J. Cosmet. Sci.! 59, 15-32 Qanuary/February 2008) A simple analysis of the changes during evaporation of a commercial emulsion of unknown composition MATEUS M. BERGAMASCHI, ORLANDO D. H. SANTOS, PEDRO A. ROCHA-FILHO, and STIG. E. FRIBERG, Department of Pharmaceutical Science, Faculty of Pharmacy, University of Sao Paulo, Ribeirao Preto, SP, 14040-903, Brazil (M.M.B, P.A.R.-F), Department of Pharmacy, School of Pharmacy, Federal University of Ouro Preto, Ouro Preto, MG, 35400-000, Brazil (0.D.H.S.), and Chemistry Department, University of Virginia, Charlottesville, VA, 22904 (S.E.F.). Accepted for publication September 27, 2007. Synopsis Optical microscopy and centrifugation were used to observe the structural changes during evaporation of a commercial skin lotion of unknown composition. The degree of evaporation was determined from the changed weight of a microscope slide with the emulsion on a defined area and thickness, the evaporation loss versus time being measured by a balance under an infrared lamp. The results revealed not only which parts of the emulsion were most prone to evaporation without chemical analysis, but also gave surprising information as to which kind of structures would appear after extensive evaporation. The importance of these changes for the action of a skin lotion is briefly discussed. INTRODUCTION The research into emulsions for skin treatment has shown an interesting divergence. On the one hand, the structures and properties of emulsions per se have been extensively analyzed (1-4). On the other hand, the research into the structure of the appropriate parts of the skin has been at an advanced state for a long time (5-9). However, the relation between the effect on the skin of emulsions and their structure on the skin has not been successful to the same degree. In fact, a conclusion some years ago that "a review of the literature shows many contradictory findings and views" (10) is not completely without validity today, even if the research in the area is extensive (11-13). Admittedly, the entire area is highly complex, with a large number of interacting factors, but one seminal factor is the changes in emulsion structure during evaporation. In itself the evaporation of emulsions has been given highly advanced treatment by Aranberri (14,15), who successfully related evaporation rate to colloidal factors in the original emulsion. These factors, albeit less directly, were also implicated by Friberg in 15

16 JOURNAL OF COSMETIC SCIENCE a series of publications on fragrance evaporation (16) and by Santos and co-workers in a study of evaporation of vegetable oil emulsion with lamellar liquid crystal phases (17). However, in spite of the rich information available, it appears that one essential factor has only received scant attention. During the evaporation of emulsions, new structures appear, even for the simplest of emulsions (18). So far these structures have not received notable attention. One possible reason for this negligence may be the complexity of the phase changes taking place during the evaporation of a typical skin lotion emulsion (19). This complexity may be the main reason for the paucity of publications utilizing phase diagrams to estimate structural changes during evaporation. The application of phase diagrams is not widely accepted within the cosmetic formulation community. However, there is no reasonable doubt that the structural changes during evaporation are of central importance for the action on the skin. With this fact in mind, we found a less elaborate process than determining phase diagrams to be of interest to obtain some essential information about evaporation changes in skin care emulsions during evapo­ ration. In order to make the investigation highly realistic, the present contribution reports the changes during evaporation in a commercial emulsion of unknown compo­ sition, utilizing only generally available equipment: an optical microscope and a cen­ trifuge. The simple experiments gave surprisingly essential information about the changes taking place. EXPERIMENT AL The present experiment followed the methodology of emulsion evaporation on micro­ scopic slides done by Santos et al. (17), who did some modifications from the one done by Friberg and his collaborators (18). A drop of the commercial emulsion was deposited onto an area on a microscope slide limited to 6.9 cm2 . The thickness of the layer was kept at 0.2 mm. The evaporation loss versus time was measured using a balance (Mettler P160N) equipped with an infrared lamp at 70°C. At each 10% of weight loss, until 90%, the microphotos of the sample were obtained in an Olympus microscope (Model BX 50) equipped with polarizers. The drop-size distribution in the emulsions was analyzed by manual measurements of the drop sizes in the microscopy photos. This information was complemented by centrifugation. In the first instance, the original emulsion was centrifuged. In a second experiment, a thin layer of the emulsion was spread in a Petri dish and, guided by the results of the microscopy, it was evaporated until 80% of the initial weight remained. The remainder was gathered into an Eppen­ dorf test tube and centrifuged at 16.100 ref (Eppendorf Centrifuge 5415R) and at 20.400 ref (Eppendorf Centrifuge 5417R) until maximum separation occurred. Each layer was photographed in the microscope both between crossed polarizers and in iso­ tropic light. The original emulsion was centrifuged in the same manner. RESULTS The results will be reported in the following order. At first the microscopy photos for the emulsion after certain percentages of evaporation will be viewed to detect the initiation of essential changes in the structure. Secondly, the gross structures after centrifugation of both the original emulsion and the one evaporated to 20% of the