Applicability of the Ferguson principle to systems of mixed preservatives

Abstract

Using solubility and distribution data for methyl and propyl p-hydroxybenzoates, the degree of saturation of the aqueous phase of a n oil-in-water emulsion has been calculated for various oil: water ratios. In emulsions of low oil content the propyl ester gives the higher degree of saturation of the aqueous phase, while in emulsions of high oil content the methyl ester gives the higher degree of saturation. Irrespective of the oil :water ratio, a mixture of the esters cannot give a higher degree of saturation or a higher thermodynamic activity of the aqueous phase, than either ester alone.The finding of many investigators that mixtures of the esters have higher biological activities than a single ester must therefore depend, not only on the degree of saturation, or thermodynamic activity of the aqueous phase, but also on some other unknown factors.ETERIORATION from mould formation or bacterial action of D cosmetic creams during storage occurs mainly in oil-in-water emulsions, and is rare in water-in-oil emulsions. Generally the organisms responsible for deterioration need an aqueous medium for growth, consequently cosmetic products contain preservatives which are effective in preventing growth in the aqueous phase. The most widely used preservatives are esters of p-hydroxybenzoic acid, the methyl and propyl esters being generally favoured.The literature contains conflicting views (Manowitz, 1962) about whether to use these preservatives singly (Atkins, 1950) or in combination (Boehm, 1959); those who advocate using the mixture claim a broader spectrum of microbiological activity than that given by their separate constituents, and also that the mixture exhibits a greater preservative action than would be expected from the individual activity (Boehm, 1959).It is well known from bacterial studies that the activity of the esters of p-hydroxybenzoic acid is related to the length of the alkyl chain; the propyl is more active. than the methyl ester but increasing the chain length reduces water' solubility and increases oil solubility, i.e. the oil : water distribution coefficients are markedly increased. As deterioration in cosmetic preparations occurs in the water phase, only that proportion of the added preservative which is dissolved in the aqueous phase can inhibit the growth of micro-organisms. But, is the greater bacterial activity of the propyl compared to the methyl ester more than offset by its lower water solubility and its higher oil solubility? Atkins (1950) claims that this is often so and that the methyl ester is then the more reliable preservative ; Atkins' data, therefore, do not support the widely held belief that a combination of the two esters is better than either alone.Since there is no apparent agreement in the published literature about whether a mixture of esters is better than a single ester in emulsion systems, calculations based on solubility and distribution data might throw