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j. Soc. Cosmet. Chem., 47, 377-386 (November/December 1996) Stability and optimization of silicone-based sunscreen formulations A. HERNANDEZ, V. GALLARDO, A. PARERA, and MaA. RUIZ, Pharmacy and Pharmaceutical Technology Department, Faculty of Pharmacy, University of Granada, E-18071 Granada, Spain. Accepted for publication February 1, 1997. Synopsis Advances in silicone chemistry have led in recent years to the development of a wide variety of compounds derived from the basic structure of siloxane bond polymers, with properties such as chemical inertia, heat resistance, and lack of systemic toxicity. Because these compounds do not cause sensitization or irritation of the skin or eyes, they are excipients of choice in skin-care products and drugs. The advantages of silicone emulsions are that they can be prepared at room temperature, have adequate extensibility, and do not leave a greasy, sticky, or unctuous film on the skin. Physical stability assays (centrifugation and aging at 25øC, 40øC, and 60øC) yielded similar results. Rheological assays, tests at different pH values, and droplet size studies were also done. Stability of the preparation was adequate: droplets remained small and homogeneous, and did not increase in size because of coalescence during the assay. pH showed minimal variations, and remained within the range of 6.5-7. Different centrifugation assays were used, none of which led to breakdown of the emulsion. Because of their hydrophobic nature, silicones impart substantiveness to the preparation. This was confirmed experimentally in washing-off tests, used because the final product was intended for use by swimmers exposed to natural sunlight. After 24 h of testing, an acceptable percentage of the initial concentration of solar filter remained present in the formulation. INTRODUCTION Among cosmetic preparations, emulsions have evolved quickly in recent years because of technological advances and research on the properties of raw materials. Emulsions are distinguished on the basis of their chemical composition and fluidity, and are generally homogeneous in appearance. To develop effective and stable formulations, complete analysis of an emulsion requires the determination of several general characteristics and the individual study of each component. The current development of silicones has led to new approaches in the preparation of emulsions moreover, the versatile siloxane derivatives also provide a number of advantages (1). 377