Spatial and Orientational Structure of the Hydration Shell of Benzene in Sub- and Supercritical Water

The spatial and orientational structure of the solvation shell of benzene in sub- and supercritical water are investigated by means of molecular dynamics simulations. The present study reveals different local organization of water molecules at different parts of the solute. The pi-hydrogen-bonding between benzene and water along the axial direction is found to exist even at supercritical conditions although to a reduced extent. The coordination number of benzene decreases substantially on increase of temperature and decrease of density. While the pi-hydrogen-bonded part in the axial region shows a slight expansion, the hydrophobically solvated part in the equatorial plane shows an opposite behavior as the temperature is increased from normal to the supercritical temperature. Two other distribution functions, namely the radial/angular and spatial orientational functions (SOFs) are calculated to explore the spatially resolved angular preferences of water molecules around the benzene solute. Water molecules located axial to the benzene are found to have strong inward orientation toward the solute, however an opposite behavior is found in the equatorial region. Although at supercritical conditions, the orientational distributions of water molecules are broadened, the preferential orientations in the axial and equatorial regions remain similar to that under ambient condition on average.