The Effect of Solvent Density Inhomogeneities on Solute Dynamics in Supercritical Fluids:  A Theoretical Perspective

The solvating properties of supercritical fluids (SCF) in the compressible region of the phase diagram may be varied dramatically with modest changes in temperature and pressure, making these solvents a promising reaction medium for chemical syntheses. However, it is in just this compressible regime that SCFs exhibit unusual behaviors, such as solvent and solute “clustering,” which complicate the prediction of SCF solvent effects on solute dynamics and reaction. Herein we describe the current understanding of solvent “clustering,” both in pure SCFs and in SCF solutions, and we discuss the effect that these average solvent density inhomogeneities may have on solvation and on activated solute reactions, as well as the methods by which these effects may be calculated. Additionally, dynamic solute processes may be sensitive to fluctuations of the solvent environment away from its inhomogeneous average, especially if the solvent fluctuations are slow. We thus present a method for examining the distribution of solvent environments experienced by solute molecules, and show that this distribution is indeed both slowly relaxing and broad, spanning a range of behaviors from vapor-like to solid-like. Finally, we point out promising new directions and outstanding questions in the area of solute dynamics in compressible SCFs.