Clathrates, Clusters and Crystals
P.M. Rodger
Department of Chemsitry
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
Molecular Sculpturing
• Form and kinetics depends onSubtle changes in molecular compositionSubtle changes in already dilute concentrations
• Need to understand mechanism in molecular detail to know how to formulate coarse-graining hierarchy
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 inhibitorsTypically comb-like polymersActivity relates to surface adsorption wide range of effects
– many small crystals in suspension– suppression of initial formation–soft, easily removed deposits
Top view of the (001) surface; cleavage planes for other surfaces are shown by the arrows
(100)
(010)
(110)
Experimental result
• 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 (m<9) acted as growth inhibitors
• Found solid solution for C26 and phase separation for C36.
• Notable reduction in crystal size
Additive Concentration
(ppm)
0
1
4
5
100
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
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
Model for inhibited wax crystal growth
Wax Wax Inhibitor
Top View
Side View
MC for crystal growth
• Gilmer and Bennema (1972)transition probabilities for addition P+ and
subtraction P-
P+ =exp(/kT) P-=exp[(2-i)2/kT] ; i=0,1,2,3,4
2 is the bond strength between 2 growth units material-related frequency; i is number of
neighbours
MC for growth inhibition
• van Enckevort and van der Berg (1998)arrays of immobile impuritiesno addition or subtraction at impurity sitesNo “bonds” to adjacent growth units
• Modification for anisotropic crystalP- = exp[((1-ix)2x+(1-iy)2y)/kT] ; ix,
iy = 0,1,2x and y are “bond” strengths in x and y
directionsParameterise from MD of solvated islands