Sergey V. Churakov

Perturbation theory based equation of state for polar molecular fluids: I. Pure fluids

Based on the thermodynamic perturbation theory an equation of state (EOS) for molecular fluids has been formulated which can be used for many fluid species in geological systems. The EOS takes into account four substance specific parameters. These are the molecular dipole moment, the molar polarizability and the two parameters of the Lennard-Jones potential. For many fluids these parameters can be evaluated directly or indirectly from experimental measurements. In the absence of direct experimental determinations, as a first approximation, for a pure fluid the parameters of the Lennard-Jones potential can be evaluated using the critical temperature and the critical density if for polar molecules in addition the dipole moment is known with reasonable accuracy. The EOS with its model potential has the appropriate asymptotic behaviour at high pressures and temperatures and can be used to calculate both vapor-liquid equilibria and thermodynamic properties of single phase fluids up to at least 10 GPa and 2000 K. Currently, parameters for 98 inorganic and organic compounds are available. In this article the EOS for pure fluids is presented. In a further communication the EOS is extended to fluid mixtures (Churakov and Gottschalk, 2003).

Size and topology of molecular clusters in supercritical water: a molecular dynamics simulation

Abstract A hybrid criterion of hydrogen bonding is applied to the analysis of molecular dynamics trajectories simulated for several near- and supercritical thermodynamic conditions. Even at vapor-like densities supercritical water is shown to contain large molecular clusters, consisting of up to 10 molecules. Relative abundances of topologically different trimers, tetramers, and pentamers are examined, and chain-like clusters are found predominant under supercritical conditions. In contrast to the results of quantum-chemical calculations for isolated water clusters, ring-like clusters are only rarely formed in supercritical water.

Perturbation theory based equation of state for polar molecular fluids: II. Fluid mixtures

The equation of state (EOS) for 98 pure organic and inorganic fluids formulated by Churakov and Gottschalk (2003) is extended to complex fluid mixtures. For the calculation of the thermodynamic properties of mixtures, theoretical combining rules from statistical mechanics are used. These mixing rules do not involve any empirical parameters. The properties of the fluid mixtures are directly derived from those of the pure constituents. As an example we show that the EOS describes accurately the thermodynamic relations in the H2O-CO2 binary at high pressures and temperatures. At subcritical conditions the EOS is able to reproduce accurately the phase relations within mixtures of non-polar fluids. In particular the EOS predicts phase separations within various fluid mixtures of polar and non-polar molecules.

DIFFUSION OF Na AND Cs IN MONTMORILLONITE

The state and dynamics of water and cations in pure and mixed Na-Cs-montmorillonite as a function of the interlayer water content were investigated in the present study, using Monte Carlo and classical, molecular-dynamics methods. While highly idealized, the simulations showed that the swelling behavior of hetero-ionic Na-Cs-montmorillonite is comparable to the swelling of a homo-ionic Na- or Cs-montmorillonite. The mixed Na-Cs-montmorillonite is characterized by intermediate interlayer distances compared to homo-ionic Na- and Cs-montmorillonites. Dry, hetero-ionic Na-Cs-montmorillonite is characterized by a symmetric sheet configuration, as is homo-ionic Cs-montmorillonite.We found that at low degrees of hydration the absolute diffusion coefficient of Cs+ is less than for Na+, whereas at greater hydration states the diffusion coefficient of Cs+ is greaterthan for Na+. An analysis of the relative diffusion coefficients (the ratio between the diffusion coefficient of an ion in the interlayer and its diffusion coefficient in bulk water) revealed that water and Na+ are always less retarded than Cs+. With large interlayer water contents, tetralayer or more, Na+ ions preferentially form outer-sphere complexes. The mobility perpendicular to the clay surface is limited and the diffusion is equivalent to two-dimensional diffusion in bulk water. In contrast, Cs+ ions preferentially form ‘inner-sphere complexes’ at all hydration states and their two-dimensional diffusion coefficient is less than in bulk water.The question remains unanswered as to why experimentally derived relative diffusion coefficients of Cs+ in the interlayer of clays are about 20 times less than those we obtained by classical molecular dynamics studies.

Thermodynamics and structure of molecular clusters in supercritical water

Abstract The extent of hydrogen-bonded cluster formation in near- and supercritical water has been studied by application of a hybrid hydrogen-bonding criterion to the analysis of Monte Carlo computer simulation results. Up to 10% of water molecules were found to constitute H-bonded clusters even in dilute supercritical water vapor ( T ∗ =1.04 , ρ ∗ =0.06 ), and the maximum size of such molecular complexes formed may be as large as seven molecules per cluster under these conditions. Relative abundance and geometric and energetic characteristics of topologically different trimers, tetramers, and pentamers were also examined. Open chain- and tree-like clusters are preferentially formed in supercritical water, while cyclic ring-like structures occur only rarely. Partitioning of molecules between clusters of the same size, but topologically different structure is found to be virtually independent of temperature and density.

Zinc adsorption on clays inferred from atomistic simulations and EXAFS spectroscopy.

Clay minerals are efficient sinks for heavy metals in the geosphere. Knowing the uptake mechanism of these elements on clays can help to protect the natural environment from industrial pollution. In this study ab initio molecular dynamics (MD) calculations were applied to simulate the uptake of Zn on the edge surfaces of montmorillonite, a dioctahedral clay, and to explain the measured K-edge extended X-ray absorption fine structure (EXAFS) spectra of adsorbed Zn. These experiments were carried out using a high ionic strength Na background electrolyte that enables one to block cation exchange processes and to restrict the Zn uptake to the sorption complexation at the edge sites of clay. The analysis of the experimental data and simulation results suggest that structurally incorporated Zn preferentially substitutes for Al(III) in the trans-symmetric sites of the octahedral layer. At low loading, Zn is incorporated into the outermost trans-octahedra on (010) and (110) edges. At medium loading, Zn forms mono- and bidentate inner-sphere surface complexes attached to the octahedral layer of (010) and (110) edge sites. The maximal site density of inner-sphere sorption sites inferred from molecular simulations agrees well with site capacities of surface complexation sites derived from macroscopic studies and modeling.

Structural position of H2O molecules and hydrogen bonding in anomalous 11 Å tobermorite

Abstract For the first time, the structure and dynamics of H2O in the interlayer of anomalous 11 Å tobermorite have been analyzed based on ab initio molecular dynamics simulations. The simulations provide detailed information on the structure of the hydrogen bonds formed by H2O molecules and OH groups. The calculated structural parameters of the tobermorite building blocks are in good agreement with the experimental model of Merlino et al. (1999), which is based on X-ray diffraction (XRD) measurements. However, in contrast to the measurements, the simulations suggest that the W1 and W3 sites are split between two general positions with 50% occupancy. It is proposed that the experimental studies provide only averaged coordinates of these sites due to the limitations imposed by the polytypic structures. Analysis of the H2O dynamics at 321 and 506 K suggest the possibility of a temperature induced order-disorder transition associated with the orientation of O6H···W1 and O6H···W3 hydrogen bonds in the structure of anomalous 11 Å tobermorite. The experimental IR and Raman spectra of 11 Å tobermorite are interpreted based on analyses of the vibrational density of states.

Mobility of Na and Cs on Montmorillonite Surface under Partially Saturated Conditions

Cs migration in soils at contaminated sites or in clay-rich backfill of waste disposal sites can take place under partially saturated conditions. To understand the molecular mechanism of Cs migration in partially saturated clays, Grand Canonical Monte Carlo simulations were applied to model adsorption of water films onto external surfaces of Cs and Na montmorillonites as function of partial water pressure. The surface complexation and diffusivity of Cs and Na at different partial water pressure was obtained by molecular dynamics simulations. The results suggest that ion mobility in adsorbed water films on external basal surfaces of clay is similar to that in the near-surface water of a saturated pore as far as the thickness of the adsorbed water film is more than two water layers. At lower partial water pressure (i.e., in thinner water films) the ion mobility dramatically decreases. In contrast, the average water mobility in thin water film is higher than in the water-saturated system due to enhanced mobility of water molecules close to vapor-film interface. The results of the simulations were applied to interpret recent laboratory measurements of tritiated water and Cs diffusivity in Callovo-Oxfordian Claystones under partially saturated conditions.

An ab initio molecular dynamics study of hydronium complexation in Na-montmorillonite

The Car–Parrinello molecular dynamics simulation technique was used to predict the structure and dynamics of hydronium solvation in mono-, bi- and trihydrated Na-montmorillonite. In monohydrated montmorillonite, hydronium ions are located within the hexagonal rings of the basal clay plane. Oxygen sites of hydronium ions point towards the clay surface and hydrogen atoms towards the water layer. In bi- and trihydrated montmorillonite, hydronium ions form water-solvated, outer-sphere complexes. Similar to the solvation mechanism in bulk water, hydronium ions donate three hydrogen bonds to interlayer water molecules. In all studied hydration states, hydronium ions do not form hydrogen bonds with the basal oxygen sites. Similar to bulk water, the free energy barrier for a classical proton transfer between interlayer water molecules is of the order of kT and therefore not the limiting factor for the proton diffusion. The diffusivity of hydrogen in the interlayer is controlled by the structural rearrangements of the solvating water molecules.

Natural fluid phases at high temperatures and low pressures

Abstract Gas phases at low pressures and high (magmatic) temperatures have certain peculiar properties. The fluid is mainly water vapour, which is usually observed during discharging of crystal magmatic melts. At >700°C and