372 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS ing, one would expect to find little difference in the stability of the emul- sions so prepared. However, the data in Fig. 5 indicate remarkable dif- ferences in the emulsion stability with respect to the initial surfactant locations. Apparently, the surfactant migration did not reach an equilibrium even after one hour of continuous mixing in the system investigated. Based on the work of Ross and coworkers on spreading coefficients and HLB values, Becher suggested a theory relating emulsion stability to spreading (8, 20). Briefly, it was argued that for an O/W emulsion, if the oil droplet spreads readily on the surface of the aqueous phase, the droplet, on rising to the surface, will lose its identity and the emulsion will not be very stable. On the other hand, if the oil droplet in an O/W emulsion does not spread, the droplet rising to the surface of the emulsion will have a chance to redisperse and thus this emulsion should be more stable than the first one. In an O/W emulsion, spreading of the oil droplet on the continuous aqueous phase surface may be predicted from the following equation: S1 = •a- (•0 + where: S• = spreading coefficient, dynes/cm •a = surface tension of the aqueous phase, dynes/cm v0 = surface tension of the oil droplet, dynes/cm visit = interfacial tension between the oil and aqueous phase, dynes/cm For an oil to spread on the aqueous phase, the S• must be greater than zero. The more negative the value of S•, the less the oil droplet will tend to spread. After studying a series of oils, Becher concluded that one of the requirements for emulsion stability is that the oil should have the "most negative spreading coefficient consistent with a low interfacial ten- sion" (8). Furthermore, he suggested that for O/W emulsions the re- quired HLB value should correspond to S• range of 0 to --5. In order to determine how the above theory would fit the experi- mental results described here, further experiments were carried out to study the effect of surfactant location on spreading. Whereas the presence of a small amount of surface active agent gen- erally has a dynamic effect on reducing the surface tension of the water, the surface tension of oil is relatively unaffected by surfactants. Ex-

SURFACTANT LOCATION 373 amination of the above equation of S• indicates that placing of the sur- factant in the aqueous phase should generally result in a smaller initial value of ,5• than the same system in which the surfactant is placed in the oil phase. For example, our surface and interfacial tension measure- ments, using a No/iy tensiometer (ring method), gave an S• value of about 33 for castor oil when the 3% Tween 80-Arlacel 80 blend (at HLB 10) was placed in the oil phase. The S• was about --8 in the same sys- tem when the same surfactant blend was placed in the aqueous phase. If the large positive S• value corresponds to a poor stability, we would then expect more separation in the e•nulsions prepared by initially plac- ing the sur[actant in the castor oil. However, the results of Figs. 4 and 5 indicate just the opposite. IqI,B requirements are next examined on the basis of the spreading model. The spreading coefficient data given in the work of Ross et al. show an increase in S• with HLB values of the surfactant blend (7). These data were obtained by dissolving 1% surfactant blends (Tween 80/Span 80) in the aqueous phase. Placing the surfactant in the oil phase in the same system is expected, at least initially, to give a higher value of S•. Therefore, on the spreading model, if the S• value has to be between 0 and --5 to give stability, it follows that the required HLB for the system prepared with the surfactant initially in the oil phase should be lower than the same system emulsified by initially placing the surfac- rant in the aqueous phase. This is, of course, just the opposite of our ex- perimental findings. There are some possible explanations of the observed apparent dis- crepancy between our results and the theory based on spreading. First ot• all, although placing of all surfactants in the oil phase will initially give a high S• value, this value will rapidly decrease after emulsification because a small amount of the surfactants migrating to the aqueous phase will reduce the surface tension of that phase greatly. For this reason, in practice, the difference in S• due to the surfactant location may be in- significant. Secondly, there are certainly a great number of factors such as droplet size distribution, viscosity of the external phase, nature of interfacial film, etc., which can influence emulsion stability (21). Whereas spreading may be one of the factors, conceivably it is not a critical factor in the sys- tems studied here. Earlier work (17) showed that initial placing of the emulsifiers in the oil phase encouraged formation of double emulsions. It is quite possible that such an effect or combination of this effect and