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 tension was measured 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.
A mathematical model for describing the bulk free‐radical polymerization of methyl‐methacrylate was developed. This model includes a novel methodology for describing the diffusive step of the kinetic rate coefficients, which is based on geometric considerations and application of the Einstein diffusion Equation. The effect of polymer content and temperature on the Rp behavior is discussed in terms of the evolution of the interdependent parameters defining the Rp. The applicability of Smoluchowski equation and the importance of the translational diffusion of short radicals in the rate of termination reaction are questioned in this context. Similitudes and differences between the model results and experimental data are discussed including minima in the Rp curves at low conversions.
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