Simon Black

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.

Stable polymorphs: difficult to make and difficult to predict

Because α-glycine grows 500 times faster than γ-glycine, the small thermodynamic preference for γ-glycine is difficult to predict, detect and exploit.

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.

17 salts of ephedrine: crystal structures and packing analysis

The structures of two neutral and 17 salt forms of the base (1R, 2S)-(-)-ephedrine are reported. These structures are discussed in the light of the conformers of the ephedrine moiety, the existence of bilayers and the structure determining role of the counterions. Overall, most of the salt structures are essentially derived from the observed packing of the neutral base and are dominated by the amphiphilic nature of the ephedrine molecular structure. In a few cases the size and hydrophobicity of the counterion disrupts this behaviour.

Morphology and polymorphism in molecular crystals: terephthalic acid

The phase characterisation and morphologies of forms I and II of terephthalic acid are presented. The structural nature of the II → I polymorphic phase transition has been explored and form II shown to be more stable at room temperature and pressure. A new mechanism is proposed for the apparent stability of multiply twinned samples of form I.

Morphology and polymorphism of terephthalic acid

Abstract The phase characterisation and morphologies of forms I and II terephthalic acid are presented. The structural nature of the II ↔ I polymorphic phase transition has been explored and form II shown to be more stable at room temperature and pressure. A new mechanism is proposed for the apparent stability of multiply twinned slamples of form I.

Simulating nucleation of molecular solids

The activation free energy for nucleation of crystals from melts and solutions is described and simulated in a different way from traditional nucleation theory. For molecules at the surface of a nucleus, the unfavourable entropy of fusion outweighs the favourable enthalpy change. The free energy barrier to nucleation in pure melts is simulated from the entropy and enthalpy of fusion. By incorporating the unfavourable entropy of unmixing, nucleation from impure melts and solutions is also simulated. The nature of the metastable zone alters and its size is estimated.

Crystallisation in polymer films: control of morphology and kinetics of an organic dye in a polysilicone matrix

The successful selection, design and synthesis of additives for the control of nucleation and crystallisation of a red anthraquinone based dye is described. This procedure is based on the crystal structure and morphology of solution grown crystals and in this study the possibility of using the same approach to control crystallisation within an amorphous polymeric matrix is explored. The work has been motivated by the desire to prepare stable dye sheets for thermal transfer printing applications, yet the parallel with natural biomineralisation processes is clear.

The influence of melt composition on the crystal morphology of m-chloronitrobenzene

The morphology of m-chloronitrobenzene (mCNB) crystals grown from pure and binary melts with its ortho and para isomers was observed. By increasing the isomer concentration the morphology was found to change from prismatic to plate-like to needle-like for the mCNB-oCNB system. For the mCNB-pCNB system lenticular crystals, changing in shape to petal-like through a paper-knife appearance, in which one end of the crystal is rounded and the other facetted, were observed. The cause of these changes is discussed and attributed to probable occupation of specific sites on the growing surface by isomer molecules.

Solvent and additive interactions as determinants in the nucleation pathway: general discussion

Sarah Price opened a general discussion of the paper by Sven Schroeder: I have been generating the thermodynamically plausible crystal structures of organic molecules for many years, and back in 2004 we did a crystal structure prediction (CSP) study on imidazole1 and found that it was relatively straightforward. Following your paper, we have reclassified the low energy structures according to the tilt within the hydrogen-bonded chain and the relative direction of the chains. Although the observed structure was the global minimum, two other structures with a displacement of otherwise identical layers are very close in energy. Do you think that if imidazole had crystallised in one of these alternative structures it would be distinguishable by NEXAFS? This would be a very sensitive test of whether NEXAFS combined with CSP could be used in characterising crystal structures.