Local density augmentation in supercritical solvents: Electronic shifts of anthracene derivatives

Fluorescence spectroscopy is used to measure local density augmentation in solution of anthracene, 9-cyanoanthacene, 9,10-dichloroanthracene, and 9,10-diphenyl-anthracene in the supercritical solvents ethane, carbon dioxide, and fluoroform. For this purpose, the relationship between density and spectral shift is calibrated using dielectric continuum models of solvatochromism together with data in the gas phase and in liquid solutions. This approach, and the uncertainties inherent in it, are discussed in detail. The effective local densities (ρ_eff) deduced from the emission shifts in all of these solute/solvent combinations are comparable at temperatures near to the critical point (T_c + 5 K or T/T_c = 1.02). Density enhancement factors ρ_eff/ρ increase with decreasing bulk density (ρ), reaching values of 5-6 at the lowest bulk densities observable. Density augmentation, the difference between the local and bulk densities (Δρ_eff ≡ ρ_eff - ρ) reaches a maximum at densities of ∼0.6 times the critical density (ρ_c). At this maximum, Δρ_eff/ρ_c takes on values between 0.6 1.0 in the different systems. Increasing the temperature leads to a substantial decrease in the density augmentation, but even 100 K above T_c, the effective local densities are still significantly larger than the bulk density. All of the effective densities observed, including this temperature dependence, can be reasonably correlated using single quantity, ΔG_uv/k_BT: the ratio of the free energy of solute-solvent association, estimated using realistic potential models, and the thermal energy k_BT. These observations suggest that proximity to the critical point is probably only of secondary importance in determining the extent of density augmentation, at least on the very local scale probed by electronic spectral shifts.