface tension of water. When the water and petrolatum are mixed, they have an interfacial tension of 57 dynes/cm. With the addition of a drop of stearic acid to water alone, the acid collects at the surface, with its polar groups projected in the water and its nonpolar groups oriented in an outward direction. The interfacial tension between the stearic acid and the water is 15 dynes/cm. When liquid petrolatum is added to the water-stearic acid mixture, the interfacial tension still remains at 15 dynes/cm. The monomolecular layer of the stearic acid between the liquid interface serves to lower the interfacial tension from 57 dynes/cm to 15 dynes/cm.
Another fundamental property of the surface-active agents is that in solution, they tend to form micelles or aggregates (see Figure 21.1). The different types of micelles formed by surfactants are shown in Figure 21.2. Micelle formation, or micellization, reduces the free energy of the system by decreasing the hydrophobic surface area exposed to water. The surfactant molecules behave differently when present in micelles compared with free monomers in solution. Micelles are a reservoir for surfactant molecules, usually in their monomer (single molecule) form. The ability of the surfactant molecules to lower surface interfacial tension and dynamic phenomena, such as wetting and foaming, is governed by the concentration of free monomers in solution. Micelles are generated at low surfactant concentration in water. The concentration at which micelles start to form is called the critical micelle concentration (CMC) and is an important characteristic for each surfactant. Table 21.1 lists the CMC and micellar aggregation numbers for the three most common types of surfactants: anionic, cationic, and nonionic (with the values recorded in water at room temperature).
The global surfactants market was estimated to be approximately $43 million in 2017 and is projected to rise to approximately $66 million by 2025. The classification of surfactants is typically made on the basis of the charge of the hydrophilic (polar) group. There are four major classes of surfactants (in order of decreasing, estimated volume demand):
A. Anionics (approximately 40%)
B. Nonionics (approximately 30%)
C. Cationics (approximately 20%)
D. Amphoterics or zwitterions (<10%)
The anionic group, which consists of alkaline salts of fatty acids, is by far the largest group used by consumers
in the form of soaps. Commercially, soaps are prepared by saponification of animal fats, vegetable oil, coconut oil, palm oil, or natural fatty acid glycerides in caustic solution. The soaps, in addition to their cleaning and detergent characteristics, are used in the pharmaceutical industry for their ability to solubilize compounds. They also act as wetting agent for solid dosage forms and emulsifying agent in emulsion formation. Monovalent soaps (Na+, K+) will give oil-in-water emulsions. Divalent soaps (Ca2+, Mg2+), due to their poor water solubility, will produce waterin-oil emulsions. The triethanolamine salts of fatty acids will give an oil-in-water emulsion. The synthetic anionic surfactants of commercial importance in this category include the following: alkyl sulfate, alkyl ether sulfate, alkyl benzenesulfonate, alkyl xylenesulfonate, dialkyl sulfosuccinate, alkyl phosphate, alkyl ether phosphate, alkyl ether carboxylate, and others.
in the form of soaps. Commercially, soaps are prepared by saponification of animal fats, vegetable oil, coconut oil, palm oil, or natural fatty acid glycerides in caustic solution. The soaps, in addition to their cleaning and detergent characteristics, are used in the pharmaceutical industry for their ability to solubilize compounds. They also act as wetting agent for solid dosage forms and emulsifying agent in emulsion formation. Monovalent soaps (Na+, K+) will give oil-in-water emulsions. Divalent soaps (Ca2+, Mg2+), due to their poor water solubility, will produce waterin-oil emulsions. The triethanolamine salts of fatty acids will give an oil-in-water emulsion. The synthetic anionic surfactants of commercial importance in this category include the following: alkyl sulfate, alkyl ether sulfate, alkyl benzenesulfonate, alkyl xylenesulfonate, dialkyl sulfosuccinate, alkyl phosphate, alkyl ether phosphate, alkyl ether carboxylate, and others.
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The nonionic group is a major class of surfactants used widely throughout the pharmaceutical industry as a result of its low toxicity, good compatibility, and excellent stability in biologic systems. The most widely used compounds are the polyoxyethylene sorbitan fatty acid esters, which are found in both internal and external pharmaceutical formulations. The polar group in this category is a polyether or polyhydroxy oxyethylene unit attached to a fatty alcohol, fatty acid, or fatty amide moiety. These compounds can be water insoluble or quite water soluble, depending on the degree of ethoxylation. Ethoxylated surfactants can be tailor-made with regard to the average number of oxyethylene units added to a specific fatty alcohol. Representative commercially available nonionic surfactants include fatty alcohol ethoxylate, fatty acid ethoxylate, fatty amide ethoxylate, fatty amine ethoxylate, alkyl glucoside, sorbitan alkanoate (SpanTM), ethoxylated sorbitan alkanoate (TweenTM), alkylphenol ethoxylate (IgepalTM CO, Nonoxynol-9 or -11). The ethers offer an advantage over the esters in that they are quite resistant to alkaline or acidic hydrolysis. Nonionic surfactants are compatible with all other classes of surfactants and have been used as stabilizers, wetting agents, dispersants, detergents, and emulsifiers.
The interfacial properties of nonionics are greatly influenced by the polarity or nonpolarity of the system. As a result of the importance of polarity, a system was developed to assign a hydrophilic-lipophilic balance (HLB) number to each surfactant. The HLB value is the percentage weight of the hydrophilic group divided by 5 to reduce the range of values. On a molar basis, a 100% hydrophilic molecule (polyethylene glycol) would have a value of 20. The Atlas Powder Company3 devised the HLB number system for nonionic surfactants used as emulsifying agents (Table 21.2). Griffin4 developed an HLB scale, which is a numeric scale that extends from 1 to approximately 50. There is a relationship between the different applications of surfactants and the HLB range (Table 21.3), and there are methods available to calculate the HLB value of new surface-active agents. Table 21.4 provides examples of how to calculate the HLB value of
nonionic emulsifying blends to form an oil-in-water or a water-in-oil emulsion and thus to assign an HLB number. The hydrophilic groups on the surfactant molecule make a positive contribution to the HLB number, and the lipophilic groups exert a negative effect. There are several members of this class of surfactants, like nonoxynol-9, with spermaticidal and some disinfecting activity (eg, against human immunodeficiency virus [HIV] and some bacteria associated with sexually transmitted diseases).5 Nonionic nonoxynol-9 can stabilize quaternary ammonium compound (QAC; types of cationic surfactant) formulations because nonionics are not sensitive to hard water. The hard-surface disinfecting activity of QACs is potentiated by the nonionics, such as nonoxynol-9. It is well documented that ethoxylated surfactants do support bacterial growth and deactivate preservatives,6 such as the parabens. Although they are predominantly used for cleaning applications, they are not widely used as disinfecting agents. They can demonstrate some bactericidal activity and are sometimes used in combination with other biocides as preservatives, demonstrating some bacteriostatic, fungistatic, and sporistatic activities. Further discussion of nonionic surfactants are outside the scope of this chapter.
nonionic emulsifying blends to form an oil-in-water or a water-in-oil emulsion and thus to assign an HLB number. The hydrophilic groups on the surfactant molecule make a positive contribution to the HLB number, and the lipophilic groups exert a negative effect. There are several members of this class of surfactants, like nonoxynol-9, with spermaticidal and some disinfecting activity (eg, against human immunodeficiency virus [HIV] and some bacteria associated with sexually transmitted diseases).5 Nonionic nonoxynol-9 can stabilize quaternary ammonium compound (QAC; types of cationic surfactant) formulations because nonionics are not sensitive to hard water. The hard-surface disinfecting activity of QACs is potentiated by the nonionics, such as nonoxynol-9. It is well documented that ethoxylated surfactants do support bacterial growth and deactivate preservatives,6 such as the parabens. Although they are predominantly used for cleaning applications, they are not widely used as disinfecting agents. They can demonstrate some bactericidal activity and are sometimes used in combination with other biocides as preservatives, demonstrating some bacteriostatic, fungistatic, and sporistatic activities. Further discussion of nonionic surfactants are outside the scope of this chapter.
TABLE 21.1 Critical micelle concentrations (CMCs) and micellar aggregation numbers of various surfactants in water at room temperature | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Cationic surfactants arise due to a cationic charge on the nitrogen atom. Although phosphonium and sulfonium cationic surfactants exist,7 this chapter primarily addresses the QACs with antimicrobial activity. This class of surfactants is hydrolytically stable and shows higher aquatic toxicity than most other classes; however, newer “soft” QACs, which were considered more environmentally friendly, have replaced dialkyl QACs as textile softening agents. They contain an ester or amide moiety in their hydrocarbon chain. Devinsky and coworkers,8 using quantitative structure-activity relationships (QSAR) methods, give a good correlation of biologic activity (minimum inhibitory concentration or MIC) and lipophilicity, which is characterized by CMC and the length of the alkyl chain (Table 21.5). This topic is discussed later in this chapter. Several molecules in this class of soft
QACs have good antimicrobial activity and low systemic toxicity. Compared with “hard” counterpart benzalkonium bromide (dimethyldodecylbenzylammonium bromide, with an oral median lethal dose [LD50] 410 mg/kg), the amides and the esters differ significantly (eg, LD50 for compound no. 4 = 976 mg/kg, whereas compound no. 15 = 1450 mg/kg orally in mice) (see Table 21.5). It was concluded that “soft” QACs are two to three times less toxic than benzalkonium bromide based on LD50 comparison. When the ester or the amide groups within a “soft” QACs are hydrolyzed, the surface activity of the QAC is lost, and so is its antimicrobial efficacy. As a result, the biodegradation order was proposed as ester, QAC >> amide >>>> benzalkonium bromide (the most resistant to biodeterioration); however, the “soft” QACs had limited application as commercial antimicrobials. Instead, they are mostly used as hair- and skin-conditioning agents in personal care industries and as fabric softeners in textile and paper industries.
QACs have good antimicrobial activity and low systemic toxicity. Compared with “hard” counterpart benzalkonium bromide (dimethyldodecylbenzylammonium bromide, with an oral median lethal dose [LD50] 410 mg/kg), the amides and the esters differ significantly (eg, LD50 for compound no. 4 = 976 mg/kg, whereas compound no. 15 = 1450 mg/kg orally in mice) (see Table 21.5). It was concluded that “soft” QACs are two to three times less toxic than benzalkonium bromide based on LD50 comparison. When the ester or the amide groups within a “soft” QACs are hydrolyzed, the surface activity of the QAC is lost, and so is its antimicrobial efficacy. As a result, the biodegradation order was proposed as ester, QAC >> amide >>>> benzalkonium bromide (the most resistant to biodeterioration); however, the “soft” QACs had limited application as commercial antimicrobials. Instead, they are mostly used as hair- and skin-conditioning agents in personal care industries and as fabric softeners in textile and paper industries.
TABLE 21.2 Approximate hydrophilic-lipophilic balance (HLB) values for emulsifying agents | ||||||||||||||||||||||||||||||||||||||||||
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TABLE 21.3 Relationship between HLB range and surfactant application | ||||||||||||||||
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TABLE 21.4 Hydrophilic-lipophilic balance group numbers | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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The amphoteric (or zwitterionic) surfactants contain both an anionic and a cationic charge on the same molecule. An amphoteric surfactant is pH dependent and can therefore be either cationic, zwitterionic, or anionic. A change in pH of an amphoteric surfactant will change its charge and naturally affects its properties like detergency, wetting, and foaming. At the isoelectric point, the physiochemical properties of these amphoterics resemble that of nonionic surfactants. Above and below the isoelectric point, there is a gradual shift toward anionic and cationic character, respectively. The amphoterics have excellent dermatologic properties and show low eye and skin irritation.
These properties are well suited for use in shampoos and other personal care products. They are compatible with all other classes of surfactants, and they are stable in acidic and basic environments. Under strong alkaline conditions, betaines retain their surfactant properties. The most common commercial representatives of amphoteric surfactants are betaine (alkyldimethyl ammonium acetate from chloroacetic acid and alkyldimethylamine), alkyldimethylamine oxide (made by reaction of alkyldimetylamine with hydrogen peroxide), amidobetaine (made from fatty acid reaction with N,N-dimethylpropane-1,3 diamine, followed by reaction with chloroacetic acid), and alkyl imidazoline (synthesized by reaction of a fatty acid with aminoethanolamine, followed by treatment with chloroacetic acid). By far, the most widely used betaine is cocoamidopropyl betaine (under many trade names). It is used by many surfactant manufacturers, such as betaine C (TEGOTM; Evonik Goldschmidt Chemical Co, Mapleton, Illinois), cocamidopropyl betaine (LexaineTM C, Inolex, Philadelphia, Pennsylvania; AmphosolTM CA, Stepan, Northfield, Illinois; MirataineTM BET C-30, Rhône-Poulenc, Collegeville, Pennsylvania; EmpigenTM BS, Albright & Wilson, Oldbury, United Kingdom; CaltaineTM C-35, Pilot Chemical Co, Houston, Texas; MackamTM 35-HP, McIntyre Group Ltd, University Park, Illinois), and oleamidopropyl betaine (LonzaineTM C; Lonza, Walkersville, Maryland).
The amphoteric surfactants are also widely used in formulations due to their surfactancy and often low associated irritation, such as a foam booster, antistatic agents, and as shampoos, but also are reported to have some antimicrobial activity. They are both used for preservative (bacteriostatic and fungistatic) and disinfectant activity (bactericidal and fungicidal) at low concentrations,9 including the yeast Pityrosporum ovale (associated with dandruff and formulations used for dandruff treatment).10
TABLE 21.5 Antimicrobial activity and critical micelle concentrations of quats | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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QUATERNARY AMMONIUM ANTIMICROBIAL COMPOUNDSThe quaternary nitrogen moiety is an essential component for many biologically active compounds. The QACs play an important role in the living process. From vitamins (vitamin B complex and thiamine) to carboxylase enzymes, which participate in the carbohydrate metabolism, to choline, which is involved in the transmethylation reaction of fat metabolism, and to acetylcholine, a mediator in the transmission of nerve impulses all play a fundamental function. There are at least four types of physiologic actions11 associated with QACs: (1) curare-like (curaremimetic or curareform) action, a muscular paralysis with no involvement of central nervous system or heart, produced by D-tubocurarine chloride used to induce muscular relaxation during surgery; (2) muscarinic-nicotinic action, which is a direct stimulation of smooth muscles and is a primary transient stimulation and secondary persistent depression of sympathetic and parasympathetic ganglia; (3) ganglia-blocking action; and (4) neuromuscular blockade. Table 21.6 illustrates the chemical structures of representative compounds responsible for these physiologic actions.
TABLE 21.6 Examples of quaternary structures with physiological actions | ||
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Medicinal chemists, using the principle of structureactivity relationships (SARs), have synthesized many QACs that will mimic certain biologic effects.12 Thus, a complex structure of D-tubocurarine chloride can be reduced to a much simpler decamethonium structure, a neuromuscular-blocking agent. Hexamethonium acts as a ganglionic blocker by preventing the receptor from responding to acetylcholine. The decamethonium is too long to fit the ganglionic receptor but acts as a neuromuscular blocker by preventing the binding of acetylcholine to muscle endplate receptors. When the number of carbon atoms separating the quaternary nitrogens is increased above 12 carbons, the autonomic nervous activity disappears and the compounds become surface active and antimicrobial; however, there are exceptions to this rule, as discussed later in the case of bis-quaternary and polymeric QACs, where two and four carbon atoms separate the quaternary nitrogen, and the products have antimicrobial properties.
Although there are many stages in the historical development of quaternary ammonium antimicrobials, there is general agreement on at least two truly historical milestones. The first is the work of Jacobs and coworkers, which examined structure, preparation, and antimicrobial activity.13 A number of papers published in 1915 described the preparation of various different series of the quaternary ammonium salts of hexamethylenetetramine.14,15,16,17,18,19,20 In 1916, the antimicrobial activity of many of these synthesized QACs and additional derivatives were described.13,21,22 In these publications, they related structure to antimicrobial activity. Although some reviews have challenged this work as the earliest investigations of QACs, their preeminence is assured by their quality, quantity, and treatment, which included antimicrobial activity and correlation between structure and antimicrobial activity. The only valid criticism may be the fact that some of the antimicrobial activity observed may be due to the release of formaldehyde from hexamethylenetetramine. Methenamine mandelate United States Pharmacopeia (USP) is still used as an antibiotic for urinary tract infection, acting by releasing formaldehyde in an acid medium.2
During the 1920s, additional information was published on the bacterial activity of quaternary derivatives of pyridine, quinoline, and other ring structures23,24,25 as well as QACs of acylated alkylene diamines.26 By 1935, with the demonstration of the antibacterial activity of long-chain quaternary ammonium salts, the second and most important milestone in the development of antimicrobial QACs took place.27 The improved bactericidal activity that occurred when a large aliphatic residue was attached to the quaternary nitrogen atom established the practicability and utility of these compounds, first in medicine and later in many other applications. This important disclosure stimulated research in the synthesis and antimicrobial testing of QACs, with the consequent frequent publications and patents up to the present. After Domagk’s discovery of the biocidal properties of cationic surface-active agents, several generations of structurally variable quaternary ammonium antimicrobials of commercial importance were developed.
The first generation was the standard benzalkonium chloride (BAC) of specific alkyl distribution, namely, C12, 40%; C14, 50%; and C16, 10%. Another version of equally commercially successful alkyl distribution in the benzalkonium series is C12, 5%; C14, 60%; C16, 30%; C18, 5% as shown in Table 21.7. The official USP recognized BAC as a pharmaceutical aid (antimicrobial preservative). The USP specification for the C12/C14 homologues components was 70% minimum of the total alkylbenzyldimethyl ammonium chloride content. This broad specification does not always give the most efficacious product. The major determining factor for biocidal efficacy is the HLB of the products. The peak for biocidal activity of the homologue series is illustrated on Table 21.8, with a carbon chain of 14 offering the best activity.28,29,30,31,32,33
Modifications in the first-generation QACs by substitution of the aromatic ring hydrogen with chlorine, methyl, and ethyl groups resulted in the second generation of the substituted benzalkonium compounds. Of this group, the product with commercial significance was the alkyldimethylethylbenzyl ammonium chloride under the trade name BTC 471 with alkyl distribution C12, 50%; C14, 30%; C16, 17%; and C18, 3%. Another product with antimicrobial activity in this group was alkyldimethyl-3,4-dichlorobenzyl ammonium chloride under the trade names of Tetrosan 3,4D and Riseptin with same alky distribution as previously described.
But by far, the greatest commercial significance was with third generation of QACs, the dual quats, initially developed in 1955 under the trade name BTC 2125M. This product was a mixture of equal proportions of alkyldimethylbenzyl ammonium and alkyldimethylethylbenzyl ammonium chlorides of specific alkyl distribution, as shown in Table 21.7. This combination of BAC with alkyl distribution (C12, 5%; C14, 60%; C16, 30%; C18, 5%) and alkyldimethylethylbenzyl ammonium chloride with alkyl distribution (C12, 68%; C14, 32%) is BTC 2125M, with claimed antimicrobial performance against vegetative bacteria. This synergistic combination of the third generation of quaternaries not only had an increased biocidal activity but also reduced the acute oral LD50 from 0.3 g/kg of BAC to an acute oral LD50 of 0.750 g/kg for BTC 2125M. The third generation of dual QACs offered improved biocidal activity, stronger detergency, and a relatively lower level of toxicity. In the early 1950s, the nonionic detergents were being developed with far greater cleaning power than natural soaps. The compatibility of QACs with nonionic detergents resulted in superior formulations that helped overcome the environmental factors, such as hard water, anionic residues of soap, and proteinaceous soils, which were found to weaken their effectiveness.34
A continual change and improvement in advancing and broadening the spectrum of biocidal activity enabled disinfectants to work under the most adverse conditions and produced safer, more economic products. In 1965, another technologic development, catalytic amination of long-chain alcohols, made commercially feasible the production of dialkylmethyl amines, which in turn can be quaternized with methyl chloride to give us the twin-chain quats, the fourth generation of quaternaries antimicrobials with high performance, unusual properties, and tolerances.35,36 The twin-chain quats, such as dioctyl dimethyl ammonium bromide and didecyldimethyl ammonium bromide, were first introduced by the British Hydrological Corporation for the British food industry (DECIQUAM 222).37 These products displayed outstanding bactericidal performance, unusual tolerance for anionic surfactants, protein loads, and hard water, and even low-foaming characteristics. Table 21.9 illustrates the bactericidal activities and pseudomonicidal, fungicidal, and hard-water tolerance (HWT) of five of the most active twin-chain
quats of 20 as measured by the official Association of Official Analytical Chemists (AOAC) procedure.36 The product of choice in this series was C8/C12DMAC because of its superior water solubility and bactericidal activity; however, the odd number chain C9/C11DMAC is an equally active product, although its commercial feasibility was not explored at that time because of high cost of the odd carbon chain alcohols. Several odd carbon chain alcohols have been offered in commercial quantities. The concept of synergistic combination in the dual quats has been applied to twin-chain quats dialkyldimethyl ammonium chlorides (dioctyl, 25%; didecyl, 25%; octyldecyl, 50%)
was combined with BACs (R = C12, 40%; C14, 50%; C16, 10%). A 60:40 blend of the previously mentioned quaternaries proved superior to the individual components tested by the AOAC use-dilution test (Official Methods of Analysis of the AOAC, 1984).38 This newer blend of quaternaries represents the fifth generation of QACs. The blend remained active under the most hostile conditions, was less toxic and less costly, and provided more convenient disinfectants.
quats of 20 as measured by the official Association of Official Analytical Chemists (AOAC) procedure.36 The product of choice in this series was C8/C12DMAC because of its superior water solubility and bactericidal activity; however, the odd number chain C9/C11DMAC is an equally active product, although its commercial feasibility was not explored at that time because of high cost of the odd carbon chain alcohols. Several odd carbon chain alcohols have been offered in commercial quantities. The concept of synergistic combination in the dual quats has been applied to twin-chain quats dialkyldimethyl ammonium chlorides (dioctyl, 25%; didecyl, 25%; octyldecyl, 50%)
was combined with BACs (R = C12, 40%; C14, 50%; C16, 10%). A 60:40 blend of the previously mentioned quaternaries proved superior to the individual components tested by the AOAC use-dilution test (Official Methods of Analysis of the AOAC, 1984).38 This newer blend of quaternaries represents the fifth generation of QACs. The blend remained active under the most hostile conditions, was less toxic and less costly, and provided more convenient disinfectants.
TABLE 21.7 Commercial antimicrobial quaternary ammonium compounds | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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TABLE 21.8 Effect of length of carbon chain on bactericidal activity: bactericidal testa | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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TABLE 21.9 Antimicrobial activity of twin-chain quaternary ammonium compounds | |||||||||||||||||||||||||||||||
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In the 1980s, the toxicity of QACs underwent scrutiny by the US Environmental Protection Agency (EPA) and other US regulatory agencies. The safety of the biocides in general received a greater priority over efficacy. A new class of biocides, the polymeric quaternaries, has emerged; these are less toxic than the standard BACs and less powerful than the dual quats or twin-chain quats. The polymeric quaternaries are milder and have found applications in pharmaceuticals as preservatives.39,40 The polymeric quaternary ammoniums are polyelectrolytes, representing the sixth generation of QACs. They are generally considered milder and safer than all other classes based on LD50 and cytotoxicity.39 More recently, other synergistic combinations, considered the seventh generation of quaternary, consisted of blends of bis-quats and polymeric quaternary (polyionenes). These offered excellent antimicrobial activity against oral flora bacteria, namely Bacteroides gingivalis, Actinomyces viscosus, and Streptococcus mutans at 1 to 5 ppm in pharmaceutically accepted polymer formulations.40 These results required further testing to confirm safety and true synergism in this series of polymeric quaternaries with bis-quats blends, and a subsequent series of patents claiming antimicrobial, low-toxicity, blend composition of bis-QACs and polyvinylpyrrolidone.40,41
CHEMISTRYThe QACs are the products of a nucleophilic substitution reaction of alkyl halides with tertiary amines. Chemically, they have four carbon atoms linked directly to the nitrogen atom through covalent bonds; the anion in the original alkylating agent becomes linked to the nitrogen by an electrovalent bond. The general formula for the QACs is represented as follows:
R1, R2, R3, and R4 are alkyl groups that may be alike or different, substituted or unsubstituted, saturated or unsaturated, branched or unbranched, and cyclic or acyclic and that may contain ether or ester or amide linkages; they may be aromatic or substituted aromatic groups. The nitrogen atom plus the attached alkyl groups forms the positively charged cation portion, which is the functional part of the molecule. The portion attached to the nitrogen by an electrovalent bond may be any anion but is usually chloride or bromide to form the salt. Depending on the nature of the R groups, the anion, and the number of quaternary nitrogen atoms present, the antimicrobial QACs may be classified in different salt groups as described in the following text.
Monoalkyltrimethyl Ammonium Salts
In these, one R group is a long-chain alkyl group, and the remaining R groups are short-chain alkyl groups, such as methyl or ethyl groups. All the compounds in this group are prepared from the reaction of a tertiary amine with an alkyl halide.27,42,43 The tertiary amine may be the long-chain alkyldimethylamine or the short-chain trimethylamine, which react with methyl halide or with the long-chain alkyl halide, respectively. Examples of commercially available products in this group are cetyltrimethylammonium bromide as CTAB; alkyltrimethyl ammonium chloride as ArquadTM 16; alkylaryltrimethyl ammonium chloride as GloquatTM C; and cetyldimethylethylammonium bromide as CyclotonTM D256B, AmmonyxTM DME, and BretolTM.
Monoalkyldimethylbenzyl Ammonium Salts
In this group, one R is a long-chain alkyl group, a second R is a benzyl radical, and the two remaining R groups are short-chain alkyl groups, such as methyl or ethyl groups. These compounds are prepared by the reaction of a longchain alkyldimethylamine with the benzyl halide.44,45,46 Examples of commercially available products in this group are alkyldimethylbenzyl ammonium chlorides as
BTCTM 824, HyamineTM 3500, CyncalTM Type 14, and CatigeneTM. In addition, there are substituted benzyl QACs such as dodecyldimethyl-3,4-dichlorobenzyl ammonium chloride, sold under the trade name of RiseptinTM. There are also mixtures of alkyldimethylbenzyl and alkyldimethyl substituted benzyl (ethylbenzyl) ammonium chlorides, such as BTC 2125M and BarquatTM 4250.
BTCTM 824, HyamineTM 3500, CyncalTM Type 14, and CatigeneTM. In addition, there are substituted benzyl QACs such as dodecyldimethyl-3,4-dichlorobenzyl ammonium chloride, sold under the trade name of RiseptinTM. There are also mixtures of alkyldimethylbenzyl and alkyldimethyl substituted benzyl (ethylbenzyl) ammonium chlorides, such as BTC 2125M and BarquatTM 4250.
Dialkyldimethyl Ammonium Salts
In this instance, two R groups are long-chain alkyl groups, and the remaining R groups are short-chain alkyl groups, such as methyl groups. These compounds are prepared by the reaction of the long-chain alkyldimethylamine with a long-chain alkyl halide or dialkylmethylamine with methyl halide.47,48,49,50 Examples of commercially available products in this group are didecyldimethyl ammonium halides, such as DeciquamTM 222 and BardacTM 22, and octyldodecyldimethyl ammonium chloride, such as BTCTM 812.
Heteroaromatic Ammonium Salts
In this group, one R chain is a long alkyl group, and the remaining three R groups are provided by some aromatic system. Thus, the quaternary nitrogen to which these three R groups are attached is part of an aromatic system such as pyridine, quinoline, or isoquinoline. These compounds are prepared by reaction of the aromatic amine with a long-chain alkyl halide.42,51 Examples of commercially available products in this group are cetylpyridinium halide (CPCTM and CeeprynTM), reaction product of hexamethylenetetramine with 1,3-dichloropropene to give cis-isomer 1-[3-chloroallyl]-3,5,7-triaza-1-azoniaadamantane (Dowicil 200), alkyl-isoquinolinium bromide (IsothanTM Q), and alkyldimethylnaphthylmethyl ammonium chloride (BTCTM 1100).
Polysubstituted Quaternary Ammonium Salts
In this group, the cation portion of the molecule is the same as that described for any of the aforementioned groups; however, the anion portion is not a small inorganic ion as previously described, but a large, high-molecular-weight organic ion. These compounds are prepared by reaction of the quaternary ammonium halides with the sodium, potassium, or calcium salt of a high-molecular-weight organic moiety, so that an exchange of the anions is effected.52,53 Examples of commercially available products in this group are alkyldimethylbenzyl ammonium saccharinate (OnyxideTM 3300 and LoroquatTM QA 100) and alkyldimethylethylbenzyl ammonium cyclohexylsulfamate (Onyxide 172).
Bis-Quaternary Ammonium Salts
In this group of compounds, there are two symmetric quaternary ammonium moieties arranged in the general formula:
Here, the R groups are as described for any of the aforementioned groups, Z is a carbon-hydrogen attached to each quaternary nitrogen via an electrovalent bond. These compounds are prepared by reaction of a bistertiary amine with alkyl halide or of a di-halo compound with a tertiary amine.54,55,56 An example of a commercially available product in this group is 1, 10-bis(2-methyl-4-aminoquinolinium chloride)-decane, sold under the trade name of DequadinTM or SorotTM. Another example of bis-quatis 1,6-bis [1-methyl-3-(2,2,6-trimethyl cyclohexyl)-propyldimethyl-ammonium chloride] hexane or triclobisonium chloride sold by trade name TriburonTM. Another commercially available bis-quat is CDQTM, used for industrial water treatment for controlling sulfatereducing bacteria (Desulfovibrio species). The CDQTM is prepared by reaction of alkyl[C12, 40%; C14, 50%; C16, 10%] dimethylamine with dichloroethyl ether. Also, reaction of 1,4-dichloro-2-butene with 2 mol of alkyldimethylamines or hexamethylenetetramine offers another example of bis-quats with broad spectrums of biocidal activity.
Polymeric Quaternary Ammonium Salts
Many different types of polymeric quaternary ammonium salts have been reported to have antimicrobial activity.57,58,59,60,61,62 The methods of preparation of these polymers are also many, from free radical polymerization of monomers containing quaternized nitrogen, to cationic, anionic polymerization, polycondensation of diamines with dihalides, or polycondensation of haloamines. The last method was used for the preparation of ionenes, which are polyelectrolytes with positively charged nitrogen atoms located in the backbone of polymeric chain. This type of polycation was first reported in 1941 and is formed by the Menshutkin reaction from ditertiary amines and dihalides.62 Although a number of patents were published since 1941 concerning the applications of ionene polymers, little information was available on the mechanism and kinetics of their formation or their solution properties. The polymerization conditions for the formation of ionenes involve a total concentration of 3.0 mol (1.5 mol
of each monomer) in a mixture of dimethylformamide/methanol solvent (1:1 or 4:1) and 5 to 7 days of reaction time at room temperature to give polyionenes with molecular weight (MW) of about 65 000. In condensation polymerization, the purity of the condensing species and stoichiometry are critical in obtaining high-molecularweight polymer. Polyionenes exhibit bactericidal action, formation of insoluble complexes with DNA and heparin, neuromuscular-blocking action, cell lysis and aggregation, and cell adhesion.62 The antimicrobial and antifungal properties of ionenes were studied by the zone of inhibition method against Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa, Bacillus subtilis, Candida sporogenes, Salmonella Typhimurium, Mycobacterium smegmatis, Pseudomonas mirabilis, Pseudomonas vulgaris, Candida globosum, M. verrucaris, Fusarium oxysporum, and Alternaria species. The MIC for five of the most active ionenes bromides against S aureus and E coli are listed in Table 21.10.
of each monomer) in a mixture of dimethylformamide/methanol solvent (1:1 or 4:1) and 5 to 7 days of reaction time at room temperature to give polyionenes with molecular weight (MW) of about 65 000. In condensation polymerization, the purity of the condensing species and stoichiometry are critical in obtaining high-molecularweight polymer. Polyionenes exhibit bactericidal action, formation of insoluble complexes with DNA and heparin, neuromuscular-blocking action, cell lysis and aggregation, and cell adhesion.62 The antimicrobial and antifungal properties of ionenes were studied by the zone of inhibition method against Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa, Bacillus subtilis, Candida sporogenes, Salmonella Typhimurium, Mycobacterium smegmatis, Pseudomonas mirabilis, Pseudomonas vulgaris, Candida globosum, M. verrucaris, Fusarium oxysporum, and Alternaria species. The MIC for five of the most active ionenes bromides against S aureus and E coli are listed in Table 21.10.
The 6,10 ionene bromide was found to be the most active in these series and, at concentrations up to 4 ppm, stimulated the growth of normal cells. This range showed inhibition and death of the transformed human cells W138, possibly by electrostatic cytotoxic interaction.63 It has been suggested that malignant cells are more electronegative than normal cells. If so, malignant cells should demonstrate a greater affinity for the electropositive ionene polymer. This hypothesis has been supported by experimental work; however, this preliminary study appears to warrant more extensive studies to evaluate this class of polymers as chemotherapeutic agents. Polyionenes and their low-molecular-weight analogues constitute a unique model system for a molecular probe of the living cell machinery because their structure, their positive-charge densities, their counterions, and their MWs can be varied systematically. These considerations apply not only to the study of toxicity or antimicrobial activity but also to the understanding of the interaction of the polyionenes with DNA or as molecular probes to elucidate the properties of cell membranes.
TABLE 21.10 Minimum inhibitory concentration (MIC) of polyionenes | ||||||||||||||||||||||||||||||||
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The high-charge density of polyionenes is responsible for their bactericidal and fungicidal activities, for the prolonged duration of the curarizing action, and for the formation of complexes with DNA and heparin. The differences in the biologic properties or stability of the complexes may be explained on the basis of electrostatic association between the negative and the positive moieties of the interacting molecules.
Because the normal and neoplastic cells show an array of different surface properties, including an increase in anodic mobility after transformation, some polyionenes (eg, 6,10 ionene bromide) may preferentially bind to cancer cells and inactivate them. This differential binding and toxicity to transformed cells conforms with topologic changes of membrane-binding sites in the transformed cells. All four products in Table 21.11 are ionenes that contain the quaternary nitrogen atom on the backbone of the polymer chain. The main difference is the MW, which is due to reactivity of the alkylating monomer, reaction time, solvent, temperature, and method of preparation. The WSCPTM or BusanTM 77 from Buckman Laboratories, which is chemically identified as poly[oxyethylene (dimethyliminio)-ethylene (dimethyliminio)-ethylene dichloride], was made by condensation of N,N,N’,N’tetramethylethylenediamine with dichloroethyl ether in water to give an MW of 2000 to 3000. An MIC at 10- to 50-ppm levels indicated broad-spectrum antimicrobial activity, with dual functionality as polymer clarifiers for swimming pools or algicides. The presence of WSCP at only 2 ppm can reduce by 80% or more the chlorine needed to kill bacteria, where it enhances the activity of all oxidizers used in swimming pools. In addition, it was used as a cooling water treatment biocide for algae, bacteria, and fungi at 20 to 40 ppm.
Another polymeric QAC was the product MirapolTM A-15, which is chemically identified as poly[N-3-dimethylammonio) propyl]N-[3-ethylneoxyethylenedimethylammonio) propyl]urea dichloride] or polyquaternium-2. This product is made by reacting a symmetrically substituted urea ditertiary amine with dichloroethylether in water to give an MW of 2000 to 3000. Initial uses were in hair-care products, with inhibitory levels of 100 ppm against P ovale, S aureus, and E coli. By a time/kill water treatment screening procedure, the product demonstrates a high-percentage kill of P aeruginosa and Enterobacter aerogenes at 10, 15, and 20 ppm of active product following a 30-minute contact period; however, these reports were debated,64 and its primary use has been as a hair-conditioning agent in shampoos.
The last structure on Table 21.11 of the polymeric QACs is chemically identified as α-4-[1-tris(2-hydroxyethyl) ammonium chloride-2-butenyl] poly [1-dimethyl ammonium chloride-2-butenyl]-ω-tris(2-hydroxyethyl)ammonium chloride or OnamerTM M or PolyquatTM or with the name polyquaternium-1. The Onamer M is made by reacting 1,4-bis[dimethylamino]-2-butene (0.9 mol),
triethanolamine (0.2 mol), and 1,4-dichloro-2-butene (1.0 mol) in water to give an average MW of 5000 to 10 000.65,66 The purpose of the triethanolamine in the preparation was randomly to terminate the polymeric chain so that we obtained a low-molecular-weight product by design. The 1,4-dichloro-2-butene alkylating agent is extremely reactive and gave a 98% conversion of organic chloride to ionic within 6 to 8 hours in water. Polyquaternium-1 was used for hair-conditioning applications but with demonstrated antimicrobial activity combined with excellent toxicologic data emerged as a preservative candidate for ophthalmic preparations.39,67 Examples of the use of this preservative included contact lenses solutions such as Opti-TearsTM, Opti-CleanTM, Opti-SoftTM, Opti-FreeTM, and Polyflex Tears-NaturaleTM II (all are trade names of Alcon). Table 21.12 compares the cytotoxic response of OnamerTM M at various concentrations and other solutions by in vitro testing of mouse L929 cells.39 The most common ophthalmic preservatives at this time were thimerosal, BAC, and chlorhexidine. These compounds can be toxic to the eye and may cause corneal erosion and corneal ulceration, resulting in pain. This problem is particularly severe with QACs that are concentrated more than 400 times by hydrophilic lenses. Chlorhexidine is concentrated as much as 100-fold by hydrophilic contact lenses, which results in the potential for injury to the eye. The comparative cellular toxicity of soft contact lenses soaked in Onamer M at various concentrations and other solutions was determined by in vitro testing. The results from Table 21.12 showed that OnamerTM M was not cytotoxic. This cytotoxic response may be related to the acute oral LD50 of these compounds (Table 21.13). Earlier and more recent studies determined that the product was bacteriostatic against a range of bacterial pathogens such as S aureus, E coli, P aeruginosa, and Streptococcus faecalis at 50 ppm and bactericidal/yeasticidal in formulations.68,69
triethanolamine (0.2 mol), and 1,4-dichloro-2-butene (1.0 mol) in water to give an average MW of 5000 to 10 000.65,66 The purpose of the triethanolamine in the preparation was randomly to terminate the polymeric chain so that we obtained a low-molecular-weight product by design. The 1,4-dichloro-2-butene alkylating agent is extremely reactive and gave a 98% conversion of organic chloride to ionic within 6 to 8 hours in water. Polyquaternium-1 was used for hair-conditioning applications but with demonstrated antimicrobial activity combined with excellent toxicologic data emerged as a preservative candidate for ophthalmic preparations.39,67 Examples of the use of this preservative included contact lenses solutions such as Opti-TearsTM, Opti-CleanTM, Opti-SoftTM, Opti-FreeTM, and Polyflex Tears-NaturaleTM II (all are trade names of Alcon). Table 21.12 compares the cytotoxic response of OnamerTM M at various concentrations and other solutions by in vitro testing of mouse L929 cells.39 The most common ophthalmic preservatives at this time were thimerosal, BAC, and chlorhexidine. These compounds can be toxic to the eye and may cause corneal erosion and corneal ulceration, resulting in pain. This problem is particularly severe with QACs that are concentrated more than 400 times by hydrophilic lenses. Chlorhexidine is concentrated as much as 100-fold by hydrophilic contact lenses, which results in the potential for injury to the eye. The comparative cellular toxicity of soft contact lenses soaked in Onamer M at various concentrations and other solutions was determined by in vitro testing. The results from Table 21.12 showed that OnamerTM M was not cytotoxic. This cytotoxic response may be related to the acute oral LD50 of these compounds (Table 21.13). Earlier and more recent studies determined that the product was bacteriostatic against a range of bacterial pathogens such as S aureus, E coli, P aeruginosa, and Streptococcus faecalis at 50 ppm and bactericidal/yeasticidal in formulations.68,69
TABLE 21.11 Polymeric polyquaternary ammonium compounds | ||||
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At least two characteristics are common to the WSCP and OnamerTM M polymeric quaternaries, making them uniquely different from other QACs. One is
the absence of foaming, even at high aqueous concentrations. The other is the remarkably lower toxicity shared by these polymeric quaternary products. As an example, for Onamer M at 30% active material, primary abraded and intact skin irritation scores were zero for all observation periods. Draize eye irritation studies recorded a mild conjunctival irritation with scores of 2 or 4 in each rabbit, which cleared on the second day of observation. The acute oral LD50 in rats was determined to be 4470 mg/kg. The product was described as nonmutagenic by the mouse lymphoma forward mutation assay and by the sex-linked recessive lethal test in Drosophila melanogaster. In addition, it was not considered carcinogenic by the in vitro transformation of Balb/3T3 cells assay.
the absence of foaming, even at high aqueous concentrations. The other is the remarkably lower toxicity shared by these polymeric quaternary products. As an example, for Onamer M at 30% active material, primary abraded and intact skin irritation scores were zero for all observation periods. Draize eye irritation studies recorded a mild conjunctival irritation with scores of 2 or 4 in each rabbit, which cleared on the second day of observation. The acute oral LD50 in rats was determined to be 4470 mg/kg. The product was described as nonmutagenic by the mouse lymphoma forward mutation assay and by the sex-linked recessive lethal test in Drosophila melanogaster. In addition, it was not considered carcinogenic by the in vitro transformation of Balb/3T3 cells assay.
TABLE 21.12 Comparative cytotoxicity of soft contact lenses soaked in preserved solutions on mouse L929 cells | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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TABLE 21.13 Acute oral LD50: commercial antimicrobial quaternary ammonium compounds | ||||||||||||||||||||||||||||||
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For reasons of structure, size, foaming, and toxicity, these polymeric quaternary compounds have encouraged the synthesis and evaluation of similar compounds for consideration. Besides the polyionene quaternary polymers, there are other classes of antimicrobial polymers in which the quaternary nitrogen is pendant away from the backbone chain of the polymer (Table 21.14). A number of quaternary monomers, homopolymers, and copolymers of N-vinyl-pyrrolidone and 2-methacryloxyethyl-N,N,N-triethyl ammonium bromide and iodide have been described to have antimicrobial activity.60 This activity increases as the content of quaternary ammonium moiety increases. The quaternary nitrogen is pendant away from backbone chain and requires the proper lipophilic groups and proper charge density of the quaternary nitrogen for biocidal activity. None of these polymers are used as antimicrobials, but they are commercially available as hair-conditioning agents for shampoos and other industrial applications such as coagulating and flocculating agents. Examples are GafquatTM (polyquaternium-11) and MerquatTM (polyquaternium-7). These products are made by the
free radical polymerization of diallyl quaternary monomer, and their MW is greater than 100 000. But some related terpolymers, with MW 10 000 to 20 000, were reported to have antimicrobial activity such as against Candida albicans at 0.1% when used as a preservative for soft contact lenses.70
free radical polymerization of diallyl quaternary monomer, and their MW is greater than 100 000. But some related terpolymers, with MW 10 000 to 20 000, were reported to have antimicrobial activity such as against Candida albicans at 0.1% when used as a preservative for soft contact lenses.70
TABLE 21.14 Free radical polymeric quaternary ammonium compounds | ||
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The antimicrobial activities (determined by MICs) of poly[trialkyl(vinylbenzyl)ammonium chloride] type of polycations against bacteria and fungi were considered more active than the corresponding monomers71,72; however, the absolute activity is low compared with commercially used quaternaries. The antibacterial assessment was based only on the conventional spread plate method, which has been widely used to evaluate the antibacterial activity of antibiotics and preservatives. But this method is subject to some variability, where the compound interacted with media negatively charged species (such as sodium caseinate of the agar plate) and produced an insoluble complex leading to inactivation of the polymer. Overall, the antibacterial activity of poly[dodecyldimethyl{vinylbenzyl} ammonium chloride] against S aureus at 0.5 ppm was effective within 30 minutes of contact, whereas the corresponding monomer was inactive. Compounds with the longest alkyl chain studied (dodecyl) exhibited high activity, which was ascribed to the contribution of the increased hydrophobicity of the compounds to the activity. In conclusion, the most significant finding was that the polymers are more active than the corresponding monomers, particularly against gram-positive bacteria. The higher activity of the polymers was interpreted as being due to favored adsorption onto the
bacterial cell surface and the cytoplasmic membrane with subsequent disruption of its integrity, although they have the disadvantage of diffusing through the cell wall, especially with the gram-negative bacteria. Antimicrobial polymeric QACs with repeated vinylbenzylammonium units have been described for preserving ophthalmic solutions.73
bacterial cell surface and the cytoplasmic membrane with subsequent disruption of its integrity, although they have the disadvantage of diffusing through the cell wall, especially with the gram-negative bacteria. Antimicrobial polymeric QACs with repeated vinylbenzylammonium units have been described for preserving ophthalmic solutions.73
An optimal MW region of 14 300 for the 6,6-ionene bromide was shown to have antibacterial activity with a minimum bactericidal concentrations (MBC) of 6.6 ppm to 10 ppm against S aureus.59 Polymeric biguanides of MW 11 900 and poly{alkyl[C2-C12] dimethyl[vinylbenzyl] ammonium chloride} of similar MW also exhibit better bactericidal action against gram-positive than gramnegative bacteria. The poly(dodecyldimethyl[vinylbenzyl] ammonium chloride) was the most active with 0.5 ppm against S aureus, which suggests that hydrophobicity plays an important role in bactericidal action. In conclusion, in all three types of polymers reported, there is a clear MW dependence of the biocidal activity; that is, polymers with low MW as well as high MW exhibit lower bactericidal activity, and there exists an optimal MW region for the biocidal action. This is a fine balance between the polymeric biocides between MW, biocidal activity (cytotoxicity), and (toxicity) acute oral LD50.
ANALYSISThe chemical structure of QACs permits a variety of quantitative procedures to determine their concentration. Various analytic methods have been reported for the determination of BACs by high-performance liquid chromatography (HPLC)74 and gas chromatography.75 Chemical ionization mass spectroscopy has been used to identify and determine the proportions of various alkyl chain lengths in commercial mixtures of BACs.30 At least two methods for determining low concentration of OnamerTM M or polyquaternium-1 antimicrobial preservative in ophthalmic solutions have been reported.66,76 A direct titration technique for the accurate, quantitative determination of cationic and anionic polyelectrolytes was reported.77 The technique is based on a direct neutralization reaction between cationic and anionic forms of the polymers. Modification of poly[vinylsulfuric acid] potassium salt method was used for determination of high concentration of OnamerTM M.78 The cationic polyelectrolytes show a light blue color in the presence of toluidine blue O dye, and the blue color turns to bluish purple when the titration end point is reached. With the many methods available, the method of choice is usually dictated by the level of QAC anticipated in the sample. For solutions estimated to contain 0.5% or more QAC, the chemist may use a diphasic or direct titration procedure.
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