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