Nucleation%20Mechanism

1
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 1980s
• Affect nucleation / growth not thermodynamics
• small volumes (parts per thousand)
• lactam polymers (PVP, PVCap copolymers)
• small is better (12 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
10.8 ns
5.4 ns
0.9 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
• Oswalds 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