ANTI-AGING EFFECTIVENESS OF RETINOL EMULSIONS 269 (a) Viscosity. The viscosity of RETI C and RETI C concentrate emulsions was investi­ gated over time after storage at 40°C. For RETI C emulsion, viscosity progressively decreased until the seventh day, but it increased again after ten days. However, RETI C concentrate emulsion showed a different behavior, with viscosity decreasing at first, increasing slightly after seven days of storage, and showing lower values after ten days (Figures 5 and 6). ( b) Rheologic determinations. Flux rheograms of RETI C and RETI C concentrate emulsions after storage at 40 ° C were performed. The curves reported in Figure 7 of RETI C emulsion indicate that the shear stress decreased over time, but the found values were higher after ten days in comparison to those of the seventh day. The thixotropic hys­ teresis area appeared smaller, and the system, initially pseudoplastic, became Newtonian. Figure 8 shows the rheograms of RETI C concentrate emulsion. Also, in this case, the shear stress decreased and the flux changed from pseudoplastic to Newtonian with a reduction of the thixotropic hysteresis area. en §. - · 0 c., · en - 0 c., ·s: 80000 70000 60000 50000 40000 30000 20000 10000 0 0 :lK. .1.., ,:, -�-\.......... - --■ *. - . - - � � :_�:=--: :=: :_ � 5 10 • at opening - • - after 1 day - • - after 3 days - ·E- • after 7 days · - iK- - · after 10 days 15 Figure 5. Viscosity of RETI C emulsion before and after storage at 40 ° C. 80000 70000 60000 • at opening 50000 - • - after 1 day 40000 - • - after 3 days 30000 - ·E- • after 7 days 20000 • - • • · after 10 days 10000 0 a 5 10 15 Figure 6. Viscosity of RETI C concentrate emulsion before and after storage at 40 ° C.

270 cu en en en .§. II) 185000 165000 145000 125000 105000 85000 65000 45000 25000 5000 185000 165000 145000 125000 105000 85000 65000 45000 25000 5000 0 0 5 JOURNAL OF COSMETIC SCIENCE 10 15 20 25 shear rate (s· 1 ) --.•--upper curve at opening - - ♦ - · back curve at opening al. upper curve after 1 day - - 11.- - · back curve after 1 day • upper curve after 3 days - - IC- - · back curve after 3 days ----upper curve after 7 days - - + - · back curve after 7 days -�e..---upper curve after 1 O days - - 0- - · back curve after 10 days Figure 7. Flux rheograms of RETI C emulsion after storage at 40 ° C. 5 10 15 shear rate (s. 1 ) 20 25 • upper curve at opening - - ♦ - · back curve at opening al. upper curve after 1 day - - * - · back curve after 1 day • upper curve after 3 days - - IC- - · back curve after 3 days ---upper curve after 7 days · - + · · back curve after 7 days -""'e--upper curve after 10 days - - 0- · · back curve after 10 days Figure 8. Flux rheograms of RETI C concentrate emulsion after storage at 40°C. Sometimes the medium in which the functional substances are dispersed could be an alter­ native substrate of degradation, increasing the stability of the active molecules. Anyway, upon observing the results of rheological studies after storage at 40 ° C, it is clear that both RETI C and RETI C concentrate emulsions go towards only a slight degradation, proved by the limited loss of viscosity over time. Therefore, it can be concluded that in this case vehicles do not take part in protecting the active substances against their degradation.