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Nucleation Mechanism

0.6 ps. Nucleation Mechanism. Most common model: Methane solvation shell is dodecahedral hydrate cage Nucleation occurs by aggregation of dodecahedral water cages Other models based on long-range ordering of methane molecules. Mechanism: Long Lived Fragments. Cluster Growth.

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Nucleation Mechanism

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  1. 0.6 ps Nucleation Mechanism • Most common model: • Methane solvation shell is dodecahedral hydrate cage • Nucleation occurs by aggregation of dodecahedral water cages • Other models based on long-range ordering of methane molecules

  2. Mechanism: Long Lived Fragments

  3. Cluster Growth • Large, rapid variations in cluster size • Not dependent on clustering distance

  4. Hydrate Inhibitors • Thermodynamic • e.g. methanol, glycols (salt) • very large volumes (50% w/w water) • Kinetic • Since very late 1980’s • Affect nucleation / growth – not thermodynamics • small volumes (parts per thousand) • lactam polymers (PVP, PVCap & copolymers) • small is better (1–2 kDa) • others include quaternary ammonium salts

  5. Fish Anti-freeze Proteins (Lal et al., Faraday Discussions, 1993) From Arctic Winter Flounder Main ice growth face

  6. Adding PVP Relax ca. 1 ns

  7. Water in Hydrate Environment

  8. Methane-Methane RDFs With PVP ; without PVP For methane within the water film

  9. Inhibitors & Nucleation? Without PVP 0.6 ns 10.5 ns 40.2 ns With PVP 0.9 ns 5.4 ns 10.8 ns

  10. Hydrate Inhibitors in Motion

  11. Inhibitors: Selected Conformations • Typically no contact between inhibitor and hydrate cluster • ca. 0.5 ns after adding inhibitor

  12. Inhibitors: Selected Conformations • Bridging between inhibitor and hydrate cluster

  13. Conclusions • Molecular Simulations can provide important information to inform coarse-graining • Parameterise models • Identify mechanisms to inform model-formulation • Identify regimes for different mechanisms – e.g. effect of different sub-coolings on crystal growth/additives • Oswald’s step rule – which unstable polymorphs never restructure? • Nature of the match between additive and crystal

  14. Acknowledgements • Modelling • Changman Moon • Mark Storr • Rob Hawtin • Dorothy Duffy • Funding • EPSRC • British Council • ICI • RF Rogaland • Computer Time • Materials Consortium • Warwick Centre for Scientific Computing

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