Local density augmentation in supercritical solvents: Electronic shifts ofanthracene derivatives

Fluorescence spectroscopy is used to measure local density augmentation in solutions of anthracene, 9-cyanoanthacene, 9,10-dichloroanthracene. and 9,1 0-diphenyl-anthracene in the supercritical solvents ethane, carbon dioxide, and fluoroform. For this purpose, the relationship between density and spe ctral shift is calibrated using dielectric continuum models of solvatochrom ism together with data in the gas phase and in liquid solutions. This appro ach, and the uncertainties inherent in it, are discussed in detail. The eff ective local densities (p(eff)) deduced from the emission shifts in all of these solute/solvent combinations are comparable at temperatures near to th e critical point (T-c + 5 K or T/T-c = 1.02). Density enhancement factors p (eff)/p increase with decreasing bulk density (p), reaching values of 5-6 a t the lowest bulk densities observable. Density augmentation, the differenc e between the local and bulk densities (Deltap(eff) equivalent to p(eff) - p)reaches a maximum at densities of similar to0.6 times the critical densit y (p(c)). At this maximum, Deltap(eff)/p(c) takes on values between 0.6 and 1.0 in the different systems. Increasing the temperature leads to a substa ntial decrease in the density augmentation, but even 100 K above T-c, the e ffective local densities are still significantly larger than the bulk densi ty. All of the effective densities observed, including this temperature dep endence, can be reasonably correlated using single quantity, DeltaG(uv)/k(B )T: ratio of the free energy of solute-solvent association, estimated using realistic potential models, and the thermal energy k(B)T. These observatio ns suggest that proximity to the critical point is probably only of seconda ry importance in determining the extent of density augmentation, at least o n the very local scale probed by electronic spectral shifts.