Vibrational Line Shifts in Supercritical Fluids

A microscopic statistical mechanical theory of solvent-induced vibrational line shifts of dilute solutes in supercritical fluids is presented. The theory is based on a simple model of a spherical solute present at infinite dilution in a fluid of spherical solvent particles. A microscopic expression for the vibrational line shift is given, which involves the solute−solvent radial distribution function and interaction potentials. The distribution function is obtained from integral equations and from Monte Carlo simulations. The theory is applied to study the experimentally observed anomalous density dependence of line shifts in supercritical fluids in the vicinity of the critical point. Model calculations of spectral shifts are performed for a range of solvent densities and temperatures and model potential parameters. In addition, a quantitative comparison of the theory with experimental data on vibrational spectral shifts is performed, and the agreement is satisfactory.