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Title: Pathways through equilibrated states with coexisting phases for gas hydrate formation

Abstract

Under ambient conditions, water freezes to either hexagonal ice or a hexagonal/cubic composite ice. The presence of hydrophobic guest molecules introduces a competing pathway: gas hydrate formation, with the guests in clathrate cages. Here, the pathways of the phase transitions are sought as sequences of states with coexisting phases, using a generalized replica exchange algorithm designed to sample them in equilibrium, avoiding nonequilibrium processes. For a dilute solution of methane in water under 200 atm, initializing the simulation with the full set of replicas leads to methane trapped in hexagonal/cubic ice, while gradually adding replicas with decreasing enthalpy produces the initial steps of hydrate growth. Once a small amount of hydrate is formed, water rearranges to form empty cages, eventually transforming the remainder of the system to metastable β ice, a scaffolding for hydrates. It is suggested that configurations with empty cages are reaction intermediates in hydrate formation when more guest molecules are available. Furthermore, free energy profiles show that methane acts as a catalyst reducing the barrier for β ice versus hexagonal/cubic ice formation.

Authors:
 [1];  [1]
  1. Boston Univ., Boston, MA (United States). Department of Chemistry
Publication Date:
Research Org.:
Boston Univ., MA (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
OSTI Identifier:
1247321
Grant/Contract Number:
SC0008810
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Journal of Physical Chemistry. B, Condensed Matter, Materials, Surfaces, Interfaces and Biophysical Chemistry
Additional Journal Information:
Journal Volume: 119; Journal Issue: 52; Journal ID: ISSN 1520-6106
Publisher:
American Chemical Society
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; 36 MATERIALS SCIENCE; 97 MATHEMATICS AND COMPUTING; hydrates; computer simulation; phase transitions; algorithm design

Citation Formats

Malolepsza, Edyta, and Keyes, Tom. Pathways through equilibrated states with coexisting phases for gas hydrate formation. United States: N. p., 2015. Web. doi:10.1021/acs.jpcb.5b06832.
Malolepsza, Edyta, & Keyes, Tom. Pathways through equilibrated states with coexisting phases for gas hydrate formation. United States. doi:10.1021/acs.jpcb.5b06832.
Malolepsza, Edyta, and Keyes, Tom. 2015. "Pathways through equilibrated states with coexisting phases for gas hydrate formation". United States. doi:10.1021/acs.jpcb.5b06832. https://www.osti.gov/servlets/purl/1247321.
@article{osti_1247321,
title = {Pathways through equilibrated states with coexisting phases for gas hydrate formation},
author = {Malolepsza, Edyta and Keyes, Tom},
abstractNote = {Under ambient conditions, water freezes to either hexagonal ice or a hexagonal/cubic composite ice. The presence of hydrophobic guest molecules introduces a competing pathway: gas hydrate formation, with the guests in clathrate cages. Here, the pathways of the phase transitions are sought as sequences of states with coexisting phases, using a generalized replica exchange algorithm designed to sample them in equilibrium, avoiding nonequilibrium processes. For a dilute solution of methane in water under 200 atm, initializing the simulation with the full set of replicas leads to methane trapped in hexagonal/cubic ice, while gradually adding replicas with decreasing enthalpy produces the initial steps of hydrate growth. Once a small amount of hydrate is formed, water rearranges to form empty cages, eventually transforming the remainder of the system to metastable β ice, a scaffolding for hydrates. It is suggested that configurations with empty cages are reaction intermediates in hydrate formation when more guest molecules are available. Furthermore, free energy profiles show that methane acts as a catalyst reducing the barrier for β ice versus hexagonal/cubic ice formation.},
doi = {10.1021/acs.jpcb.5b06832},
journal = {Journal of Physical Chemistry. B, Condensed Matter, Materials, Surfaces, Interfaces and Biophysical Chemistry},
number = 52,
volume = 119,
place = {United States},
year = 2015,
month =
}

Journal Article:
Free Publicly Available Full Text
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