258 JOURNAL OF COSMETIC SCIENCE Abil WE 09 1% (b) Abil WE 09 5% Figure 2. Microphotographs of the formulations with Abil WE 09 at 1% (a) and 5% (b). Photographs of the study on the influence of temperature show slight changes in the 5 % surfactant preparations after one month at 40øC, consisting of a scant increase in droplet size, which was somewhat greater in the Abil WE 09 formula (Figure 4) than in the Abil EM 90 formula (Figure 5). At 60øC, however, the destruction of the system was greater and affected both preparations equally (Figure 6). RHEOLOGICAL STUDY Rheological studies are of great importance in evaluating physicochemical properties, as lOO 8o 40 1 - 9 gm 10 - 30 tam Figure 3. Graph comparing droplet size of the preparations with l•, Abil EM 90 5% El, Abil EM 90 1% •], Abil WE 09 5% and 1 Abil WE 09 1%.

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