G. Clydesdale's research while affiliated with University of Leeds and other places

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The crystal morphology of adipic acid (AA) and its mediation by the action of the homologous additives, caproic acid, glutaric acid, and succinic acid, has been investigated via computational molecular modeling-based simulation techniques (Clydesdale, G.; Docherty, R.; Roberts, K. J. HABIT95, Quantum Chemistry Program Exchange (QCPE), Bloomington, IN 47405, Program Number 670, 1996). With the aid of intermolecular and interatomic energetic analyses, the experimental effect of these impurities has been rationalized. The predicted morphologies are in good agreement with sublimation-grown experimental data. This is not so for solution growth data, reflecting the possible adsorption of water molecules onto exposed carboxylic acid groups via hydrogen bonding on the {100} faces, thus allowing this form to become dominant. Modeling the adsorption of the additives reveals preferential adsorption onto the {302̄} faces, consistent with good additive/host templating at the growth interface, as demonstrated experimentally. Intermolecular bonding interactions examined those that disappeared in the presence of the additives. It was shown that impurity incorporation disrupted the hydrogen-bonding network within the system, due to an increase in interaction distances and in atom−atom repulsions. Despite its industrial significance, this is the first detailed study of the intermolecular interactions involved in impurity incorporation within adipic acid crystallization, and so any conclusions concerning the effect of impurities within the crystallizing mixture will be most useful.

A molecular modelling capability for predicting the purity and morphology of solids manufactured via crystallisation is important in the realisation of a ‘molecule-up’ or product-centred, approach for the optimisation of chemical processes. Recent work on producing the morphology of molecular solids based upon the molecular and crystal chemistry is presented. The approach is based on the atom-atom method for calculating surface attachment energies, and from these relative crystal growth rates as a function of crystallographic growth direction. The computations have been recently [22. Clydesdale , G. , Hammond , R. B. , & Roberts , K. J. ( 2003 ). Molecular modeling of bulk impurity segregation and impurity-mediated crystal habit modification of naphthalene in the presence of heteroimpurity species . J. Phys. Chem. B , 107 , 4826 – 4833 . [CROSSREF] View all references 13-15View all references] developed to be able to treat the effects of impurity species that influence morphology. Molecular modelling procedures include an optimisation of the position and orientation of the adsorbed additive within the host lattice hence, simulating the crystal chemistry of the hetero-species within the host lattice. Case studies include naphthalene and phenanthrene in the presence of biphenyl impurity and caprolactam in the presence of synthesis impurities.

A study of impurity incorporation into host crystal surfaces, using molecular modeling techniques, is presented for c-caprolactam, illustrating a rapid method for predicting the effects of additives on the purity and shape of particles formed through crystallization. Optimum positions, in terms of calculated lattice energy, were located for the impurity molecules within the host crystal lattice. Differential binding energies and modified attachment energies were calculated using the program HABIT95 (Clydesdale, G.; Roberts, K. J; Docherty, R. Quantum Chemistry Program Exchange 1996, 16, 1) for each impurity molecule for all the important growth forms. From the differential binding energies, values were calculated for the equilibrium segregation coefficients. The impact of the impurities cyclohexane, cyclohexanol, cyclohexanone, and the imino-tautomeric form of epsilon-caprolactarn (or caprolactim) was considered in the study. Excellent agreement was found between calculated equilibrium segregation coefficients for the impurity cyclohexanone in the {110} and {11 (1) over bar} forms of c-caprolactam and reported experimental values for crystals grown from the melt in the presence of a wide range of cyclohexanone impurity concentrations, 0.1-30 mol %, for supersaturations of the melt, 2 x 10(-3) to 6 x 10(-3) (van den Berg, E. P. G.; Bogels, G.; Arkenbout, G. J. J. Cryst. Growth 1998, 191, 169-177). It was calculated that the incorporation of cyclohexanol into c-caprolactam would be most significant for the {101} form and that the extent of partitioning of cyclohexane into the most important growth forms is 10-100 times smaller than those in the cases of cyclohexanol and cyclohexanone. The effect of impurity incorporation on the habit of c-caprolactam crystals was calculated from the modified attachment energies.

The ability to predict particle morphology (shape) in the presence of habit-modifying impurities is of great use in allowing the optimization of growth conditions to produce a required crystal habit. Previous morphological modeling techniques developed by the authors1,2 utilized the attachment energy method, which assumes knowledge of the solid-state structure, to simulate the shape of organic particulate solids while keeping the position of the impurity in the lattice fixed. These techniques are improved upon here by allowing the orientation of the adsorbing impurity molecule to be relaxed with respect to the intermolecular forces of the crystal bulk using a restricted molecular mechanics approach. This results in far more accurate attachment energiesand hence morphological simulationsthan could be achieved before. The calculations also allow improved binding energies for additives in different crystal faces to be ascertained; these can be used as a measure of the equilibrium impurity segregation coefficient for liquid/solid solidification. The method is illustrated by considering the morphological impact of a variety of host/additive systems, including naphthalene doped by biphenyl and phenanthrene crystallizing in the presence of biphenyl or anthracene. A forward look to the development of this approach for more complex systems is also provided.

The development of structural models for predicting the external morphology of crystalline materials is presented and discussed in terms of their applications to molecular crystals. The predicted crystal morphologies of a number of molecular materials including alpha -sulphur, naphthalene, benzoic acid and hexamine are presented using the Bravais-Friedel-Donnay-Harker, attachment energy and Ising models. The results of the various models are compared both against each other and against the experimentally observed morphologies.

A morphological prediction of the polar crystal morphology of the molecular ionic solid sodium chlorate is presented. This prediction uses interatomic potential calculations that employ surface-specific attachment energy calculations associated with an ab initio calculation of surface charges via a Hartree-Fock calculation using periodic boundary conditions. The data predicts assignment of the absolute polarity of the crystal with respect to the published crystal structure (Burke-Laing, M. E.; Trueblood, K. N. Acta Crystallogr. 1977, B33, 2698), which reveals the chlorate-rich {-1 -1 -1} to be the observed form rather than its sodium-rich Freidel opposite, {111}. The predicted crystal morphology is in reasonable agreement with observed morphologies, although there is an underestimation of the dominant {200} form. The latter is rationalized with experimental data in terms of a face-specific solvent binding model.

Molecular modeling techniques using attachment energy calculations have been applied, for the first time to our knowledge, to simulate the morphology of an organic hydrate: alpha-lactose monohydrate. Calculation of the strong intermolecular forces using the atom-atom approximation and the potential parameters of Nemethy et al. (Némethy, G.; Pottle, M. S.; Scheraga, H. A. J. Phys. Chem. 1983, 87, 1883-1887) reveals the crystallization to be dominated by intermolecular interactions between lactose molecules rather than lactose-water interactions, suggesting that water of hydration plays a space-filling role in the growth process. The simulated crystal shows a tomahawk-like morphology with the polar effect of the monoclinic space group P21 correctly reproduced by the occurrence of only one of the [010] forms. Confrontation of these results with experimental work of our own, involving crystals precipitated from aqueous solutions at various degrees of undercooling examined by scanning electron microscopy, and that of Visser and Bennema (Visser, R. A.; Bennema, P. Neth. Milk Dairy J. 1983, 37,109-137), who assigned faces to a crystal, gives good agreement, suggesting the suitability of the force field and atom-atom approaches to model the crystallization of organic hydrates.

Differential thermal analysis is used to examine the kinetics of nucleation associated with the melt phase crystallization of n-alkanes in the homologous series C13H28 to C32H66. Crystallization studies from stagnant melt samples reveal a direct correlation between structural type and nucleation behavior, notably demonstrating an alternating behavior between the even and odd carbon number homologues which decreases in extent as a function of increasing chain length. This effect is mirrored in calculations of the lattice energies based on the crystallographic structures with greater lattice stability being found for the even carbon number n-alkanes which crystallize in the triclinic crystal structure. The data are consistent with a heterogeneous nucleation mechanism associated with preferential nucleation at the free surface of the sample associated with surface freezing.

Using the internal structure as a starting point, the computer program HABIT95 uses the atom-atom approximation to determine the intermolecular interactions in a molecular crystal. Summing all the atom-atom interactions yields the lattice energy. Calculating the interactions along specific crystallographic directions allows the slice and attachment energies to be calculated. The attachment energy is a measure of the relative growth rate along specific directions and consequently the growth morphology can be modelled. The effects of solvents and impurities are considered by calculating modified attachment energy terms leading to simulated habit-modified morphologies. The program is written in standard Fortran 77 and is available for Unix-based platforms.