J. Soc. Cosmet. Chem., 21, 393-415 (May 27, 1970) Prediction and Evaluation of Emulsion Stability with Ultracentrifugal Stress* EDWARD R. GARRETT, Ph.D.* Presented in part September 9, 1969, Seminar, St. Louis, Mo. Synopsis--The hypothesis that the SVEDBERG RELATIONS for the FLOTATION of OIL PARTICLES in high ratio OIL/WATER EMULSIONS do not hold is substantiated. UL- TRACENTRIFUGATION of such emulsions introduccs a nonlinearity into log oil cicarance rs. time plots which is due to the subsequent sweep-out of the heterogencous discontinuous phase since slippage and stratification of particles are blocked. NONSTRATIFIED CREAMS demonstrate an initially apparent first-order process of large particle drainage from. the cream concomitant with continuous drainage of heavier surfactant. The final rate of oil separa- tion is constant and reflects that rate-determining coalescence is at the cream-oil interface, that the ultimate cream is a tightly packed, homogeneously particulate mass with mininmm continuous phase. This was substantiated by microscopic examination. In addition to reproducible preparative methods, sensitive analytical methods have been devcloped to assay the SURFACTANT and oil concentrations in various cmulsions. INTRODUCTION Although centrih•gation has been considered in a casual manner in the evaluation of the stability and properties of emulsions, little quan- titative study has been effected until recent use of the modern analytical * Work supported by Grant GM-12099-01, 02, 03 from the National Institutes of Health, U.S. Public Health Service, Bethesda, Md. t College of Pharmacy, J. Hillis Miller Health Center, University of Florida, Gainesville, Fla. 32601. 393

394 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS ultracentrifuge (1-6). A recent review (7) considers in detail the prior uses of centrifugation (8-11) and other methods [or studies on emulsion deterioration. Methods of measuring the stability of emulsions in terms of changes in interfacial area or drop volume may be very insensitive to small but important changes occurring in the emulsion and may be useful only in unstable emulsions (3, 4). Emulsion stability frequently has been de- termined by measuring the formation ot5 cream and the separate oil phase as a function of time, but has been largely restricted to normal gravitational situations (12-17). The enormous time lags involved and the uncertainties of complete phase separation are definite disadvantages o15 such methods, and, in general, they are only applicable to poor and unstable emulsions. Garrett (1), Void and Groot (3, 5, 6), and Rehfeld (2) have demon- strated that the analytical ultracentri15uge is an excellent tool 15or the evaluation of emulsion stability. The determination of rates ot• fioccula- tion, flotation, or creaming o15 a dispersed oil phase can be correlated with the Svedberg relations applied to the sector-shaped cell o15 the analytical ultracentrifuge (18, 19). The clearing rate of dilute emulsions with low oil/water ratios ad- heres to the Svedberg equation and is valid for the prediction of such rates at stresses approaching that of normal gravity (1). This rate does not necessarily permit classification as a "good" or "bad" emulsion, how- ever. It is most probable that the flotation of the smallest particles is ß what is measured in both emulsion types. Such definitions of emulsions are peculiarly arbitrary and frequently operationally defined. A consistent model for these emulsion phenomena in the ultracentri- fuge may be the more ready flotation of large oil particles so that the cream is a grading of large to small oil particles from top to bottom in low oil/water ratios (1). This may not occur in the case of high oil/water ratios (3) -where the phenomenon o15 "sweep-out," i.e., large particles forcing the smaller ones upward without slippage, may occur (1). Higher centrifugal speeds 15or a given oil/water ratio may also pro- mote this "sweep-out" phenomenon and the cream may not have any stratification (3). The continuous phase has to drain from the packed globules which will be de15ormed by the centrifugal stress (1). It has been postulated (20) that the more an interface curves away 15rom a drop, the longer the lifetime of a drop. It is consistent with this premise that the more readily de15ormable large drops should coalesce first.