MC Results - Growth Rate  y /kT = 0.7 kJ mol -1  y /kT = 1.0 kJ mol -1

MC Results for  /kT = impurities Length=10 units Length = 15 units Length = 20 units

MC results - summary Sudden change in slope at  /kT = 0.2 signals transition from step growth to island nucleation Þ glass ceiling for low dosage (kinetic) inhibitors? For  x /kT = 0.7  (  /kT)* = 0.1 for defects of length = 10 units  (  /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 polyhedra ¯diameter 8–9 Å type I type II

Clathrate Hydrates: Guests hydrophobic (natural gas) Acid Gases (H 2 S, CO 2 ) 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” energy ¯Unfavourable interfacial energy ¯Critical “cluster” size Classical Nucleation Theory Fundamentally Stochastic ¯Long and random induction times (~ days) Favourable Unfavourable

Nucleation in Other Aqueous Systems Matsumoto, Saito & Ohmine, Nature 2002 MD Mild subcooling (ca 10 K) Multiple  s 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)