Encyclopedia of Environmental Science and Engineering, Volume I and II

234 DISINFECTION

group, and the other a water-attracting (hydrophilic or polar)

group. Depending on the basis of the charge or absence of

ionization of the hydrophilic group, surface-active agents are

classified into (James, 1965):

1) Anionic agents, which usually have strong deter-

gent, but weak antimicrobial properties, e.g.,

sodium lauryl sulphate.

2) Cationic agents, which have strong bactericidal,

but weak detergent, properties. The term “cationic

detergent” usually signifies a quarternary ammo-

nium compound (QAC, onium compound), but

this is not strictly accurate, as the concentration

at which a QAC is germicidal is so low that its

detergent activity is negligible.

3) Non-ionic agents, which consist of a hydrocar-

bon chain attached to a non-polar water-attracting

group, which is usually a chain of ethylene oxide

units, e.g., cetomacrogols. Non-ionic agents have

no antimicrobial properties.

4) Ampholytic (amphoteric) agents, which are com-

pounds of mixed anionic–cationic character, in

which the charge can be similarly positive and

negative. They are effective antimicrobial com-

pounds, which are extensively used in the dairy

industry.

The QACs are organically substituted ammonium com-

pounds, in which the nitrogen atom has a valency of 5,

four of the substituent radicals (R^1 –R^4 ) are alkyl or het-

erocyclic radicals, and the fifth is a small anion. The sum

of the carbon atoms in the 4R groups is more than 10.

For a QAC to have a high activity, at least one of the four

organic radicals must have a chain length in the range C 8

to C 18. The germicidal activities of the QACs were origi-

nally recognized in 1916, but they did not attain promi-

nence until Domagk’s work in 1935. They are primarily

active against Gram-positive bacteria, with concentrations

as low as about 1 in 200,000 being lethal; higher concentra-

tions ( c. 1 in 30,000) are lethal to Gram-negative bacteria,

although Pseudomonas aeruginosa is highly resistant. The

QACs possess antifungal properties, are sporostatic and

not sporicidal (Russell, 1971) and are inactive against the

tubercle bacillus. Their antimicrobial activity is markedly

affected by organic matter, and they are incompatible with

anionic surface-active agents, some of the non-ionic agents

(such as Lubrols and Tweens) and the phospholipids, e.g.,

lecithin and other fat-containing substances (Russell, 1982;

Russell et al. , 1979). The QACs exert their maximal bac-

tericidal activity under alkaline conditions. Although the

QACs are effective bactericidal and fungicidal agents, it

has been found (Grossgebauer, 1970) that viruses are rather

more resistant than bacteria or fungi.

R^1

R^4 R^3

R^2

N

+

X–

Although several hundred QACs have been prepared

and tested for antimicrobial activity, only a few are regularly

used. These are:

1) Cetrimide (cetyltrimethylammonium bromide,

CTAB), a mixture of dodecyl, tetradecyl- and

hexadecyl-trimethylammonium bromide. In addi-

tion to it uses for pre-operative skin disinfec-

tion and treatment of seborrhoea of the scalp,

cetri mide is also employed, in conjunction with

sodium nitrite, which delays or prevents rusting,

for the storage of sterilized surgical instruments,

although this practice should be discontinued

(Rubbo and Gardner, 1965).

2) Benzalkonium chloride (Zephirol, Zephiran) is

the active constituent of a general purpose and

skin sterilizing agent, “Roccal.” It is also used to

alleviate or prevent napkin rash caused by urea-

splitting organisms, as a preservative in eye-drop

preparations (Pharmaceutical Codex, 1979) and

for the disinfection of blankets.

Other important QACs are domiphen bromide and cetyl-

pyridinium chloride.

The value of QACs in the disinfection of woollen blan-

kets has been observed by various authors (Frisby, 1957;

International Wood Secretariat, 1961), although they cannot be

incorporated in the wash with an anionic compound, because

of incompatibility. In addition, it must be emphasized that res-

idues of anionic agents on blankets from previous washings

could interfere with the action of QACs. To overcome this,

blankets may be washed in an appropriate non-ionic deter-

gent, with a final prolonged rinse in a QAC, or they may be

treated with a non-ionic detergent incorporating a QAC.

The QACs are also of considerable value as disinfectants

in food and dairy plant. If an alkali detergent containing

anionic surface-active wetting agents is used prior to sanitiz-

ing, then utensils and equipment must be thoroughly rinsed

(Barrett, 1969). The QACs at their normally used concentra-

tions are odourless and noncorrosive, but many are not free-

rinsing, and undesirable traces may remain on equipment or

even be present in dairy food (Clegg, 1967, 1970). However,

a combination of a free-rinsing type of QAC and a suitable

non-ionic agent may be usefully employed for washing

instruments and cutlery, etc. (Barrett, 1969).

The cytoplasmic membrane of bacteria and fungi is the

site of action of the QACs. The membrane is composed of

lipoprotein, and it is considered (Russell, 1971) that the lipid

moiety is involved in the lethal action of these compounds

(for more recent information, see Hugo, 1976a,b).

There appears to be a clear relationship between the ther-

modynamic and antibacterial activities of QACs, with solu-

tions having equal antimicrobial activity against an organism

having surface concentration values of the same order of

magnitude. Thermodynamic activities of QACs at two bac-

terial survivor levels (1% and 0.01%) have been shown to

be sufficiently constant (Laycock and Mulley, 1970) to sup-

port the Ferguson (1939) principle that compounds with the

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