174 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS ultracentrifuge. Where larger quantities of emulsion were required, two batches of 270 ml each were prepared separately as described above, and blended together before addition of the final portions of aqueous surfactant solution. Ultracentrifugation Technique and Treatment of Data Unless otherwise indicated, all the emulsions were centrifuged in a Beckman Model E ultracentrifuge at 39,460 rpm at 25øC. The technique of ultracentrifugation and the treatment of the data to deduce the vol- ume of separated oil from the experimental observations were the same as used previously (8, 11). Generally, determinations were made at least in duplicate to establish the reproducibility of the behavior observed. RESULTS In previous ultracentrifugal studies of Nujol-water emulsions sta- bilized with SDS (8, 9, 11, 12), the rate at which oil separated was found to be constant independent of time except for a transitory more rapid separation at the beginning of the experiment. It now appears that this behavior is the exception rather than the rule, since most of the systems studied in the present work showed a decreasing rate of separation of oil with increasing time of centrifugation approaching zero as a limit. Such behavior is also found with Nujol-water emulsions themselves, provided centrifugation is carried out for much longer periods of time, i.e., result- ing in reduction of the residual emulsified oil to smaller fractions of the original amount present (Fig. 1). Figure 1 shows the per cent of oil separated from a 50% Nujol-50% water-0.2% SDS emulsion as a [unction of time of centrifugation at 39,460 rpm at 25øC over a period of 7 hours. The curve has been divided into four regions. In region I at the beginning of centrifugation, the rate of oil separation is relatively much [aster and changes rapidly with time. In region II, the rate is constant, i.e., a linear relation between amount separated and time of centrifugation. In region III, this relation is no longer linear, but the rate of separation decreases progressively with increasing time of centrifugation. With some systems, including Nujol- water-SDS emulsions, this behavior is accurately represented by an em- pirical equation (11). t 1 t - + % oilse•, b. % oilmax % Oilmax

ULTRACENTRIFUGAL STABILITY OF EMULSIONS 175 8O •64- 16 I I I I I I I I I I 0 40 80 120 160 :00 :40 280 520 560 400 TIME OF CENTRIFUGATION, MINUTES Figure 1. Separation of oil from 50% Nujol-50% water-0.2% sodium do,decyl sulfate emul- sion (M 060669). Regions shown: I, initial rapid separation of oil II, rate of oil separation independent of time III, rate of oil separation decreasing with time, III-A if (as with Nujol) conforms to empirical eq 1, otherwise III-B IV, rate of oil separati.on is zero independent of time In such cases, it will be referred to as region III-A. In other systems, no simple analytical function could be found, and in such cases the region of decreasing rate of oil separation with increasing time is called III-B. In region IV, there is no further separation of oil, the rate having become zero, although a certain fraction of the original oil still is present in the emulsified state. Unfortunately, it is not possible to deduce the compo- sition (or oil-water volume ratio) of the residual emulsion unambigu- ously from the per cent oil separated, as this will depend on whether the separation of oil occurs exclusively at the interface between bulk oil and emulsion or whether it occurs also throughout the body of the emulsion (12). In more stable systems, whether due to the intrinsic character of the oils and surfactants or simply to use of a higher concentration of emulsi- fier, there was frequently an induction period of 5 to 20 rain after ultra- centrifugation was started before any free oil appeared. In such cases, the total curve of per cent oil separated rs. time of centrifugation usually was made up of regions I, III-B, and IV as described above, with no linear portion and no fit to the empirical equation developed to describe the rate of separation of oil in region III-A. Figure 2 shows the quantity of oil separated as a function of time of centrifugation at 39,460 rpm at 25øC for 50% oil-50% water-0.2% sur-