Formation and Growth of Water-in-CO₂ Miniemulsions

We report nonflocculated dilute water-in-CO₂ (W/C) miniemulsions stable for 24 h in contrast with flocculated unstable macroemulsions reported previously. The surfactants, poly(1,1-dihydroperfluorooctyl methacrylate)-b-poly(ethylene oxide) (PFOMA-b-PEO), were synthesized by atom transfer radical polymerization to achieve the proper hydrophilic-CO₂-philic balance (HCB) and a low interfacial tension (~0.2-2 mN/m) between water and CO₂. The average particle diameter, ranging from 70 to 140 nm, was measured with multiwavelength turbidimetry utilizing Mie theory, and the interfacial tensionwasmeasured with high pressure pendant drop tensiometry. Because flocculation and coalescence were suppressed nearly completely, it became feasible to investigate emulsion droplet formation and droplet growth by Ostwald ripening. Droplet formation was characterized as a function of the mechanical energy at various CO₂ densities and temperatures and was correlated quantitatively to the interfacial tensions. The molar water/surfactant ratios reached 1170, in contrast with values of only about 5-60 for W/C microemulsions. The ability to stabilize nanometer-sized miniemulsion droplets with large water/surfactant ratios is of great practical interest in reactions, separations, and materials formation processes in CO₂.