408 .JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS EMULSION ß -REFERENCE -_4_-- -- - BAR 20 50 I00 250 SEC, SEC. SEC. SEC, TOP OF CELL IToword Rotor Center} _•/Z7••-- CELL WALL / / CLEARED I I •AQUEOUS PHASE COUNTER BOTTOM OF CELL (A•ey From Rotor Center) CELL WALL• ..... AIR -- •'-•N T ER F•'C• OIL -- - •l'• ANSLUCENT CREAM TOP OF CELL (Toward Rotor Center) 3,000 SEC. I0,000 SEC. SEC. • REFERENCE --HOLE IN - -- COUNTER BOTTOM OF CELL [Awoy Fram Rotor Center) '•00,000 SEC, Figure 6. Example of photographic monitoring of ultracentrifugation of an oil-in-water emulsion. The 50-50 toluene-water-l% G-2151 surfactant emulsions were ultracentrifuged at 50,740 rpm. Black areas in the photographs represent opaque phases white areas, trans- parent phases gray areas, translucent phases. The photographs of the ultracentrifugal cell, 20-250 sec, were obtained during the period of bringing the ultracentrifuge to speed but are typical of the creaming process. Sharp lines of demarcation are observed in the cell photo- graphs. These boundaries separate transparent aqueous phase from the opaque cell wall (bottom) and the opaque florating emulsion, and separate the top of the opaque emulsion from the transparent air and the air from the opaque upper cell wall. The compressing emulsion is represented (upper right cell) as grading particle sizes in the initial creaming (top) and with particle sizes remaining ungraded as the creaming progresses (bottom). The photographs of the ultracentrifugal cell, 3,000-600,000 sec, are typical of the oil-separation process. A sharp line is observed that separates the opaque bottom cell wall from the opaque sedimented surfactant micelies. Sharp lines of demarcation separate opaque surfactant micelles horn the transparent aqueous phase, separate transparent aqueous phase from opaque cream, and separate translucent cream from transparent oil. The separation of opaque cream from translucent cream (or separated oil) is definite but slightly diffuse. The interface between the transparent oil and the transparent air was observed as a sharp opaque narrow bar. The translucent cream is represented in the lower cell diagram as the micro- scopically observed compressed polygonal oil particles separated by surfactant-adsorbed mem•oranes. The opaque cream is represented as the microscopically observed toluene oil particles entrappeal in precipitated surfactant. The surfactant micelle, and oil and water phase increased with time of ultracentrifugation. The translucent cream phase ultimately disappeared and the opaque cream phase compressed to a finite value

EMULSION STABILITY 409 h, and --7.5% in the apparent terminal zero-order rate. Variations in spraying pressures (E through H) do affect the rates of oil separation and must be held constant for the preparation of reproducible emulsions. The difference between the apparent rate constants for emulsions aged one and 11 days (Table I) indicates that this degree of aging decreased the rates of oil separation under equivalent ultracentrifugal stress. DISCUSSION Ultracentrifugation of 50/50 Toluene-Water-G-21 51 Surfactant Emulsion The observed ultracentrifugal phenomena can be correlated with changes in the body of the emulsion. The emulsions made by the present spray technique have reasonably reproducible rates of oil separation in the ultracentrifuge (Table I). The ultracentrifugal behavior of emulsions can be divided into four parts. Flotation The flotation of oil particles in the toluene emulsion of low surfactant concentration (1%) was too rapid at 50,000 rpm for a quantitative study. The studies were possible with the 7% surfactant and plots of the dis- tam:e of the water phase-cream boundary from the center of rotation vs. time (Fig. 3) based on photographic monitoring (Fig. 6, 20-250 second photographs) suggest at least two distinct phenomena. The first represents an initial rapid flotation of particles which obeys Svedberg's law (Eqs. 1 and 2, Figs. 3 and 7). It seems likely that this is the centrif- ugal movement of the relatively small unhindered particles. This rate should not depend on the oil/water ratio, but rather on the size of the smallest particles (1). However, the duration of this phase should be dependent on the oil/water ratio since compressed oil particles would hinder free flotation (26). The second distinct phase of flotation (Fig. 3) is a relatively slow movement over short distances in the ultracentrifugal cell and can be interpreted as packing of the oil particles with concomitant drainage of residual water from the cream (26). When the emulsion was treated at relatively low centrifugal speeds (13,000 rpm), plots such as Fig. 3 showed intermediate transition rates between the two phases which can be assigned to the movement of particles as aggregates before packing.