Microscopic solvent structure of subcritical and supercritical methanol from ultraviolet/visible absorption and fluorescence spectroscopies

Ultraviolet/visible absorption and fluorescence spectroscopies at different temperatures and pressures were applied to investigate the microscopic sol vent structures of subcritical and supercritical methanol using 4-nitroanis ole, ethyl-(4-dimethylamino)benzoate, Reichardt's dye, and anthracene as th e probe molecules. It was found that at temperatures higher than 150 degree s C the long winding chains of sequentially hydrogen-bonded methanol molecu les were probably broken, but the small hydrogen-bonded aggregates possibly existed in methanol even at higher temperature. It was also found that the solvation process of the anthracene molecule in the S-0-ground state obeye d the Langmuir adsorption model. However, in the case of fluorescence measu rements in supercritical methanol, we detected deviations from the simple L angmuir adsorption model. These deviations were explained in terms of prefe rential solvation of the solvent molecules around photoexcited anthracene. Judging from the experimental results, it was concluded that the local dens ity augmentation of the supercritical methanol around the nonpolar solute w as a short-ranged effect, which did not correspond directly to the large is othermal compressibility of fluid near the critical point. (C) 1999 America n Institute of Physics. [S0021-9606(99)51233-6].