UV absorption shifts of benzenes in supercritical carbon dioxide

UV spectral shifts of benzene, naphthalene, and chlorobenzene in supercritical 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 spectral shifts to lower energy as a function of the solvent density. The shift was quantitatively explained by dielectric theory, which indicated that the local density of the solvent surrounding the solute molecule was approximately the same as the bulk density. In contrast, the spectral shifts of naphthalene and chlorobenzene-CO2 systems near their critical points indicated that the local density was augmented. The relationship of the spectral shifts vs. solvent density for benzene, naphthalene, and chlorobenzene could be explained by the Langmuir adsorption model. The Langmuir adsorption model 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 degree of the local density augmentation was explained as a function of solute-solvent interaction. (C) 1999 Elsevier Science B.V. All rights reserved.