ANTIOXIDANT POWER OF DERMOCOSMETIC CREAMS 87 10 - .0 -1 -1.'0 , -0.5 00 0.5 1.0 1 5 I -5 -10 -15 -20 potential (V /MSE) Figure 4. Linear sweep voltammograms obtained with platinum electrode introduced in the bulk of the depilatory cream containing 0 (-), 10 (-), 361 (-·-),and 1246 (---) pmol of H202 per gram of cream. Potential scan rate: 50 mV/s- 1 . scavenge significant amounts of hydrogen peroxide as a reactive oxygen species. They also point to the possibility of further developing the method in a way to measure quantitatively the amount of reduced species that show efficient antioxidant activity, expressed by an equivalent of hydrogen peroxide added to consume them. INFLUENCE OF OXYGEN AND NATURAL LIGHT In order to evaluate the stability of the cream, the influence of two sources of "natural" oxidative stress-oxygen in air and natural light-on its antioxidant properties was studied. An air or a nitrogen flux was maintained in the electrochemical cell containing the cream sample, and the experiment was performed in the dark or in daylight. Cyclic voltammograms were performed at the surface of the cream, and the anodic peak current was recorded as a function of time. Figure 5 represents the evolution of the anodic peak current (ip) compared to the initial peak current (ip0) for the different experimental conditions. First, results revealed in all cases a general decrease in the ratio ip/ip 0 with time. This evolution was observed even when the cream was protected from light and oxygen. In

88 JOURNAL OF COSMETIC SCIENCE 120 ,----------------------------------, 100 � 80 ..... -� 60 40 20 0 300 time (min) 600 Fi g ure 5. Influence of natural light and/or air on the global antioxidant capacity of a depilatory cream. Evolution of the ratio (peak current density)/(initial peak current density) as a function of time (working electrode: platinum disk potential scan rate: 50 m V/s): - ◊ exposed to nitrogen and protected from light · - · f:o., exposed to air and protected from light ... □, exposed to nitrogen and light - - x, exposed to air and light. this latter case, the decrease of 20% in the peak current density after 500 min cannot be due to oxidative stress. It can better be assumed that the surface of the cream dried, resulting in the modification of the diffusion properties and a decrease in the electro­ chemical reaction rate. However, this phenomenon, if correct, remained the same for all the conditions studied because the flux was controlled by a flow meter (flow: 1 1/h). Second, the comparison of the different curves highlights an influence of oxygen and light on the antioxidant properties. An important decrease in the antioxidant properties was observed when the cream was exposed to oxidative stress the fastest decrease was recorded for the creams exposed to daylight. The loss of the antioxidant properties of the cream exposed to light and nitrogen was 30% in the case where the cream was exposed to oxygen only, it was 25% (compared to 20% when the cream was protected from oxidative stress). Consequently, daylight and oxygen played an important role in the loss of the antioxidant properties, but the influence of daylight appeared more significant. However, the most important decrease was observed when the cream was exposed to both factors (air and daylight) simultaneously: 40% of its antioxidant properties was lost after 500 min. Otherwise, the modification of the cream properties was mainly situated at the surface of the cream (data not shown). CONCLUSION Cyclic voltammetry and linear sweep voltammetry are reliable methods to evaluate the