Abstract
Gas hydrates have an important role in environmental and astrochemistry, as well as in energy materials research. Although it is widely accepted that gas accumulation is an important and necessary process during hydrate nucleation, how guest molecules aggregate remains largely unknown. Here, we have performed molecular dynamics simulations to clarify the nucleation path of methane hydrate. We demonstrated that methane gather with a three-body aggregate pattern corresponding to the free energy minimum of three-methane hydrophobic interaction. Methane molecules fluctuate around one methane which later becomes the central gas molecule, and when several methanes move into the region within 0.8 nm of the potential central methane, they act as directional methane molecules. Two neighbor directional methanes and the potential central methane form a three-body aggregate as a regular triangle with a distance of ~6.7 Å which is well within the range of typical methane-methane distances in hydrates or in solution. We further showed that hydrate nucleation and growth is inextricably linked to three-body aggregates. By forming one, two, and three three-body aggregates, the possibility of hydrate nucleation at the aggregate increases from 3/6, 5/6 to 6/6. The results show three-body aggregation of guest molecules is a key step in gas hydrate formation.
Highlights
IntroductionGas hydrates have an important role in environmental and astrochemistry, as well as in energy materials research
Against from labile cluster hypothesis, the local structuring hypothesis describes the origin of gas hydrate nucleation as the local ordering of guest molecules caused by thermal fluctuation[19]
As the number of three-body aggregates increases in the initial system, the hydrate nucleation induction time becomes shorter and the hydrate grows at a faster rate
Summary
Gas hydrates have an important role in environmental and astrochemistry, as well as in energy materials research. The classical nucleation theory is challenged when handling excess free energy and critical radius of hydrate nucleus It does not provide the detailed nucleation pathway as well as the hydrate structure. For the hydration layer compression/shedding hypothesis, the water hydration layers of the neighbor methane molecules are compressed to form ternary water-ring aggregations which is recognized as the fundamental structures in gas hydrate nucleation[24]. With these hypotheses, the main debate focuses on whether the gas hydrate nucleation is originally triggered by guest molecules or water molecules. How the guest molecules aggregate and their aggregation patterns remains unknown
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