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    Review date: July 24, 2003
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    Micelles and Microemulsions
    Publication list, More applications

    Microemulsions are clear, thermodynamically stable solutions that generally contain water, a surfactant, and an "oil". The "oil" in this case is the supercritical fluid phase. The 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.

    Recently, microemulsions formed in liquid and supercritical CO2 have emerged as a new type of solvent for industrial-scale processes. These CO2 microemulsions, consisting of macromolecular surfactant aggregates, have profound implications for enabling new separations and reactions with virtually no detrimental environmental effects and with significantly reduced life-cycle energy content.

    The implications of this technology are far ranging and could have a large impact on US industry in the 21st century, including:

    • use of an environmentally benign, low cost solvent, with greatly reduced energy costs through efficient separations and reactions and through the minimization or elimination waste
    • new solvent for a broad range of industrial reactions and extractions.

    Microemulsions dramatically improve the solvent properties of CO2 and other supercritical fluids to allow the dissolution of a wide range of polar species. PNNL is already a recognized leader in the study of microemulsions in supercritical fluids and PNNL researchers were the first to address the solubility limitations by using microemulsions in supercritical fluids. CO2 is environmentally benign, non-toxic, and is the second least expensive solvent available, next to water. Its use by industry has been limited because, by itself, it will dissolve only a small number of compounds having low polarity and low molecular weight. A microemulsion overcomes the major limitations of CO2 or other supercritical fluids by making it possible to dissolve highly polar, ionic, high molecular weight species.

    Extraction of a biomolecule using a propane microemulsion
    We have recently reported results, in a collaboration with researchers at University of Idaho, where Ag nano-crystals (5 - 15 nm size) were synthesized and stabilized in CO2. This finding opens a broad range of potential synthesis routes for a variety of different nanoparticles that provides a simple, effective means for removal of the solvent (CO2) from the particles. In addition to developing the particle chemistry suitable for a CO2 solvent, the colloid stability is controlled by the proper choice of surfactants, co-surfactants or capping agents.

    Ultra-small silver particles synthesized and stably suspended in carbon dioxide
    Ultra-small silver particles synthesized and stably suspended
    in carbon dioxide.

    PNNL is developing other applications of the micelle technology in the following areas:

    • Separation and reaction processes involving polar solutes
    • Polymerizations
    • Parts/garment cleaning
    • Processing and purification of pharmaceutical compounds
    • Solute deposition (UV stabilizers, dyes, flame retardants, chemical resistance)
    • Hazardous waste cleanup.

    For information about supercritical fluid capabilities at PNNL,
    please contact Clement Yonker, at (509) 372-4748, clem.yonker@pnl.gov.