Andriy Plugatyr

Molecular Diffusivity of Phenol in Sub- and Supercritical Water: Application of the Split-Flow Taylor Dispersion Technique

The binary diffusion coefficient of phenol in aqueous solution was examined from ambient to supercritical water conditions by using the developed split-flow Taylor dispersion technique. The technique significantly simplifies diffusivity measurements in high-temperature and supercritical water, as the sample injection and detection are performed ex situ at ambient conditions. The binary diffusion coefficient of phenol increases from 1.013 × 10(-9) m(2) s(-1) at 298.7 K and 25 MPa to about 34.71 × 10(-9) m(2) s(-1) at 672.9 K and 30 MPa and follows Arrhenius behavior with an activation energy of 15.09 kJ/mol. The diffusion coefficient of phenol in infinitely dilute solution was also calculated by means of molecular dynamics (MD) simulations over a wide temperature and density range (298-773 K and from 0.07 to 1.0 g/cm(3), respectively). A dramatic increase in the diffusivity was observed upon transition into the low density supercritical region. The obtained experimental data agrees well with available literature values and the MD results. At subcritical conditions the experimentally obtained binary diffusion coefficients generally follow the predictions from the Stokes-Einstein equation, with the estimate for the hydrodynamic radius of the solute taken from MD data.

Spatial hydration structures and dynamics of phenol in sub- and supercritical water

The hydration structures and dynamics of phenol in aqueous solution at infinite dilution are investigated using molecular-dynamics simulation technique. The simulations are performed at several temperatures along the coexistence curve of water up to the critical point, and above the critical point with density fixed at 0.3 g/cm3. The hydration structures of phenol are characterized using the radial, cylindrical, and spatial distribution functions. In particular, full spatial maps of local atomic (solvent) density around a solute molecule are presented. It is demonstrated that in addition to normal H bonds with hydroxyl group of phenol, water forms pi-type complexes with the center of the benzene ring, in which H2O molecules act as H-bond donor. At ambient conditions phenol is solvated by 38 water molecules, which make up a large hydrophobic cavity, and forms on average 2.39 H bonds (1.55 of which are due to the hydroxyl group-water interactions and 0.84 are due to the pi complex) with its hydration shell. As temperature increases, the hydration structure of phenol undergoes significant changes. The disappearance of the pi-type H bonding is observed near the critical point. Self-diffusion coefficients of water and phenol are also calculated. Dramatic increase in the diffusivity of phenol in aqueous solution is observed near the critical point of simple point-charge-extended water and is related to the changes in water structure at these conditions.

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.

Spatial hydration maps and dynamics of naphthalene in ambient and supercritical water.

The hydration structures and dynamics of naphthalene in aqueous solution are examined using molecular-dynamics simulations. The simulations are performed at several state points along the coexistence curve of water up to the critical point, and above the critical point with the density fixed at 0.3 g/cm(3). Spatial maps of local atomic pair-density are presented which show a detailed picture of the hydration shell around a bicyclic aromatic structure. The self-diffusion coefficient of naphthalene is also calculated. It is shown that water molecules tend to form pi-type complexes with the two aromatic regions of naphthalene, where water acts as the H-bond donor. At ambient conditions, the hydration shell of naphthalene is comprised, on average, of about 39 water molecules. Within this shell, two water molecules can be identified as pi-coordinating, forming close to one H-bond to the aromatic rings. With increasing temperature, the hydration of naphthalene changes dramatically, leading to the disappearance of the pi-coordination near the critical point.

Diffusivity and hydration of hydrazine in liquid and supercritical water through molecular dynamics simulations and split-flow pulse injection experiments

The diffusion properties and hydration structure of hydrazine in an aqueous solution are investigated through molecular dynamics simulations and split-flow pulse injection experiments. The simulations are performed from ambient conditions along the liquid side of the liquid-vapor coexistence curve, up to the critical point, and in the supercritical region at temperatures of 673, 773, 873, and 973 K and at densities ranging from 0.1 to 0.8 g cm(-3). The spatial distributions functions for hydrated water are presented. At ambient conditions, hydrazine is hydrated by 24 water molecules with about 1.6 H-bonds being donated to each nitrogen atom. The hydration number decreases with temperature along the coexistence curve and is seen to increase with system density in the supercritical region. At low density supercritical conditions, hydrazine has no appreciable hydration structure and is surrounded by only 2 water molecules at 873 K and 0.1 g cm(-3). The diffusion coefficients for hydrazine at subcritical state conditions are found to be in agreement with Stokes-Einstein and Wilke-Chang predictions. The diffusion coefficients in the supercritical region are found to correlate more closely with the overall fit to the Dymond equation.