Optimization of a surfactant-free antioxidant formulation using response surface methodology.

Topical application of natural antioxidants has proven to be effective in protecting the skin against ultraviolet radiation-mediated oxidative damage. In previous studies, a Castanea sativa leaf ethanol:water (7:3) extract exhibited scavenging activity against different reactive oxygen species that are thought to contribute to oxidative damage in the skin. Its stability was shown to be enhanced in the presence of glycerine, and therefore a glycerine-based formulation with Carbopol 940 and liquid paraffin (LP) was developed as base. In this work, the influence of the glycerine and LP contents on the textural properties of the topical base and on the antioxidant activity of the formulation with C. sativa extract was evaluated using response surface methodology after 30 d storage at 20 °C and 40 °C. The textural analysis was performed in a texturometer, by carrying out a spreadability test. Paretto charts showed that both glycerine and LP contents significantly influenced the textural properties of the formulations (p < 0.05). LP presented the major influence. DPPH scavenging activity was not related to any of the studied ingredients. These conclusions were valid both for 20 °C and 40 °C storage. This optimization study provided valuable information to support the development of a semisolid base for C. sativa extract leading to the conclusion that the selection of these ingredients contents can be guided exclusively by the desirable textural properties.