DOI: 10.1021/cg1010727
2010, Vol. 10 4203–4205
Virtual Special Issue on British Association for Crystal Growth 40th Anniversary Conference 2009
2009 marked the 40th anniversary of the British Association for Crystal Growth (BACG), which was celebrated at an anniversary conference, held at Wills Hall, Bristol in September 2009. Historically, the British Association for Crystal Growth (BACG) was one of the first national associations for Crystal Growth, holding its Foundation Meeting in 1969. With Charles Frank as the founding president, the first annual conference was held in Bristol in 1970. Following the Association’s rapid growth in the 1970s and 1980s and its subsequent restructuring in the 1990s,1 the BACG has continued to be a focus for the crystal growth community in the U.K. Drawing together researchers from seemingly wide-ranging disciplines including industrial crystallization, semiconductors, pharmaceuticals, mineralogy, food science, and biomineralization, the BACG has always sought to provide the community with an opportunity for sharing a common interest in crystal growth. The series of articles published in the virtual special issue (http://pubs.acs.org/page/cgdefu/vi/3) of Crystal Growth & Design reflect the wide range of subject areas discussed at the 2009 anniversary conference. Highlighted as the cover art of the virtual special issue, Weaver et al.2 describe a study of the inhibition of the step-growth of calcium oxalate monohydrate (COM) crystals by an aspartic-acid rich peptide. COM is the principal inorganic component of human kidney stones, whose formation is inhibited in vivo by acidic urinary molecules including osteopontin, a complex protein. Through use of an aspartic-acid rich peptide which mimics some of the key structural features of osteopontin, this work provides insight into the mechanism by which the formation of COM is inhibited within the body. In situ atomic force microscopy (AFM) was used to investigate the influence of the selected peptide on COM growth, and it was shown that the magnitude of the inhibition of step growth depends on the exposure time of the terraces to the peptide solution. These results were interpreted by developing a time-dependent step-pinning model in which the average impurity spacing depends on the terrace lifetime. The model enables the characteristic peptide adsorption time to be determined and also predicts that bistable growth dynamics will occur when there is a crossover in the time scales for impurity adsorption and terrace exposure. This effect was observed experimentally with a sharp transition from a freely growing to an inhibited state upon decrease of the supersaturation. Observation of such an effect relies on an additive displaying slow absorption dynamics, as would be expected for the macromolecular additives, which are widely considered to act to control biomineralization processes. Such molecules would therefore appear to be uniquely suited to producing significant changes in growth rates over small ranges of supersaturation and additive concentrations in biomineral systems. Continuing with the theme of inorganic crystals, the synthesis of powders of bismuth orthosilicate (Bi4Si3O12) is described
by Marina et al.3 Bismuth orthosilicate (eulytite) has applications as scintillators in high-energy physics, computer tomography, and dosimetry, but precipitation from the melt offers many challenges. In this article, hydrothermal methods are described to produce fine precipitates of bismuth orthosilicate from fluoride solutions at temperatures of 260 °C and pressures of 500 bar. Selection of the solution composition and reaction time yielded powders of bismuth orthosilicate with particle sizes ranging from a few tens up to a few hundreds of micrometers. In their article, de Menezes et al.4 investigate the strain in two InGaP/GaAs (001) epitaxial structures with different thicknesses. Ternary alloys of In1-xGaxP are being explored for use in electronic and optical devices, and there is considerable interest in the alloy In0.49Ga0.51P, as it can be grown epitaxially on a GaAs substrate. Strain in such heteostructures induces changes in the band structure and thus in the optical and electronic properties. Examination of the strain in In0.49Ga0.51P/GaAs (001) structures was performed using a synchrotron X-ray multiple diffraction scan (XRMD), which revealed layer tetragonal distortion in these epitaxial structures. The thickness, composition, and perpendicular and parallel lattice parameters of the two epitaxial ternary layers were obtained which enabled the determination of the perpendicular and in-plane strains of the layers in the samples. This article therefore demonstrates the application of the XRMD technique to the structural analysis of semiconductor heterostructures. Our final article focusing on inorganic crystals is provided by Di Tommaso and de Leeuw,5 who describe simulations of the properties of hydrated alkali earth metal ions and solvated metal carbonates. The data presented show differences in the coordination behavior and rate of water exchange of complexed Mg2þ, Ca2þ, and Sr2þ ions and also suggest that the hydration structure and water dynamics of the first and second hydration shell of these metals are influenced by the presence of simple anions such as fluoride, chloride, and bromide. Knowledge of the solvation structure and dynamical behavior of ions is essential to the development of an understanding of the nucleation and growth of minerals, and these results suggest that the reactivity of metals ions can be affected by the presence of foreign species due to changes in both the complexation and solvation environments of the metal ions. The other articles in this virtual special issue describe studies of the crystallization of organic compounds. The first two papers examine the use of physical techniques to study batch cooling crystallization processes. Batch cooling crystallization is a key step in the production of pharmaceuticals and fine and specialty chemicals and determines the quality of the final product, which is dictated by features such as the crystal size, morphology, polymorphic composition, and purity. Control of the product quality rests on careful control of the crystallization
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process, which in turn depends on knowledge of the solution supersaturation and thus concentrations. Kadam et al.6 compare the use of attenuated total reflectance (ATR) Fourier transform infrared (FTIR) in the mid-IR (MIR) region and Fourier transform (FT) near infrared (NIR) spectroscopy as a means for monitoring the batch cooling crystallization of four systems: R-lactose monohydrate in water, ammonium sulfate in water, ibuprofen in hexane, and L-glutamic acid in water. ATRFTIR was shown to perform better statistically and in the presence of crystals than FT-NIR, which suffered a problem with fouling of the probe when crystals were present. These data therefore suggest that ATR-FTIR is better suited to in situ crystallization measurements than FT-NIR. Bakar et al.7 in turn investigate the effect of temperature cycling on the surface structures of crystals of sulfathiazole precipitated during seeded batch cooling crystallization, using focused beam reflectance measurement (FBRM) and ex situ optical microscopy, scanning electron microscopy (SEM), and atomic force microscopy (AFM) to study the surfaces of the crystals. Temperature cycling, where alternate cycles of heating and cooling are applied, is used to control features such as the crystal size distribution and to produce seeds for further cycles. It can also control the crystal surface properties, leading to smoother crystals with superior flow properties. Examination of the crystals using FBRM revealed that the crystal surfaces became smoother on heating, while roughening, which is associated with the appearance of surface features, occurred during cooling. The sensitivity of this technique to such nanoscale surface features was attributed to a combination of beam-spreading resulting from changes in the surface structure and signal/chord splitting due to the formation of sharp edges. This study therefore demonstrates that focused beam reflectance measurements can be used to study the surfaces of crystals and, thus potentially, crystal growth mechanisms. The following articles consider the important topic of polymorphism, considering the problem from a range of angles. Considering first the formation and characterization of novel polymorphs, Seton et al.8 investigate the solid state forms of the compound theophylline and present a new anhydrous polymorph, termed Form IV, which was shown to be the most thermodynamically stable anhydrous polymorph. Studying the hydration behavior of theophylline showed that the monohydrate form of theophylline becomes thermodynamically stable at water activities of a g 0.7 at 25 °C. An et al.,9 in contrast, obtain polymorphic control over the active pharmaceutical compound adefovir dipivoxil (AD) by precipitating it from an ionic liquid environment. On precipitating AD from the ionic liquid 1-ally-3-ethylimidazolium tetrafluoroborate (AEImBF4), new anhydrous and hemihydrate crystals of AD were obtained according to the ionic liquid fraction present and the crystallization temperature. It is well-recognized that polymorphism can be controlled according to the solvent-solute molecule interactions. Exploitation of ionic liquids, which can be expected to induce unique intermolecular interactions, may therefore provide a powerful route to accessing novel crystal polymorphs. The production of new solvates provides the focus for the articles by Klimakow et al.10 and Bernardes et al.11 In such compounds, the solvent molecules can be entrapped in channels or voids within the lattice, or they can interact specifically with the solute molecules. Inclusion of solvent molecules within the crystal lattice is anticipated to induce changes in the structure and physical properties of a compound and, as such, provides a basis for manipulation of properties. The
Meldrum and Ristic
compound nifedipine, which is used in medical treatments as a dihydropyridine calcium antagonist, is studied by Klimakow et al.10 Crystallization under slow-evaporation conditions was monitored using synchrotron-XRD single-crystal X-ray structural analysis and Raman spectroscopy. While rapid evaporation gave rise to the thermodynamically most stable R-polymorph of nifedipine, slow evaporation from DMSO yields a novel pseudopolymorph as a 1:1 solvate. Only one solvate of nifedipine, formed as a 2:1 compound of nifedipine to dioxane solvate, has been previously reported. A new hydrate of 40 -hydroxyacetophenone (HAP) with a H2O/ HAP molar ratio of 1.5 is described by Bernardes et al.11 Structural determination by single crystal X-ray diffraction showed that the water molecules line up in chains that reside in lattice channels (channel hydrate) and are sustained by hydrogen bonds. Thermochemical characterization of the dehydration of this solvate was consistent with the observations that HAP 3 1.5H2O is stable at ambient laboratory conditions but will tend to decompose above ≈331 K. This article therefore provides a useful insight into how solvate formation can be used to control the structure/property relationships of suitable compounds. The mechanisms by which polymorphic transformations occur are studied in the next two articles. Moynihan12 describes a study of the characterization and transformations of polymorphs of 2-iodo-4-nitroaniline, in which a new monoclinic polymorph of this compound is identified. Full characterizations of the three crystal polymorphs of 2-iodo-4nitroaniline were carried out using XRD, DSC, and IR, and all were structurally distinct using these techniques. The solvent-mediated transformations of the orthorhombic polymorph and mixtures of polymorphs were also studied by monitoring changes in crystal morphologies and data from an in situ laser probe. Transformation to the monoclinic form was observed in all cases, illustrating the thermodynamic stability of this phase. Continuing with the theme of polymorph recrystallization, the final article in this virtual special issue is by Croker et al.,13 who provide a review of the current understanding of polymorphic phase transitions that are mediated by a solvent, addressing in particular the role of surfaces in mediating these processes. Polymorphic transformations are initiated by nucleation of the second phase, an event which tends to be mediated by the surface of the parent phase. A number of published examples of this phenomenon are discussed in this review article, which rather than giving an exhaustive description of the field, is intended to provide a starting-point for further research in the field. The articles presented in this virtual special issue of Crystal Growth & Design therefore provide a flavor of the 40th anniversary meeting of the BACG. Together they serve to illustrate the vibrant range of topics brought together under the subject of crystal growth, and they show how the discipline has developed since the early days of the BACG, when the focus of the association was on the U.K. semiconductor industry and its associated technologies. The subsequent growth in interest in pharmaceuticals can be attributed to changes in the working practices of the large pharmaceutical companies, and it is expected that the BACG will continue to evolve to reflect topical developments in crystal growth in both academia and industry. Finally, we would like to thank all of the authors and reviewers who have contributed to the quality of the virtual special issue. We also wish to express our thanks to Mihaela Rogers, Coordinating Editor of Crystal Growth & Design, for
Crystal Growth & Design, Vol. 10, No. 10, 2010
Editorial
all the help that she has provided to bring this virtual special issue to its final form.
References (1) Cockayne, B.; Hurle, D. T.; Roberts, K. J.; Bambrook, T. A History of the British Association for Crystal Growth 1969-2009; Leeds University Press: 2009; ISBN 978 0 85316 284 1. (2) Weaver, M. L.; Qiu, S. R.; Friddle, R. W.; Casey, W. H.; De Yoreo, J. J. Cryst. Growth Des. 2010, 10, 2954–2959. (3) Marina, E.; Marin, A.; Balitsky, V. In preparation. (4) de Menezes, A. S.; dos Santos, A. O.; Almeida, J. M. A.; Bortoleto, J. R. R.; Cotta, M. A.; Morelh~ao, S. L.; Cardoso, L. P. Cryst. Growth Des. 2010, 10, 3436–3441. (5) Di Tommaso, D.; de Leeuw, N. H. Cryst. Growth Des., http://dx.doi. org/10.1021/cg100055p. (6) Kadam, S. S.; van der Windt, E.; Daudey, P. J.; Kramer, H. J. M. Cryst. Growth Des. 2010, 10, 2629–2640. (7) Bakar, M. R. A.; Nagy, Z. K.; Rielly, C. D. Cryst. Growth Des. 2010, 10, 3892–3900. (8) Seton, L.; Khamar, D.; Bradshaw, I. J.; Hutcheon, G. A. Cryst. Growth Des. 2010, 10, 3879–3886.
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(9) An, J.-H.; Kim, J.-M.; Chang, S.-M.; Kim, W.-S. Cryst. Growth Des. 2010, 10, 3044–3050. (10) Klimakow, M.; Rademann, K.; Emmerling, F. Cryst. Growth Des. 2010, 10, 2693–2698. (11) Bernardes, C. E. S.; Piedade, M. F. M.; da Piedade, M. E. M. Cryst. Growth Des. 2010, 10, 3070–3076. (12) Kelly, D. M.; Eccles, K.; Elcoate, C.; Lawrence, S. E.; Moynihan, H. A. In preparation. (13) Croker, D.; Hodnett, B. K. Cryst. Growth Des. 2010, 10, 2806– 2816.
Fiona C. Meldrum* Guest Editor University of Leeds, U.K. Radoljub I. Ristic Guest Editor University of Sheffield, U.K.
