176 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 5C 48 46 44 40 38 36 34 c• t• 30 r 28 1.• 24 0 2o 16 14 I0 8 6 N U.lol - Water - SDS Ohve Od-Water-SDS 4 2 o /x Nujol-Water-Tween 20 Nujol-Water -Triton X-I00 15 30 45 60 75 90 105 TIME IN MINUTES Figure 2. Separation of oil at 39,460 rpm from 50% oil-50% water-0.2% surfactant emul- sions. Nuj•ol-SDS emulsions M 111968 olive oil-SDS emulsions M 060669 Nujol-Tween 20 emulsions A 091069 Nujol-Triton X-100 emulsions A 090969 factant Nujol-water emulsions with SDS, Tween 20, and Triton X-100, and olive oil with SDS. Experiments with emulsions stabilized with Tween and Triton were carried out for 10 min and 85 min longer than the results shown in the figure with no change from the smooth curve found for shorter periods. For the most significant comparison of the behavior of the different systems, it would have been desirable to have equal drop size distribution in all, and to make comparisons at constant equilibrium rather than initial concentration of surfactant, or better yet at constant fraction of the oil-water interface covered by adsorbed emul- sifier in all cases. To accomplish this, it would be necessary to have adsorption iso- therms for each of the surfactants, in order to make appropriate adjust- •nents of the initial concentrations so as to obtain the same equilibrium concentration in the different cases, and to change the extent of mechani- cal processing during emulsification so as to obtain emulsions of equal

ULTRACENTRIFUGAL STABILITY OF EMULSIONS 177 36 32 2• 24 • 2o -- 0 e 4 o • 4o do d• •6o ' •20 TIME OF ULTRACENTRIFUGATION, MINUTES Figure 3. Separation of oil from 50% Nujol-50% water-0.1% cetyl pyridinium chloride emulsion (A 112669) specific interfacial area and hence of equal average drop size (8, 12). However, although a good chemical method is available for determina- tion of SDS, permitting determination of the amount adsorbed by differ- ence between the initial and equilibrium concentrations in the aqueous phase, no satisfactory methods were found for determination of Triton X-100 or Tween 20 in the equilibrium aqueous phase separable from the emulsions. Absorbance methods for Triton failed because of inability to obtain totally transparent equilibrium solutions from the emulsions free of turbidity due to residual traces of unseparated oil, while neither surface tension lowering (13) nor chemical methods (14, 15) based on the formation and estimation of a cobaltothiocyanate complex could be made to give accurate results for the concentration of Tween 20 (16). Nevertheless, the differences in behavior between the different systems are so extreme that it is easily possible to draw significant conclusions from the results with equal initial concentrations of the surfactants. Figure 3 presents the results obtained with 50% Nujol-50% water- 0.1% CPC emulsions. The curve obtained is very similar to that ob- tained with many of the other emulsifying agents at higher concentra- tions, and shows a decrease in rate of separation of oil with increasing time of centrifugation not conforming to the empirical equation (III-B).