SILICONE VEHICLES FOR ALOE VERA 259 Figure 4. Microphotograph of droplets of the formula with Abil WE 09 5% at 40øC. well as for quality control both during and after the formulation. The two formulas with 5% surfactant were therefore tested rheologically as a function of temperature (20 ø, 30 ø, and 40øC) using a digital rotary viscometer (Brookfield DV-II, spindle no. 27). Figure 7 gives the viscosity of these formulations at the afore-mentioned temperatures. It can be seen that an increase in temperature causes a decrease in viscosity. This phenomenon occurs in most pharmaceutical preparations since a rise in temperature produces greater mobility in the dispersive phase and thus a decrease in viscosity (13). The rheograms obtained reveal an almost non-existent "hysteresis loop," with decreasing values (slightly higher in the Abil EM 90 formulations) as the temperature increases. The flow curves (Figure 8) of the two preparations show how fluidity increases with temperature and the shear stress decreases. That is, as the temperature rises, less force is required for the fluid to flow. The shear stress values vary with the deformational velocity values, although not proportionally as would occur in a Newtonian or a Bing- ham body, and so we have catalogued them as fluids with a plastic behavior (14). The data provided by the rheological study of the two formulations may be explained by the fact that Abil WE 09 is a less powerful surfactant than Abil EM 90. The former is a second-generation silicon surfactant recommended for use at 5% with a complemen- tary surfactant and liquid wax. However, Abil EM 90 is a third-generation silicone Figure 5. Microphotograph of droplets of the formula with Abil EM 09 5% at 40øC

260 JOURNAL OF COSMETIC SCIENCE Figure 6. Microphotograph of the formula with Abil EM 90 5% at 60øC. surfactant recommended for use at 2.5 % with no complementary surfactants or additives (15). One of the aims of this work was to find a vehicle for Aloe vera in a suitable excipient that would both protect it and release it gradually. We therefore added Aloe vera at 2% to the two latexes with the best properties in the above studies (Table III). The timing of the incorporation of Aloe vera during the mixing process is crucial, as it can affect its diffusion in the excipient. The two variables during preparation were: 1. The gradual incorporation of Aloe vera into the prepared latex by continuous stirring to homogeneity. 2. Adding Aloe vera to the aqueous phase of the silicone latex and then incorporating that in the oily phase slowly and stirring continually. Nevertheless, whichever of the above two techniques is used, the most important vari- 10000 90OO 8OOO 7000 • 6000 ,-• 5000 o ß • 4000 3000 2000 1000 0 5 10 15 20 25 30 35 11sec Figure 7. Viscosity curves of the two preparations as a function of temperature: Abil EM 90 at 20øC ( ß ), Abil EM 90 at 30øC (&), Abil EM 90 at 40øC ('), Abil WE 09 at 20øC (O), Abil WE 09 at 30øC (/•), and Abil WE 09 at 40øC (,•).