Carbon dioxide in the liquid and supercritical fluid states is useful as a replacement for toxic organic solvents. However, nonvolatile hydrophilic substances such as proteins, ions, and most catalysts are insoluble. This limitation was overcome by the formation of aqueous microemulsion droplets in a carbon dioxide-continuous phase with a nontoxic ammonium carboxylate perfluoropolyether surfactant. Several spectroscopic techniques consistently indicated that the properties of the droplets approach those of bulk water. The protein bovine serum albumin (BSA) with a molecular weight of 67,000 is soluble in this microemulsion and experiences an environment similar to that of native BSA in buffer.

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.

IntroductionIn the supercritical fluid state, the solvent properties (such as density, viscosity, and dielectric) are highly sensitive to small changes in pressure or temperature, especially in the immediate vicinity of the critical point. These "tunable" properties make supercritical fluid (SCF) solvents attractive for both separation (McHugh and Krukonis, 1986) and reaction processes (Wu et al., 1991). Commercially successful processes have been developed for SCF extraction. Applications of SCF solvents in reaction processes, however, are only beginning to emerge, and there are not many data available on reactions. Design and control of systems for reactions under supercritical conditions will require not only the same pure-component and mixture thermophysical properties needed for extractions (such as equations of state and mixing rules), but also additional mechanistic information on SCF solvent effects on reactions.Several studies of reactions revealed large solvent effects on the rate constants which have been explained in terms of "cage effects" or "clustering" of the solvent about solute molecules Johnston and Haynes, 1987;Wu et al., 1991;Combes et al., 1992). Solvatochromic (Yonker and Smith, 1988;Kim and Johnston, 1987;Johnston et al., 1989) and fluorescence spectroscopy (Brennecke and Eckert, 1989;Brennecke et al., 1990a;Betts et al., 1992) experiments among others have shown solute-solvent interactions higher than those in homogeneous liquids, leading to the conclusion that effective local solvent or cosolvent densities can be several times higher than the bulk densities. Reaction rates are affected simultaneously through both the rate constants and the local concentrations of the reactants. Because these effects may exhibit opposite pressure dependencies, understanding of the phenomena involved at the molecular level can greatly improve the choice of physical conditions and prediction of reaction rates, and allow the tailoring of SCF solutions for specific applications. Despite the experimental and computational evidence for their existence, solvent-solute clusters have been the subject of a recent debate (Economou and Donohue, 1990;Brennecke et al., 1990b pressibilities result from long-range effects, while local density measurements by various spectroscopic techniques reflect shortrange order. The two types of effects may not necessarily be coincident; we will show data from two systems which suggest the contrary.

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Bimolecular rate constants for the Heisenberg spin-exchange reaction between nitroxide free radicals are reported in nearcritical and supercritical ethane. Rate constants are independent of radical concentration over the range investigated. Measured activation volumes for the diffusioncontrolled reaction are as large as 7.5 L mol-' and are well-predicted from Stokes-Emtein hydrodynamics. Reaction probabilities, however, show the effect of local density augmentation of solvent molecules around the radicals, particularly in the immediate vicinity of the critical point. Local density augmentation increases the length of time that the radicals are in contact with one another, thus increasing the probability of reaction per collision. The disparate time scales for diffusion, cluster lifetime, and collision lifetime are used to explain the effects of local density augmentation on the reaction rate constants.