Prediction of the Crystal Morphology of β‑HMX using a Generalized Interfacial Structure Analysis Model

At sufficiently low supersaturations such that the spiral growth mechanism dominates, β-cyclotetramethylenetetranitramine (HMX) grows from acetone into a polyhedron surrounded mainly by the (020) and (011) faces. In order to elucidate the morphology, a generalized form of the interfacial structure analysis model is suggested. In this method, the molecular order parameters of crystals are defined to identify the orientation and conformation of the adsorbed growth unit at the interface. This presents a robust method to calculate the orientational and conformational free energy surfaces that are utilized for the spiral growth model of centrosymmetric growth units with polygonal spiral edges. From the metadynamics simulation using these order parameters as collective variables, the free energy surfaces with respect to the collective variables revealed that high conformational free energy of the chair conformation discouraged preordering of the growth units into crystal-like orientation and conformation. The resulting morphology was consistent with the previous experimental and theoretical results, indicating that the anisotropic local concentrations of the growth units at the interface play a critical role in the different relative growth rates of the slow-growing faces

Interfacial Structure Analysis for the Morphology Prediction of Adipic Acid Crystals from Aqueous Solution

Adipic acid crystals grown from aqueous solutions have a hexagonal plate morphology with a dominant (100) face, where the hydrogen-bonding carboxylic acid groups are exposed. In the present work, the crystal morphology was investigated by interfacial structure analysis to obtain the relative growth rates for the spiral growth model. The concentration of effective growth units at the interface was found to be the key external habit-controlling factor by molecular dynamics simulations at the crystal–solution interface. The differences between the experimentally observed faces of (002), (100), and (011) and unobserved faces of (111̅), (102̅), and (202̅) were explained by two concepts from the interfacial structure analysis that determine the concentration of the effective growth units. The observed faces were characterized by larger values of both the surface scaling factor and molecular orientation factor, implying low anisotropic local concentrations at the interface and high free energy barriers for reorientation on these faces, respectively. Furthermore, the number of turns and the length of one complete spiral rotation and the number of unsaturated bonds were incorporated into the original approach. This consideration of the spiral geometry resulted in a close resemblance to the experimental morphology