j. Soc. Cosmet. Chem., 44, 23-34 (January/February 1993) Evaporation from a complex emulsion system BRUNO R. C. LANGLOIS and STIG E. FRIBERG, Center for Advanced Materials Processing, Department of Chemistry, Clarkson University, Potsdam, NY 1699-5814. Received September 30, 1992. Synopsis The phase equilibria were analyzed in a typical skin lotion system of water, decane, triethanolamine, and isostearic acid, and the evaporation rate was determined. The system showed two isotropic solutions, one of decane/isostearic acid, and a water/triethanolamine combination. In addition, a large region of lamellar liquid crystal phase was found plus a small region of liquid crystal that consisted of close-packed cylinders. Evaporation of water and decane initially led to a transfer of triethanolamine from the aqueous solution to the decane/isostearic acid solution. With sufficient depletion of water and decane, a three-phase region was entered with the third phase, the lameIlar liquid crystal. INTRODUCTION Cosmetic emulsions (1) are of significant interest because of their stringent stability requirements, combined with a pleasing appearance as well as appealing feel upon application. The phase changes that occur during evaporation of volatile components are important because they influence the evaporation rate per se (2-4). In addition, the structure remaining on the skin after evaporation has ceased is of equal importance. This structure may be an oil phase, in which case excellent occlusivity may be found (5), or a liquid crystal with its interesting structural interactions with the stratum comeurn lipids (3,6). We have earlier (7) analyzed the conditions when an emulsion is applied to a surface with a hydrophobicity similar to that of human skin. That analysis was concerned with phase changes and inversion of the emulsion as well as with its flocculation and coalescence. In the current article we present an analysis of the phase equilibria related to the evaporation of water and decane from the system. The changes in the equilibria show a more complex behavior than expected. EXPERIMENTAL CHEMICALS The decane 99.7% (Fisher Scientific), the triethanolamine 99.9%, (Fisher Scientific), 23

24 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS the isostearic acid (Sigma), and a mixture of approximately 30% normal and methyl branched isomers and their homologs were used as obtained without further purifica- tion. The water was doubly distilled. EMULSION PREPARATION FOR EVAPORATION A mixture of isostearic acid/triethanolamine (65/35 weight ratio) was used as an emul- sifier. The emulsions were prepared by a method previously described (7), with different W/O ratios and 10% emulsifier counted on total weight. After emulsification on a vortex mixer, the droplet size distribution was checked by microscopy and found similar for the different W/O ratios (Figure 1). EVAPORATION STUDIES Previously we have noted (7) that the essential factor for the evaporation is the area exposed to the atmosphere. The monitoring of the area was made possible by use of a watch glass to avoid interruption of the film during evaporation, and the atmosphere was controlled by a constant air flow to ensure identical convection. The relative humidity was ambient and varied between 24.0% and 27.0%. This small variation had no influence on our results. The exposed area of the emulsion was slowly decreased by evaporation. To obtain the rate of evaporation per unit area, the slope of the weight loss as a function of time is divided by the exposed area. The areas measured during evaporation were divided by the largest initial area obtained for the emulsion system of different W/O ratios, giving an evaporation rate per arbitrary unit area. All the initial areas were nearly identical, since we started with similar quantities of material. Since water and decane, which are the only volatile components of our systems at room temperature, have similar vapor pressure, it is possible to follow the evaporation rate versus the composition of each emulsion with different W/O ratios. Finally, for some experiments, the evaporation was followed gravimetrically, while changes in phase structure that occurred during evaporation were observed under a microscope with the sample between crossed polarizers. PHASE DIAGRAM DETERMINATION The phases in the ternary phase diagram for the decane/water/(isostearic acid/ triethanolamine, wt. ratio 65/35) were determined. The liquid crystalline phase, the isotropic phase, and the three-phase area boundaries were identified by optical micros- copy. The boundaries of the lameliar phase were also obtained by small-angle X-ray diffrac- tion. The mixture was drawn into fine glass capillaries of 0.7-mm diameter, sealed at both ends, and placed into a brass sample holder with a 2-mm diameter opening. The X-ray radiation was K= copper filtered by a nickel foil to give a wavelength of 0. 1542 nm at 40,000 V and 18.10 -3 A. The X-Ray equipment (Siemens Crystalloflex 4) consisted of a sensitive detector system (Tennelec PSD 100), a flow proportional