Physical stability, centrifugation tests, and entrapment efficiency studies of carnauba wax–decyl oleate nanoparticles used for the dispersion of inorganic sunscreens in aqueous media

Aqueous nanoscale lipid dispersions consisting of carnauba wax–decyl oleate mixtures acting as carriers or accompanying vehicles for inorganic sunscreens such as barium sulfate, strontium carbonate, and titanium dioxide were prepared by high pressure homogenization. For the manufacture of these nanosuspensions, three pigment concentrations (%wt), namely 2, 4, and 6, and two carnauba wax–decyl oleate ratios, 1:1 and 2:1, were used, being some of these combinations chosen for stability studies. Six-month physical stability tests at 4, 20, and 40 °C selecting the mean particle size and the polydispersity index of the nanosuspensions as reference parameters were performed. Centrifugation tests of the nanosuspensions assessed by transmission electron microscopy and by the determination of the content of pigments and carnauba wax in the separated fractions were done. The mean particle sizes and the polydispersity indices of the nanosuspensions were not altered after six-month storages at 20 and at 40 °C. However, the storage of those at 4 °C considerably increased the particle size and polydispersity of the systems, particularly when wax–oil ratios (2:1) were used for the entrapment of the pigments. Transmission electron micrographs of centrifuged samples denoted the presence of three major fractions showing the different types of particles integrated into the nanosuspensions. Furthermore, it was observed that not all the carnauba wax participated in the entrapment of the pigment. Regarding the amount of pigment being encapsulated or bonded by the wax–oil matrices, entrapment efficiencies higher than 85.52% were reported.