EFFECT OF SOLUTE-SOLUTE CORRELATIONS ON RAPID REACTIONS IN SUPERCRITICAL FLUIDS

Brownian and molecular dynamics simulations are used to study rapid bi molecular reactions at near-infinite dilution in near-critical and sup ercritical fluids. We probe the dynamics of both nonreactive and react ive collisions and measure rate constants for reaction and collision. Collision rate constants are nearly independent of bulk solvent densit y, but affected by local solute-solute density enhancements at a given density: their magnitudes depend on the length scale for molecular en counters (cybotactic radius) in the reaction through the equilibrium s olute-solute radial distribution function. In contrast, reaction rate constants asymptotically approach the gas-kinetic limit at low densiti es and the Smoluchowski liquid-like limit at high densities. They also display the same radial dependence as collision rate constants at low er densities and a direct dependence on the cybotactic radius at highe r densities (as in the Smoluchowski theory). Their behavior is explain ed in terms of a transition from a collision-limited regime at low den sities to a diffusion-limited regime at higher densities. The transiti on between these regimes depends on the cybotactic radius and the dens ity of the system, the interplay of which causes shifts in the transit ion region which depend not only on the properties of the near-critica l solvent: they differ for different reactions, even at the same solve nt density. This explains some of the apparent inconsistencies among p revious experimental and computational studies of reactions in supercr itical fluid media.