This paper reports on the solvent effect on energy transfer reactions in supercritical CO2. The energy transfer reactions are studied by steady-state and time-resolved fluorescence spectroscopy and the fluorophor/quencher reaction pairs are chosen to vary the reactions from diffusion-controlled to kinetically-controlled. In particular, the fluorescence quenching of anthracene by CBr4, 1,2-benzanthracene by CBr4, and anthracene by C2H5Br in supercritical CO2 at 35 °C has been reported. Experimental rate constants for the first two reaction pairs, anthracene/CBr4 and 1,2-benzanthracene/CBr4, follow the predicted diffusion control limit at all pressures from 77.9 to 160.6 bar, indicating that local solvation does not enhance the reaction rate nor substantially impede the diffusion process in supercritical CO2. The rate constants for the third reaction, the quenching of anthracene by C2H5Br, are several orders of magnitude below the diffusion control limit, indicating that the reaction is kinetically controlled, as it is in liquids. In supercritical CO2 the apparent rate constants (i.e., those based on bulk concentrations of the reactants) for the anthracene/C2H5Br reaction decrease dramatically with increasing pressure. We believe that this apparently large pressure effect on the reaction rate is primarily due to the local composition enhancement of the quencher molecules around the dilute anthracene solute. This analysis is supported by fluorescence spectra and solvatochromic shift data of anthracene in pure CO2 and in mixtures of CO2 with C2H5Br at 35 °C that indicate both local density augmentation and local composition increases around the anthracene.

We report on the solvent effect on a kinetic-controlled energy-transfer reaction in super- and subcritical ethane. The reaction chosen, the quenching of naphthalene fluorescence by C2H5Br, occurs well below the diffusion-controlled limit in liquid solvents and in ethane. The reaction was studied in ethane over a wide range of pressures, extending from 34.8 to 111.6 bar at 40 °C, which corresponds to reduced densities (ρ r = ρ/ρ c, where ρ c is the critical density of ethane) between 0.27 and 1.76. The rate constants in super- and subcritical ethane range from 2.42 × 108 to 1.36 × 109 M-1 s-1, which is several orders of magnitude below the diffusion-controlled limit (which is on the order of 1011 M-1 s-1 at these conditions). As in several previous reports, the apparent rate constants, based on bulk concentrations of the reactants, increase dramatically with decreasing pressure in the supercritical region. More importantly, at reduced densities below about 0.44, the apparent rate constants based on bulk concentrations decrease with decreasing pressure. Because the rate constants are solvent insensitive in a variety of liquid solvents, we attribute the behavior of the apparent rate constants in ethane to local composition enhancement of C2H5Br around naphthalene. In addition, we present compelling theoretical (based on integral equation calculation) results that confirm a maximum in the local composition of C2H5Br around naphthalene near a reduced density of 0.44. This is the first experimental study of a simple, bimolecular reaction over such a wide range of densities. Moreover, it is the first to show a maximum in the reaction rate, which corresponds to the expected maximum in the local composition enhancement.

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