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

Abstract

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.

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

“…As the desolvation corresponds to the breaking of the solvation shell on the solute, it is independent of the identity of the face, and therefore considered isotropic. The original interfacial structure analysis and further studies ,− , assumed that the value of this desolvation barrier is close to the thermal fluctuation, k B T , but this could be significantly different from the actual value. If the actual value is much larger than the assumed value as suggested by Vekilov, where the expected values are approximately 28 kJ/mol for the enthalpy part and near zero for the entropy part, the effect of reorientation barrier could be negligible.…”

“…The reason is that kinks are the only sites for the attachment and detachment of the growth units that (1) maintain the surface free energy of the edge as constant and (2) create a new kink site upon incorporation . For solution growth of small organic molecules with layered growth mechanisms, ignoring the surface diffusion and assuming the kink incorporation as the rate-determining step whose barrier corresponds to that of the desolvation of the solute molecule have become a consensus in mechanistic models. − …”

“…This desolvation barrier is usually assumed isotropic for all the faces. In the approaches referred to as the interfacial structure analysis model, ,,− two additional concepts were addressed: (1) the local concentration of the growth units at the interface and (2) the barrier related to the reorientation of the growth units at the interface, introduced as “the orientational and conformational entropy barrier” . Before desolvation for incorporation into a kink, the adsorbed growth units are assumed to go through an reorientation process with a free energy barrier between its current orientational/conformational state (called an F2-unit state) and the crystal-like orientational/conformational state (called an F1-unit state). , A generalized interfacial structure analysis model for the Kossel-like crystal systems of centrosymmetric growth units, , based on the kink rate expressions by Shim and Koo, is presented here.…”

“…Subsequently, the relative growth rate can be calculated by collecting the anisotropic factors of G hkl . where n hkl is the number of nearest neighbors in the two-dimensional lattice and α̅ hkl is the angle between the regular spiral edges. As for the values of a̅ p, hkl and a̅ e, hkl , the previously used assumptions were a̅ e, hkl ≈ a̅ p, hkl ≈ a = ( v m ) 1/3 where v m is the molecular volume. ,,− In the present work, 1 and √3/2 were assigned as the values of a̅ p, hkl / a̅ e, hkl for the cases of square and regular hexagon, respectively to account for the different polygonal spirals. Canceling of the terms results in the final expression of The average kink energy is obtained from the experimental dissolution enthalpy and other habit-controlling factors through the surface scaling factor C l( hkl ) * and molecular orientational factor t hkl , The surface anisotropy factor ξ hkl = E hkl slice / E latt and n hkl are calculated from the PBC analysis.…”

Morphology Prediction Methods and Their Applications in Energetic Crystals

“…Santiso et al proposed a method for constructing order parameters to study molecular crystallization and polymorphic transformations . Seo et al introduced this method into a growth mechanistic model, defined the molecular order parameter of a crystal to determine the orientation and conformation of attachment growth units at interfaces, and proposed an interface structure analysis model. , This model takes into account the local concentration of growth units at the interface and the energy barrier of their reorientation as the orientational and conformational entropy barrier. One characteristic of the interface structure analysis model is that it can distinguish between crystallike (ordered) and solutionlike (disordered) states of the crystal by defining molecular order parameters.…”