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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 supercritical fluid what amounts to a second solvent environment, which is highly polar and which may be manipulated by changing the pressure. Polar or ionic species partition into the aqueous microphase of the microemulsion. As a result, these microemulsions greatly expand the range of applications of supercritical fluids for cleaning processes.
Although it is not quite a decade since the discovery of microemulsion formation in supercritical fluids^11 there has been a great deal of interest in understanding their properties121'171 in order to efficiently apply this new technology. Microemulsions formed in conventional liquid solvents such as alkanes have been known for a much longer time and a variety of applications have been explored including use for general separations, enhanced oil recovery,[81 as reaction media for catalytic,^101 or enzymatic reactions,11111121 for general catalytic and semiconductor applications^131^151 as a mobile phase in chromatographic separations,I161 and for the separation of proteins and biomolecules from aqueous solution.11711181 Many similar applications of supercritical microemulsions have been explored.119^201
The technique may be viewed as an alternative to the addition of cosolvents or modifiers (sometimes termed entrainers) that are commonly used in supercritical fluid technology to enhance the polarity of the fluid. For cleaning processes, however, these cosolvents may be toxic or detrimental in various ways to the substrate. In addition, these modifiers are usually more difficult to separate downstream from the process due to their high volatility. In contrast, surfactants typically have very low volatility and thus interact to a much lesser degree with the substrate. Furthermore, they often dramatically improve the solubility of polar species, well beyond that of simple modifiers.
Because of the obvious importance of a C02-based system, there has been much work in the area of developing this type of supercritical fluid microemulsion.PP1H24] Qn the surface, the strategy is simple and interesting parallels can be made to the first soap or surfactants used in water thousands of years ago. For instance, with
water, we start with an abundant, benign solvent and add surfactants that will aid in the solubilization of oils in order to improve the cleaning efficacy. For a C02 microemulsion we reverse the thinking. We start with the least expensive nonpolar solvent and add surfactants to increase the solubility of polar species—again dramatically improving the cleaning properties. As discussed herein, there are many applications where there are distinct advantages in using a C02-based microemulsion over aqueous- or liquid-based systems.
The application of supercritical microemulsions to cleaning is a new area and there have been no published results to date applying these systems to cleaning operations but the combination of surfactants with fluids would be a natural choice for many systems. The information in this chapter is to be used as a guide to the development of cleaning applications based upon supercritical fluid-based microemulsions.
2.0 MICROEMULSION STRUCTURE
It is helpful to begin by describing and defining the basic properties of microemulsion systems. Microemulsions are clear, thermodynamically stable solutions that generally contain water, a surfactant, and an oil. An example of the structure of a microemulsion phase is shown in Fig. 1. The “oil” in this case can be a nonpolar or low polarity liquid solvent or a supercritical fluid phase. The surfactant molecules form organized molecular aggregates and preferentially occupy the interface between the oil and water phases. Although microemulsions are thermodynamically stable, they are not static and are continuously breaking and reforming. The oil and water microdomains have characteristic structural dimensions between 5 and 100 nm. Aggregates of this size are poor scatterers of visible light and hence these solutions are optically clear. Water-in-oil (w/o) microemulsions can have a multitude of different microscopic structures including sphere, rod, or disc shaped aggregates. Micelles are the smallest aggregate entity and are generally defined as binary systems containing a surfactant that forms organized clusters in either water (normal micelles) or in oil (reverse or inverted micelles). When
small amounts of water are added to a reverse micelle system, the water preferentially partitions into the hydrophilic core, causing it to swell and yielding (at some arbitrary water content) a water-in-oil microemulsion. At low water concentrations, such inverted microemulsions typically contain spherical, nanometer-sized droplets of water. These microdomains of water are dispersed into a nonpolar solvent that is sometimes referred to as the continuous phase solvent.