144 JOURNAL OF COSMETIC SCIENCE Table II Mean Diameters of Microemulsions in the Presence of 0.05% w/w Fragrance after One-Week Repeated Freeze-Thaw Cycles Mean diameter (nm) System 24 h after preparation 4 h (40°C) 4 h (40°C) + 4 h (-20°C) Microemulsion alone Coconut Fragrance Orange Tea Violet Standard deviations are in brackets. 27.3 (8.2) 27.3 (8.0) 29.6 (6.9) 27.3 (7.3) 27 .3 (9.2) 29.2 (5.6) 23.6 (6.2) 23.9 (8.1) 28.6 (7.6) 26.0 (8.5) 27.9 (7.4) 27.0 (8.0) 25.2 (7.5) 28.3 (8.2) 30.8 (6.9) 27.2 (8.2) 27.6 (8.0) 29.3 (8.3) acid being detectable in any aqueous solution after two hours, except in pH 5 .0 aqueous solution, in which 32% of the initial amount of kojic acid was still present (Table III). Higher concentrations (9.2 x 10- 3 M arbutin and 1.7 x 10- 2 M kojic acid) were chosen as they corresponded to 0.25% w/w, an amount usually employed in cosmetics as a whitening agent. In this case, arbutin was slightly less stable than at the lower concen­ tration, while kojic acid was more stable: after five-hour UVB irradiation, non-degraded arbutin at pH 7 .0 was at 81.0% (Figure 1), and non-degraded kojic acid at pH 5.0 was at 51.2% (Figure 2). In the presence of microemulsion components (sol. a), a slight increase in the stability of arbutin was noted, while kojic acid underwent somewhat faster degradation, particu­ larly at the start of irradiation. For both whitening agents, maximum photostability occurred when they were dissolved in the microemulsion (see Figures 1 and 2), indi­ cating some protective effect, probably due to the presence of a complex interfacial structure. This result is particularly significant, if one considers the destabilizing effect of the microemulsion components in solution on kojic acid, and might indicate the importance of the microstructure of the disperse system in increasing photostability. Since the microemulsions possessed a rather unpleasant smell due to the surfactants, which would make them unfit for cosmetic use, five aromatic mixtures were used to Table III UVB Photodegradation of Kojic Acid and Arbutin (5 x 10- 3 M) in Aqueous Solutions pH 3.0 pH 5.0 pH 7.0 pH 8.0 Time Kojic Kojic Kojic Kojic (h) acid* Arbutin* acid* Arbutin* acid* Arbutin* acid* Arbutin* 0 100 100 100 100 100 100 100 100 48.2 (2.25) 88.1 (0.6) 66.6 (1.1) 88.5 (4.2) 99.6 (2.9) 99.8 (3.2) 2 83.4 (1.9) 32.2 (3.1) 87.9 (1.5) 99.8 (1.7) 94.9 (3.0) 3 81.9 (4.0) 86.3 (1.4) 99.1 (1.6) 94.1 (1.9) 4 80.6 (2.2) 84.8 (2.6) 98.6 (2.5) 88.0 (2.3) 5 74.5 (2.8) 83.2 (3.0) 98.4 (2.0) 83.9 (2.8) Standard deviations are in brackets (n = 4). * Non-degraded percentages.

C :.;:::. 90 L... m Z"' "'C C Cl) Cl) 0 m L... L... Cl) e a. 70 Cl) - C 0 60 C 50 100 "'C 90 ·u nJ -� 80 "a'.-. � ... C "'C a a 0 70 "'C I... nJ a a. C)._, 60 a "'C 50 C 40 0 0 PHOTOSTABILITY OF WHITENING AGENTS 145 1 2 3 4 5 6 time (hours) Figure 1. Photodegradation of 0.25% w/w arbutin. 1 2 3 4 5 6 time (hours) Figure 2. Photodegradation of 0.25% w/w kojic acid. -+-aqueous sol. ......,._sol.a --+-m.e. -e-m.e. citral ---m.e. linalool � aqueous sol. ___..,._sol.a -+-m.e. �m.e. citral �m.e. linalool overcome this problem. Since many odorous molecules are photounstable ( 15, 16) and can also negatively influence the photostability of cosmetic ingredients, the influence of these perfumed compositions on the stability of kojic acid and arbutin in the micro­ emulsion was investigated.