Interaction induced Raman light scattering as a probe of the local densitystructure of binary supercritical solutions

Interaction induced Raman light scattering is presented as a unique tool fo r the understanding of solvation processes from the solute's point of view in weakly interacting solute-solvent systems. A review of pertinent literat ure shows that this technique should be useful at least in single-phase bin ary mixtures such as supercritical solutions. Methane is used here as a pro be molecule at 10 mol% concentration (as the solute) and 90 mol% CO and CO2 are the solvents. The light scattering results, i.e., the dependence of th e anisotropic intensities divided by density (I/d) on the density, are inte rpreted by use of the Duh-Haymet-Henderson closure (bridge) function of the Ornstein-Zernike integral equation. These data, together, are examined in the context of known supercritical solution thermodynamics and statistical mechanical results. It is shown that the light scattering I/d data versus d ensity yield maxims in both attractive and repulsive solute-solvent systems . The local number density maxims were found near these same densities by t he integral equation calculations for both methane + carbon monoxide or car bon dioxide using only Lennard-Jones single-centre parameters as input. The methane + carbon monoxide system is identified as weakly attractive (augme nting), whereas the methane + carbon dioxide system is identified as repuls ive (avoidance).