Vladimir R. Belosludov

Theoretical study of phase transitions in Kr and Ar clathrate hydrates from structure II to structure I under pressure.

The theory developed in our earlier papers is extended to predict dynamical and thermodynamic properties of clathrate structures by accounting for the possibility of multiple filling of cavities by guest molecules. The method is applied to the thermodynamic properties of argon and krypton hydrates, considering both structures I (sI) and II (sII), in which the small cages can be singly occupied and large cages of sII can be singly or doubly occupied. It was confirmed that the structure of the clathrate hydrate is determined by two main factors: intermolecular interaction between guest and host molecules and the configurational entropy. It is shown that for guests weakly interacting with water molecules, such as argon or krypton, the free energy of host lattices without the contribution of entropy is the main structure-determining factor for clathrate hydrates, and it is a cause of hydrate sII formation at low pressure with these guests. Explicit account of the entropy contribution in the Gibbs free energy allows one to determine the stability of hydrate phases and to estimate the line of structural transition from sII to sI in P-T plane. The structural transition between sII and sI in argon and krypton hydrates at high pressure is shown to be the consequence of increasing intermolecular interaction and the degree of occupancy of the large cavities.

Accurate description of phase diagram of clathrate hydrates at the molecular level

In order to accurately estimate the thermodynamic properties of hydrogen clathrate hydrates, we developed a method based on the solid solution theory of van der Waals and Platteeuw. This model allows one to take into account the influence of guest molecules on the host lattice and guest-guest interactions--especially when more than one guest molecule occupies a cage. The free energies, equations of state, and chemical potentials of hydrogen and mixed propane-hydrogen clathrate hydrates of cubic structure II with different cage fillings have been estimated using this approach. Moreover, the proposed theory has been used for construction p-T phase diagrams of hydrogen hydrate and mixed hydrogen-propane hydrates in a wide range of pressures and temperatures. For the systems with well defined interactions the calculated curves of guest gas-hydrate-ice I(h) equilibrium agree with the available experimental data. We also believe that the present model allows one not only to calculate the hydrogen storage ability of known hydrogen hydrate but also predict this value for structures that have not yet been realized by experiment.

Thermal expansion and lattice distortion of clathrate hydrates of cubic structures I and II

Abstract Thermal expansion of clathrate hydrates of argon, krypton, and propane with cubic structure II (CS-II), methane and xenon hydrates of cubic structure I (CS-I) and empty lattices of CS-I and CS-II at zero pressure have been investigated within the framework of lattice dynamics approach in quasiharmonic approximation. For all hydrates a good agreement with experiment for lattice parameters at some fixed temperatures have been obtained. In the case of the CS-II, it is found that inclusion of sufficiently small molecules such as argon and krypton into the water framework results in effective compression of empty hydrate lattice. In the case of large propane molecules included only in the large cavities the lattice is expanded relative to the empty lattice. The thermal expansion coefficients of hydrates with large enclathrated molecules are less than for hydrates formed by small guest molecules and the smallest value of thermal expansion coefficient is obtained for the empty lattice. By comparison of the data obtained for xenon and methane hydrates of CS-I and the empty lattice of CS-I it is found that the same behavior is observed also in the case of hydrates of CS-I. The effect of lattice stretching due to guest size on the reference chemical potential between the empty lattices of CS-I and ice Ih and empty lattice of CS-II and ice Ih is calculated too.

Solvation Mechanism of Task-Specific Ionic Liquids in Water: A Combined Investigation Using Classical Molecular Dynamics and Density Functional Theory

The solvation behavior of task-specific ionic liquids (TSILs) containing a common, L-histidine derived imidazolium cation [C20H28N3O3](+) and different anions, bromide-[Br](-) and bis(trifluoromethylsulfonyl)amide-[NTF2](-), in water is examined, computationally. These amino acid functionalized ionic liquids (ILs) are taken into account because of their ability to react with rare earth metal salts. It has been noted that the TSIL with [Br](-) is more soluble than its counterpart TSIL with [NTF2](-), experimentally. In this theoretical work, the combined classical molecular dynamics (CMD) and density functional theory (DFT) calculations are performed to study the behavior of the bulk phase of these two TSILs in the vicinity of water (H2O) molecules with different concentrations. Initially, all the constructed systems are equilibrated using the CMD method. The final structures of the equilibrated systems are extracted for DFT calculations. Under CMD operation, the radial distribution function (RDF) plots and viscosity of TSILs are analyzed to understand the effect of water on TSILs. In the DFT regime, binding energy per H2O, charge transfer, charge density mapping, and electronic density of states (EDOS) analyses are done. The CMD results along with the DFT results are consolidated to support the hydrophilic and hydrophobic nature of the TSILs. Interestingly, we have found a strong correlation between the viscosity and the EDOS results that leads to an understanding of the hydration properties of the TSILs.

Microscopic model of clathrate compounds

The major generalization of the existing theory of clathrate hydrates, so that it can account for phenomena such as multiple occupancy of individual cages and mutual guest-host couplings and guest-guest interaction, are suggested. The new model allows taking into account the influence of guest molecules on the host lattice. Atomistic modeling of structural, dynamical and thermodynamic properties of ices and different hydrates at high pressures and a range of temperature were performed. The influence of guest molecules (argon, methane and xenon) on the host lattice of hydrate of cubic structures I and II was investigated. Results of these calculations agree with known experimental data.

Ab Initio Study of Hydrogen Hydrate Clathrates for Hydrogen Storage within the ITBL Environment

Recently, for the first time a hydrate clathrate was discovered with hydrogen. Aside from the great technological promise that is inherent in storing hydrogen at high density at modest pressures, there is great scientific interest as this would constitute the first hydrate clathrate with multiple guest molecules per cage. The multiple cage occupancy is controversial, and reproducibility of the experiments has been questioned. Therefore, in this study we try to illucidate the remarkable stability of the hydrogen hydrate clathrate, and determine the thermodynamically most favored cage occupancy using highly accurate ab initio computer simulations in a parameter survey. To carry out these extraordinary demanding computations a distributed ab initio code has been developed using the SuperSINET with the Information Technology Based Laboratory (ITBL) software as the top-layer.

Crystal-like low frequency phonons in the low-density amorphous and high-density amorphous ices

The structure and vibrational properties of high- and low-density amorphous (HDA and LDA, respectively) ices have been determined using reverse Monte Carlo, molecular dynamics, and lattice dynamics simulations. This combined approach leads to a more accurate and detailed structural description of HDA and LDA ices when compared to experiment than was previously possible. The water molecules in these ices form well connected hydrogen-bond networks that exhibit modes of vibration that extend throughout the solid and can involve up to 70% of all molecules. However, the networks display significant differences in their dynamical behavior. In HDA, the extended low-frequency vibrational modes occur in dense parallel two dimensional layers of water that are approximately 10 nm thick. In contrast, the extended modes in LDA resemble a holey structure that encapsulates many small pockets of nonparticipating water molecules.

Local pressure and density distribution in methane hydrate - ice Ih system

Effect of self-preservation of gas hydrates was explored over many years but there is no complete understanding how can hydrates exist in their thermodynamic instability region. We are suggesting the microscopic-level model of methane hydrate clusters immersed in ice matrix. Due to differences in thermal expansion of methane hydrate and Ice Ih the additional pressure appears in the hydrate phase and this moves it into its stability field. MD simulations were performed to find local pressure and density profiles. Results are well confirming our assumption.

THEORETICAL DESCRIPTION OF METHANE HYDRATE EQUILIBRIUM IN A WIDE RANGE OF TEMPERATURE AND PRESSURE

Theoretical approach was developed for description of clathrate hydrates stability area at low and modestly high temperatures on molecular level. The approach permits to account for interactions between guest molecules and dependence of unit cell structure upon guests sort, temperature and pressure. Non-ideality of the gas phase was accounted using Van-der-Waals equation of state. Within this approach, methane hydrate calculations have been performed both for low temperature and low pressure region (in equilibrium with ice Ih) and for modestly high temperatures and pressures (in equilibrium with liquid pure water). Calculations show the notable dependence of hydrates thermodynamic properties upon the guest-guest interactions. For methane hydrates, the guest-guest interaction energy can reach 10% of the guest-host value. The results of calculations are in good agreement with available experimental data. This method is applicable for accurate prediction of clathrate hydrate stability in a wide range of P–T conditions.

PECULIARITIES OF VIBRATION CHARACTERISTICS OF AMORPHOUS ICES

Dynamic properties of low (LDA), high (HDA) and very high (VHDA) density amorphous ices were investigated within the approach based on Lattice Dynamics simulations. In this approach, we assume that the short-range molecular order mainly determines the dynamic and thermodynamic properties of amorphous ices. Simulation cell of 512 water molecules with periodical boundary conditions and disordering allows us to study dynamical properties and dispersion curves in the Brillouin zone of pseudo-crystal. Existence of collective phenomena in amorphous ices which is usual for crystals but anomalous for disordered phase was confirmed in our simulations. Molecule amplitudes of delocalized (collective) as well as localized vibrations have been considered.