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Review date: July 24, 2003
PNNL-SA-27883

Structure and Reactions in Microemulsions Formed in Near-critical and Supercritical Fluids

JL Fulton. In Microemulsion: Fundamentals and Applied Aspects. P. Kumar, Ed. Marcel Dekker, New York, 1996 (in press).

Abstract: Microemulsions tremendously expand the potential applications of supercritical fluids as reaction media for chemical reactions. By themselves, near-critical and supercritical fluids are much weaker solvents than the typical organic liquid. However, microemulsions create a highly polar region that is capable of solvating polar catalysts, reactants, or products. The unique aspects of the near-critical or supercritical continuous phases offer many advantages over their liquid-phase analogs.

The microstructure is wide and varied, and it appears that the fascinating variety of geometries found in liquid-phase microemulsions will be available for supercritical microemulsions. Thus far, several different water-in-"oil" microemulsions containing spherical droplets have been identified in fluids using cationic, anionic, and non-ionic surfactants. Further, long, extended-rod structures have been formed with a zwitterionic surfactant in near-critical propylene. Finally, an "oil"-in-water microemulsion can be formed in which cores of the spherical droplets are filled with a near-critical or supercritical fluid. Recently, new surfactants are appearing for use in CO2. This system has exciting potential for "solvent-free" organic synthesis reactions.

Supercritical microemulsions have thus far been used for particle synthesis, polymerizations, and hydrogenation reactions. These types of systems represent a potentially rich area of high industrial importance. Studies of these systems should expand in the coming years.