UV absorption solvatochromic shift of 4-nitroaniline in supercritical water

Abstract

The solvation structure in supercritical (SC) water was studied by measuring UV spectral shifts of 4-nitroaniline (1) at 380, 390 and 410 °C. The red shift was observed with increasing water density. The spectral shifts were plotted against the polarity parameter, (ε − 1)/(2ε + 1) (ε: the dielectric constant of the solvent). The data at 410 °C show an excellent straight line whereas the data at 380 and 390 °C deviate from the line, which implies the occurrence of solvent clustering around the solute 1 near the critical temperature (T_c = 374 °C). The temperature range where significant clustering occurs in SC water is quite limited (T_r < 1.05) in contrast to the temperature dependence of the solvation in SC CF₃H and SC CO₂. This fact implies that the critical inhomogeneity of pure fluid significantly contributes to the clustering in the case of SC water.