A MOLECULAR-DYNAMICS STUDY OF SOLVENT AND IMPURITY INTERACTION ON THECRYSTAL HABIT SURFACES OF EPSILON-CAPROLACTAM

The influence of impurities on the morphology of epsilon-caprolactam c rystals was examined in terms of molecular recognition processes at th e crystal/solution interface. Summing the intermolecular interactions, using the atom-atom approximation together with the DREIDING force fi eld provided a convergence in the lattice energy summation with a good correlation to thermodynamic data. Attachment energy calculations usi ng the HABIT95 program predicted a growth morphology dominated by {200 }, {110}, and {11(1) over bar} crystal forms in good agreement with kn own experimental vapor growth data. Molecular dynamics studies of acet one, ethanol, toluene, water, and hexane solvent adsorption on the hab it faces predicted the effect of solvent on the crystal morphology and in particular the development of the {31(1) over bar} growth form. Su rface visualization modeling revealed the {31(1) over bar} surface to be much rougher, on the molecular scale, than the other major habit fa ces, consistent with easier impurity incorporation. Parallel studies o f the binding of the impurity cyclohexanone revealed significantly str ong binding on all habit faces, particularly on the solvent-dependent {31(1) over bar} face. Implications for attaining a high-purity produc t when crystallizing caprolactam in the presence of this impurity are discussed.