COSMETIC PRESERVATION 667 the phases. In any specfic emulsion, the equilibrium distribution of a stable prese. rvative in the different phases will depend on the nature of the phases (i.e.,: on the partition coefilcient), the solubility limits of the preservative, the volumes of the different phases, and the pH. Methyl paraben, for example, is soluble in water to the extent of about 0.25% at room temperature. It is possi- ble to make up an aqueous phase containing 0.25% methyl paraben, but if that phase comes into contact with an equal volume of vegetable oil, the methyl paraben will gradually migrate into the oil phase. Eventually, the methyl paraben will partition approximately 10 to i in favor of the oil phase. The aqueous phase now contains not 0.25% methyl paraben, but only about 0.02 or 0.03%. The emulsion is probably not adequately protected against micro- bial attack. Migration of preservatives is a complex phenomenon, and attempts to pre- dict final distributions in actual emulsions are made difficult by the diverse nature of each phase. Each ingredient of a phase affects the solubility of the preservative in that phase. Since a preservative system must be able to protect a cosmetic product throughout the period of its storage and use, the migration of a preservative out of the water phase can be as serious a problem as the loss of the preservative by evaporation, reaction with other components, or chemical breakdown. Only microbiological challenge testing over extended periods of product storage can prove that migration of preservatives is not a problem. Although the presence of water is essential for microbial growth, anhydrous cosmetics should also be protected by a preservative system. If anhydrous oils or powders are kept completely water-free, preservation problems are minor. Unfortunately, a film of water can form on such a product just by exposure to moist air, and in the high local concentrations of water, microorganisms will grow. It is a wise precaution, therefore, to have a water-soluble preservative in an anhydrous powder or oil, so that when water does condense onto the prod- uct, the preservative can dissolve in the water and prevent microbial growth. The final factor, which is probably the most common source of preservative failure, is the effect of other components of the formulation. The components of a• cosmetic may interfere with or may help the antimicrobial action of a preservative. For this reason, s,creening experiments (serial dilutions, mini- mum inhibitory doses, etc.) in laboratory media are practically worthless to the cosmetic chemist. It is essential that preservative testing be carried out on the cosmetic itself. A preservative that functions well in a screening experi- ment with microbiological growth media may be usless in the actual cosmetic because of interference by some component of the cosmetic. Conversely, a compound that behaves poorly in a screening test with microbiological media can have high antimicrobial activity in the cosmetic, not only because of en- hancement of the preservative action by other components, but also because the cosmetic is usually not an ideal culture medium for microorganisms. The

668 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS microorganism is often more susceptible to antimicrol)ial action in the cosmet- ic than it is in a carefully selected nutrient medium. For these reasons• cos- metic chemists aud microbiologists should test what really •natters: the cos- metic itself. Nonionic emulsifiers are commonly used in eosinerie ereams. Unfortunately, they are also one of the most common causes of preservative failure. When used in low concentrations (e.g., below ca. 1%), they do not significantly in- terfere with preservative action. Usually, they are employed in concentrations higher than the critical mieelle concentration. Above this critical mieelle con- centration, the nonionic emulsifier forms aggregates, called "micelies," which constitute a new phase in the system and actually extract preservatives out of the other phases. If 5% nonionic emulsifier is added to a water-oil enmlsion, as much as 75% of the total methyl paraben preservative will locate in the nonionic emulsifier mieelle, leaving only 25% of the preservative to distribute between the oil and water phases (1). Some organic compounds form a coating around the microorganism and give the cell protection against chemical attack. Many components of cosmet- ics inaetivate or lower the activity of antimierobials by reacting with them, absorbing them, or dissolving them. Although the inaetivation is sometimes complete, often it is not, and some residual activity renmins. In the following comments the term inaetivated is meant to include both complete and partial inaetivation. Thus, anionie surface-active agents inaetivate eationies (i.e., quaternary ammonium compounds), phenolies, and mercurials. Proteins inae- tivate most preservatives: quaternary ammonium compounds, phenolies, mer- curials, and parabens. Antimicrobial agents are inaetivated by many different materials. Quater- nary ammonium compounds lose activity in the presence of lanolin, methyl cellulose, leeithin, tartaric add, silicates, kaolin, zinc oxide, and other com- pounds. Although quaternary ammonium compounds are effective antimicro- bial agents in screening experiments, they are not of general use in cosmetics. Parabens are bound up by various macromolecules, such as methyl cellulose and gelatin. Nonionic emulsifiers, especially those containing polyethyleneoxy groupings (e.g., Tweens ©*, Myrj©*, etc.), and many other coralnon ingredi- ents in cosmetics also tie up parabens and render them less available for anti- microbial action. The lesson to be drawn from all these possible complications is that a pre- servative system must be tested in the specific cosmetic product being studied. A related problem is absorption of preservative by the container used. Rub- her and plastics, especially polyethylene containers or caps, are ahvays suspect because lipid-soluble preservatives are capable of migrating into them. These effects can sometimes be predicted, but only microbiological challenge tests *Registered trademarks of ICI America Inc., Wihnington, Del.