•26 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS The reaction requires an excess of pyridine and some secondary reactions invariably occur such that isolation of a pure product can only be achieved by crystallising from a suitable solvent. The alkylpyridinium bromides, in common with other quaternary bromides, are prepared more easily than the corresponding chlorides. In each of the examples given the hydrophobic radical is united directly to the basic nitrogen atom. Some compounds are used in which the hydro- phobic group is united to the ionic group by means of ester, amide or ether linkages, as with many anionic surface-active agents. One such compound is p-tert.octylphenoxyethoxyethyldimethylbenzylammonium chloride• (IV.) GENERAL PROPERTIES With the infinite number of compounds which may be produced by vary- ing the nature of the radicals R•--R4 attached to the quaternary nitrogen atom it is to be expected that markedly different properties will reside in the various compounds produced. Thus generalisation as to properties of the group as a whole is difficult, and failure to recognise this basic truth has led to many patently inaccurate statements in the literature. It is generally true to say that most cationics will react with anionics and other anions of high molecular weight to form poorly ionised, insoluble compounds, yet marked differences are shown in terms of stlch "incompatibilities," and one cannot argue from the specific to the general case as has been done by several authors (e.g., Lawrence3). The influence of modest variation of the groups R•-R4 on the properties of a quaternary ammonium compound is illustrated in the series: cetyltrimethylammonium bromide stearyltrimethylammonium bromide cetyldimethylbenzylammonium bromide stearyldimethylbenzylammonium bromide stearyldimethylethylammonium bromide wherein the visual appearance alone of aqueous solutions of the materials shows obvious differences to exist in the physical properties of these com- pounds. With anionic detergents the nature of the non-surface-active ion affects the properties of the molecule as a whole, as, for example, the well-known cosmetic differences between the calcium, magnesium, potassium and amine soaps. So also with cationics the nature of the associated anion contributes to the properties of the compound as a whole. Where a quater- nary ammonium compound is used in the presence of a particular anion, the properties are dictated by the ionic equilibrium which results. This has been used as the basis of a technique whereby it is possible to prepare

CATIONIC SURFACE-ACTIVE AGENTS 227 the quaternary salts of any acid, including those of weak or unstable acids, e.g., nitrites, bicarbonates, sulphites, etc. • Patents have been granted to workers who have "discovered" that the artion affects the properties of a particular cation, as in the case of the claim that the alkyldimethylbenzylammonium bromides and iodides are more powerful bactericides than the corresponding chlorides. 5 Apart from the fact that such differences have been reported previously, 6,7 these disappear in practice when other substances are present which can displace and control the ionic equilibrium established by solution of the original quaternary salt in water alone. The test technique influences the results obtained also, and it is not uncommon to find that in vitro results bear no relation to those obtained in vivo. Cationic surface-active agents possess similar properties to the anionics in respect of their ability to act as detergents, emulsifying agents, to foam, to reduce interfacial tension, etc., but they are rarely used for these proper- ties alone. The development of the cationics has been conditioned by those properties of many compounds of this class which are not possessed by anionics to any marked extent, principally their: (a) Bactericidal activity. (b) Adsorptive properties. (c) Fixation of direct dyestuffs. Bactericidal Activity Not all surface-active quaternary ammonium compounds are strongly bactericidal the activity varies considerably with the structure of the compound in a manner which cannot be directly related to physical proper- ties such as surface tension reduction. Table 1 illustrates the variation of a•[tivity observed using one test technique for some closely related com- pounds. For any one compound the bactericidal activity may be made to appear good or poor simply by varying the bacteriological test technique employed. This frequently makes comparison of results from different authors difficult. TABLE 1 Method: United States Department of Agriculture, Circular 198. Results expressed as concentration of substance which will kill organisms of standard phenolic resistance in 10 minutes but not in 5. Substances: Lauryldimethylbenzylammonium bromide (L.D.B.A. 13.) Cetyldimethylbenzylammoniurn bromide (C. D. 13. A. 13. ) Lauryldimethylbenzylammonium chloride (L.D.13.A.C.) Myristyldimethylbenzylammonium chloride (M.D.13.A.C.) Cetyldimethylbenzylammonium chloride (C.D.13.A.C.) Stearyldimethylbenzylammonium chloride (S.D.B.A.C.) Lauryltrimethylammonium bromide (L. T. A. 13. ) Myristyltrimethylammonium bromide (M. T. A. 13. ) Cetyltrimethylammonium bromide (C. T. A. 13. ) Stearyltrimethylammonium bromide (S.T.A.B)