Electrodissociation of clathrate-like structures

Acoustic-propagation properties of methane clathrate hydrates from non-equilibrium molecular dynamics

Given methane hydrates' importance in marine sediments, as well as the widespread use of seabed acoustic-signaling methods in oil and gas exploration, the elastic characterization of these materials is particularly relevant. A greater understanding of the properties governing phonon, sound, and acoustic propagation would help to better classify methane-hydrate deposits, aiding in their discovery. Recently, we have published a new nonequilibrium molecular-dynamics (NEMD) methodology to recreate longitudinal and transverse perturbations, observing their propagation through a crystalline lattice by various metrics, to study the underlying S- and P-wave velocities (achieving excellent agreement with experiment) [Melgar et al., J. Phys. Chem. 122(5), 3006-3013 (2018); ibid.150, 084101 (2019)]. Here, we apply these NEMD methods to methane-clathrate systems to study acoustic-propagation characteristics, as well as the lattice elastic behavior. In so doing, we determine S- and P-wave velocities in excellent accord with experiment; we also ascertain the allowable magnitude range of acoustic perturbation and establish a threshold for lattice breakup and hydrate decomposition. Interestingly, upon dissociation, we observe the formation of methane nanobubbles, which agrees with previous studies on the microscopic fundamentals of hydrate dissociation by various means.

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 modeling study of methane hydrate decomposition in contact with the external surface of zeolites

Methane hydrate dissociates on the external surface of siliceous zeolites with methane absorbed by the solid and water forming a liquid-like phase.

Encaged molecules in external electric fields: A molecular "tug-of-war"

Response of polar molecules CH3OH and H2O2 and a non-polar molecule, CO2, as "guests" encapsulated in the dodecahedral water cage (H2O)20 "host," to an external, perturbative electric field is investigated theoretically. We employ the hybrid density-functionals M06-2X and ωB97X-D incorporating the effects of damped dispersion, in conjunction with the maug-cc-pVTZ basis set, amenable for a hydrogen bonding description. While the host cluster (cage) tends to confine the embedded guest molecule through cooperative hydrogen bonding, the applied electric field tends to rupture the cluster-composite by stretching it; these two competitive effects leading to a molecular "tug-of-war." The composite remains stable up to a maximal sustainable threshold electric field, beyond which, concomitant with the vanishing of the HOMO-LUMO gap, the field wins over and the cluster breaks down. The electric-field effects are gauged in terms of the changes in the molecular geometry of the confined species, interaction energy, molecular electrostatic potential surfaces, and frequency shifts of characteristic normal vibrations in the IR regime. Interestingly, beyond the characteristic threshold electric field, the labile, distorted host cluster fragmentizes, and the guest molecule still tethered to a remnant fragment, an effect attributed to the underlying hydrogen-bonded networks.

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.

Perspectives on external electric fields in molecular simulation: progress, prospects and challenges

In this review, the application of a wide variety of external electric fields in molecular simulation shall be discussed, including time-varying and electromagnetic, as well as the utility and potential impact and prospects for exploitation of such simulations for real-world and industrial end use. In particular, non-equilibrium molecular dynamics will be discussed, as well as challenges in addressing adequate thermostatting and scaling field amplitudes to more experimentally relevant levels. Attention shall be devoted to recent progress and advances in external fields in ab initio molecular simulation and dynamics, as well as elusive challenges thereof (and, to some extent, for molecular dynamics from empirical potentials), such as timescales required to observe low-frequency and intensity field effects. The challenge of deterministic molecular dynamics in external fields in sampling phase space shall be discussed, along with prospects for application of fields in enhanced-sampling simulations. Finally, the application of external electric fields to a wide variety of aqueous, nanoscale and biological systems will be discussed, often motivated by the possibility of exploitation in real-world applications, which serve to underpin our molecular-level understanding of field effects in terms of microscopic mechanisms, and possibly with a view to control thereof.

A theoretical study of the dissociation of the sI methane hydrate induced by an external electric field

Molecular dynamics simulations in the equilibrium isobaric-isothermal (NPT) ensemble were used to examine the strength of an external electric field required to dissociate the methane hydrate sI structure. The water molecules were modeled using the four-site TIP4P/Ice analytical potential and methane was described as a simple Lennard-Jones interaction site. A series of simulations were performed at T = 260 K with P = 80 bars and at T = 285 K with P = 400 bars with an applied electric field ranging from 1.0 V nm(-1) to 5.0 V nm(-1). For both (T,P) conditions, applying a field greater than 1.5 V nm(-1) resulted in the orientation of the water molecules such that an ice Ih-type structure was formed, from which the methane was segregated. When the simulations were continued without the external field, the ice-like structures became disordered, resulting in two separate phases: gas methane and liquid water.