Water Core within Perfluoropolyether-Based Microemulsions Formed in Supercritical Carbon Dioxide

Abstract

We report new experimental data on the ability of a perfluoropolyether-based surfactant (PFPE) to form stable reverse micelles in supercritical CO 2 . Previous work from our groups showed that PFPE reverse micelles formed in CO 2 can host a wide variety of hydrophiles and even provide an environment capable of solubilizing large proteins [Johnston, K. P.; Harrison, K. L.; Clarke, M. J.; Howdle, S. M.; Heitz, M. P.; Bright, F. V.; Carlier, C.; Randolph, T. W. Science 1996, 271, 624-626].In the current work we report cloud point data for PFPE in CO 2 , X-band EPR studies, and time-resolved anisotropy measurements. The cloud point data show that a one-phase water-in-CO 2 microemulsion can be formed with a nominal molar water-to-surfactant ratio (R) of 20.7 at 45°C and 158.1 bar. EPR experiments on PFPE (with 4-hydroxy-TEMPO) and Mn-(PFPE) 2 show that PFPE aggregates in CO 2 at pressures below which a water pool can be formed. Stable Mn(PFPE) 2 micelles can also be formed in supercritical CO 2 , and the internal water pool within these micelles is able to ionize manganese, demonstrating that the water within this pool differs significantly from water within the CO 2 bulk phase. EPR results also suggest that these micelles exist in a nonspherical form. The rotational reorientation kinetics of two model fluorescent probes, rhodamine 6G and lissamine rhodamine B sulfonyl hydrazine, are described well by a biexponential decay law. The faster rotational reorientation time (φ fast ) is approximately 100 ps and remains constant regardless of CO 2 continuous phase density or R. We interpret the fluorophore rotational dynamics using three established models: a wobbling-in-a-cone model in which the fluorophore precesses about its emission transition dipole, a lateral diffusion model wherein the probe diffuses along the reverse micelle headgroup/water core interface boundary, and an anisotropic rotor model where the micelle shape itself is nonspherical.

Dissolution of Trihexyltetradecylphosphonium Chloride in Supercritical CO2

“…A broadened spectrum could also result from increased solution heterogeneity, which is conceivable if C153 is partitioning into distinctly different solvent environments. Dissolution of amphiphilic molecules into a bulk supercritical fluid phase, including scCO 2 , has demonstrated the formation of a micellar environment in supercritical fluids [67][68][69][70][71][72]. While we are not suggesting micellization of [P 6,6,6,14 ] + Cl − , these reports show that molecules similar to this IL have been solubilized in supercritical fluids.…”

Characterization of Water/Supercritical CO2 Microemulsion by UV-visible Spectroscopy and Dynamic Light Scattering

“…A number of studies have been directed toward the development of a surfactant system for stable formation of a W/scCO 2 mE, and more than 150 surfactants have been examined in these studies [7][8][9][10][11][12][13][14][15][16][17][18][19] . However, only a few have succeeded in developing a W/scCO 2 mE with W 0 c > 0 (W 0 c :water-to-surfactant molar ratio corrected by subtracting the moles of water soluble in CO 2 ).…”

Synthesis and Evaluation of CO2 Thickeners Designed with Molecular Modeling

“…Although crystalline, high molecular weight peracetylated polysaccharides such as cellulose triacetate are CO 2 insoluble, we have 67 determined that P(AcGIcVE), a polymer with a polyethylene backbone, a flexible -OCH 2 Ospacer and pendent sugar acetates, is more CO 2 soluble that any polymer other than PVAc.…”

“…EPR identified the micelle as having a bulk-like water core able to solvate ionic species while time-resolved fluorescence depolarization revealed an anisotropic/nonspherical reverse micelle. [67] FTIR and UV-vis spectroscopy by Johnston and coworkers [68,69] showed that the w/c microemulsion consists of regions of "bulk" hydrogen bonded water, "interfacial" water, and "free" water dissolved in sc-CO 2 . The UV-vis spectroscopic studies using methyl orange as a probe indicated that the PFPE/sc-CO 2 /water reverse micelle cores have a polar environment as seen in dry PFPE reverse micelles, a bulk-like water region, and an acidic environment resulting from carbonic acid formation.…”