Molecular modeling of bulk impurity segregation and impurity-mediated crystal habit modification of naphthalene and phenanthrene in the presence of heteroimpurity species

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 authors(1,2) 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 energies-and hence morphological simulations-than 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.