Crystals grown from solutions

Fig. 3.15. Growth rates of (110) and (200) sectors of polyethylene single crystals grown from solution in tetradecanol (courtesy of S. Organ)...

A major weakness in the calculations described above is that they can only be used to represent vapor grown crystals. In crystals grown from solution, the solvent can greatly influence the crystal habit as can small amounts of impurities. Several investigators (68. 69 accounted for discrepancies between observed crystal habit and those obtained using attachment energies by assuming preferential solvent (or impurity) adsorption on crystal faces. 

Fig. 4 a The structure of [Co (rt -C5H4COO)2] [NH4]-3H20 as obtained from single crystals grown from solution, b The calculated powder diffraction pattern matches the one measured on the powder material recovered from the solid-gas reaction... 

The most commonly encountered distribution of dislocations in crystals grown from solution or vapor phase (dilute ambient phases) by natural nucleation and without seed may be observed as dislocation bundles starting from the center of a crystal and running nearly perpendicular to the habit faces. In addition to these dislocations, smaller dislocation bundles originating from inclusions may be observed. See Figs. 6.1(e) and 6.5 for examples. 

Crystals grown from solutions containing Mg, Ca or Zn also form hexagonal platelets similar to the undoped samples, but crystals containing Mg grow slightly more slowly in directions perpendicular to the c-axis and faster into c-directions, resulting in relatively smaller, but thicker, platelets. 

The equilibrium shape of a crystal is that of its minimum energy. This is called the Wulff condition and indicates that the area of faces present will be such as to minimize the Gibbs free energy of the crystal. Unfortunately, the observed habit of crystals grown from solutions is often quite different from the prediction by the Wulff condition. 

Based on the thermodynamics of solid and liquid solutions, a fairly general expression has been derived by Rosenberger and Riveros (1974) for determining impurity segregation in crystals grown from solution... 

There has been a series of systematic studies of the effect of impurities and tailor-made additives on the shape of crystals grown from solution (Addadi et al. 1982 Berkovitch-Yellin 1985 Weissbuch et al. 1999.) They used amino acids and amino acid derivatives as model solutes. By combining habit observations with crystal structure and molecular orientation, they were able to rationalize a number of effects, including resolution of steroisomers. Hopefully, further studies in this area will provide a rational basis for the many impurity and additive effects found during crystallization of biochemicals. 

Morphology of Polymer Single Crystals Grown from Solution... 

Sato, K. (1988) Observations of spatial correlation between growth spirals and inclusions in stearic acid crystals grown from solution. Japanese Journal of Applied Physics, 19, 1257 1264 and 1829-1836. 

Polyethylene Crystals Grown from Solution in o-Xylene... 

Following their nucleation, crystals grown from solution typically exhibit regular, planar facets characterised by their Miller indices. Although appearing flat to the naked eye, these crystalline surfaces are rarely so at the molecular level. The various features which make up the nanoscale surface topography of crystal faces are intimately involved in the mechanisms by which crystals grow [48]. 

Careful examination of single crystals grown from solution at 70 °C in xylene indicated that they are lozenge shaped and have surfaces that are 110 planes. 

Whilst much of the initial research has focused on the structures generated from crystals grown from solutions of 0.01-0.001 wt%, it is the solid-phase morphology that is the more challenging to understand. A quantitative discussion of crystallization from dilute solution is attempted in Chapter 6 as an introduction, the factors that influence crystallization will be considered. 

Fig. 7. Scanning electron microscopy images of L glutamic acid crystals (scale bar=200pm) (A) crystal morphology of crystals grown on the surface of L-AAPP SAMs and (B) crystal morphology of crystals grown from solution. (Dressier and Mastai, 2007)...

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