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 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 (φ_fast) 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.