A fluorescence lifetime and integral equation study of the quenching of naphthalene fluorescence by bromoethane in super- and subcritical ethane

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 degrees C, which corresponds to reduced densities (rho(r) = rho/rho(c), where rho(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 x 10(8) to 1.36 x 10(9) M-1 s(-1), which is several orders of magnitude below the diffusion-controlled limit (which is on the order of 10(11) 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.