EMULSION STABILITY 807 5 -O- TWEEN 80 IN OIL •- TWEEN 80IN WATER ( 100 DAYS) ( 50 DAYS) 0 0 20 40 60 80 100 120 ' EMULSIFICATION TIME (MIN) Figure 9. Effect of emulsification time on stability of isopropy] myristate emulsion (% separation after 108 and 59 days) The stability data of the mineral oil and isopropyl myristate are shown in Figs. 8 and 9. The stability was evaluated in terms of the percentage of the separated phase after 2 to 4 months' storage at 37øC. In these systems, sep- aration took place on the bottom of the container. By comparing these stability data against Co data presented before, it can be seen that for the emulsions tested, no further stability improvement was obtained after the system was m xed long enough to achieve a surfactant equilibrium. Moreover, it is to be noted that under a high-speed mixing, the initial location of the migrating sur- factant did not appear to be an important factor as far as the droplet size and emulsion stability were concerned. Slow Mixing Emulsification The results of fl•e stability study of the emulsion prepared under a slow mix- ing speed (170 rpm) using a paddle mixer were very different from the re- sults obtained with a high speed Homomixer presented above. For example, as shown in Fig. 10, the stability of the oleyl alcohol emulsion was much greater when the surfactant was initially placed in the oil phase. A similar trend was observed in the oleic acid, octyl dodecyl triglyceride emulsions shown in Figs. 11 and 12. To investigate this trend further, 26 oils which are commonly used in cos- metics were selected and emulsions were made with a Tween 80-Arlacel 80

808 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 6o[ 1 30- / INOIL 0 20 40 60 80 100 ø1o TWEEN 80 IN AQUEOUS PHASE Figure 10. Effect of initial surfactant location on emulsion stability for oley] alcohol system prepared at slow mixing speed (170 rpm, 18-hour mixing, 15-min standing) 60 50 z o_4 o • 30 • 20 10 TWEEN 80 INITIALLY IN WATER , TWEEN 80 INITIALLY IN OIL 0 .... 0 20 40 60 80 100 % TWEEN 80 IN AQUEOUS PHASE Figure 11. Effect o• initial surfactant location on emulsion stability for oleic acid system prepared at slow mixing speed (170 rpm, 18-hour mixing, 15-min standing)