QUASICRITICAL BEHAVIOR OF DENSE-GAS SOLVENT-SOLUTE CLUSTERS AT NEAR-INFINITE DILUTION

We compare molecular dynamics and electron paramagnetic resonance spec troscopic experiments on very dilute solutions of nitroxide free radic als in nearcritical and supercritical ethane. The measurement of the e ffective local density of solvent in the cybotactic region of the prob e solute shows that maxima in local density enhancements occur in both computer and spectroscopic experiments at about half of the critical density, well removed from the maximum in the solvent's isothermal com pressibility. Remarkably, at the bulk density where the maximum enhanc ement occurs, we observed long-lived clusters in the vicinity of the s olute molecule. Geometrically defined clusters persist for at least 10 0 picoseconds of simulated time. The persistence of these clusters is corroborated by the behavior of the solute diffusion coefficient, whic h shows fluctuating values over simulated time periods as long as 1.3 nanoseconds. We argue that fluctuations in the local solvent density a bout a solute molecule are not a long-ranged solvent critical effect. Rather, they represent a localized, short-ranged quasicritical phenome non induced by the solute, which may be described in terms of a transi tion between weakly attractive and attractive behavior. We discuss the implications of time-varying microstructure for reactions in supercri tical fluid media.