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j. Soc. Cosmet. Chem., 34, 73-81 (March/April 1983) Microemulsions with esters . STIG E. FRIBERG and LI GAN-ZUO,* Chemistry Department, University of Missouri-Rolla, Rolla, Missouri 65401. Received August 16, 1982. Synopsis The region of stability was determined for water/ester microemulsions stabilized by sodium dodecyl sulfate and pentanol. Increased content of short chain esters gave larger microemulsion areas, while long chain ones gave smaller areas than for the corresponding hydrocarbon systems. Both effects could be explained from the enhanced interfacial activity of the esters versus hydrocarbons, using the microemulsion/liquid crystal equilibrium that is the limiting factor for the microemulsion ar•as. The presence of short chain esters destabilized the lameliar liquid crystal giving larger microemulsion areas long chain esters had the opposite effect. INTRODUCTION Microemulsion stability and structure form a problem that has intrigued researchers for a long time. The discussion has lasted from the early introduction in patent literature (1,2), over the scientific introduction about forty years ago (3), to the present, rather intense research efforts. The development has shown the early emphasis on interfacial tension (4,5) as the only factor for stability to be an oversimplification (6). The latest research in the area has demonstrated the stability of a microemulsion system to be a rather complex phenomenon (7). In spite of this development into more complete coverage of the thermodynamic factors, it should be realized that the central theme of any stability theory always harbors the concept of an extremely low interfacial tension. In this respect, the Schulman approach (8) has made a lasting contribution. The present contribution is not aimed at being an argumeht in the discussion of microemulsion structure and stability. The goal is to enlarge the area of microemul- sions from hydrocarbons to other organic compounds. So far, the literature in the field has been confined to hydrocarbons, a natural trend with regard to the importance of microemulsions in the area of tertiary oil recovery (9,10). However, esters are an important group of compounds in cosmetics and in pharmacy, and an evaluation of the conditions to prepare microemulsions from this class of compounds should fill an important need. *Permanent address: University of Shandong, Jinan, Shandong, Peoples' Republic of China. 73