A molecular dynamics study of solvent and impurity interaction on the crystal habit surfaces of ε-caprolactam

The influence of impurities on the morphology of epsilon-caprolactam crystals was examined in terms of molecular recognition processes at the crystal/solution interface. Summing the intermolecular interactions, using the atom-atom approximation together with the DREIDING force field provided a convergence in the lattice energy summation with a good correlation to thermodynamic data. Attachment energy calculations using the HABIT95 program predicted a growth morphology dominated by {200}, {110}, and {11(1) over bar} crystal forms in good agreement with known experimental vapor growth data. Molecular dynamics studies of acetone, ethanol, toluene, water, and hexane solvent adsorption on the habit faces predicted the effect of solvent on the crystal morphology and in particular the development of the {31(1) over bar} growth form. Surface visualization modeling revealed the {31(1) over bar} surface to be much rougher, on the molecular scale, than the other major habit faces, consistent with easier impurity incorporation. Parallel studies of the binding of the impurity cyclohexanone revealed significantly strong binding on all habit faces, particularly on the solvent-dependent {31(1) over bar} face. Implications for attaining a high-purity product when crystallizing caprolactam in the presence of this impurity are discussed.