228 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Organism Author Temperature Substance Staph. aureus E. typhosa -,. Shelton, et al. 7 37* C. L.D.B.A.B. 1: 32,000 1: 30,000 C.D.B.A.B. 1:76,000 1:32,000 -- Valko and 37* C. L.D.B.A.C. 1:40,000 -- Dubois ø C.D.B.A.C. 1: 15,000 -- 3/[ixture of L.D.B.A.C. + C.D.B.A.C. 1: 27,000' • ,. Valko and 20 ø C. L.D.B.A.C. 1: 20,000 1: 16,000 Dubois 8 M.D.B.A.C. 1:21,000 1: 17,000 C.D.B.A.C. 1: 14,000 1: 11,000 S.D.B.A.C. 1: 5,000 1: 7,000 __ __ Shelton, et al. 7 37 ø C. L.T.A.B. 1: 4,000 1: 9,0C0 M.T.A.B. 1: 38,000 1: 36,000 C.T.A.B. 1: 80,000 1: 40,000 S.T.A.B. 1:64,000 1: 8,000 ,. Valko and 20 ø C. L.T.A.B. 1: 6,500 1: 5,000 Dubois 8 M.T.A.B. 1: 12,000 1: 11,000 C.T.A.B. 1: 14,000 1: 10,000 S.T.A.B. 1: 9,000 1: 5,000 * --"the killing dilution is that of L.D.B.A.C. alone." The precise mechanism by which micro-organisms are destroyed is not fully understood, although this problem has attracted considerable research (see, e.g., 9,•o). The best known and most widely used bactericidal quaternary ammoni%rn compounds are those employed in medical and pharmaceutical practice, i.e., Cetrimide B.P. and Benzalkonium Chloride B.P.C. and U.S.P. Cetrimide is a mixed alkyltrimethylammonium bromide containing the C•-C•0 homologues, while benzalkonium chloride is a mixed alkyldimethylbenzyl- ammonium chloride principally C•-C•4. Regrettably, neither compound is subject to exact chemical specification, leading to considerable variations among materials which separately conform to the pharmaceutical requirements. • In most practical applications, the bactericidal quaternary ammonium compounds are employed in concentrations of the order of 50 to 500 p.p.m. In special cases, for example, when used as detergents with disinfectant properties and also when employed as emulsifying agents in cream prepara- tion, the concentration is increased to 0.5 to 1-0 per cent. Several reviews of the bactericidal properties and applications of quater- nary ammonium compounds have appeared, e.g., those of Lawrence. TM

CATIONIC SURFACE-ACTIVE AGENTS 229 Adsorptive Properties Many quaternary ammonium compounds are strongly adsorbed on solid surfaces. This property has been utilised in many fields ranging from the flotation of minerals to antistatic treatments of plastic goods. They are adsorbed particularly by proteins, notably keratin. This property is one of the most valuable in relation to cosmetic uses of quaternary ammonium compounds, particularly in hair preparations of all types. The most spectacular effect of the adsorption of quaternary ammonium compounds on the hair is to eliminate scroop from hair which has been washed with anionics, bleached, or permed the adsorbed film gives body to fine hair and yet softens coarse hair, is lubricant, and considerably reduces static. A recent paper on this subject is due to Mills, Ester and Henkin, • while earlier reviews of this application of quaternary ammonium compounds have been published by Hilfer. TM The adsorption on skin is of value also in the formulation of barrier creams. The extent to which any given quaternary is adsorbed is a function of the surface involved, the concentration of the solution, pH and temperature. With any one surface and constant conditions different quaternary ammon- ium compounds are adsorbed in differing degree. In general the maximum adsorption for cosmetic purposes is found with compounds containing a hydrophobic group comprising 16 to 22 carbon atoms. Dyestuff Fixation Direct dyes suffer from the disadvantage that they bleed in water the quaternary ammonium compounds prevent or reduce this effect. With these and other dyes, quaternary ammonium compounds may also act as levellers and restrainers, and as a result have come to be fairly widely used in colour rinses, particularly those of the shampoo type. Since actual chemical combination between the dye and the quaternary ammonium compound may occur, it is frequently found that the quaternary ammonium causes a change of shade which must be anticipated when formulating. The alkyl pyridinium salts appear to be the most popular for this applica• tion, but several other quaternary ammonium compounds are also used. L'Oreal TM have claimed the preparation of creams with dye content which contain both cationic and anionic surface-active agents. ANALYSIS It is clear from the foregoing discussion that comparatively small variations in either structure or purity of a quaternary ammonium compound can result in large variations in properties. Unfortunately many commer- cially available quaternary ammonium compounds are supplied under rnisleading names and frequently without specification. Thus "lauryl