Microemulsions in Near-Critical and Supercritical Fluids
Although the discovery that micelles and microemulsion phases could be stabilized in supercritical fluid (SCF) solutions was made in the authors’ laboratory less than 5 years ago, our knowledge of these systems has progressed rapidly. The combination of the unique properties of SCFs (e.g., viscosity, diffusion rates, solvent properties, etc.) with those of a dispersed microemulsion (or reverse micelle) phase creates a whole new class of solvents. The microemulsion phase adds to the properties of the SCF what amounts to a second solvent environment, which is highly polar and which may be manipulated using pressure. This second phase can itself manifest a wide range of solvent properties. Although SCFs are very attractive for separation and reaction processes owing to their density-dependent properties, they are limited at moderate temperatures and pressure by their inability to appreciably solvate most moderately polar solutes and nearly all ionic materials. 1 The addition of a dispersed droplet phase (forming a microemulsion) provides a convenient means of solubilizing highly polar or ionic species into the low polarity environment of the SCF phase. Hence, the combination of supercritical solvents with microemulsion structures provides a new type of solvent with some unusual and important properties of potential interest to a range of technologies.