Characterization of supercritical fluid solvents using solvatochromic shifts

Solvatochromic probe molecules, with absorbances in the UV region, were used to examine the cybotactic region of pure and mixed supercritical fluid solvent systems. This approach allowed determination of the relative polarity-polarizability of these fluids and comparison with conventional solvents. The position of maximum absorbance was seen to be density dependent. The solvatochromic shift for the most polar fluid studied, supercritical ammonia, was the most sensitive to changes in solvent density, and at moderate densities it approached that observed for tetrahydrofuran. Supercritical CO/sub 2/, N/sub 2/O, and CCl/sub 3/F showed smaller spectral shifts over a comparable density range. In mixed fluid solvents (CO/sub 2/-methanol) the spectral shift approached that seen for pure methanol as the percentage of methanol was increased.