STRUCTURAL INVESTIGATIONS OF OINTMENTS 23 Literatur l l) Ffihrcr, C., Jtmgi•gcr, H., Fribcrg, S., 1. S()c. C(•sn•ct. Chc• ...... (2) Kicssig, H., Kolloid Z. 152, 62 {1957}. (3) Brooks, 1. H., "Spreading behaviour and crystalline phases, part 1II, thermal curves" in "Retardation of Evaporation by M(•t•laycrs", Hrsg. V. K. L,• Met, Academic l•ress, New Yt•rk tu•d London {1962). (4) Stewart, F. H. C., Aust. J. Appl Sci. 2, 157

J. Soc. Cosmet. Chem., 30, 25-38 (January/February 1979) Selection of parabens as preservatives for cosmetics and toiletries JOHN J. O'NEILL, PHILIP L. PEELOR, ARTltUR F. PETERSON and CHARLES tt. STRUBE Avon Products, Inc., Suffern, NY 10901. Received July 31, 1978. Presented at Annual Scientific Meeting, Society of Cosmetic Chemists, December 1977, New York, New York. Synopsis Theoretical extension of Evans' application of the Ferguson principle to PRESERVATION of simple emulsions leads to the conclusion that mixtures of PARABENS are never more efficient than an equal weight of one of their components. This applies to both initial potency and capacity, the latter according "kinetic" expressions also derived from the Ferguson principle. We propose that regardless of the complexity of the product, the most efficient paraben can be identified by making direct measurements of solubilities in the complete product the least soluble paraben is the most efficient in initial potency and, probably, in capacity as well. Microbiological and solubility data on model systems and finished COSMETICS are presented and discussed in terms of the theory. INTRODUCTION Many cosmetics and toiletties are preserved with mixtures ooe parabens, the esters of parahydroxybenzoic acid. A combination of the methyl and propyl esters in a ratio of about two to one is often used. There are many papers in the technical and trade literature in which such mixtures are recommended (1-6) but we find no clear experimental demonstration that any particular binary mixture exhibits synergism in the strict sense of being better, on an equal weight basis, than its more potent component. Neither do we find any theoretical argument that such synergy is to be expected. On the contrary, both theory and data seem to indicate that simple additivity of efficiencies is the rule. The gist of this paper is that the Ferguson principle (7) implies that the paraben of least solubility in the product to be preserved is the best choice in both initial potency and capacity and that no combination can be better in either respect. INITIAL POTENCY IN SIMPLE EMULSIONS In 1965, W. P. Evans (8) showed on theoretical grounds that it is impossible for a binary mixture of parabens to be better than both of its components in a simple 25