An ab initio molecular dynamics study of benzene in water at supercritical conditions: Structure, dynamics, and polarity of hydration shell water and the solute.

Anisotropic structure and dynamics of the hydration shell of a benzene solute in supercritical water are investigated by means of ab initio molecular dynamics simulations. The polarity and structural distortion of the benzene solute in supercritical water are also investigated in this study. Calculations are done at 673 K for three different densities of the solvent. The simulations are carried out using the Becke-Lee-Yang-Parr (BLYP) and also the Becke-Lee-Yang-Parr functional including dispersion corrections of Grimme (BYLP-D). The structural anisotropy is found to exist even at supercritical conditions as elucidated by the radial distribution functions of different conical regions and also by angular and spatial distribution functions. The benzene-water πH-bond and also the water-water hydrogen bonds are found to exist even at the supercritical temperature of 673 K. However, the numbers of these hydrogen bonds are reduced substantially with a decrease in water density. The water molecules in the axial region of benzene are found to be preferably oriented with one OH vector pointing toward the benzene ring, whereas the water molecules located in the equatorial region are found to orient their dipoles mostly parallel to the ring plane. The orientational distributions, however, are found to be rather broad at the supercritical temperature due to thermal fluctuations. Although the water molecules have faster dynamics at these supercritical conditions, a slight difference is observed in the dynamics of the solvation shell and bulk molecules. The conformational flexibility of the ring is found to be enhanced which causes an increase in polarity of the benzene solute in water under supercritical conditions.