CHANGES DURING EVAPORATION OF EMULSION 25 Figure 7. The emulsion evaporated to 80% and subsequently centrifuged. that the ultra-small and irregular particles, which are infrequent in Figure la, are prevalent and separated from the oil drops in Figure 5a. The fraction of dispersed phase is estimated to a volume fraction of 0.22. This value was obtained by a summation of the volume of the drops in the original emulsion divided by the volume of the layer for which the drop size was determined. As a consequence, the fraction of dispersed phase of the emulsion layer in Figure 4a is estimated at approximately 0.5 from a visual inspection of the figure. As a conclusion, it may be stated that the emulsion contains approximately 20 volume-% oil and that the initial evaporation to 40% does not include a significant fraction of the oil, but leads to precipitation of water-soluble or finely dispersed material in the aqueous phase. Secondly, the results convincingly confirmed the early discovered (17) and recently emphasized fact (18) that the original structure of the emulsion is not the entity to which the action on the skin may be related. The results at 80% evaporation (Figure 8) demonstrate the original structure of the emulsion to be completely deteriorated. Not only does the original drop pattern not exist, but, in addition, a new structure has appeared, which is birefringent and, hence, optically anisotropic. Since there is no presence of drops in the photos in Figure 8 and since the original top layer of emulsion in Figure 3 is not present in Figure 7, it must be concluded that the original oil phase has evaporated. The bottom layer emulsion in Figure 8 is obviously similar in structure to the bottom layer in Figure 3, but now enlarged by more material from the original two top layers. The middle layer in Figure 3 after centrifugation contains some solid material, as is the case-and even more so-with the bottom layer. A conservative estimate would be that between 5% and 10% of these layers consist of this material. In addition to this presence of material in the aqueous layer, one must assume that the emulsions are stabilized by a surfactant at an approximate level of 5 % . A combination of this direct visual information with the conclusion of the initial evaporation to be limited to water indicates that the formation is an amphiphilic association structure. Such a structure is most likely formed from the surfactant com-

26 JOURNAL OF COSMETIC SCIENCE (a) (b) Figure 8. Microphotographs of the emulsion 80% evaporated after centrifugation. (a) Top layer, without crossed polarizers. (b) Top layer, with crossed polarizers.