FREE-RADICAL REACTIONS IN SUPERCRITICAL ETHANE - A PROBE OF SUPERCRITICAL FLUID STRUCTURE

Bimolecular rate constants for the Heisenberg spin-exchange reaction between nitroxide free radicals are reported in near-critical and supercritical ethane. Rate constants are independent of radical concentration over the range investigated. Measured activation volumes for the diffusion-controlled reaction are as large as 7.5 L mol-1 and are well-predicted from Stokes-Einstein 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.