I. Soc. Cosmet. Chem., 26, 105-117 (February 1975) The Influence of pH, Emulsifier, and Accelerated Ageing upon Preservative Requirements of OIW Emulsions GENE JACOBS, M.S., S. M. HENRY, Ph.D., and V. F. COTTY, Ph.D.* Presented December 11, 1973, New York City Synopsis-Twenty-nine individual PRESERVATIVES and 16 combinations of two or more preservatives were tested in an ANIONIC and a NONIONIC OIL/WATER LOTION, each of which was formulated at an acid and an alkaline pH. MINIMUM INHIBITORY CONCENTRATIONS against four representative challenge MICRO- ORGANISMS were determined for the preservatives and/or combinations. Less than 35% of the preservatives or systems tested in lotions were effective. Anionic emulsions were somewhat easier to preserve than formulas made with nonionic emulsifiers. INTRODUCTION A recent survey by Gucklhorn (1) reviews data on a large number of anti- bacterial and antifungal compounds. He has gathered information on these compounds from a variety of sources and provides an invaluable review of the pertinent published information. There are, however, serious gaps in available data due to the variation in test methods employed to evaluate the preservatives. It was our purpose to study those compounds of most interest in a uniform system (or systems). In addition, we were interested in developing a proce- dure for screening new preservatives that gave more significant results than the traditional serial dilution in broth. Oil-in-water emulsions, known to be highly susceptible to microbial degradation, were chosen as the model sys- tems for this study. * Bristol-Myers Products, Hillside, N.J. 07207. 105

106 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Representative oil-in-water emulsions were devised to study the following aspects of preservation: 1. Inaetivation of preservatives in anionie and nonionic enmlsifier systems. 2. The effect of pH on preservation potential. 3. Stability of preservatives upon ageing at elevated temperatures. 4. Determination of Minimum Inhibitory Concentration (MIC) of satis- factory preservatives. 5. Augmentation of activity by inclusion of two or more preservatives. A survey of the literature on cosmetic preservation shows that several in- vestigators have studied the inaetivation of preservatives by nonionic emulsi- fiers and/or other components of a formulation. Most investigators use an aqueous system to evaluate binding of preservatives by nonionics. Some ex- cellent papers of this genre are by Patel (2), Patel and Kostenbauder (3), deNavarre (4), Wedderbum (5), and Barr and Tice (6). A number of papers have been published which deal with interactions between anionies and preservatives. Many of these state that anionie emulsi- fiers may enhance the activity of some preservatives. However, Schuster and Modde (7) demonstrate binding of some preservatives by anionie moieties in aqueous systems. Entrekin (8) and Wiekliffe and Entrekin (9) evaluated the effects of pH on preservatives in a broth medium. Optimal pH conditions were ascertained for 17 preservatives and 4 preservative mixtures. Oil-in-water emulsions have been used by a few authors to evaluate pres- ervation phenomena. Boehm (10) used emulsions to evaluate synergism be- tween preservatives against a variety of test organisms. Barr and Tiee (6) studied the preservation of aqueous solutions containing nonionic surfae- rants. Those preservatives showing satisfactory results were incorporated into emulsions to compare preservative activity in aqueous systems with emul- sions. The effect of the hydrophilie-lipophilie balance (HLB) of nonionic sur- factants on preservatives in emulsions was studied by Tilbury (11). Ten preservatives were incorporated into simple emulsions to evaluate HLB on preservation. Bean et al. (12) studied the effects of the partition coefficient of a preservative in cosmetic emulsions. The present study differs from most of the above investigations in that it is concerned with the simultaneous evaluation of pH, emulsifier type, and accelerated ageing of complete oil-in-water emulsions prepared with the great variety of preservatives now available. Further, it is an extension and up- dating of several facets of the preservative studies conducted by others. Pre- servatives no longer usable in cosmetics because of regulatory action have been excluded from evaluation. The authors, instead, have concentrated on those currently available to us including many new compounds with little or no history of use in oil-in-water emulsions. The tables of effective levels