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 CO2. Previous work from our groups showed that PFPE reverse micelles formed in CO2 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 CO2, X-band EPR studies, and time-resolved anisotropy measurements. The cloud point data show that a one-phase water-in-CO2 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 CO2 at pressures below which a water pool can be formed. Stable Mn(PFPE)2 micelles can also be formed in supercritical CO2, 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 CO2 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 (φfast) is approximately 100 ps and remains constant regardless of CO2 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.