LOW-SURFACTANT EMULSIFICATION 177 PROCESS OPTIMIZATION Having found the most efficient surfactant combination for a given oil, one can further reduce the total amount of surfactants through process optimization. It is well known that the methods of processing an emulsion often have a marked effect on the quality of the emulsion produced (10). In a batch processing, an O/W emulsions can be made by either adding the oil to water or water to oil. The latter process involves phase inversion and is generally regarded as a better way of producing emulsions with fine droplet sizes. In addition, initial placing of the surfactants in the oil phase is generally more effective than placing the surfactants in the aqueous phase, as far as the droplet size is concerned. Table I shows the approximate mean droplet sizes of an O/W formulation stabilized with polysorbate-20 and sorbitan monolaurate at i: 1 ratio, prepared by four different methods. In all cases, the total surfactant mixture was 5% and mineral oil madeup 20% of the emulsion. The rate of addition was set at 40 ml/min in making all emulsions. Sorbitan monolaurate was initially placed in the oil phase but the location of polysorbate-20 was varied. None of the emulsions prepared had very fine droplet size, but placing of the hydrophilic surfactant in the oil phase combined with a phase inversion technique produced the best result. The effect of the initial hydrophilic surfactant location was studied further by dividing the polysorbate-20 in different proportions between the oil phase and the aqueous phase. The results obtained by emulsifying the predivided mixtures are shown in Figure 7. All sorbitan monolaurate was placed in the oil phase. It is seen here that emulsions tend to become coarse as more polysorbate-20 is initially placed in the aqueous phase. However, it is surprising that placing 20 to 40% of the hydrophilic surfactant in the aqueous phase initially, produced a finer emulsion than one obtained by placing all the surfactant in the oil phase. This effect is probably due to the fact that the hydrophilic surfactant is made more available to the interface by transferring a part of the surfactant from the oil phase in which it is poorly soluble to the aqueous phase in which it is completely soluble. In addition to surfactant location, the rate of aqueous phase addition was found to be a very important factor. Figure 8 shows a series of experiments with varying rates of aqueous phase addition ranging from 10 to 80 ml/min. It is clearly seen here that for the system studied, a significant improvement of emulsion was possible by simply lowering the rate of aqueous phase addition. For the experimental run at the lowest addition rate (R = 10 ml/min), the total mixing time had to be extended to 8 rain from the standard 5-rain mixing period. However, it was clear that the improvement of the emulsion droplet size was not due to the Table I Mean Droplet Sizes of O/W Emulsion Prepared by Four Different Methods Methods of Preparation Approximate Mean Droplet Size in Microns Polysorbate-20 in oil, water to oil Polysorbate-20 in water, water to oil Polysorbate-20 in oil, oil to water Polysorbate-20 in water, oil to water 12

178 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS i I' / ] ' I ' I ' I . - POLYSORBATE -20 õ I- / HINERALO, I ,,• - 0 o 20 40 60 80 10o % HYDROPHILIC SURFACTANT INITIALLY IN WATER Figure 7. Effect of initial location of the hydrophilic surfactant. (Emulsions contain 75% distilled water, 20% mineral oil, 2.5% polysorbate-20 and 2.5% sorbitan monolaurate.) n 0 C• z ' I [ I ' I [ I [ I ' I [ I [ I [ o o POLYSORBAT E -20 $ORBITAN MONOLAURATE MINERAL OIL I I I [ I i I I I , I I I [ I • I0 20 50 40 50 60 70 80 90 RATE OF AQUEOUS PHASE ADDITION, R. (ML/MIN) Figure 8. Effegt of the rate of aqueous phase addition. (Emulsions contain 75ø23 distilled water, 20% mineral oil, 2% polysorbate-20, and 3% so'rbitan monolaurate.)