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.
All Science Journal Classification (ASJC) codes
- Physical and Theoretical Chemistry
- Surfaces, Coatings and Films
- Materials Chemistry