344 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 11o z 100 • 90- 80- 70- 60- 50- 40- 30- 20 10 o + [] _ D [] .. I I I I I I I 1 I I 0 0.4. 0.8 1.2 1.6 2 TIME (DAYS) Figure 10. Sedimentation curves at room temperature. Emulsions contained 1% emulsifier and various methylcellulose concentrations. [] 0%, + 0.25%, O 0.5%, /• 1.0%. In Figure 13, the relationship between sedimentation rate and viscosity is explored. For each of the polymers, an increase in viscosity results in a decrease in sedimentation rate. However, the relationship is not a simple one. At the same viscosity, the sedimentation rate for emulsions containing xanthan gum was less than that of emulsions containing the other polymers by one or two orders of magnitude. This can be explained in terms of the importance of the rheological behavior at very low shear, the condition within the emulsions at rest. Our viscosity values were obtained at moderate shear. The viscosity in an emulsion at rest may be quite different depending on its rheological character. For pseudoplastic fluids which obey the relation q^ = qs n-• (Eq. 3) in which 'q^ is the apparent viscosity and •ls is the viscosity at a shear rate of 1 sec-1, the constant n varies from 0 to 1 (20). The smaller the value of n, the higher the degree of pseudoplasticity and the greater the effect of shear rate on apparent viscosity. Of the three polymers studied, xanthan gum is the most pseudoplastic, followed by carboxymethylcellulose and then methylcellulose. Therefore, xanthan gum would have the highest effective viscosity under the shear conditions found in a standing emulsion. The existence of a gel-like state in xanthan gum solutions (7) is another factor contrib- uting to the high resistance to creaming in emulsions containing this gum.

EMULSION STABILIZATION BY GUMS 345 Ld -1 -2 -3 o 0.1 0.2 0.3 0.4 XANTHAN GUM CONC. (% W/W) Figure l l. Effect of xanthan gum on sedimentation rate at different emulsifier concentrations at room temperature. [] 0.25%, + 0.5%, O 1.0%. STORAGE AT 45 DEGREES Samples of the emulsions, stored in capped cylindrical vials in an oven at 45 degrees, were monitored to obtain data on phase changes and creaming. No oil separation was observed in emulsions containing 0.25 % emulsifier. Emulsions containing 0.5 % emul- sifier and no polymer creamed rapidly, but no oil was observed at the top. In emulsions containing xanthan gum, a film of oil (too thin to be measured) was observed. The time required for appearance of this oil film was longer the higher the gum concentration. In all of the emulsions containing 1% emulsifier, an oil layer was observed after storage, and several eventually separated into three phases: oil, a surfactant-rich phase of density intermediate between that of the oil and aqueous phases, and an underlying, relatively clear aqueous phase. Some oil separation was also observed in emulsions containing higher emulsifier concentrations. The greater tendency fbr oil to separate as the emulsi- fier concentration was increased is probably related to the decrease in phase inversion temperature with concentration that has been observed in commercial surfactants (21). A typical set of sedimentation curves for emulsions with 1% emulsifier is presented in Figure 14. With many of the emulsions, creaming was extremely rapid and separation