Stability of vitamins C and E in topical microemulsions for combined antioxidant therapy.

An interesting strategy for protecting skin from excessive exposure to free radicals is to support the skin endogenous antioxidant system. As the balance between different skin antioxidants is very important, a combined therapy using at least two antioxidants is desirable. In the present work, o/w, w/o, and gel-like microemulsions (ME), all composed of the same ingredients, were selected as carrier systems for dermal delivery of vitamins C and E. Gel-like ME was found to offer the best protection for both vitamins, although other ME also significantly increased their stability compared with that solution. In the presence of vitamin C no decrease in vitamin E content occurred. To obtain ME appropriate for dermal use, their viscosity was increased by adding thickening agents. On the basis of visual examination of viscosity and physical stability of thickened systems, several thickeners were selected. The addition of thickener significantly increased the viscosity of ME and changed the behavior of systems from ideal Newtonian to thixotropic. Finally, the stability of both vitamins was examined as a function of thickening agent and of the location of vitamins in the ME. The addition of thickeners changed the stability of at least one vitamin, but the systems generally still protected vitamins better than solutions. It is likely that the changes in internal organization of ME resulting from the addition of thickener, confirmed by thermal analysis and changes in solubility of oxygen in the outer phase, were the most important factors that influenced the stability of vitamins in thickened systems.