Unraveling nucleation pathway in methane clathrate formation

Abstract

Methane clathrates are widespread on the ocean floor of the Earth. A better understanding of methane clathrate formation has important implications for natural-gas exploitation, storage, and transportation. A key step toward understanding clathrate formation is hydrate nucleation, which has been suggested to involve multiple evolution pathways. Herein, a unique nucleation/growth pathway for methane clathrate formation has been identified by analyzing the trajectories of large-scale molecular dynamics (MD) simulations. In particular, ternary water-ring aggregations (TWRAs) have been identified as fundamental structures for characterizing the nucleation pathway. Based on this nucleation pathway, the critical nucleus size and nucleation timescale can be quantitatively determined. Specifically, a methane hydration layer compression/shedding process is observed to be the critical step in (and driving) the nucleation/growth pathway, which is manifested through overlapping/compression of the surrounding hydration layers of the methane molecules, followed by detachment (shedding) of the hydration layer. As such, an effective way to control methane hydrate nucleation is to alter the hydration layer compression/shedding process during the course of nucleation.

Elucidating the critical oligomeric steps in secondary organic aerosol and brown carbon formation

“…These organic carbonyls engage in a variety of reactions, including new particle formation, condensation / equilibrium partitioning, particle-and aqueous-phase reactions, leading to the formation of additional products in the particle phase (Loeffler et al, 2006;Kroll et al, 2005;Nozière et al, 2009; Published by Copernicus Publications on behalf of the European Geosciences Union. Marrero-Ortiz et al, 2019;Tuguldurova et al, 2019;Kua et al, 2011;Li et al, 2020;Shi et al, 2019;Xia et al, 2021).…”

Multistep nucleation of anisotropic molecules

“…Molecular simulations have thus provided an alternative and promising means to elucidate the microscopic mechanisms of phase transition phenomena, in particular nucleation processes. Indeed, the questions of how a crystal nucleus forms in a liquid phase have been addressed for diverse systems 14 , 15 , 20 , 24 , 25 , 35 – 41 . However, there have been few numerical studies that successfully demonstrate the actual non-classical nucleation dynamics together with the energetic stability, statistics and dynamics of clusters and transient structures involved in the nucleation process.…”

Anti-gas hydrate surfaces: perspectives, progress and prospects

“…Great efforts have been concentrating on revealing the homogeneous nucleation mechanism of hydrates starting in the early 1990s, 71–74 which has yielded significant progress, especially in the last decade. The labile-cluster and local-structuring hypotheses (LCH & LSH, respectively) are two representative classical theories which attempt to explain the homogeneous nucleation of hydrates.…”

“…Other recent studies provided further understanding and more detailed explanations on the “blob” model. 74,76 It's fair to say that the “blob” mechanism combines LSH and LCH, which lays the basis of much current understanding about homogeneous hydrate-nucleation mechanisms. It should be noted that the homogeneous hydrate nucleation hypotheses assume evenly mixed states of guest and water molecules, which could deviate from reality.…”