Roger J. Davey

Nucleation of organic crystals - A molecular perspective

The outcome of synthetic procedures for crystalline organic materials strongly depends on the first steps along the molecular self-assembly pathway, a process we know as crystal nucleation. New experimental techniques and computational methodologies have spurred significant interest in understanding the detailed molecular mechanisms by which nuclei form and develop into macroscopic crystals. Although classical nucleation theory (CNT) has served well in describing the kinetics of the processes involved, new proposed nucleation mechanisms are additionally concerned with the evolution of structure and the competing nature of crystallization in polymorphic systems. In this Review, we explore the extent to which CNT and nucleation rate measurements can yield molecular-scale information on this process and summarize current knowledge relating to molecular self-assembly in nucleating systems.

The morphology of α-Al2O3 and α-Fe2O3: The importance of surface relaxation

Current methods for predicting crystal morphology from structure ignore the fact that real crystal surfaces are not necessarily exact terminations of the bulk structure. Instead, surfaces relax and in so doing minimise the total surface energy. This lowering of the surface energy and its influence on crystal morphology are here calculated and illustrated for α-Al2O3 (corundum) and α-Fe2O3 (haematite). The results are compared with previous studies based on Hartman-Perdok theory.

Crystal engineering - nucleation, the key step

In the world of crystal engineering in which the focus of effort is on the assembly of molecular species by crystallisation, surprisingly little attention has been paid to the actual nucleation and crystallisation processes involved. This Highlight explores the structural aspects of the nucleation process in a range of small molecule systems. It uses a combination of thermodynamic, structural and modelling approaches in order to progress our understanding of the link between liquid phase molecular assemblies, which constitute crystal growth units, and their solid state counterparts, the supramolecular synthons.

Crystal chemistry and solvent effects in polymorphic systems: Sulfathiazole

Sulfathiazole, a compound that forms four known crystal structures, has been examined with a view to understanding its polymorphism. A graph set approach was used to classify the structural differences and similarities of the polymorphs, the results of which indicated packing motifs common to three of the four structures. By combining this analysis with experimental morphological data, it has been possible to examine the origins of the observed solvent dependence of polymorph appearance in this system. In particular, the possible link between the observed hydrogen-bond motifs of each form and the associated processes of nucleation and crystal growth from n-propanol, nitromethane, ethanol, water and ammonia solution, have been considered.

Interactions at the organic/inorganic interface: binding motifs for phosphonates at the surface of barite crystals

Synthetic barite crystals, prepared under conditions chosen to mimic those encountered in off-shore oil fields, have a simple rhombic plate-like morphology in which large (001) faces are bound by smaller (210) sides. The addition of certain stereochemically related diphosphonates produces a characteristic morphological change leading to disc and elliptical morphologies. These results are shown to be consistent with a binding model in which the diphosphonate ion replaces two sulphates in the (011) surface. The implications of these results to the case of gypsum precipitation is also considered.

Oriented crystallization of CaCo3 under compressed monolayers. Part 1.—Morphological studies of mature crystals

The crystallization of CaCO3 under compressed Langmuir monolayers of stearic acid [CH3(CH2)16CO2H], octadecylamine [CH3(CH2)17NH2], octadecanol [CH3(CH2)17OH] and cholesterol (C27H45OH) has been studied over a range of supersaturation conditions by optical and scanning electron microscopy and X-ray diffraction. At total [Ca]= 9 mmol dm–3, negatively charged stearate monolayes induced the oriented nucleation of calcite. The crystals were of two related morphological types, one of which was formed through the physical realignment of the oriented crystals at the organic surface during growth. Reducing the total [Ca] to 4.5 mmol dm–3, resulted in oriented vaterite nucleation on the stearate monolayers. Vaterite was the major product on positively charged octadecylamine films, independent of [Ca]. Unlike the vaterite crystals of stearate films, these crystals were of two distinct morphological forms which represented two specific nucleation orientations. Neutral monolayers of octadecanol inhibited crystallization and those of cholesterol gave random non-oriented calcite deposition analogous to the control experiments. Morphological analyses indicated that the crystals were oriented with their [1text-decoration:overline1.0](calcite) and [00.1] and [11.0](vaterite) axes perpendicular to the monolayer surface.

Disappearing polymorphs and the role of reaction by-products: the case of sulphathiazole

Abstract A combined modelling and experimental strategy has been applied to the problem of reaction by-product influence on the appearance of sulphathaizole polymorphs from aqueous solution. From pure aqueous solution after 24 h the most stable phase, form IV was isolated. This work shows for the first time that a reaction by-product, ethamidosulphathiazole, from the final hydrolysis stage at concentrations as low as 1 mol.% stabilises the metastable modification, form I. In the presence of 1.0–0.5 mol.% forms II and III are stabilised. Only at concentration below 0.5 mol.% does the transformation proceed to form IV as in the pure solution. The role of the impurity was accounted for from an analysis of the respective hydrogen bond networks and crystal morphologies of each phase.

Calculated bulk and surface properties of sulfates

Atomistic simulation techniques are used to model a range of sulfates. Two widely different sets of potentials have been developed. The first is based on shell model, electron-gas potentials; the second is a rigid ion model which treats inter- and intra-molecular forces differently. The success of the potential models has been demonstrated by comparing calculated and experimental lattice parameters and elastic constants. The structures and energetics of surfaces of barite (BaSO4) are examined in detail, allowing for the effects of surface relaxation. The two lowest-energy surfaces are {001} and {210} which dominate the calculated equilibrium morphology.

The utility of a ternary phase diagram in the discovery of new co-crystal forms

The construction of the ternary phase diagram urea–glutaric acid–water has led to the discovery of a new co-crystal form.

Theoretical analysis of the polar morphology and absolute polarity of crystalline urea

Changes to the molecular polarisation of urea associated with its crystallisation are considered through ab initio quantum-mechanical calculations starting with the isolated molecule with increasing complexity up to a full 3D calculation using periodic boundary conditions. The calculations accurately reveal the electrostatic nature of the intermolecular forces associated with the hydrogen-bonding network in the solid state. The resulting charge densities are used together with the force field of Lifson, Hagler and Dauber (J. Am. Chem. Soc., 1979, 101, 5111) in an attachment energy calculation to predict the polar morphology of the material. The resultant simulations are in good agreement with the morphology of crystals prepared from the vapour phase and are used to absolutely assign the polar forms as {1 1 1}. Habit modification effects associated with re-crystallisation from polar solvents are also discussed from a structural perspective.