5 - Surfactants and Microemulsions in Supercritical Fluids

Abstract

This chapter describes surfactants and microemulsions in supercritical fluids. The chapter discusses a new class of cleaning solvents in which the unique solvent power of a supercritical fluid is united with the broad solvation capabilities of a reverse microemulsion. The application of these systems in cleaning processes is discussed. It is mentioned that a reverse microemulsion has a structure similar to that of a conventional water based surfactant system with the exception that the nanometer-sized structures are inverted—the core of the micelle is an ultra small droplet of water and the exterior is the oil or supercritical fluid phase. The advantages of supercritical microemulsions over conventional liquid or aqueous based systems are mentioned as—energy savings, environmental benefits since the fluids may be benign and the contaminants or surfactants can more easily be recovered, microemulsions have a high capacity for oils or other lipophilic materials, the selectivity for the contaminants may be adjusted by density manipulation, and improved cleaning or extraction efficiency because of the high diffusivities and low viscosities inherent in supercritical fluids. A carbon dioxide (CO2 )-based microemulsion is especially attractive since CO2 is very abundant, relatively inexpensive, and environmentally benign at this scale of use.. The range of industrial applications of supercritical microemulsions is extensive and includes areas such as separations, reactions, and spray coatings. Applications to cleaning could include dry cleaning, the separation of dyestuffs, the extraction of contaminants from soils, the regeneration of activated catalysts, and the removal of strongly polar or ionic species from printed circuit boards, polymers, foams, aerogels, porous ceramics, and laser optics. The combination of colloidal technology with the technology of supercritical fluids greatly enhances potential applications in cleaning processes. It concludes by mentioning research continuation with development of new surfactants with CO2-philic groups and with possible alternatives to carbon dioxide as the continuous phase.