Consistent Valence Force Field Research Articles

Theoretical process of rubber nano-coating on calcium silicate hydrate film for durable cementitious materials

The lifetime of building materials is a significant challenge and is strongly related to their surface state and impermeability. This work is dedicated to an in-depth analysis of rubber coating process on Calcium-Silicate-Hydrate (CSH) surface. By means of molecular dynamics simulations and using a combination potential of a general force field (CLAYFF) and the consistent-valence force field (CVFF), it is found that the efficiency of coating with TPI (1,4-trans-Polyisoprene) and CPB (1,4-cis-Polybutadiene) than CPI (1,4-cis-Polyisoprene) and TPB (1,4-trans-Polybutadiene). The obtained surface energy of TPI and CPB coated CSH drastically drops from the range of 698–740 mJ/m2 to 46.3–48.4 mJ/m2. Consequently, the averaged contact angle between water nanodroplet and rubber coated CSH surface is obtained in the range of 92.85° and 98.11°, making hydrophobic the nano-coated CSH surface. The interfacial adhesion analysis between rubber film and CSH surface gives a work of adhesion in the range of 49.42 mJ/m2 to 92.76 mJ/m2, in agreement with experimental measurements.

Molecular dynamics insight into the adsorption and distribution of bitumen subfractions on Na-montmorillonite surface

How bitumen works with clay minerals is an essential matter for the application of nano-clay minerals (e. g., montmorillonite, vermiculite) in bitumen modification. In this paper, the combination of ClayFF and consistent-valence force field (CVFF) is developed to characterize the bitumen-clay interaction. The adsorption configuration, distribution pattern of bitumen subfractions on the Na-montmorillonite surface and thermal effect are systematacially investigated by examining the evolution of bitumen model at different temperatures based on the molecular dynamics simulation. For comparision, 12 isolated bitumen monomers and bitumen SARA fractions (saturates, aromatics, resins, asphaltenes) are attached to Na-montmorillonite surface respectively. The results show that asphaltenes have preference for parallel adsorption near bitumen-clay interface, but the tightly adsorbed configuration can be disrupted by their self-aggregation and light fractions. The distribution pattern is highly influenced by the adsorption strength and molecular diffusion, despite non-negligible spatial effect. Herein, the interaction energy of isolated monomers with clay platelet shows the adsorption strength of SARA fractions in the order of asphaltenes > resins > aromatics > saturates, while the diffusion can be ranked as: Saturates > Resins > Aromatics > Asphaltenes. The distribution pattern of bitumen inferred from the synergy of interaction strength and diffusion suggests that the bitumen exhibits stratified property on clay surface, the asphaltenes and resins content is likely to be higher in the range (1.0～1.5 nm) close to the surface. As for the thermal effect, it is observed that the asphaltenes and resins desorb from the clay surface as the temperature rises as aromatics dominates the adsorption zone, also supported by the finding that asphaltenes dimers are formed and the interaction energy of bitumen with clay platelet decreases to 72.98% at elevated temperature, which implies the thermal effect can reverse the distribution by inconsistently increasing the diffusion of bitumen subfractions. Overall, these findings are helpful for understanding bitumen-clay interaction, optimizing processing temperature and screening surfactant to improve compatibility of nano-clay modifiers with bitumen.

RETRACTED ARTICLE: Modeling and molecular simulation of natural gas hydrate stabilizers

ABSTRACT Uncontrolled decomposition of natural gas hydrate may lead to serious marine geological disasters and air pollution. The model of natural gas hydrate and lecithin was established. The stability mechanism of lecithin to structure hydrate was studied by molecular dynamics simulation. The consistent valence force field (CVFF) and TIP3P potential models are used to define the interaction between CH4-CH4 and water-water species, respectively. The simulations are performed on a combination of a 2 × 2 × 4 unit cell of sI hydrate and a water liquid phase with lecithin. The results of the simulations indicate that lecithin molecules adsorb on the hydrate surface with their hydrocarbon chains crossing and forming a net structure, easily producing the hydrate memory effect, which will narrow the available space for hydrate methane and water movement. Compared to the pure water-hydrate model, the mean square displacement (MSD) values of hydrate methane and water molecules are much lower, indicating that the hydrate dissociates more slowly.

Molecular scale modeling approach to evaluate stability and dissociation of methane and carbon dioxide hydrates

A comprehensive knowledge and precise estimation of the dynamic, structural, and thermodynamic characteristics of hydrates are needed to assess the stability of gas hydrates. Thermodynamic model and experimental studies can be utilized to compute the physical and dynamic properties of hydrate structures. The use of molecular dynamic (MD) simulation is a well-established approach in gas hydrate studies at the atomic level where the properties of interest are obtained from the numerical solution of Newtonian equations. The present work uses MD simulations by employing the constant temperature-constant pressure (NPT), constant temperature-constant volume (NVT) conditions, and the consistent valence force field (CVFF) to monitor the stability and decomposition of methane and carbon dioxide gas hydrates with different compositions. The effects of temperature and composition on the hydrate stability are investigated. In this study, we also compute the radial distribution function, mean square displacement, diffusion coefficient, lattice parameter, potential energy, dissociation enthalpy as well as the density of methane and carbon dioxide under various thermodynamic and process conditions. The formation of methane and carbon dioxide bubbles is studied to investigate bubble evolution during hydrate dissociation. The sizes of methane and carbon dioxide bubbles are not the same due to different solubility conditions of methane and carbon dioxide in liquid water. In addition, the influences of pressure and temperature on the lattice parameter and density of clathrate hydrates are discussed. The obtained results are consistent with previous theoretical and experimental findings, implying that the methodology followed in this work is reliable. The most stable arrangement of methane and carbon dioxide molecules in the gas hydrate is found. The insights/findings of this study might be useful to further understand detailed transport phenomena (e.g., molecular interactions, gas production rate, carbon dioxide replacement, and carbon dioxide capture) involved in the process of carbon dioxide injection into gas hydrate reservoirs.

Molecular dynamic simulations to evaluate dissociation of hydrate structure II in the presence of inhibitors: A mechanistic study

The present work aims to investigate stability and decomposition of hydrate structure II of methane, propane, and isobutane systems at various temperatures, pressures, and compositions in the absence and/or presence of inhibitor molecules. To assess the stability of gas hydrates, a comprehensive knowledge of the structural, thermodynamic, and dynamic properties of the hydrates is needed. The structure II of gas hydrates is embedded in a molecular dynamic (MD) approach and the simulations are carried out under constant temperature-constant volume (NVT) and constant temperature-constant pressure (NPT) conditions by employing the consistent valence force field (CVFF). In this work, first, the mean square displacement (MSD) and diffusion coefficient are evaluated to demonstrate the movement of host molecules. The radial displacement function is then utilized to display the characteristic configurations of structure II under different process and thermodynamic conditions. In addition, other vital properties including lattice parameter and potential energy are determined. The effect of inhibitors on stability and/or decomposition of hydrate structure II is investigated. The achieved results are in good agreement with previous theoretical and experimental outputs, confirming reliability and appropriateness of the simulation method. The inhibition capability of different inhibitors based on the simulation results has the following order: methanol > ethanol > glycerol. The findings of this study can help for better understanding of hydrate dissociation in molecular scale as well as for proper selection and design of effective inhibitors for hydrates with different structures and characteristics.

Vapor sensing and interface properties of reduced graphene oxide–poly(methyl methacrylate) nanocomposite

Synthesized reduced graphene oxide–poly(methyl methacrylate) (RGO–PMMA) nanocomposites were characterized by differential scanning calorimetry, thermogravimetric analysis, and probed for volatile organic compounds (VOC) sensing. A molecular dynamics simulation is performed to investigate the interaction between PMMA and a graphene surface. The condensed-phase optimized molecular potentials for atomistic simulation studies (COMPASS), polymer consistent force-field (PCFF) and consistent valence force-field (CVFF) are used to describe the interaction of the graphene–PMMA. None of the three simulated force fields COMPASS, PCFF, and CVFF exhibits a distinctive behaviour of interaction between graphene and PMMA, but CVFF predicts a higher interaction energy in comparison with the simulated force fields COMPASS and PCFF. Experimentally, the selective response for different VOC has been analysed and the highest response together with the fastest recovery is obtained for tetrahydrofuran. A model is introduced explaining observed features.

Prediction of Hydrophobic Reagent for Flotation Process Using Molecular Modeling.

The interaction or nonbonded energies of base organic ions and water molecules during the flotation process of minerals have important meanings for organizing hydrophobic and stable collectors. Furthermore, the interaction, cross-term, and valence energies of optimized structures are important for understanding the properties and structures of selective collectors. The simulation of pure scheelite mineral (PSM) surfaces with four different negative ions, using an adsorption locator module is demonstrated. The interaction energies for base organic ions and water molecules were resolved and detected by shaping the best hydrophobic interaction and the most stable suspension over the PSM surface (112) and (101). The adsorption locator results for base organic ions and water molecules on PSM surfaces (112) and (101) using buffer width 0.5 Å and temperature range from 318.15 to 283.15 K confirmed the results obtain from Forcite calculations. The results have demonstrated that the possibilities of using consistent valence force field implemented by Forcite and adsorption locator modules in the selection of flotation reagents are cost saving. Furthermore, hydrophobicity of the main negative ions in soaps were solved by the simulation methods and results are in a good agreement with the experimental methods that proved that mustard soap is more selective on the mineral surfaces than sunflower soap when used as a collector. Increasing the molecular weight of negative ions increases the interaction energy between base collector ions and PSM surfaces (112) and (101) significantly.

Molecular weight-dependent physisorption of non-charged poly(9,9-dioctylfluorene) onto the neutral surface of cuboidal γ-alumina in toluene

To understand how polymers physisorb onto solid surfaces, we investigated the physisorption behavior of non-charged, semiflexible poly(9,9-dioctylfluorene) (PF8) with three different number-average degrees of polymerization (DPn) as photoluminescent and chromophoric probes onto cuboidal γ-alumina in toluene at 5, 25, and 50 °C. PF8 revealed noticeable DPn and temperature dependencies in its physisorption behaviors. Molecular mechanics (MM)/molecular dynamics (MD) simulations [consistent valence force field (CVFF)] and Moller–Plesset second-order perturbation theory (MP2) with 6-31 G(d,p) calculations suggested that the PF8 in toluene has multiple interactions from CH/π to C–H/O interactions on the (110) surface of γ-alumina. The competition between multiple intermolecular CH/π and C–H/O interactions was crucial for the spontaneous physisorption of PF8 to occur in the presence of a solvent quantity of toluene. Calculations by time-dependent density functional theory (TD-DFT) with Becke three parameter Lee-Yang-Par (B3LYP) method and 6–31 G(d,p) basis set of a model fluorene 9-mer indicated that the π–π* absorption wavelength largely depends on the regularity of the dihedral angles between fluorene rings, while the intensity and spectral width of the π–π* absorption band are largely influenced by the regularity of the dihedral angles. Solution-phase physisorption systems are a result of the inherent nature of several competitive weak intermolecular interactions coexisting among the polymers, surface, and solvents. Noticeable molecular weight and temperature dependency for physisorption behavior of semiflexible, non-charged poly(9,9-n-dioctylfluorene) (PF8) onto cuboidal γ-alumina in toluene were found. MM/MD simulations (CVFF) and quantum mechanical (MP2/6-31G(d,p)) calculations suggested that PF8 swaps interacting toluene for (110) surface of γ-alumina by changing the interaction from CH/π interactions to C-H/O interactions of γ-alumina in toluene. The competition between multiple intermolecular CH/π and C-H/O interactions was crucial whether the spontaneous physisorption of PF8 occurs in place of solvent quantity of toluene. The solution-phase physisorption systems should be the consequence of several weak attractive intermolecular interactions coexisting between polymers, surface, and solvents

Dissociation mechanism of propane hydrate with methanol additive: A molecular dynamics simulation

Employing NPT molecular dynamics method with consistent valence force field, dissociation processes of propane hydrate with and without methanol additive are simulated at different temperatures and a constant pressure of 50 MPa. We analyze structural snapshots, radial distribution functions, density distributions, angle distributions, change of energies, mean square displacements and diffusion coefficients of two comparative models and find that encaging methanol molecules in the 512 cavities could enhance the diffusion behaviors of H2O and C3H8 molecules and shorten the decomposition time of propane hydrate. The hydroxyl group of methanol could form hydrogen bonds with the water cage skeleton, and destroy the original hydrogen bond balance of the hydrate simultaneously. Our theoretical results could provide a useful guideline to understand gas hydrate blocking wellbore in the production and transportation of oil and gas.

Molecular dynamics simulations of the buckling behavior of defective carbon nanotubes embedded in epoxy nanocomposites

Many investigations have identified buckling of CNTs as a common mode of failure of CNT-reinforced nanocomposites. Several studies have also focused on the influence of defects on the buckling properties of freestanding CNTs. However, it is found that molecular dynamics (MD) based investigations concerned with the buckling of embedded CNTs are rare in the literature. In this research, the effects of various vacancy and Stone-Wales (SW) defect configurations on the buckling behavior of freestanding and embedded single-walled CNTs (SWCNTs) are investigated using MD. The simulations are based on the consistent valence forcefield (CVFF) and performed with the aid of LAMMPS. Our findings revealed different degradation effects and buckling modes for freestanding and embedded SWCNTs. The defects generally have the same influence on the buckling strain of the freestanding and embedded SWCNTs. However, it is found that increasing the number of missing atoms generally reduces the buckling load of freestanding SWCNTs, while increasing it for embedded armchair SWCNTs.

Molecular dynamics simulations of PAMAM dendrimer-encapsulated Au nanoparticles of different sizes under different pH conditions

Molecular dynamics (MD) simulations were carried out to investigate the conformations of fourth generation polyamidoamine (G4 PAMAM) dendrimer-encapsulated Au nanoparticles (AuNPs) of different sizes at different pH conditions. The consistent valence force field (CVFF) was used to model the interaction between the atoms of G4 PAMAM dendrimer and water molecules as well as their interactions with the Au atoms of AuNP. The many-body tight-binding potential was adopted to describe the interaction between Au atoms. The simulation results show a single G4 PAMAM dendrimer can only partially cover the AuNP when its diameter is larger than 2.3nm. According to the variations of radius of gyration (Rg) values, the conformations of PAMAM dendrimers become more compact after covering the AuNP at neutral pH. According to the Stokes–Einstein (SE) relation and the diffusion coefficients obtained by the Einstein equation, it was proposed that both the PAMAM dendrimer conformation and the interaction strength between the PAMAM/AuNP and water molecules have significant influences on the diffusion behaviors of PAMAM/AuNP systems.

Full-atomistic simulations of poly(ϵ-caprolactone) diol models with CVFF and CGenFF

Poly(ϵ-caprolactone) (PCL) diol, with good biodegradation and biocompatibility, is one of the widely used soft segments (SSs) in composing bio-polyester-urethanes (Bio-PUs), which show great potential in both biomedical and tissue engineering applications. Properties of Bio-PUs are tunable by combining SS monomers with different molecular weights, structures, modifications, and ratio of components. Although numbers of research have reported many Bio-PUs properties, few studies have been done at the molecular scale. In this study, we use molecular dynamic (MD) simulation to construct atomistic models for two commonly used PCL diol SSs with different molecular weights 1247.58 Da and 1932.42 Da. We compare the simulation results by using two widely used classical force fields for organic molecules: Consistent Valence Force Field (CVFF) and CHARMM General Force Field (CGenFF), and discuss the validity and accuracy. Melt density, volume, polymer conformations, transition temperature, and mechanical properties of PCL diols are calculated and compared with experiments. Our results show that both force fields provide accurate predictions on the properties of PCL diol system at the molecular scale and could help the design of future Bio-PUs.

Molecular simulation of shale gas adsorption onto overmature type II model kerogen with control microporosity

ABSTRACTWe use an all-atom molecular dynamics simulation to generate the dense porous structures of overmature type II kerogen with control microporosity. The structures mimic the organic part of Barnett shale under a typical reservoir condition of 365 K and 275 bar. First, we build an atomistic model of kerogen unit using the consistent valence force field and chemical structure proposed by Ungerer and his colleagues. Second, we generate kerogen structures by gradual cooling and compression of the initial low-density random configurations of kerogen units. During the structure generation, we use a dummy particle of varying size to introduce microporosity into the kerogen structures. We systematically characterise the microporous kerogen structures by calculating the geometric pore size distribution, pore limiting diameter, maximum pore size, accessible surface area, and pore volume and by analysing the pore network accessibility. Third, we employ grand canonical Monte Carlo (GCMC) to study the adsorption of two proxies of shale gas (pure methane and mixture of 82% of methane, 12% of ethane and 6% of propane) in the kerogen structures. The shale gas adsorptions are compared with GCMC simulation of CO2 adsorption in the kerogen structures. Hydrocarbons are modelled using the all-atom optimized potential for liquid simulations (OPLS) force field while carbon dioxide is represented by the EPM2 model. We complement the adsorption studies by exploring accessibility of pore space of the kerogen structures using molecular dynamics simulation. Finally, we introduce a mesoscale pore void into a microporous kerogen structure and probe the adsorption behaviour of the hydrocarbon mixture in such a multiscale kerogen model.

Molecular simulations of the influence of defects and functionalization on the shear strength of carbon nanotube-epoxy polymer interfaces

Unlike earlier studies, which focused on thermoplastics, in this work attention is devoted to carbon nanotube (CNT) reinforced thermoset polymer composites. Specifically, extensive molecular dynamics (MD) simulations were conducted to study the effect of defects and functionalization on the interfacial shear strength (ISS) of carbon nanotube-epoxy composites before and after cross-linking. Three main types of defects (vacancies, adatom, and Stone-Wales (SW)) as well as phenyl functional group were considered in a representative volume element (RVE). The RVE comprised a single-walled CNT embedded in an epoxy matrix, and consistent valence force field (CVFF) was used in all MD simulations. Our results reveal that (i) vacancy defects degrade the ISS, and that the degradation effects depend predominantly on the number of missing atoms and not on the type of vacancy defects, (ii) adatom defects have a marginal effect on the ISS, (iii) SW defects promote greater ISS of cured systems, (iv) phenyl groups dramatically increase the ISS of cured systems, and (v) uncured systems experience lower ISS which is not affected by the number and type of defects and functionalization. The findings of this study will assist in improving our understanding of the operating toughening/weakening mechanisms and the load transfer capability in epoxy-based nanocomposites.

Atomic Understanding of the Swelling and Phase Transition of Polyacrylamide Hydrogel

A polymer network can imbibe copious amounts of solvent (water) and swell, and the resulting state is known as a hydrogel. In this study, we have made the modification for the all-atom consistent valence force field (CVFF) to investigate the swelling property of polyacrylamide (PAM) hydrogel by molecular dynamics simulation. We have built 21 hydrogel models with different solvent contents and calculate the average chemical potential and diffusion coefficient of solvent molecules in PAM hydrogel. We find that when the mass fraction of solvent is about 90%, PAM hydrogel reaches its free swelling limitation and loses the motivation of absorbing solvent. Furthermore, it is also found that PAM hydrogel has a phase transition phenomenon when the values of solvent chemical potential are between [Formula: see text][Formula: see text]kcal/mol and [Formula: see text][Formula: see text]kcal/mol. This study will provide insight into the basic parameters which are widely used in continuum mechanics analysis of hydrogels from atomic point of view and help researchers to improve the continuum mechanics model for neutral hydrogel.

Evaluating electronic structure of quinazolinone and pyrimidinone molecules for its corrosion inhibition effectiveness on target specific mild steel in the acidic medium: A combined DFT and MD simulation study

The quantum chemical calculations, based on density functional theory, have been implemented to explore the corrosion inhibition mechanism and the corresponding inhibition effectiveness of quinazolinone and pyrimidinone compounds, viz., 6-chloroquinazolin-4(3H)-one (Q1A); 2,3-dihydro-3-phenethyl-2-thioxopyrido[2,3-d]pyrimidin-4(1H)-one (Q1B) and 6-chloro-2,3-dihydro-3-phenethyl-2-thioxoquinazolin-4(1H)-one (Q1C) for mild steel in acidic solution. Global reactivity of the molecules related to the quantum chemical parameters such as EHOMO, ELUMO, energy gap (ΔE), softness (S), hardness (η) and fraction of electron transferred (ΔN) between the inhibitor molecule and the metal surface atom have been calculated and explored. In order to describe the reactive sites of the inhibitor molecules Fukui indices analysis has been performed. To mimic the real environment of corrosion inhibition, molecular dynamic (MD) simulations have also been modelled consisting of all concerned species (inhibitor molecule, H2O, H3O+ ion, SO42− ion and Fe surface) and thereby simulated by the consistent-valence force field (CVFF).

Investigation on the Structural and Thermal Behaviors of Poly(amidoamine) Dendrimer-Encapsulated Au Nanoparticles of Different Sizes

The dynamical and thermal behaviors of Au nanoparticle (AuNP) and fourth generation poly(amidoamine) dendrimer (G4 PAMAM) of the dendrimer-encapsulated Au nanoparticle system under the dry environment were investigated by the molecular dynamics (MD) simulation. The consistent valence force field (CVFF) was used to describe the interaction of the G4 PAMAM dendrimer and the interaction between the dendrimer and the AuNP. The many-body tight-binding potential was adopted to describe the atomic interaction between the Au atoms of an AuNP. Three AuNPs, Au116, Au201, and Au405, with the diameters of 1.45, 1.8, and 2.3 nm were concerned, respectively. Au116 and Au201 are nearly completely covered by G4 PAMAM dendrimer, but the branching chains of the dendrimer are no longer able to wrap around the Au405.The conformation of G4 PAMAM dendrimer still possesses its unique spherical structure after encapsulating the AuNP, and the irregularity of the outer surface decreases as the size of the AuNP increases. For the t...

Molecular Dynamic Simulation of Oxaliplatin Diffusion in Poly(lactic acid-co-glycolic acid). Part A: Parameterization and Validation of the Force-Field CVFF

This study focuses on the estimation and validation of some interaction parameters of the Consistent Valence Force-Field (CVFF), which are required for the calculation of thermodynamic and transport properties of oxaliplatin (a colorectal anticancer drug) in poly(lactic-co-glycolic) acids (PLGAs) matrices. Our methodology to validate the parameters for PLGAs consisted on calculation of glass transition temperature and correlations between structural properties as: fractional free volume, polymer density, and cohesive energy density using Molecular dynamic simulations. For the oxaliplatin, metal-dependent and independent interaction parameters were included into CVFF and validated with an ab-initio method (RHF/LanL2DZ). The results achieved in the present work showed that the CVFF has been wellparameterized.

Multiscale modeling of functionalized interface effects on the effective elastic material properties of CNT–polyethylene nanocomposites

The effects of functionalization of the interface between carbon nanotubes (CNTs) and the polymer matrix on the macroscale elastic mechanical material properties have been investigated using a multiscale model. Molecular dynamic (MD) simulations are used to investigate the influence of functionalization on the nanoscale load transfer ability of the interface between the CNT and the polymer matrix material in CNT–polyethylene (PE) nanocomposites. Graphene–polymer interface models with variable numbers of functional groups grafted between the graphene sheet and the polymer chains are adopted to represent the interface between the CNTs and the polymer matrix. Normal mode and sliding mode separations are performed using MD simulations in order to characterize the load transfer of the functionalized graphene–polymer interface in terms of force-separation responses. The influence of the functionalization density on the load transfer at the interface is studied parametrically. Two force field potentials, the consistent valence force field (CVFF) and adaptive intermolecular reactive empirical bond order (AIREBO) potential, are used to simulate the interactions between the atoms in the polymer matrix and the functional groups. An energy based bond breaking criteria is introduced into the CVFF potential such that both potentials allow for bond breaking. Comparison of the two potentials is conducted by comparing the MD simulation results where it is found that two potentials give different force-separation responses, but that in both cases, there is significant increase in load transfer capability and toughness of the interface with increasing degree of functionalization of the interface. With cohesive zone laws derived from MD results, a finite element implementation of the cohesive zone models is used to calculate the effective elastic mechanical properties of the CNT–PE nanocomposites in order to study the influence of functionalization at the nanoscale on their macroscale mechanical performance.

Dissociation mechanism of gas hydrates (I, II, H) of alkane molecules: a comparative molecular dynamics simulation

Employing NPT molecular dynamics method with consistent valence force field, the dissociation processes of sI, sII and sH gas hydrates are simulated at different temperatures and at a constant pressure of 100 MPa. The dissociation mechanisms of gas hydrates are revealed by analysing the structural snapshots, radial distribution functions and diffusion coefficients at different temperatures. As temperature increases, the diffusion rates of water molecules and guest molecules increase; thus the clathrate skeleton formed by water molecules with hydrogen bonds distorts and breaks down; meanwhile the guest molecules encapsulated in the water cavities are released. The size of guest molecules affects the dissociation behaviour of gas hydrate. In addition, the dissociation behaviour also relies on the structural phase of gas hydrates.