A model for the evaluation and prediction of preservative activity in oil-in-water emulsions.

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Abstract
A simple mathematical model was derived to evaluate the antimicrobial activity of p-hydroxybenzoates in emulsified systems containing nonionic surfactants of the polyoxyethylene type. The model consists of preservative bound within the surfactant micelles, species partitioned into the oil phase, the ionized form in the water phase and the free effective form in the aqueous phase. The validity of the derived equations was confirmed by ultrafiltration experiments. The total concentration of a preservative necessary to give the desired concentration of free preservative in the aqueous phase of the emulsion can be calculated using this model. The amount of bound preservative, within a mixture of several surfactants, could be predicted from the binding parameters which characterize the interaction with each surfactant constituting the mixture, the binding parameters of a preservative for a surfactant mixture could be estimated from the hydrophile-lipophile balance value of the mixture and the binding parameters of some appropriate surfactants. The limitations of this simple mathematical model are discussed.

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