Formation and Growth of Water-in-CO₂ Miniemulsions

Abstract

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.

Critical flocculation density of dilute water-in-CO2 emulsions stabilized with block copolymers

“…The temperature was controlled to ±0.1 • C by immersing both the view cell and the homogenizer into water baths equipped with temperature controllers (Julabo, Inc.). The morphology of the emulsion was determined via electrical conductivity measurements, as described elsewhere [12].…”

“…As a result of these beneficial properties, carbon dioxide has emerged as an alternative to traditional organic solvents in the preparation of colloidal dispersions [2][3][4][5][6], including microemulsions [3,[7][8][9], miniemulsions [10][11][12], macroemulsions [13][14][15][16][17][18], and polymer latexes [19,20]. Water-in-CO 2 (W/C) miniemulsions [10,12], with droplets ranging from 50 to 500 nm in diameter, and macroemulsions [14,15,18,21], consisting of droplets greater than 500 nm, are formed by shear and are stabilized kinetically, whereas microemulsions are thermodynamically stable. Examples of applications for these liquid or solid dispersions include dry cleaning [22], photoresist drying [23], metal extraction [24], nanoparticle synthesis [25,26], and enzymatic catalysis [27].…”

“…As a result of this poor solvation, the forces between CO 2 -philic tails are often attractive or only weakly repulsive and thus insufficient to counteract the attractive core-core forces between approaching droplets and prevent flocculation. To accommodate CO 2 's relatively weak van der Waals forces, emulsions are often stabilized by polymers with low cohesive energy densities, including polydimethysiloxanes, polyfluoroethers, and polyfluoroacrylates [12,18,20,28,29]. Smallangle neutron scattering (SANS) measurements indicate that both poly(1,1-dihydroperfluorooctyl acrylate) (PFOA) and poly(dimethyl siloxane) posses a positive second virial coefficient (repulsive interactions) in dense CO 2 , indicating that CO 2 is a good solvent for these polymers [30,31].…”

“…For this study, the nonionic surfactant, poly(1,1-dihydroperfluorooctyl methacrylate)-b-poly(ethylene oxide) (PFOMAb-PEO) was utilized, since it has been shown to form and stabilize nonflocculated, dilute W/C miniemulsions for greater than 24 h [12]. The CFD for these miniemulsions was measured using turbidimetry as a function of PFOMA molecular weight, average core size, temperature, and surface coverage of the stabilizer.…”

Microscopic phase behavior of supercritical carbon dioxide+non-ionic surfactant+water systems at elevated pressures

“…In addition to investigations of engineering applications utilizing both W/CO 2 microemulsion and macroemulsion, basic studies from the viewpoint of thermodynamic stability and interfacial behavior were reported by Johnston et al [15][16][17][18][19][20][21][22][23]. Moreover, a phase diagram is also important for the development of novel processes utilizing W/CO 2 microemulsion and macroemulsion, therefore, the phase behaviors of the scCO 2 + surfactant + water systems were also measured.…”