Water-in-carbon dioxide microemulsions and emulsions: Formation, stability, and media for chemical reactions

Together, water and carbon dioxide (CO₂) constitute the two most abundant and environmentally benign solvents on earth. Liquid or supercritical CO₂ exhibits solvent properties that are tunable with pressure, and is essentially nontoxic and nonflammable. Dense CO₂ is non-polar (unlike water) and has weak van der Waals forces (unlike oils) and as such may be considered a third type of fluid phase in nature, somewhat similar to fluorocarbons. Dispersions of water-in-CO₂ (W/C), whether on the nanometer (microemulsions) or micrometer (emulsions) scale, offer new possibilities for separations on the basis of polarity, and as media for reactions between polar and nonpolar molecules. For the first time, stable W/C emulsions are formed containing 70 vol% water. Optical microscopy is used to measure drop size and the degree of droplet aggregation. Two mechanisms of emulsion stability are identified, namely Marangoni-Gibbs and steric stabilization. With small changes in pressure, the W/C emulsions become unstable, allowing easy product recovery. In contrast to emulsions, concentrated W/C microemulsions have been formed containing equal amounts of water and CO₂ using greater amounts of surfactant. As a result of the dramatic increase in droplet volume fraction obtained in this study, droplet interactions dominate the microemulsion phase behavior and are able to be studied for the first time. Electrical conductivity measurements are used to identify regions of strong droplet interactions. A dynamic percolation phenomenon is observed whereby the clustering of discrete droplets and not the formation of bicontinuous structures accounts for the increased values of the microemulsion conductivity. Small-angle neutron scattering (SANS) experiments are performed allowing for the quantification of droplet interactions in W/C microemulsion systems, and it is revealed that droplet interactions are greater in these microemulsion as compared to traditional oil-continuous microemulsions. SANS is also utilized to verify the formation of a new class of W/C microemulsions formed with cationic surfactants. For the first time, tuning of drop size with a field variable, in this case temperature, is demonstrated. Finally, reactions occurring at the water-CO₂ interface of W/C emulsions between a hydrophilic substance in water and a hydrophobic substance dissolved in CO₂ are studied.