Igor M. Svishchev

Dielectric response of concentrated NaCl aqueous solutions: Molecular dynamics simulations

Molecular dynamics simulations are performed to study the dielectric response of concentrated NaCl aqueous solutions. The extended simple point charge interaction potential for water molecules and the Higgis–Mayer potential for ion–ion interactions are used. The ion–ion and ion–water distributions are examined for 1 M solution at 298, 373 and 473 K. The solvate-separated ion pairs with the Na+–Cl− separation of about 5.0 A are found to form preferentially at ambient temperature. The close contact pairs with the ion–ion separation of 2.9 A tend to form in high temperature solution. The 3D water–ion arrangements are revealed with the aid of the spatial distribution function. The Na+–O–Cl− angle in the solvate-separated ion pairs is found to be close to 106°. The correlation times of translations, τT, and rotations, τR, for water molecules in the solvation shells of the cation and anion are determined. The frequency-dependent dielectric permittivity and absorption coefficient for the concentrated NaCl soluti...

Phase coexistence properties for the polarizable point charge model of water and the effects of applied electric field

Extensive efforts are currently devoted to the development of water models for computer simulations that explicitly incorporate molecular polarizability. Liquid–vapor coexistence properties for the polarizable point charge (PPC) model of water are examined in this contribution with the aid of molecular dynamics calculations. An accurate analytical equation of state for the high-temperature states of the PPC model is presented, including its critical region. The liquid–vapor coexistence curve and the critical point parameters for the PPC water are extracted from its equation of state. The critical temperature, pressure, and density for the PPC water are found to be within 10% of their experimental values and its critical compression factor is within 2%. The effect of applying an external electric field to the system on the coexistence properties is investigated. A shift of the critical point of water to higher temperature and pressure is observed, a result similar to that of the introduction of an electrol...

Equations of state and phase coexistence properties for simulated water

Abstract There is a practical need in modeling both ambient and high-temperature water near and above the critical point with a single transferable intermolecular potential. Thermodynamic properties for two water potentials, the simple point charge (SPC/E) and the polarizable point charge (PPC) models, are examined with the aid of molecular dynamics simulations. Analytical equations of state, liquid–vapor coexistence properties and critical point parameters are given. The effect of applying an external electric field to the system on the coexistence properties is investigated. A shift of the critical point of water to higher temperature and pressure is observed, a result similar to that of the introduction of an electrolyte to water.

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.

Nucleation of NaCl Nanoparticles in Supercritical Water : Molecular Dynamics Simulations

Formation of NaCl nanoparticles in supercritical water is studied using molecular dynamics simulation method. We have simulated particle nucleation and growth in NaCl-H2O fluids, with salt concentration of 5.1 wt %, in the temperature and density range of 673-1073 K and 0.17-0.34 g/cm(3), respectively. The cluster size distributions, the size of critical nuclei and cluster lifetimes are reported. The size distribution of emerging clusters shows a very strong dependence on the systems density, with larger clusters forming at lower densities. Clusters consisting of approximately 14-24 ions appear critical for the thermodynamic states examined. The local structures of critical clusters are found to be amorphous. The lifetime values for clusters containing more than 20 ions are in the range of 10-50 ps. We have calculated the NaCl nucleation rates, which appear to be on the order of 10(28) cm(-3) s(-1).

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.

Three-dimensional picture of dynamical structure in liquid water

This paper presents a methodology with which to study the local density distributions in molecular liquids and their fluctuations in any spatial direction. The distinct part of the van Hove density–density correlation function for liquid water is calculated in molecular dynamics simulations. Because of the pronounced nonspherical intermolecular interactions this pair-density function is direction dependent in the local molecular frame. We explicitly resolve the distinct van Hove function in the local frame of water molecules. The dynamics of the tetrahedrally coordinated (hydrogen bonded) and the interstitial molecules in liquid water are examined. The spectrum of the pair-density fluctuations for the tetrahedrally coordinated molecules in supercooled and ambient water exhibits a well-known translational mode at 200 cm−1 and a collective relaxation mode at lower frequencies, at approximately 10 cm−1 at 263 K. The correlation time of this relaxation process decreases with temperature, from 2.2 ps at 238 K ...

Rotational Dynamics in Liquid Water: A Simulation Study of Librational Motions

The rotational dynamics in liquid water have been studied. The power spectra of the single-molecule orientational autocorrelation function (ACF) have been calculated in molecular dynamics simulations with the SPC/E potential and have been used to characterize various rotational motions of water molecules. Both ordinary and heavy water have been examined at temperatures of – 10 and 25 °C. For liquid H2O at 25 °C the power spectra of the second-order (Raman) orientational ACFs contain three intense bands, centred at ca. 500, ca. 560 and ca. 670 cm–1, and a less intense high-frequency shoulder at ca. 820 cm–1. Two intense librational bands with maxima at ca. 570 and ca. 650 cm–1 are present in the power spectrum of the single-dipole orientational ACF for the same system. The average temperature coefficient for the librational frequencies of SPC/E water is found to be about –0.65 cm–1 K–1, which agrees well with experimental estimates. A well resolved rototranslational band centred at ca. 55 cm–1 is observed in the low-frequency region of the power spectra of the single-molecule orientational ACFs. This band is relatively insensitive to temperature variations and shows no isotopic effect.

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.