Preparation and structure of a water-in-oil cream containing lipid nanoparticles.

To obtain a topical dermatological product with a high degree of occlusivity combined with attractive cosmetic properties, a water-in-oil (w/o) cream containing small particles of solid paraffin was developed. Dynamic light scattering, freeze-fracture electron microscopy, polarization microscopy, and differential scanning calorimetry were used to characterize the cream. The preparation method essentially consisted of two steps. First, an aqueous dispersion of solid paraffin particles, with a mean diameter of 200 nm, was prepared with an oil-in-water (o/w) emulsifier. The aqueous dispersion proved to be extremely stable, and the particles had a spherical shape. Second, the aqueous dispersion was incorporated into the water phase of the cream during its production. After production of the cream, 68% of the paraffin was present as particles in the dispersed water phase. The size and shape of these particles did not change by the mechanical treatment during the production of the cream. At least 28% of the paraffin was present in the continuous oily phase, either as solid particles or in the form of a gel structure. At most, 4% of the paraffin was dissolved in this oily phase. The excess o/w emulsifier present in the aqueous phase of the w/o cream did not cause physical instability.