Morphology and Associated Surface Chemistry of l-Isoleucine Crystals Modeled under the Influence of l-Leucine Additive Molecules

Molecular modeling techniques using both the empirical atom–atom and ab initio quantum mechanical methods are used to simulate the morphology of l-isoleucine. The lattice energy calculated by using a selected potential function and an atomic charge set is in excellent agreement with experimental data where the percentage difference between the calculated and experimental lattice energies is less than 5%, hence confirming the suitability of the potential functions and methods chosen to calculate the partial atomic charges. Calculation of the atom–atom interactions also shows that the energy contribution to the lattice energy is dominated by the interatomic interactions between the carbonyl oxygens and the amino hydrogens, consistent with the large calculated electrostatic contribution to the lattice energy. The simulated crystal morphology shows an elongated hexagonal platelike morphology with dominant crystal facets of (001) and (001̅) together with minor (100), (1̅00), (110), (1̅1̅0), (1̅10), (11̅0), (011̅), and (01̅1) faces. Experimental studies of the closely related amino acid l-leucine as an additive tol-isoleucine reveal that the addition of l-leucine alters l-isoleucine morphology, forming a more isometric hexagonal shape crystal by reducing the growth along the b-axis of the l-isoleucine crystal. This observation is supported by modeling through assessment of binding of l-leucine on preferential sites of the crystal habit surfaces of l-isoleucine where additive binding is found to be most preferred on the (100), (1̅00), (110), and (011̅) facets.