Solvatochromism in supercritical fluids

Solvatochromic shifts of the five probe indicators 2-nitroaniline, 4-nitroaniline, 4-nitroanisole, 4-nitrophenol, and N,N-dimethyl-4-nitroaniline have been used to specify the solvatochromic solubility parameters pi* and beta of the five pure fluids CO2, N2O, CClF3, SF6, and NH3 according to Kamlet, Abboud, and Taft by means of UV/VIS spectroscopy. Three isotherms, nearcritical and supercritical, were examined, for the first time from zero pressure up to 100 MPa. Except NH3 with beta approximate to 0.70 (293 K), 0.65 (333 K), and 0.55 (413 K) at 100 MPa, respectively, all other fluids do not show significant basicity at any temperature and pressure conditions. Plotting pi* vs reduced density of any fluid results in steeply increasing curves with maximum enhanced pi* data at reduced densities of about 0.6 which continuously level out and finally rise again asymptotically as straight lines.