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Contributed by Joseph S. Francisco, August 21, 2020 (sent for review June 11, 2020; reviewed by Jianwen Jiang and Sotiris S. Xantheas)
Significance
Understanding the clathrate formation mechanism has important implications on natural gas exploitation, storage, and transportation. A key toward clathrate formation is the hydrate nucleation, which involves complex interplay between water and guest molecules at nanoscale and multiple kinetic stages. Herein, by tracing the kinetic evolution of multiple ternary water-ring aggregations based on trajectories of large-scale molecular dynamics simulations, we identified a generic nucleation pathway to clathrate formation. This observed nucleation pathway is driven by numerous compression/shedding processes among the methane hydration layers, and it can explain the widely accepted “blob” model of clathrate nucleation. The molecular insight into the nucleation pathway allows us to quantitatively assess the nucleation timescales as well as the critical nucleus sizes in the 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.
Footnotes
↵1L.L. and J. Zhong contributed equally to this work.
- ↵2To whom correspondence may be addressed. Email: zhangjun.upc{at}gmail.com, xjiafang{at}upc.edu.cn, frjoseph{at}sas.upenn.edu, or xzeng1{at}unl.edu.
Author contributions: L.L., J. Zhong, Y.Y., J. Zhang, J.X., J.S.F., and X.C.Z. designed research; J. Zhong, Y.Y., J. Zhang, and X.C.Z. performed research; L.L., J. Zhong, Y.Y., J. Zhang, J.X., J.S.F., and X.C.Z. analyzed data; and L.L., J. Zhong, Y.Y., J. Zhang, J.X., J.S.F., and X.C.Z. wrote the paper.
Reviewers: J.J., National University of Singapore; and S.S.X., Pacific Northwest National Laboratory.
The authors declare no competing interest.
This article contains supporting information online at https://www.pnas.org/lookup/suppl/doi:10.1073/pnas.2011755117/-/DCSupplemental.
Data Availability.
All study data are included in the article and SI Appendix.
Published under the PNAS license.
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Unraveling nucleation pathway in methane clathrate formation
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