UV absorption solvatochromic shift of 4-nitroaniline in supercritical water

The solvation structure in supercritical (SC) water was studied by measuring UV spectral shifts of 4-nitroaniline (1) at 380, 390 and 410degreesC. The red shift was observed with increasing water density. The spectral shifts were plotted against the polarity parameter, (epsilon - 1)/(2epsilon + 1) (e: the dielectric constant of the solvent). The data at 410degreesC show an excellent straight line whereas the data at 380 and 390degreesC deviate from the line, which implies the occurrence of solvent clustering around the solute 1 near the critical temperature (T-c = 374degreesC). 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.