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MC Results - Growth Rate

MC Results - Growth Rate. f y /kT = 0.7 kJ mol -1. f y /kT = 1.0 kJ mol -1. MC Results for Dm /kT = 0.2. 0 impurities. Length=10 units. Length = 20 units. Length = 15 units. MC results - summary.

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MC Results - Growth Rate

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  1. MC Results - Growth Rate fy/kT = 0.7 kJ mol-1 fy/kT = 1.0 kJ mol-1

  2. MC Results for Dm/kT = 0.2 0 impurities Length=10 units Length = 20 units Length = 15 units

  3. 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 fx/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 fx has little effect on growth rate • increasing fy reduces growth rate significantly

  4. 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 II type I

  5. Clathrate Hydrates: Guests • hydrophobic (natural gas) • Acid Gases (H2S, CO2) • cyclic ethers (DMO, TMO, THF) • NOT ionic or strongly polar • Size matches/determines structure

  6. Typical of sub-sea pipelines, continental shoulders, tundra Hydrates: Phase Diagram • Favoured by low temperatures and high pressures

  7. Favourable Unfavourable Crystal Nucleation • Activated process: • Favourable “bulk” energy • Unfavourable interfacial energy • Critical “cluster” size • Classical Nucleation Theory • Fundamentally Stochastic • Long and random induction times (~ days)

  8. 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

  9. Nature of the nucleus • Non-spherical & not compact for ice (from Matsumo et al, Nature 2002)

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