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Clathrates, Clusters and Crystals

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

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Clathrates, Clusters and Crystals

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  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 (m<9) 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

  7. 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+ =nexp(Dm/kT) P-=nexp[(2-i)2f/kT] ; i=0,1,2,3,4 • 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

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