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

0.6 ps

Mechanism: Long Lived Fragments

Cluster GrowthSize of Biggest Cluster

0

20

40

60

80

100

120

140

160

0 0.2 0.4 0.6 0.8 1t / ns

nu

mb

er o

f h

ydra

te w

ater

s 2.75 A3.3 A3.5 A

• Large, rapid variations in cluster size

• Not dependent on clustering distance

Hydrate Inhibitors

• Thermodynamic e.g. methanol, glycols (salt)very large volumes (50% w/w water)

• KineticSince very late 1980’sAffect nucleation / growth – not thermodynamicssmall volumes (parts per thousand)lactam polymers (PVP, PVCap & copolymers)small is better (1–2 kDa)others include quaternary ammonium salts

Fish Anti-freeze Proteins

From Arctic Winter Flounder

Main ice growth face

(Lal et al., Faraday Discussions, 1993)

Relax

ca. 1 ns

Adding PVP

Water in Hydrate Environment

Fraction of Hydrate-Water

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0 10 20 30 40

time / ns

Control1

Control2

Inhib1

Inhib2

Control3

Methane-Methane RDFs

For methane within the water film

With PVP ; without PVP

Inhibitors & Nucleation?

Without PVP

With PVP

0.6 ns 10.5 ns 40.2 ns

10.8 ns5.4 ns0.9 ns

Hydrate Inhibitors in Motion

Inhibitors: Selected Conformations

• Typically no contact between inhibitor and hydrate cluster

• ca. 0.5 ns after adding inhibitor

• Bridging between inhibitor and hydrate cluster

Inhibitors: Selected Conformations

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

Acknowledgements

• ModellingChangman MoonMark StorrRob HawtinDorothy Duffy

• FundingEPSRCBritish CouncilICIRF Rogaland

• Computer TimeMaterials ConsortiumWarwick Centre for

Scientific Computing