406 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Rates of Oil Separation of Toluene-l% G-2151-Water Emulsions The rates ot• oil separation ot• the 1% surt•actant emulsions prepared by the spray technique (emulsions A through H) at 50,740 rpm were determined from a plot of the per cent volume of oil separated rs. time. Typical examples ot• such plots are given in Fig. 4. Each point was taken from an individual photograph ot• the four simultaneously ultra- centrifuged samples (25). A slight induction period may be observed. 6O 55 5O 45 .= 40 •o :55 15 5 Odd• 19,200 36,400 57,600 76,800 192,000 211,200 230,400 249,600 268,6½ Time In Seconds Figure 4. Volume per cent of available oil separated as monitored from photographs vs. time of ultracentrifugation at 50,740 rpm. The four spray-prepared 50% toluene-water emulsions with 1.0% surfactant (G-2151) were identical in composition but were prepared at different spraying pressures Subsequently, the curves ot• Fig. 4 can be analyzed into a fraction of oil separated by an apparent first-order process and a subsequent slower zero-order or constant rate of separation ot• oil. The per cent volume ot• oil separated at 115,000 sec was chosen as an estimate ot• the asymptote of the initial first-order separation and the apparent first-order rate con- stants were estimated from the expression: Log [% vol oil at 115,000 sec -- % vol oil at time t] = --kt + constant (3) where the flotation rates, k, in sec -• are given in Table I. Typical plots for emulsions according to Eq. 3 are given in Fig. 5 and show reasonable adherence to linearity. The subsequent zero-order rate constants were estimated from the terminal slopes after 115,000 sec (Fig. 4). Table I summarizes the data. The observed oil separation rates (Figs. 4 and 5) for all the spray- prepared emulsions of the same composition and pressures (A through D) show high variability, ___ 1.5% in the apparent first-order rate constant,

EMULSION STABILITY 407 Table I Apparent Rate Constants for Oil Separation of Toluene-Water-1 c•0 G-2151 Surfactant Emulsions by Ultracentrifugation at 50,740 rpm One Day After Preparation Eleven Days After Preparation Emulsion'* 10Sk(sec-1) b 105ko(%/scc)c 105k(scc-•)b s ' 10 ko(%/sec • A 2.26 6.9 2.08 4.5 B 2.26 6.4 2.08 4.3 C 2.34 6.1 1.95 4.7 1) 2.34 7.1 1.95 4.7 E 2.46 6.0 ...... F 2.85 5.4 ...... G 2.78 6.3 ...... H 2.63 5.8 ...... a Emulsions A through D were prepared to be identical in every respect. Emulsions E through H were identical except that four different spraying pressures were used. • These apparent first-order rate constants for oil separation were obtained from the slopes of the In [amount of oil separated at 115,000 sec minus amount of oil separated at any prior time, t.] rs. t. c These apparent zero-order rate constants represent the constant rate of oil separation after 115,000 seconds. 60 • 30 •-• • 20 * •o / • 9 I 8 7 g o 19,•oo •8.4oo 57•6oo 76,600 Time, t, In •econd• four different sp•[• p•essu•es. •he fi•s[-o•de• As•mptote •as estimated At ]]5,000 sec