Forster, Th., B. Jackwerth, W. Pittermann, W. von Tybinskin and M. Schmitt. Properties of Emulsions. Cosmetics & Toiletries 1997. 112:73-82.

Most of today's cosmetics contain active ingredients that will ameliorate skin conditions and functions only if they penetrate the skin. A systematic in vitro study of penetration into the stratum corneum and the living skin was performed with the isolated perfused bovine udder skin model. The results substantiate the influence of emulsion type and structure on the degree of penetration of water-soluble and oil-soluble vitamins into the skin. Emulsions are a more preferable application form than simple solutions, because of their universal solubilizing behavior and the higher effective concentration of the vitamins in the oil or water phase for the same initial added amount. The oil-soluble vitamin E penetrates more strongly from w/o than from o/w emulsions. This is mainly attributable to the lamellar gel network blended into the o/w emulsions to impart better consistency; this network impedes the free diffusion of the vitamin from the oil phase into the skin. The water-soluble panthenol penetrates more strongly than the oil-soluble vitamins, which can be explained by increased thermodynamic activity when the emulsion dries on the skin.