FLUORESCENCE SPECTROSCOPY STUDIES OF DILUTE SUPERCRITICAL SOLUTIONS

Much of the unusual behavior in supercritical fluids (SCFs), including enhanced solubilities, synergistic effects of mixed solutes, and entrainer effects, may be explained by a region around the solute that has a higher density than the bulk. We present a new technique to investigate SCFs using fluorescence spectroscopy to probe the local interactions. In contrast to thermodynamic measurements, this technique determines the strength of interactions on a molecular scale, which is important in the development of more accurate thermodynamic models. The results indicate stronger solute/solvent interactions near the critical point and local densities that are liquid-like, even when the bulk density is quite low. In addition, we determine a quantitative value of the local density enrichment from the spectroscopic measurements. The local solvent density enhancement around the solute appears to correlate reasonably well with the isothermal compressibility, so we speculate that it may be possible to model solute/solvent clustering by knowing only pure solvent properties. We also present evidence of the importance of solute/solute interactions in dilute supercritical fluid solutions. © 1990, American Chemical Society. All rights reserved.