776 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS are applied transiently and then rinsed away (1). Phenylmercuric nitrate may be useful for preserving shampoos having a pH value below 7.0. At higher pH values it is less stable and may be irritating. The phenylmercuric salts are generally used in preparations at levels of 0.001-0.004%. 4.35 Sorbic acid (Hexa-2,•l-dienoic acid) 4.351 Properties Sorbic acid is a white crystalline powder with a mildly acrid odour, slightly soluble in water (1:600). It functions as an antimicrobial preserv- ative, active against moulds and yeasts and to a lesser degree against bacteria. Antimicrobial activity is retained in the presence of nonionic surfactants. 4.352 Disadvantages Sorbic acid is only effective in the form of the undissociated acid and therefore only of use in formulations below pH 6.5-7. In high concen- trations, the sorbates oxidize under the influence of sunlight, which can result in product discoloration. Sorbic acid itself may cause slight itching and erythema, but there is little evidence that it has any adverse cutaneous effect at normal use-levels. 4.353 Uses Sorbic acid is used primarily as an antifungal agent, with an optimum pH about 4.5. Its fungistatic activity is increased by the addition of acids and sodium chloride. It may be used in preparations at 0.1-0.2ø//o. 4.36 Dioxin (6-acetoxy-2, •t-dimethyl-m-dioxane) 4.361 Properties Dioxin occurs as a yellow to light amber liquid, soluble in or miscible with water and many organic solvents. It has a rather strong characteristic odour. Dioxin is active against gram-negative and gram-positive bacteria, yeasts and fungi at concentrations in the range of 0.03-0.1% and is active over a wide pH range. Activity is not reduced in the presence of nonionics. It has good bactericidal and fungicidal properties in proteinaceous cosmetic systems. 4.362 Disadvantages The odour may be objectionable and difficult to mask in some formu- lations. Dioxin may cause some darkening or discoloration of a protein-

HYGIENIC MANUFACTURE AND PRESERVATION 777 aceous product, •vhich increases with time and is accelerated by elevated temperatures. 4.3t33 Uses For preservation of emulsions, liquid soap and some proteinaceous cosmetic systems, Dioxin may be used at a level of about 0.1%. 4.37 Phenols Phenolic compounds in general interact to form complexes with poly- vinylpyrrolidone and certain glycols such as polyethylene glycol 13 000 or polypropylene glycol 1 200, resulting in precipitation or separation of oil, and loss of antimicrobial activity. These compounds have a high oil:water partition coefficient and are usually more active in acid than in alkaline conditions they are unstable to light, sometimes causing product dis- coloration. Although many of these compounds are too irritant for use as preservatives in cosmetics, the halogenated phenols in particular have been employed for this purpose. Thus, p-chlor-m-cresol, p-chlor-m-xylenol, and dichlor-m-xylenol have been used in cosmetic preparations. o-Phenyl- phenol has been recommended for impregnation of paper liners for cosmetic jars. It should be borne in mind that the p-hydroxybenzoates are phenolic compounds, having similar physico-chemical properties to the phenols generally. 4.38 Quaternary ammonium compounds The use of these compounds in cosmetics is limited by the frequency of inactivation by other ingredients, especially nonionic and anionic sur- factants they also interfere with the stability of some cosmetic formulations. However, they are readily water-soluble and inhibit bacterial growth at high dilution. Benzalkonium chloride has been found useful at 0.1% con- centration. Whilst low concentrations of quaternary ammonium compounds appear to be quite safe to use, high concentrations are usually severe eye irritants. 4.3•) Alcohols Where appropriate, ethanol and isopropanol may be incorporated in liquid and other preparations in order to impart preservative activity. Concentrations of alcohol from 5-10ø//o upwards generally inhibit moulds and bacteria, and give useful protection against further microbial con- tamination.