Molecular dynamics study of CO2 hydrate dissociation: Fluctuation-dissipation and non-equilibrium analysis

Large-Scale Nonequilibrium Molecular Studies of Thermal Hydrate Dissociation

The energy content of methane hydrate reservoirs (MHRs) is at least twice that of conventional fossil fuels. So, there is considerable interest in their commercial development by heating, among other dissociation mechanisms. However, a few researchers have highlighted the potentially uncontrollable release of methane from MHRs, which could occur because of global warming. Therefore, it is crucial to understand the kinetics of thermal hydrate dissociation to safely develop these resources and prevent the release of this greenhouse gas into the environment. Although there have been several molecular studies of thermal dissociation, most of these use small simulation domains that cannot capture the transient nature of the process. To address this limitation, we performed coarse-grained molecular dynamics (CGMD) simulations on a significantly larger domain with a hundred times more hydrate unit cells than those used in previous studies. We monitored the kinetics of dissociation using an image-processing algorithm and observed the dynamics of the process while maintaining a thermal gradient at the dissociation front. For the first time, we report the formation of an unstable secondary dissociation path that triggers gas bubbles within the solid hydrate. The kinetics of thermal dissociation appears to occur in three stages. In the first stage, the energy of the system increases until it exceeds the activation energy, and dissociation is initiated. Consistent dissociation occurs in the second stage, whereas the third stage involves the dissociation of the remaining hydrates across a nonplanar and heterogeneous interface.

All-Atom Molecular Dynamics of Pure Water-Methane Gas Hydrate Systems under Pre-Nucleation Conditions: A Direct Comparison between Experiments and Simulations of Transport Properties for the Tip4p/Ice Water Model

(1) Background: New technologies involving gas hydrates under pre-nucleation conditions such as gas separations and storage have become more prominent. This has necessitated the characterization and modeling of the transport properties of such systems. (2) Methodology: This work explored methane hydrate systems under pre-nucleation conditions. All-atom molecular dynamics simulations were used to quantify the performance of the TIP4P/2005 and TIP4P/Ice water models to predict the viscosity, diffusivity, and thermal conductivity using various formulations. (3) Results: Molecular simulation equilibrium was robustly demonstrated using various measures. The Green–Kubo estimation of viscosity outperformed other formulations when combined with TIP4P/Ice, and the same combination outperformed all TIP4P/2005 formulations. The Green–Kubo TIP4P/Ice estimation of viscosity overestimates (by 84% on average) the viscosity of methane hydrate systems under pre-nucleation conditions across all pressures considered (0–5 MPag). The presence of methane was found to increase the average number of hydrogen bonds over time (6.7–7.8%). TIP4P/Ice methane systems were also found to have 16–19% longer hydrogen bond lifetimes over pure water systems. (4) Conclusion: An inherent limitation in the current water force field for its application in the context of transport properties estimations for methane gas hydrate systems. A re-parametrization of the current force field is suggested as a starting point. Until then, this work may serve as a characterization of the deviance in viscosity prediction.

Environmental Exploration of Ultra-Dense Nanobubbles: Rethinking Sustainability

Nanobubbles are nanoscopic gaseous domains than can exist on solid surfaces or in bulk liquids. They have attracted significant attention in the last decade due to their long-time (meta)stability and ready potential for real-world applications, especially in environmental engineering and more sustainable ecosystems, water treatment, irrigation, and crop growth. After reviewing important nano-bubble science and activity, with some of the latest promising results in agriculture, we point out important directions in applications of nano-bubble phenomena for boosting sustainability, with viewpoints on how to revolutionise best-practice environmental and green sustainability, taking into account economic drivers and impacts. More specifically, it is pointed out how nanobubbles may be used as delivery vehicles, or “nano-carriers”, for nutrients or other agents to specific targets in a variety of ecosystems of environmental relevance, and how core this is to realising a vision of ultra-dense NBs in shaping a positive and lasting impact on ecosystems and our natural environment.

Exploring CO2 @sI Clathrate Hydrates as CO2 Storage Agents by Computational Density Functional Approaches

The formation of specific clathrate hydrates and their transformation at given thermodynamic conditions depends on the interactions between the guest molecule/s and the host water lattice. Understanding their structural stability is essential to control structure-property relations involved in different technological applications. Thus, the energetic aspects relative to CO₂ @sI clathrate hydrate are investigated through the computation of the underlying interactions, dominated by hydrogen bonds and van der Waals forces, from first-principles electronic structure approaches. The stability of the CO₂ @sI clathrate is evaluated by combining bottom-up and top-down approaches. Guest-free and CO₂ guest-filled aperiodic cages, up to the gradually CO₂ occupation of the entire sI periodic unit cells were considered. Saturation, cohesive and binding energies for the systems are determined by employing a variety of density functionals and their performance is assessed. The dispersion corrections on the non-covalent interactions are found to be important in the stabilization of the CO₂ @sI energies, with the encapsulation of the CO₂ into guest-free/empty cage/lattice being always an energetically favorable process for most of the functionals studied. The PW86PBE functional with XDM or D3(BJ) dispersion corrections predicts a lattice constant in accord to the experimental values available, and simultaneously provides a reliable description for the guest-host interactions in the periodic CO₂ @sI crystal, as well as the energetics of its progressive single cage occupancy process. It has been found that the preferential orientation of the single CO₂ in the large sI crystal cages has a stabilizing effect on the hydrate, concluding that the CO₂ @sI structure is favored either by considering the individual building block cages or the complete sI unit cell crystal. Such benchmark and methodology cross-check studies benefit new data-driven model research by providing high-quality training information, with new insights that indicate the underlying factors governing their structure-driven stability, and triggering further investigations for controlling the stabilization of these promising long-term CO₂ storage materials.

Computational density-functional approaches on finite-size and guest-lattice effects in CO2@sII clathrate hydrate

We performed first-principles computations to investigate guest-host/host-host effects on the encapsulation of the CO₂ molecule in sII clathrate hydrates from finite-size clusters up to periodic 3D crystal lattice systems. Structural and energetic properties were first computed for the individual and first-neighbors clathrate-like sII cages, where highly accurate ab initio quantum chemical methods are available nowadays, allowing in this way the assessment of the density functional (DFT) theoretical approaches employed. The performance of exchange-correlation functionals together with recently developed dispersion-corrected schemes was evaluated in describing interactions in both short-range and long-range regions of the potential. On this basis, structural relaxations of the CO₂-filled and empty sII unit cells yield lattice and compressibility parameters comparable to experimental and previous theoretical values available for sII hydrates. According to these data, the CO₂ enclathration in the sII clathrate cages is a stabilizing process, either by considering both guest-host and host-host interactions in the complete unit cell or only the guest-water energies for the individual clathrate-like sII cages. CO₂@sII clathrates are predicted to be stable whatever the dispersion correction applied and in the case of single cage occupancy are found to be more stable than the CO₂@sI structures. Our results reveal that DFT approaches could provide a good reasonable description of the underlying interactions, enabling the investigation of formation and transformation processes as a function of temperature and pressure.

Gas hydrates in sustainable chemistry

This review includes the current state of the art understanding and advances in technical developments about various fields of gas hydrates, which are combined with expert perspectives and analyses.

Fluctuation-dissipation analysis of nonequilibrium thermal transport at the hydrate dissociation interface

Herein, a nonequilibrium molecular-dynamics simulation in an NVE ensemble was performed to investigate the spontaneous dissociation of methane-hydrate when it came in contact with liquid water. The nonequilibrium in the interface region is linked to the dissociation process of the hydrate near the interface according to the Onsager's hypothesis. The simulated thickness of the interface was found to be close to the acoustic phonon mean path of methane hydrate and agreed with the reference value. The normalized heat flow autocorrelation function was introduced to study fluctuation-dissipation in terms of the thickness and moving velocity of the interface and the Stefan number. This helped to clearly identify three distinct hydrate-decomposition regimes dominated by sensible heat, latent heat and an intrinsically unstable lattice framework. It was found that the fluctuation-dissipation theory could express the nonequilibrium nature in the front two stages before the threshold was reached, and the dissociation rate increased in the latter stage; this was different from the case of thermal-driven dissociation. The Stefan number decreased rapidly with dissociation in the initial stage and then fluctuated in the intermediate stage; this was analogous to the fluctuation characteristics of the heat flow autocorrelation function. The Stefan number effect shows that thermal dissipation drives the hydrate dissociation and correlates fluctuation to the nonequilibrium nature. It was also found that a small Stefan number was enough to break up the residual hydrate soon after the threshold was achieved. The transient interfacial thermal resistance of the interfacial region was obtained as a typical value in the range of 10^-7-10^-9 m² K W^-1. This justifies that fluctuation-dissipation exists in the nonequilibrium process of hydrate dissociation either in terms of heat flux, as observed in this study, or the diffusion of guest molecules, as reported in other studies.

Magnetic-field effects on methane-hydrate kinetics and potential geophysical implications: Insights from non-equilibrium molecular dynamics

We have conducted non-equilibrium molecular-dynamics (NEMD) simulation to show that externally-applied magnetic fields, including their reversals in direction, have important effects on gas-release dynamics from methane hydrates. In particular, we apply fluctuation-dissipation analysis in the guise of Onsager's hypothesis to study hydrate kinetics at lower applied-field intensities, including temporary hydrate destabilisation in the wake of field-polarity switch; we scale down to the lowest practicable field intensities, of the order of 1 T. We conjecture, that these NEMD-based findings, particularly those involving polarity switch, may have ramifications for superchron-related Earth's magnetic-field polarity swaps affecting methane release into the geosphere, although a good deal of further work would be needed to provide a more definitive causal link.

Non-equilibrium molecular-dynamics study of electromagnetic-field-induced propane-hydrate dissociation

Non-equilibrium molecular-dynamics simulations have been performed for dissolution of planar propane-hydrate/water interfaces in externally-applied electromagnetic (e/m) fields in the microwave to far infrared range (∼2.45-200 GHz) at electric-field intensities up to 2.0 V/nm and at roughly 20 K over/under temperatures vis-à-vis the zero-field propane-hydrate melting point. Upon e/m-field application, there is a field-frequency threshold above which the dissociation rate drops significantly, with a plateau therein for larger-frequencies. It was found that higher intensity and lower frequency facilitates dissociation. Except in the presence of a thermal driving-force, the 10 GHz frequency shows more substantial rate-enhancement effect vis-à-vis static electric fields or, indeed, lower-frequency e/m fields.

A Systematic Protocol for Benchmarking Guest-Host Interactions by First-Principles Computations: Capturing CO2 in Clathrate Hydrates

Clathrate hydrates of CO₂ have been proposed as potential molecular materials in tackling important environmental problems related to greenhouse gases capture and storage. Despite the increasing interest in such hydrates and their technological applications, a molecular-level understanding of their formation and properties is still far from complete. Modeling interactions is a challenging and computationally demanding task, essential to reliably determine molecular properties. First-principles calculations for the CO₂ guest in all sI, sII, and sH clathrate cages were performed, and the nature of the guest-host interactions, dominated by both hydrogen-bond and van der Waals forces, was systematically investigated. Different families of density functionals, as well as pairwise CO₂ @H₂ O model potentials versus wavefunction-based quantum approaches were studied for CO₂ clathrate-like systems. Benchmark energies for new distance-dependent datasets, consisting of potential energy curves sampling representative configurations of the systems at the repulsive, near-equilibrium, and asymptotic/long-range regions of the full-dimensional surface, were generated, and a general protocol was proposed to assess the accuracy of such conventional and modern approaches at minimum and non-minimum orientations. Our results show that dispersion interactions are important in the guest-host stabilization energies of such clathrate cages, and the encapsulation of the CO₂ into guest-free clathrate cages is always energetically favorable. In addition, the orientation of CO₂ inside each cage was explored, and the ability of current promising approaches to accurately describe non-covalent CO₂ @H₂ O guest-host interactions in sI, sII, and sH clathrates was discussed, providing information for their applicability to future multiscale computer simulations.

Molecular-dynamics study of propane-hydrate dissociation: Fluctuation-dissipation and non-equilibrium analysis

Equilibrium and non-equilibrium molecular-dynamics (MD) simulations have been performed to investigate thermal-driven break-up of planar propane-hydrate interfaces in contact with liquid water over the 260-320 K range. Two types of hydrate-surface water-lattice molecular termination were adopted, at the hydrate edge with water, for comparison: a 001-direct surface cleavage and one with completed cages. Statistically significant differences in melting temperatures and initial break-up rates were observed between both interface types. Dissociation rates were observed to be strongly dependent on temperature, with higher rates at larger over-temperatures vis-à-vis melting. A simple coupled mass and heat transfer model, developed previously, was applied to fit the observed dissociation profiles, and this helps us to identify clearly two distinct hydrate-decomposition régimes; following a highly temperature-dependent break-up phase, a second well-defined stage is essentially independent of temperature, in which the remaining nanoscale, de facto two-dimensional system's lattice framework is intrinsically unstable. Further equilibrium MD-analysis of the two-phase systems at their melting point, with consideration of the relaxation times gleaned from the auto-correlation functions of fluctuations in a number of enclathrated guest molecules, led to statistically significant differences between the two surface-termination cases; a consistent correlation emerged in both cases between the underlying, non-equilibrium, thermal-driven dissociation rates sampled directly from melting with that from an equilibrium-MD fluctuation-dissipation approach.

Computational study of the interplay between intermolecular interactions and CO2 orientations in type I hydrates

Carbon dioxide (CO₂) molecules show a rich orientation landscape when they are enclathrated in type I hydrates. Previous studies have described experimentally their preferential orientations, and some theoretical works have explained, but only partially, these experimental results. In the present paper, we use classical molecular dynamics and electronic density functional theory to advance in the theoretical description of CO₂ orientations within type I hydrates. Our results are fully compatible with those previously reported, both theoretical and experimental, the geometric shape of the cavities in hydrate being, and therefore, the steric constraints, responsible for some (but not all) preferential angles. In addition, our calculations also show that guest-guest interactions in neighbouring cages are a key factor to explain the remaining experimental angles. Besides the implication concerning equation of state hydrate modeling approximations, the conclusion is that these guest-guest interactions should not be neglected, contrary to the usual practice.

A Theoretical Study of the Hydration of Methane, from the Aqueous Solution to the sI Hydrate-Liquid Water-Gas Coexistence

Monte Carlo and molecular dynamics simulations were done with three recent water models TIP4P/2005 (Transferable Intermolecular Potential with 4 Points/2005), TIP4P/Ice (Transferable Intermolecular Potential with 4 Points/ Ice) and TIP4Q (Transferable Intermolecular Potential with 4 charges) combined with two models for methane: an all-atom one OPLS-AA (Optimal Parametrization for the Liquid State) and a united-atom one (UA); a correction for the C–O interaction was applied to the latter and used in a third set of simulations. The models were validated by comparison to experimental values of the free energy of hydration at 280, 300, 330 and 370 K, all under a pressure of 1 bar, and to the experimental radial distribution functions at 277, 283 and 291 K, under a pressure of 145 bar. Regardless of the combination rules used for σ_C,O, good agreement was found, except when the correction to the UA model was applied. Thus, further simulations of the sI hydrate were performed with the united-atom model to compare the thermal expansivity to the experiment. A final set of simulations was done with the UA methane model and the three water models, to study the sI hydrate-liquid water-gas coexistence at 80, 230 and 400 bar. The melting temperatures were compared to the experimental values. The results show the need to perform simulations with various different models to attain a reliable and robust molecular image of the systems of interest.

A molecular dynamics study of model SI clathrate hydrates: the effect of guest size and guest–water interaction on decomposition kinetics

One of the options suggested for methane recovery from natural gas hydrates is molecular replacement of methane by suitable guests like CO₂ and N₂.