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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