SOLVATOCHROMIC CHARACTERIZATION OF THE LIQUID-PHASE IN LIQUID-SUPERCRITICAL CO2 MIXTURES

The solvatochromic dye phenol blue (N,N-dimethylindoaniline) is used t o characterize the solvent strength (polarity) of the saturated liquid phase in a series of solvent-carbon dioxide binary mixtures. Data wer e obtained at 35 and 55 degrees C and at pressures up to similar to 70 bar: Five solvents were investigated-acetone, cyclohexane, methanol, THF, and toluene. The polarity of the liquid phase decreases significa ntly with increasing pressure due to the increasing carbon dioxide con tent of this phase at equilibrium. For example, the polarity of aceton e saturated with carbon dioxide at 35 degrees C and similar to 60 bar is equivalent to the polarity of pure cyclohexane at ambient pressure. The local environment about the dye is significantly richer in the po lar liquid component than the bulk composition would indicate. The deg ree of enrichment reflects concentration effects at low pressure, and both concentration and pressure effects at high pressure where the mix tures are highly compressible. The NRTL model of Renon and Prausnitz i s able to predict these local compositions with reasonable accuracy ex cept at CO2-rich conditions where compressibility effects are importan t.