410 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS ii / 42 \ / ß--4• 43----. i / "----47 / ß/•o / / ß --55 i i i • i i i i 3 4 5 G '7 8 9 I0 I0 e Figure 7. Initial flotation rates in sec -• as a function of ultracentrifugal (rpm? for a 50% toluene-in-water emulsion stabilized with 7% G-2151. The numbers indicate age in days of the emulsions at lhe time of centrifugation Sedimentation oI SurIactant Micelles The polyoxyethylene ester G-2151 is the substance of highest density in these emulsions. Even at a concentration of 1% in the total emulsion, the concentration of G-2151 in the water phase should be greater than the cmc. The resultant micelles should be very heavy and would be expected to sediment toward the bottom of the cell. Their rate of sedi- mentation can be followed since, as they accumulate on the cell bottom, they either absorb or scatter light. They do not precipitate. This was observed as an increase in the height of an opaque layer from the bottom of the cell with time (e.g., 3,000 to 600,000 sec photographs in Fig. 6). This accumulation of surfactant micelles is a first-order process and was complete in 15,000 sec at 50,740 rpm. This was demonstrated by the linearity of the logarithm of the difference between the ultimate levels of surfactant micelles and the level at any time plotted against time. Evidence that this can be assigned to the sedimentation of sur[actant micelles was obtained from the similar behavior of aqueous surfactant so- lutions and the absence of this behavior in toluene-sur[actant solutions where surfactant micelle formation probably does not occur. These phenomena were not observed in the tetradecane-water emul- sions with the Igepal Co-610 sur[actant of different solubility and proper- ties.

EMULSION STABILITY 411 Characteristics o[ Creams Shortly at:ter the completion ot: flotation in emulsions of low surt:ac- tant concentrations, the optical properties ot: the cream changed. Ini- tially no light is transmitted (0-250 sec photographs in Fig. 6). Subse- quently, the transmittance of a percentage of the incident light is ob- served. Generally, about 80% of the cream shows partial light trans- mittance with 2,000 sec of centrifugation at 50,740 rpm. The amount of cream exhibiting this phenomenon remains constant for approxi- mately 250,000 sec and then very slowly increases (Fig. 6). This phe- nomenon has not been observed in high 7% surfactant concentration emulsions where the excess surfactant has been shown microscopically to precipitate in the cream and prevents coalescence. Any toluene which separates most probably comes from this semitransparent cream since the amount of opaque cream remains constant. The fact that discrete toluene particles exist in the cream was shown by their rapid dispersibility in water, but not in toluene. Also, discrete oil particles in the cream were observed under the microscope. The translucent cream may represent toluene particles packed tightly to- gether and covered with monomolecular surfactant films from which the continuous medium (water) has been eliminated to minimize light scat- tering optical properties. The small fraction of the cream which never transmits light, i.e., opaque cream (Fig. 6), is probably a region of tightly packed particles with a relatively high concentration of trapped or precipitated surfac- tant. The prepared tetradecane emulsion showed rapid creaming and no such variations in opacity of the cream phase. Separation of Oil Toluene appeared at 3,000 sec at the top of the cell (Fig. 6) when the 1% G-2151 toluene-in-water emulsions were centrifuged at 50,740 rpm. The rate of oil separation could be analyzed, after an initial lag period, into an initial accumulation at a first-order rate for approximately 115,000 sec with a subsequent constant rate of oil separation (Figs. 4 and 5). In one experiment continued for 11 days of ultracentrifugation, only slight terminal diminution was noted. The initial lag period before the ap- pearance of toluene demonstrates that certain phenomena must occur in the body of the cream before oil separation. There may be a surfactant- film strength that must be exceeded, i.e., a "yield" value (1). There may