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Brownian and molecular dynamics simulations are used to study rapid bimolecular reactions at near-in finite dilution in near-critical and supercritical fluids. We probe the dynamics of both nonreactive and reactive collisions and measure rate constants f o r reaction and collision. Collision rate constants are nearly independent of bulk solvent density, but affected by local solute-solute density enhancements at a given density: their magnitudes depend on the length scale for molecular encounters (cybotactic radius) in the reaction through the equilibrium solute-solute radial distribution function. In contrast, reaction rate constants asymptotically approach the gas-kinetic limit at low densities and the IntroductionThere are certain classes of extremely rapid reactions where mass transfer is the rate-limiting step. Almost all free-radical reactions require so little activation energy that they are diffusion-controlled at liquid-like densities; examples of such reactions include atom recombinations, low molecular-weight free-radical reactions, and macroradical reactions such as freeradical polymerization (North, 1964). While these limitations could be relieved by carrying out these reactions in the gas phase, gas-phase reactant solubilities are often extremely low. The supercritical fluid phase provides a balance between the high solubilities provided by liquids and the high mass-transfer rate achievable in gases.Correspondence concerning this article should be addressed to J . A. O'Brien 346 February 1995The use of supercritical fluids (SCFs) as reaction media is an area of current technological development. Some of the unique features that characterize SCFs are liquid-like densities, gas-like viscosities, and diffusivities that are intermediate between typical gas and liquid values. Perhaps the most important feature is the ability to vary widely these properties of a SCF using small changes in temperature and pressure. Despite these potential advantages, fundamental understanding of the effect of supercritical fluid structure on reactivity is still incomplete, Reactivity of a solute in a supercritical solvent is strongly dependent on its local environment (Johnston and Haynes, 1987). Studies of reactions in supercritical fluid media have shown that local solvent structure has a substantial effect on reaction rate constants, which has been explained in terms of local density variations of the solvent in the environment sur- Vol. 41, No. 2 AIChE Journalrounding the solute (Johnston and Haynes, 1987). Local density augmentation of solvents around solute molecules has been measured in various spectroscopic studies (Kim and Johnston, 1987; Kajimito et al., 1988;Yonker and Smith, 1988;Johnston et al., 1989; Eckert, 1988, 1989;Betts et al., 1992;Carlier and Randolph, 1993). Computational approaches and integral equation theories have provided further evidence of these local density variations. Molecular dynamics (MD) simulations of Knutson et al. (1992) showed local density enhancements of supercritical carbon dioxid...

We present a general methodology that can be used to study the structural effects of supercritical solvents on fast reactions. Using a hierarchy of theoretical, computational, and spectroscopic methods, we investigate the effect of local solvent structure on a model free-radical reaction. Our results from theory and experiments indicate the existence of locally high solvent densities around reactants at subcritical bulk densities. By comparing simulations and experiments, we show how these local structural correlations enhance the rate constant for Heisenberg spin exchange in supercritical fluids.