Solvation and solvatochromism in CO₂-expanded liquids. 2. Experiment-simulation comparisons of preferential solvation in three prototypical mixtures

Electronic absorption and emission spectra of 10-bis(phenylethynyl) anthracene (PEA) and coumarin 153 (C153) are measured as functions of composition along the bubble-point curve at 25 °C in CO₂-expanded cyclohexane (c-C₆H₁₂), acetonitrile (CH₃CN), and methanol (CH₃OH). The nonlinear dependence of the spectral frequencies on composition suggests substantial preferential solvation of both solutes by the liquid components of these mixtures. Estimates of enrichment factors (local mole fraction of a component divided by its bulk value) based on the assumption that spectral shifts are linearly related to local composition are quite large (∼10) in the cases of the C153/CH₃CN + CO ₂ and C153/CH₃OH + CO₂ systems at high x _CO2. Computer simulations of anthracene, the chromophore of PEA, and C153 in these three CO₂-expanded liquids are used to clarify the relationship between local composition and spectral shift. A semiempirical model consisting of additive electrostatic and dispersive interactions is able to capture the main features observed experimentally in all six solute/solvent combinations. The simulations show that the commonly used assumption of a linear relation between spectral shifts and local compositions grossly exaggerates the extent of preferential solvation in these mixtures. The collective nature of electrostatic solvation and the composition dependence of the solute's coordination number are shown to be responsible for the breakdown of this assumption.