125 • e 100 - ,, 0 '- 75 - CD i 50 25 0 0 CHANGES DURING EVAPORATION OF EMULSION 1 2 Diameter, microns 3 19 4 Figure 2. The number size distribution of the original emulsion (6) and after evaporation to 60% of 40% of the initial weight (■). crystal, but the particles are too small to provide an answer. However, when one realizes that the emulsion remains at this stage are the essential element in order to understand the relation between the emulsion components and its action on the skin, it is apparent that additional information is obviously of essence. Some of this informational needed is offered by the results of the centrifugation (Figure 7). The bottom layer emulsion is now increased to 73% of the emulsion Figure 3. Original emulsion after centrifugation.

20 JOURNAL OF COSMETIC SCIENCE Table I Volume Fractions after Centrifugation of Original Emulsion and after Evaporation After evaporation After evaporation Layer Original emulsion (fractions) (weights) Top emulsion 0.46 0 0 Liquid 0.52 0.27 0.05 Bottom emulsion 0.02 0.73 0.15 (Table I), the rest being the liquid part. These numbers provide a comparison of the changes in the emulsion, but the numbers may be more informative if they are corrected for the change in the total amount of the emulsion. The "weights" column in Table I shows that of the original liquid layer and the emulsion layer only a small fraction remains, while of the amount in the bottom emulsion layer a significant increase is found. The microscopy photos of the centrifuged emulsion shed some light on the reason for this change. Figure Sa shows the liquid layer not to contain any drops. Instead, there is some dispersed material in the form of large particles, and the photo of the sample between crossed polarizers reveals this material to be birefringent (Figure 86). In addi­ tion, the lower emulsion layer contains large amounts of birefringent material. The bottom layer (Figure Sc) pictures the same structure as the entire emulsion (Figure 6c). It now consists of an incomplete cover of the surface, and nothing is left of the original emulsion drop structure. The photo between crossed polarizers (Figure Sd) illustrates the fact that birefringent material now constitutes a majority of the emulsion. DISCUSSION The results are of interest for the behavior per se of a skin care emulsion during evapo­ ration, but they also contain information relevant to the action by the emulsion on the skin. The following discussion will concentrate on the first matter the latter aspects will be discussed interspersed with the former ones when appropriate. The results leave surprisingly complete information about the structure and behavior of the emulsion, in spite of the fact that only unsophisticated and common laboratory equipment was used in the investigation. At first the results reveal that the emulsion has excellent stability against coalescence, as demonstrated by the results in Figures 1, 2 and 4. The microscopy photos show the size distribution of macroscopic drops to remain intact after centrifugation as well as after evaporation to 40% of the total. The creation of smaller drops during evaporation (Figures 1 and 2 versus Figure 5) is a result of a precipitation of compounds dissolved in one of the liquid phases during evaporation. Evaporation of the oil phase at this early stage must be excluded because the similarity of the drop size distribution photos in Figures la, 2, and 5a convincingly proves the oil phase not to evaporate initially. Hence, the particles formed during evaporation are formed in the aqueous phase, a conclusion that is supported by the fact