CATIONIC SURFACE-ACTIVE AGENTS By P. A. LINCOLN, M.Sc., M.R.S.H.* A lecture delivered to the Society on Friday, April 5th, 1957. SURFACE-ACTIVE agents, however these may be defined, fall into two classes: Ionic surface-active agents, which are substances capable of ionisation in aqueous solution, and Nonionic surface-active agents, which do not so ionise. Where the surface-active properties of ionic compounds are associated with the anion the substances are termed anionic surface-active agents or simply anionics where these properties reside in the cation, cationic surface- active agents or cationits. Some compounds can behave both as cationic and anionic agents according to the pH of the medium, and this zwitterionic class is referred to generally as ampholytic. The majority of surface-active ions are characterised by a chemical structure which is essentially linear, one end of these linear surface-active ions being composed of radicals which are hydrophilic and the other of hydrophobic radicals. The ionic surface-active agents may be subdivided according to the nature of the hydrophilic or solubilising group contained by the active ion. Whereas the principal anionics utilise the carboxy (--COOH), sulphonic acid (--S03H) or sulphuric ester (--OS03H) solubilising groups, all cationic agents of major importance contain the basic nitrogen atom and are salts of primary, secondary or tertiary amines, or of quaternary ammonium bases. Structure Cationics may therefore be represented by the type structure R2 R• -- N -- R3 I R, X- (I) where R•, R2, R•, R, are natural or synthetic radicals or hydrogen, at least one of these substituents containing 8 to 22 carbon atoms. Only rarely is more than one substituent hydrophobic, and the remaining groups are commonly H, CH•, C2H5 or benzyl groups except where the compound is derived from a heterocyclic base, as, for example, with pyridinium and * Milton Antiseptic Ltd., London, W. 1. 222

CATIONIC SURFACE-ACTIVE AGENTS 223 morpholinium salts. Three typical structures of fully substituted cationics are illustrated by cetyltrimethylammonium chloride, cetylpyridinium chloride, and p-tert-octylphenoxyethoxyethyldimethylbenzylammoni chloride. H• + C.H • •--N--CH • I C1- (II) Cetyltrimethylammonium chloride CH = CH C•6H33 -- N CH CH -- CH C1- (III) Cetylpyridinium chloride c 8H. OC2H 40C2H 4N (CH CH• C1- (IV) p-tert-Octylphenoxyethoxyethyldimethylbenzylammonium chloride Cationics which contain one or more hydrogen groups, i.e., non-quate• nary compounds, are characterised in that the free base is formed and normally precipitated in alkaline media and so can only be employed effectively in acid solutions. Quaternary ammonium bases are sufficiently strong, however, for their salts to be stable in both acid and alkaline solution, and are only unstable in the presence of alkalis in those instances (e.g., quaternary pyridinium salts) where the free base undergoes decomposition. For this reason, together with the fact that many free amines are primary skin irritants, the surface-active quaternary ammonium compounds are the sole class of cationic agent employed in cosmetic, toilet and pharmaceutical preparations, and discussion will be limited to this type of compound.