802 JOUBNAL OF THE SOCIETY OF COSMETIC CHEMISTS emulsions subjected to various lcngfl•s of mixing times, T•, during elnulsifica- tion. As indicated, these curves were extrapolated to zero time to obtain Co, the concentration of Tween 80 in the aqueous phase of the undisturbed, stable enmlsion. Two centrifuge speeds (4,000 and 5,000 rpm) were employed in this series of experiments. The emulsions made at longer mixing times (lg0 and 180 rain) were more stable .and required a higher speed to separate the aqueous phase for analysis. 2.0 1.5 ml.0 z z i.1.1 I.iJ •:0.5 CASTOR OIL IPI• mINERAL OIL OLE IC ACID o o 20 40 60 80 lOO 12o ¾.,o 183 18o EMULSIFICATION TImE , Te (MIN ) Figure 2. Effect of emulsification time on Co, concentration of Tween 80 in the continuous phase of stable emulsion, prepared with 4 different ()ils (Tween 80 initia]ly placed in the oil phase)

EMULSION STABILITY 803 The values of Co were then plotted against T. •o study the change in surfac- tant concentration as the mixing time increased. An example of such a plot is shown in Fig. 2 for oleic acid, mineral oil, isopropyl myristate (IPM), and castor oil systems in which the migrating surfactant, Tween 80, was initially placed in the oil phase. Figure 3 shows the curves for the similar systems in which Tween 80 was initially placed in the aqueous phase. The fiat portion of the curve indicates that no further migration is taking place, i.e., the system has apparently reached an equilibrium with respect to the surfactant move- ment. 2.5 z ,1.5 z 8 z 0.5 0 C ASTOR O I L IPM MINERAL OIL OLEIC ACID I i I i 40 60 80 100 EMULSIFICATION TIME, I I I 120 140 160 T e (MIN) 180 Figure 3. Effect of emulsification time on C., concenh'ation cf Tween 80 in the cor•tinuous phase of stable emulsion, prepared with 4 different oils (Tween 80 initially placed in the water phase)