EMULSION PHASE INVERSION 193 Z 76 74 72 70 68 66( i I I I 20 40 60 80 I00 X h, W'T. % TWEEN 80 INITIALLY IN AQ. PHASE Figure 5. Effect of initial surfactant location on q• (mineral oil emulsion stabilized with Tween 80 - Arlacel 80 at HLB 8) By determining the values of 4 for a series of identical emulsions pre- pared with varying initial surfactant distribution, the effects of initial surfac- tant location were examined. Figure 5 presents a typical result obtained with nfineral oil emulsions stabilized with Tween 80 and Arlacel 80 at HLB 8. X/, is the weight per cent of the total hydrophilic surfactant (Tween 80) initially present in the aqueous phase. X/, = 100 means that the entire Twccn 80 was initially' in the aqueous phase and X•, --• 0 corresponds to a run where the entire Twccn 80 (as well as all Arlaccl 80) was placed in the oil phase prior to emulsification. It is clear that the effect of the initial surfactant location was very pronounced in this system. The increase of ½ with X/, means that as more hydrophilic surfactant is

194 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS I I I I • SOLULAN 98 - AMERCHOL L-IOI 7z • 68 • 66 0 20 40 60 80 I00 Xh, WT.% SOLULAN 98 INITIALLY IN All. PHASE Figure 6. Effect of initial surfactant location on qo (mineral oil and isopropyl myristate stabilized with Solulan 98 - Amerchol L-101 at HLB 8) placed in the aqueous phase prior to emulsification, the stronger will be the tendency for the system to form an O/W emulsion. This observation is qualitatively in agreement with equation 1 of Davies. The marked difference in the values of 4 observed seems to indicate that, for the system studied, the rate of surfactant migration from one phase to another is not instantaneous. The dependency of 4• on 2f• was found to be related to the type of surfactant as well as the oil used. Figure 6 shows mineral oil and isopropyl myristate stabilized with a mixture of Solulan 98 and Amerchol L-101 at HLB 8. From a comparison of these results with those of Fig. 5, it is apparent that the effect of the initial surfactant location in these two systems is much less significant than the mineral oil system stabilized with the Tween 80--Arlaccl 80 combination. The difference may be partially due to the difference in the solubilities of the surfactants in the oils. How- ever, as before, placing of the more hydrophilic surfactant in the aqueous phase favored formation of an O/W emulsion.