jp0c02675_si_003.mpg (23.45 MB)
Propane and Water: The Cooperativity of Unlikely Molecules to Form Clathrate Structures
media
posted on 2020-05-22, 15:44 authored by Yong Chen, Chen Chen, Amadeu K. SumMany
unanswered questions still exist at the molecular level to
understand the nucleation process and mechanism of clathrate hydrates,
especially for larger guest molecules that would result in the structure
II crystal. Here, we report on molecular dynamics simulations for
propane and water to describe the molecular mechanism leading to a
structure II system. Through a large number (30) of long (5 μs)
and coupled annealing (20 μs) simulations, we detail the prenucleation,
nucleation, growth, and annealing of propane clathrate hydrate structures
at 250 K and 1800 bar. The results demonstrate the equal importance
of the empty and occupied cages in the nucleation of propane hydrates.
The critical nucleus size is identified to be eight cages. While separate
distinct clusters may exist during the prenucleation period, only
one survives to grow beyond the critical nucleus size, with the others
remaining subcritical. From the annealing simulations, it is clear
that solid rearrangement is a very slow process, and 20 μs is
still not long enough to capture long-range ordering resembling the
structure II crystal. These results, along with the developed analysis
method, have a significant impact in advancing our understanding of
the nucleation process for unlike molecules.