Clathrates,%20Clusters%20and%20Crystals

1
Clathrates, Clusters and Crystals

• P.M. Rodger
• Department of Chemsitry

2
Crystal Modifiers

• Growth Morphology Control
• Biomineralisation complete control of
  morphology, polymorph size (e.g. using
  polysaccharides)
• Inhibition of crystallisation
• Suppression of nucleation
• Reduction in growth rate
• Polycrystalline suspension

3
Molecular Sculpturing

• Form and kinetics depends on
• Subtle changes in molecular composition
• Subtle changes in already dilute concentrations
• Need to understand mechanism in molecular detail
  to know how to formulate coarse-graining hierarchy

4
Waxes

• Polycrystalline soft solids
• Lamellar structure found in n-alkanes persists in
  waxes
• Growth rates controlled by (110) and (010)
  surfaces
• Growth is defect-driven
• Low dosage inhibitors
• Typically comb-like polymers
• Activity relates to surface adsorption
• wide range of effects
• many small crystals in suspension
• suppression of initial formation
• soft, easily removed deposits

(010)
(110)
(100)
Top view of the (001) surface cleavage planes
for other surfaces are shown by the arrows
5
Experimental result
Additive Concentration (ppm)

• R. Kern and R. Dassonville J.Cryst. Growth
  116 (1992) 191
• Crystallized C26 and C36 from heptane solution
  with varying concentrations of polyalkylacrylate
• High degree of polymerization acted as growth
  promoter
• Low polymerization (mlt9) acted as growth
  inhibitors
• Found solid solution for C26 and phase
  separation for C36.
• Notable reduction in crystal size

0
1
4
5
100
6
Wax Inhibitors Strategy

• Simulate in series of steps of increasing
  complexity
• wax growth in vacuum
• wax growth with inhibitor
• wax growth with inhibitor and oil
• Identify key factors that determine activity
• Develop coarse-grained simulations to encompass
  these factors

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Wax Inhibitors key factors

• Match to surface is affected by size of polymer
• Inhibitor targets growth surface only for octamer
  or larger

(001) Favoured for dimer, but strained in octamer
(110) Surface favoured for oligomers

• Subsequent growth is incommensurate with wax
• Shear defects remove lamelar structure

Four alkane layers grown on an inhibited (110)
surface
8
Model for inhibited wax crystal growth
Top View
Wax
Wax
Inhibitor
Side View
9
MC for crystal growth

• Gilmer and Bennema (1972)
• transition probabilities for addition P and
  subtraction P-
• P n exp(Dm/kT) P-n
  exp(2-i)2f/kT i0,1,2,3,4
• 2f is the bond strength between 2 growth units
• n material-related frequency i is number of
  neighbours

10
MC for growth inhibition

• van Enckevort and van der Berg (1998)
• arrays of immobile impurities
• no addition or subtraction at impurity sites
• No bonds to adjacent growth units
• Modification for anisotropic crystal
• P- n exp((1-ix)2fx(1-iy)2fy)/kT
  ix, iy 0,1,2
• fx and fy are bond strengths in x and y
  directions
• Parameterise from MD of solvated islands