MC Results - Growth Rate
y/kT = 0.7 kJ mol-1 y/kT = 1.0 kJ mol-1
0.0 0.1 0.2 0.3 0.40.0000
0.0005
0.0010
0.0015
0.0020
0.0025
0.0030
0.0035
0.0040
0.0045
0.0050
0.0055
0.0060
0.0065
ndefx=2 : ndefy=2 : x/kT=5.0 :
y/kT=0.7 : sizey=2
0 defects sizex=10 sizex=15 sizex=20
Gro
wth
Rat
e
Dm/kT
0.0 0.1 0.2 0.3 0.40.0000
0.0005
0.0010
0.0015
0.0020
0.0025
0.0030
0.0035
0.0040
0.0045
0.0050
0.0055
0.0060
0.0065 ndefx=2 : ndefy=2 : x/kT=5.0 :
y/kT=1.0 : sizey=2
0 defects sizex=10 sizex=15 sizex=20
Gro
wth
Rat
eDm/kT
MC Results for Dm/kT = 0.2
0 impurities Length=10 units
Length = 15 units
Length = 20 units
MC results - summary
• Sudden change in slope at Dm/kT = 0.2 signals transition from step growth to island nucleation
glass ceiling for low dosage (kinetic) inhibitors?
• For x/kT = 0.7
(Dm/kT)* = 0.1 for defects of length = 10 units
(Dm/kT)* = 0.2 for defects of length = 15 and 20 units
• decreasing defect width has little effect on growth rate
• increasing x has little effect on growth rate
• increasing y reduces growth rate significantly
Gas Hydrates (Clathrate Hydrates)
• Two components open tetrahedral water lattice hydrophobic “guest”
molecules
• Three main crystal structures; probably two important for
environment & industry
• Each built from two water polyhedradiameter 8–9 Å
type I
type II
Clathrate Hydrates: Guests
• hydrophobic (natural gas)
• Acid Gases (H2S, CO2)
• cyclic ethers (DMO, TMO, THF)
• NOT ionic or strongly polar
• Size matches/determines structure
Hydrates: Phase Diagram
• Favoured by low temperatures and high pressures
• Typical of sub-sea pipelines, continental shoulders, tundra
Crystal Nucleation
• Activated process:Favourable “bulk” energyUnfavourable interfacial energyCritical “cluster” size
• Classical Nucleation Theory
• Fundamentally StochasticLong and random induction times (~ days)
FavourableUnfavourable
-20
-10
0
10
0 5 10r
delta-G
3 244
3G r r m D D
Nucleation in Other Aqueous Systems
• Matsumoto, Saito & Ohmine, Nature 2002
• MD
• Mild subcooling (ca 10 K)
• Multiple ms trajectories
• Critical nucleus built around on long-lived (1-2 ns) hydrogen bonds
Nature of the nucleus
• Non-spherical & not compact for ice
(from Matsumo et al, Nature 2002)
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