Toward a Fundamental Understanding of Molecular Recognition:  A Synthetic and Computational Study of Morphological Control of Ca₃Al₂(OH)₁₂

Abstract

Molecular modeling has been used to predict both the structure and morphology of Ca3Al2(OH)12. The calculated structure is in excellent agreement with the experimentally determined one with a less than 1% decrease in the lattice parameter. The morphology of Ca3Al2(OH)12 was predicted to be the 24-sided deltoidal icositetrahedron formed from the {112} face, and while the experimental morphology of Ca3Al2(OH)12 is generally poorly defined, the smaller crystals in the sample did display this morphology. This experimental morphology of Ca3Al2(OH)12 has been shown to be strongly dependent on the presence of crystal growth modifier anions in the reaction solution with each additive studied favoring a specific crystal face to the exclusion of all others. It was found that sulfate stabilizes the {111} face, leading to the formation of crystals with an octahedral morphology, oxalate favors the {110} face, forming rhombic dodecahedral crystals, and EDTA interacts preferentially with the {210} face, generating crystals with a tetrakis hexahedral morphology. These experimental observations have been explained by molecular modeling, which demonstrated that the lowest replacement energy for each of the additive anions was for the experimentally observed crystal face.

Computing Shapes of Nanocrystals from X-ray Diffraction Data

“…equilibrium crystal shapes and Wulff's construction) or the relative rates of growth of different faces. For example, the growth of copper nanocrystals as cubes, 24 the perovskite BaZrO 3 25 as rhombic dodecahedrons, and the three different shapes of tricalcium aluminate 26 were traced, respectively, to chloride ion adsorption on (100) faces, solvent-induced polarity variation, and the presence of the three growth modifiers, sulfate, oxalate, or EDTA, each one having a strong affinity to a different face of the crystal.…”

Layered Double Hydroxides Precursor as Chloride Inhibitor: Synthesis, Characterization, Assessment of Chloride Adsorption Performance

“…Figure 4 presents the calculated structures of CaAl LDH precursor and CaAl-Cl LDH visualized by Material studio. The cubic structure of CaAl LDH precursor was presented as the 24-side deltoidal icositetrahedron from {010} faces [28]. Furthermore, hydroxyl groups is served as a part of Al(OH) 4 − groups in the bulk structures.…”

“…However, with the adsorption of chloride ions by CaAl LDH precursor, it can be noticed that the atomic structure of CaAl-Cl LDH was shown as the layered crystals with octahedral structure as the skeleton and chloride ions as the interlayered anion from {110} faces. The transformation of defined morphology to newly formed layered polyhedral structure is ascribed to as the molecular recognition between the crystal surfaces and the adsorption anions [28]. Taking the above analysis into account, the adsorption mechanism during the chloride ions adsorption process can be ascribed to the co-precipitation interacted between Cl − and the hydrolysis product of CaAl LDH precursors.…”

Empirical molecular modelling of crystal growth modifiers

“…This has the drawback that it does not investigate the possible interaction of the protonated nitrogen with the surface but is a worthwhile first approximation. The potentials and parameters for these molecules were the same as those used by Wilson (15) and Fogg et al, (16). Briefly, these potentials were derived from CVFF in InsightII (17), whilst the charges were obtained from Spartan (18) via fitting to the electrostatic potential from a PM3 (19) optimisation run.…”