218 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS •D • * oo

PRESERVATION OF TOILET PREPARATIONS CONTAINING NONIONICS 219 TABLE IV SORBIC ACID As in the case of benzoic acid the pH of all test media was adjusted to pH $.$. Sorbic Acid Sorbi½ Acid + 6% Nonionic 0.2 0.15 0.1 0.05 0.2 0-15 0-1 0.05 % % % % % % % % staph. a -- -- +++ +++ -- + +++ +++ albus b -- -- +++ q-++ -- -- +++ +q-q- Strep. a -- -- ++ +++ -- ++ ++ +++ bovis , . b -- -- + +++ -- ++ ++ Ps. a -- -- +++ +++ -- ++ ++ +++ escens b -- -- +++ +++ -- + ++ +++ -- . . Proteus a -- -- -- + + -- -- + + + + vulgaris -- b -- -- -- q-q- -- -- q- q-q-q- Gram + These tables show that all these preservatives which remained active during the tests reported in Table I, that is, where the ratio of nonionic surfactant to preservative was lower, are in fact inactivated to some extent at higher ratios. The bacteriostatic titre for each preservative naturally varies for each test organism, but it is interesting to note that this titre is invariably lower when 6 per cent of mixed nonionic is added to the test medium containing preservative. This effect is particularly noticeable in the case of the organic mercurial compounds. Effect of Ratio of Nonionic to Preservative Some preservatives are effective at certain concentrations, but when nonionic surfactants are present, their effective concentrations are higher. This suggests the existence of a critical ratio of preservative to nonionic. The effect of altering the ratio of various nonionic surfactants to several preservatives has been studied and Table VI shows that there is a critical ratio at which the preservative becomes ineffective. As an example the results of an experiment with Tween 80 and Benz- alkonium chloride are given. Tween 80 and Benzalkonium Chloride 0.1 per cent of Benzalkonium chloride was used throughout and the amount of Tween 80 was varied.