J. Soc. Cosmetic Chemists 16 15-30 (1965) ¸ Io6• Society o! ½osmai• ½•mists o! G,eat Britain The Activity of Antibacterials Two-Phase Systems H. S. BEAN, S. M. HEMAN-ACKAH and J. THOMAS* Presented at the Symposium on "Preservatives and Antioxidants", organised by the Pharmaceutical Society of Great Britain and the Society of Cosmetic Chemists of Great Britain, in London on I7th November 1964. l•lynop$1$--Bactericides in oil/water systems are partitioned between the two phases, the concentration in the aqueous phase being controlled by the overall concentration and the oil:water ratio. The antibacterial activity of the systems is determined both by these factors and by the enhanced concentration of bactericide at the oil/water interface. A chaaxge in temperature of the dispersions changes their antibacterial activity by virtue of the normal effect of temperature on bactericides, and by a change in the partition coefficient which may, in some cases, be appreciable. During the past decade or so an increasing number of failures of so-called "preservatives" to protect creams from microbial spoilage has been reported. This period coincides with that during which, in general, there has been a change from the use of anionic emulsifiers to nonionic emulsifiers, and this reformulation unwittingly modified the resistance of the products to microbial attack. Many factors influence the effectiveness of a preservative in any formulation and there are some notable reviews on the subject (1,2,3,4). Emulsions and creams contain many substances which collectively form excellent substrates for the growth of micro-organisms (5,6). Under some conditions the oil phase may be metabolised. Fungi have been reported to grow on fixed oils (7,8,9), and some oxidative and lipolytic bacteria can break down fixed oils and fats (10,11,12), while hydrocarbon oils can be *School of Pharmacy, Chelsea College of Science and Technology, London, S.W.3. 15

16 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS metabolised by pseudomonads and other bacteria (13,14,15). Many emulsi- fying agents are liable to microbial attack. In low concentration many anionic emulgents can be used as energy sources (16,17), but the anio,nic soaps are normally used in creams in sufficient concentration to produce an unfavourably high pH for growth or indeed to be bactericidal. Sodium stearate is bactericidal at a concentration of 1 per cent, and other saturated and unsaturated fatty acid soaps at concentrations far below this level (18). Many nonionic emulgents, particularly the fatty acid ester type, can be utili. zed by a variety of organisms (19), but it is not certain whether this is due to the greater facility with which they can be metabolised or to their aqueous solutions being near to neutrality and less likely than aqueous solutions of anionic emulgent s to interfere with the functioning of bacterial enzymes. It is now well established that nonionic emulgents can inactivate a wide variety of commonly-used preservatives including the phydroxybenzoates (20,21), and phenols (22). The mechanism of inactivation may be complex formation due to hydrogen bonding (22) or solubilization within micelles (23), the latter process being established in the case of inactivation of bactericides by anionic soaps (24,25,26). Other important factors affecting the activity of preservatives in o/w systems are those controlling the availability of the preservative in the aqueous phase and in particular the o/w partition coefficient of the preservative, the phase-volume ratio and the temperature. It has long been known that phenols dissolved in oils and fats possess no antimicrobial activity except when the oil is in contact with water (27), and that the inclusion of water in phenol ointment markedly increases the activity of the phenol (28,29). These observations indicate that the activity of phenol in the presence of both oil and water is controlled by its partitioning between the two phases. Indeed, Solution of Chloroxy- lenol B.P.C. which contains 5 per cent chloroxylenol has a Rideal-Walker coefficient of about 3, but 5 per cent chloroxylenol has been found in- adequate to prevent mould growth in emulsified ointments (30). Whenever preservatives are more soluble in oil than in water, enough must be added to an o/w system to obtain a sufficient concentration in the aqueous phase (31). Thus a knowledge of the partition coefficient of the preservative is essential to ensure that the forementioned condition is met. A very useful table of partition coefficients of methyl phydroxybenzoate in oils commonly used in creams has been prepared by Hibbott and Monks ($2) but there is comparatively little data of this type in the literature.