The hydration of aniline: Analysis of spatial distribution functions

Molecular dynamics simulations of aniline in aqueous infinitely dilute solution are performed from ambient to supercritical conditions. Spatial hydration structures of aniline are examined along the liquid branch of the liquid-vapor coexistence curve of the simple point charge/extended water model at 298, 373, 473, and 573 K and in the supercritical region at 633, 733, and 833 K with density fixed at 0.3 g/cm(3). The coordination and H-bond numbers of aniline are calculated. The self-diffusion coefficient of aniline is also evaluated. At room temperature the solvation shell of aniline is comprised of approximately 32 water molecules. At 298 K, the amino group is hydrated by three water molecules with which it forms one strong and two weak (0.6) H bonds acting as an acceptor and donor, respectively. In addition, approximately 1.5 water molecules are identified as pi-coordinated, forming close to 0.75 H bonds with the aromatic ring of aniline. The features of the hydration shell structure of aniline diminish with temperature and decreasing density. The disappearance of pi-coordinated water molecules is noted at around 473 K, whereas the loss of the hydrophobic solvent cage is observed near the critical point of water. At supercritical conditions aniline is hydrated by approximately eight water molecules with the amino group coordinated to roughly two of them, forming less than one H bond in total.