SILICONE-BASED SUNSCREENS 381 ABSORPTION ABSORPTION _ 0,6 - 0,4 - 311 rim. • 3•9 urn. -1 270 290 310 :320 340 359 370 400 WAVELENGTH (nm) Figure 2. Absorption spectrum of the formulation. -0,8 0,6 0,4 - 0,2 experimentally with microscopic examination that confirmed the stability of the samples (12): droplet size remained small and homogeneous. During the final days of the ex- periment, samples at 60øC showed a slight reduction in particle size, with a slight loss of homogeneity in comparison with the beginning of the experiment. Decreased droplet size was accompanied by increased viscosity, as found in the previous assay. Figures 6-8 show droplets from samples kept at room temperature, 40øC, and 60øC. Resistance to centrifugation was also used to test stability. This procedure is useful for comparing formulas of similar composition and density on the basis of phase separation, creaming, or exudation. In accordance with the AFNOR T73, 409, 1976 standard, samples were spun at 15,000 rpm for 10 min at 30øC. We also ran tests according to the recommendations of Merril (11), i.e., samples were centrifuged at 3000 rpm for 30 min. None of the tests caused separation of the components in the final formulation.

382 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS ABSORPTION .... ß • •'•. , ABSORPTION 311 nm. ! 1 .,_•----•_ 0.8 0.6 - 0,4- 0.2 - + • 3S9 rim. 0 15 30 45 60 75 TIME (days) 0.6 0.4 0,2 o 9O ß 25"C 311nm I 40•C 311nm • 60"C 311nm ß 25"C 359nm I 40•C 359nm ' • 60"C 359nm Figure 3. Absorbance of the formulation as a function of time for three temperatures and two wavelengths. pH PH • _ _ _ o 20 35 50 65 80 TIME (days) -õ -6 -4 -2 • 0 ß 2fi.C I 40oc * 60.C Figure 4. Effect of time on the pH of the formulation for three temperatures.