WATER CORE WITHIN PERFLUOROPOLYETHER-BASED MICROEMULSIONS FORMED IN SUPERCRITICAL CARBON-DIOXIDE

We report new experimental data on the ability of a perfluoropolyether -based surfactant (PFPE) to form stable reverse micelles in supercriti cal CO2. Previous work from our groups showed that PFPE reverse micell es formed in CO2 can host a wide variety of hydrophiles and even provi de an environment capable of solubilizing large proteins [Johnston, K. P.; Harrison, K. L.; Clarke, M. J.; Howdle, S. M.; Heitz, M. P.; Brig ht, F. V.; Carlier, C.; Randolph, T. W. Science 1996, 271, 624-626]. I n the current work we report cloud point data for PFPE in CO2, X-band EPR studies, and time-resolved anisotropy measurements. The cloud poin t 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 degre es C and 158.1 bar. EPR experiments on PFPE (with 4-hydroxy-TEMPO) and Mn(PFPE)I show that PFPE aggregates in CO2 at pressures below which a water pool can be formed. Stable Mn(PFPE)2 micelles can also be forme d in supercritical CO2, and the internal water pool within these micel les is able to ionize manganese, demonstrating that the water within t his pool differs significantly from water within the CO2 bulk phase. E PR results also suggest that these micelles exist in a nonspherical fo rm. The rotational reorientation kinetics of two model fluorescent pro bes, rhodamine 6G and lissamine rhodamine B sulfonyl hydrazine, are de scribed well by a biexponential decay law. The faster rotational reori entation time (phi(fast)) is approximately 100 ps and remains constant regardless of CO2 continuous phase density or R. We interpret the flu orophore rotational dynamics using three established models: a wobblin g-in-a-cone model in which the fluorophore precesses about its emissio n transition dipole, a lateral diffusion model wherein the probe diffu ses along the reverse micelle headgroup/water core interface boundary, and an anisotropic rotor model where the micelle shape itself is nons pherical.