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Methane hydrate: interfacial nucleation

Methane hydrate: interfacial nucleation. Crystal. Melted under vacuum (300 K), then pressurised under methane (30 atm). Potential Energy (rolling average over 10 ps) (n.b. should divide by 1654 to quote per mole of water. Density profile across interfaces I = 0–0.3 ns II = 9–10 ns.

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Methane hydrate: interfacial nucleation

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  1. Methane hydrate: interfacial nucleation Crystal Melted under vacuum (300 K), then pressurised under methane (30 atm)

  2. Potential Energy (rolling average over 10 ps) (n.b. should divide by 1654 to quote per mole of water Density profile across interfaces I = 0–0.3 ns II = 9–10 ns Time Evolution

  3. Hydrate Formation: Analysis upper half of water film (0 – 20 Å) lower half of water film(-20 – 0 Å) Methane-Methane radial distribution functions, g(r)

  4. Order parameters: 3-body • Fluctuations from tetrahedral network • Average over all triplets, based on central oxygen and “bonding” radius

  5. Order parameters: “4-body” • Locate a three H-bond chain • Calculate torsion angle and triple product from “bond” vectors • Mimic by two-molecules • Average over coordination shell

  6. Local Phase of Water Molecules • Define local order parameters that distinguish between bulk phases • Determine standard deviations, , within stable bulk phases (hydrate/ice) • Assign individual molecule as hydrate/ice if all its order parameters agree with bulk values to within 2  H-bond network angles H-bond network torsions

  7. Order parameters & melting • Analysis of melting crystal shows order parameters are consistent • Analysis of covariance matrix (bulk) shows they are independent

  8. Characterising Molecular Order • Define vector of three order parameters (f) • Calculate covariance matrix for each molecule (C–1) for stable phases • Eigenvalue analysis to de-correlate (y)

  9. Local Phase Assignment • Calculate f for each molecule in arbitrary system • Project onto eigenvectors (components of y) • Compare with : assign “local phase” if all three components within 2(?) standard deviation of  for that phase

  10. Water in Hydrate Environment

  11. Distribution of order parameters 1 ns Difference: 22 ns - 1 ns

  12. Animated Nucleation

  13. Simulated Nucleation [ hydrate-waters ) 0.6ns 1.5ns 2.4ns 3.3ns 4.2ns 5.1ns 6.0ns 6.9ns 7.8ns 10.5ns 20ns 40ns

  14. Which hydrate structure? type I type II • Best signature is arrangement of dodecahedra

  15. Which hydrate structure? • Early appearance of face-sharing dodecahedra  type II • Oswald’s step rule: form the unstable polymorph first • Experimental verification: time resolved X-ray powder study (Kuhs, 2002)

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