800 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS aqueous and oil phases accordiug to the experimental requirement. The com- bination of Arlaccl 80 and Tween 80 gave an HLB value of 10 in the formula- tion. Carbopol 934 was added to improve the stability of the emulsion since it would be difficult to make accurate measurements if the emulsion were mrsta- ble. The oils tested were cosmetic grade mineral oils, oleyl alcohol, isopropyl myristate, castor oil, and others commonly used for cosmetics. To make a test emulsion, the aqueous phase was first placed in the beaker and the Homomixer was placed at the center of the beaker with the clearance between the mixer tip and the bottom of the beaker set at exactly 20 mm. The predispersed oil phase was then carefully placed on the top of the aqueous phase and the Homomixer was turned on to start emulsification. The Homo- mixer speed was kept constant at 11,900 -100 rpm and all operations were carefully controlled to avoid air entrapment and to assure good reproducibil- ity. To determine the distribution of Tween 80 in the prepared emulsion, a set of samples of emulsion were subjected to centrifuge for various lengths of time, usually in six stages sta•'ting h'om 1 min up to 10 min. The rpm of the centrifuge used for a given set of samples was such that a reasonable amount of the aqueous phase could be separated at each stage for analysis. In most cases the rpm ranged from 4,000 to 6,000. The separated aqueous phase was carefully withdrawn with a hypodermic needle and the Tween 80 concentra- tion was determined by a sodium tetraphenylborate titration method de- scribed bv Kasai et al. (4). By plotting the Tween 80 concentrations in the separated aqueous phases at various stages of centrifugal separation, the data were extrapolated to zero separation in order to obtain the surfactant concentration in the unseparated sample of the emulsion. The experiments were repeated until a consistent re- sult was obtained. The snrfactant concentration in the aqueous phase was de- termined as .a function of emulsification time in order to follow surfactant mi- gration as the emulsification progressed. The stability of the emulsions was measured by placing the samples at 37øC for several months and observing any sign of creaming or separation. Photo- graphs of the emulsion were also taken under a microscope to measure the droplet size distribution. For emulsification under slow mixing, several laboratory mixers with identi- cal straight paddle-type impellers were used. The length of the paddle was 60 mm and the height was 20 mm. The batch size of the test emulsions was 400 g and emulsifications were made in 500-ml beakers at speeds ranging from 170 to 650 rpm. The formula used for slow mixing emulsification was the same as the one used for the rapid mixing experiments except Carbopol 934 and the neutraliz- er were taken out and replaced with water to allow a relatively quick separa- tion. In most cases, six emulsions with varying initial surfactant location were

EMULSION STABILITY 801 made simultaneously, and the emulsions were placed in graduated cylinders for stability observation. Photographs were taken under the microscope to al- low observation of change in droplet size distribution. .• I•IgSULTS AND DISCUSSION Rapid Mixing Emulsification Emulsions prepared under rapid mixing with the Homomixer were general- ly stable and had average droplet sizes ranging from 1 to 4/2. Figure 1 shows typical curves obtained by successive centrifugation of a group of oleic acid 1.5 z [ I I I I I I I I '[[' -- id"•ENTRIFUC-• RPM - •' 180 5.000 •" 120 5,000 •:[ 60 4.000 • 3o 4,0oo o- lO 4,00o 0 1 2 3 4 5 6 7 8 9 10 TIME SUBJECTED TO CENTRIFUSE (MIN.) Figure 1. Extrapolation of Tween 80 concentrations in centrifuged emulsions to obtain Co (oleic acid system, Tween 80 initially in oil phase)