Temperature dependence of local density augmentation for acetophenone N,N,N′,N′-tetramethylbenzidine exciplex in supercritical water

Local density augmentation around exciplex between acetophenone and N,N,N′,N′-tetramethylbenzidine in supercritical water was measured by observing the peak shift of transient absorption spectrum at temperatures from 380 to 410°C and at pressures from 6 to 37 MPa. Large local density augmentation was observed at lower solvent densities. Local density augmentation was evaluated by the excess density, which was defined as the difference between local density and bulk density, and the density enhancement factor, which was defined by the ratio of the local density to the bulk density. The number of solvating molecules was estimated with a Langmuir adsorption model. The excess density was found to exhibit a maximum at approximately 0.15 g cm^-3, which decreased with increasing temperature. The density enhancement factor was found to decease with increasing temperature; however, its value was much greater than unity at 410°C, which provides evidence that exciplex - water interactions still exist at these conditions. The temperature dependence of local density augmentation around the exciplex in supercritical water was comparable with that in supercritical carbon dioxide, which suggests that the ratios of the solute - solvent and solvent - solvent interactions are comparable between these two systems.