ANTI-AGING EFFECTIVENESS OF RETINOL EMULSIONS 271 (c) Conductivity. Conductivity significantly changed after storage at 40°C. The relatively high values indicated the presence of a continuous aqueous phase. The data are reported in Table I. (d) Centrifugation assay. The phase separation of RETI C and RETI C concentrate emul­ sions stored at 40°C was evaluated. RETI C emulsion showed a partial separation after three days of storage, but the separation progressively increased over time, being evident after ten days. RETI C concentrate emulsion appeared less stable at high temperature, compared to RETI C emulsion. In fact, RETI C concentrate emulsion showed a little phase separation after only a week. After ten days it appeared considerably separated. STABILITY OVER TIME AT 25 ° C The organoleptic features of RETI C emulsion stored at room temperature changed over time. Particularly, their color changed after seven and ten days, becoming dark yellow. RETI C concentrate emulsion appeared pale yellow after one day, but the color inten­ sified over time. After ten days of storage the emulsion was dark yellow in color. As regards concentration stability, RETI C emulsion did not show any retinal degra­ dation after one day at room temperature, but after three days the retinal concentration decreased by 10.3% in comparison to its initial concentration. After a week at 25°C, the percentage of degradation was 16.8%, and after ten days 23.9%. RETI C concentrate emulsion followed a similar trend. The concentration of retinol began to decrease after three days of storage and showed a decrease of retinal equal to 15. 5 % , while after seven and ten days the percentage of retinol degraded was 19.4% and 24.0%, respectively. In Figure 9 the percentage of retinol degraded after storage at 25°C is reported. (a) Viscosity. In Figure 10 the viscosity values of RETI C emulsion before and after storage at 25°C are plotted. They decreased over time, indicating a low instability in the system. RETI C concentrate emulsion showed an anomalous behavior. In fact, the viscosity decreased after one and three days at room temperature, increased after a week, and decreased again after ten days. The curves recorded at different shear rates are reported in Figure 11. ( b) Rheologic determinations. The evolution of rheological properties of emulsions stored at room temperature were investigated by plotting shear stress versus shear rate values. The shear stress values of RETI C emulsion showed a clear decrease over time. At the beginning of the test, the preparation had a pseudoplastic rheological behavior with very small thixotropic hysteresis areas. After seventh day, the system showed a flux inclining to Newtonian. The flux rheograms of RETI C emulsion after storage at 25°C are reported in Figure 12. Table I Conductivity of RETI C and RETI C Concentrate Emulsions After Storage at 40°C Days of storage RETI C RETI C concentrate 1 2.89 mS 1.42 mS 3 1.53 mS 1.30 mS 7 1.55 mS 0.64 mS 10 1.16 mS 1.23 mS

272 100 90 "O 80 "O 70 cu 60 "O 50 40 cu 30 ... � 0 20 10 0 JOURNAL OF COSMETIC SCIENCE y = 2,4 706x + 1,4034 R2 = 0,8665 RETI C cone 0 2 4 6 8 time (days) ♦RETI C ■RETI C cone y = 2,4477x - 0,0805 R2 = 0,9655 RETI C 10 Figure 9. Percentage of retinal degradation in RETI C and RETI C concentrate emulsions after storage at 25 ° C. 80000 70000 en 60000 50000 .§. • at opening - • - after 1 day � 40000 - - • - ·after 3 days 30000 - -E- • after 7 days 20000 ·s;: - -Jf - after 10 days 10000 0 a 5 10 15 Figure 10. Viscosity of RETI C emulsion before and after storage at 25 ° C. RETI C concentrate emulsion showed an irregular trend. In fact, after the first and the third day of storage a decrease in shear stress was observed at increasing shear rates. After seven days the shear stress values increased, and after ten days they decreased again (Figure 13). (c) Conductivity. Both emulsions stored at 25 ° C showed progressively a decrease in con­ ductivity over time, nevertheless maintaining values in the order of mS, characteristic of emulsions having external acqueous phase. The results are reported in Table II. (d) Centrifugation assay. The studied systems were submitted to an accelerated techno­ logical test to assess their stability at room temperature. RETI C emulsion remained unvaried and showed a little separation after only ten days. On the other hand, RETI C concentrate emulsion demonstrated high stability because it didn't show any phase separation over time.