Simple Creams, Complex Structures

Creams intended for topical application are formulated as oil-in-water emulsions; such creams have been widely studied and widely used in pharmaceutical,cosmetic,and personal care products.The scientific understanding of their internal molecular structures is still far from complete, however, with the result that they are still formulated only in a semi-empirical manner. To rectify this deficiency and work toward a structure-based rationale for topical formulations, we report here a study wherein we sought to secure a detailed understanding of the molecular structure of creams based on the medicinal product Aqueous Cream BP. Each cream was formulated with liquid paraffin as the oil and hexadecanol and octadecanol as cosurfactants, but with the surfactant varied among sodium dodecyl sulfate (SDS), distearoylphosphatidylcholine (DSPC),and dodecyltrimethylammonium bromide(DTAB).The macroscopic properties of the creams were studied using rheology and microscopy,and their molecular architectures were determined by small-angle neutron scattering exploiting the technique of hydrogen/deuterium contrast variation.The creams prepared with the different surfactants were found to have quite different internal architectures.The DSPC-based cream contains relatively large oil droplets interspersed with unilamellar DSPC vesicles and oil-containing cosurfactant lamellae. The SDS-and DTAB-based creams contain smaller oil droplets,with the former also containing mixed SDS/cosurfactant bicelles along with oil-containing cosurfactant lamellae and the latter containing only oil-containing DTAB/cosurfactant lamellae. The different creams also exhibit differences in the surfactant/cosurfactant layers surrounding the oil droplets.The combination of these various microstructural differences leads to differences in the creams’ rheology that impact their long-term stability.