UV absorption shifts of benzenes in supercritical carbon dioxide

UV spectral shifts of benzene, naphthalene, and chlorobenzene in supercriti cal CO2 were measured as a function of solvent density and temperature. The UV absorption spectrum (A B-1(2u) <-- X (1)A(1g)) Of benzene exhibited spe ctral shifts to lower energy as a function of the solvent density. The shif t was quantitatively explained by dielectric theory, which indicated that t he local density of the solvent surrounding the solute molecule was approxi mately the same as the bulk density. In contrast, the spectral shifts of na phthalene and chlorobenzene-CO2 systems near their critical points indicate d that the local density was augmented. The relationship of the spectral sh ifts vs. solvent density for benzene, naphthalene, and chlorobenzene could be explained by the Langmuir adsorption model. The Langmuir adsorption mode l was also applied successfully to the spectral shifts reported by previous researchers for anthracene, 2-nitroanisole, 4-aminobenzophenone and 4-(N,N -dimethylamino)benzonitrile in supercritical CO2. Through this model, the d egree of the local density augmentation was explained as a function of solu te-solvent interaction. (C) 1999 Elsevier Science B.V. All rights reserved.