Nucleation rate analysis of methane hydrate from molecular dynamics simulations

Abstract

Clathrate hydrates are solid crystalline structures most commonly formed from solutions that have nucleated to form a mixed solid composed of water and gas. Understanding the mechanism of clathrate hydrate nucleation is essential to grasp the fundamental chemistry of these complex structures and their applications. Molecular dynamics (MD) simulation is an ideal method to study nucleation at the molecular level because the size of the critical nucleus and formation rate occur on the nano scale. Various analysis methods for nucleation have been developed through MD to analyze nucleation. In particular, the mean first-passage time (MFPT) and survival probability (SP) methods have proven to be effective in procuring the nucleation rate and critical nucleus size for monatomic systems. This study assesses the MFPT and SP methods, previously used for monatomic systems, when applied to analyzing clathrate hydrate nucleation. Because clathrate hydrate nucleation is relatively difficult to observe in MD simulations (due to its high free energy barrier), these methods have yet to be applied to clathrate hydrate systems. In this study, we have analyzed the nucleation rate and critical nucleus size of methane hydrate using MFPT and SP methods from data generated by MD simulations at 255 K and 50 MPa. MFPT was modified for clathrate hydrate from the original version by adding the maximum likelihood estimate and growth effect term. The nucleation rates calculated by MFPT and SP methods are within 5%, and the critical nucleus size estimated by the MFPT method was 50% higher, than values obtained through other more rigorous but computationally expensive estimates. These methods can also be extended to the analysis of other clathrate hydrates.

Overcoming time scale and finite size limitations to compute nucleation rates from small scale well tempered metadynamics simulations

“…In Fig. 6c values of J N are reported as a function of the supersaturation S, clearly showing a significant extension of the accessible nucleation rates domain, which is increased up to ten orders of magnitude compared to the domain typically accessible by standard molecular dynamics 12,23,24,33,34 . The surface tension γ obtained from the fitting of the J N values computed from WTmetaD corresponds to γ = 16.9 mN/m, a value that nicely extrapolates the data of Goujon et al 35 for the same system in the temperature range between 85 K and 135 K as shown in the Supplementary Information (SI).…”

Rate Prediction for Homogeneous Nucleation of Methane Hydrate at Moderate Supersaturation Using Transition Interface Sampling

“…In this work, we apply transition interface sampling (TIS) 40 to compute the nucleation rate for realistic undercooling 41 ( T = 280 K) at a relevant pressure ( P = 500 bar), employing an accurate atomistic force field (Tip4P/ICE). 42 Recently, we performed extensive transition path sampling (TPS) 43 simulations on hydrate nucleation in a methane/water mixture with an sI stoichiometry at moderate temperatures between 270 and 285 K at a pressure of 500 bar using the same system setup as in refs ( 2 , 19 , 21 , 22 ). The melting point for the model used is 303 ± 2 K. 44 In the metastable liquid state, the system is phase separated into a (super)saturated water–methane mixture in equilibrium with a (spherical) methane gas reservoir.…”

Numerical Simulation on the Dissociation, Formation, and Recovery of Gas Hydrates on Microscale Approach

“…In addition, MD has been considered a robust technique used to investigate crystal growth mechanisms [ 102 , 103 ], the solid/liquid and gas/liquid interfaces [ 104 , 105 , 106 , 107 ]. Several parameters were observed, such as potential energy changes, MSD of molecules, the number of methane molecules close to the solid/liquid interface, and the position of liquid/solid interfaces with time.…”