Morphology, Polymorphism, and Co-Crystallization of Molecular Compounds

Abstract

In classical terms, crystallization from solution can be considered a three-step mechanisms: nucleation, growth, and growth cessation, 1 as schematized in Fig. 5.1. These steps are governed by supersaturation, and the way in which each of them develops, is associated with several macroscopic crystal properties. 2 When spontaneous crystallization starts, the process can follow several pathways that inevitably result in the formation of crystals of various sizes, size distribution, shapes, purity, and structure (polymorphism); these pathways depend on the relative rates of the nucleation and the growth stages. In general, the rates of nucleation and growth increase with supersaturation at a given temperature. At high supersaturation, nucleation is the dominant mechanism, showing proportionality with the extension of the crystalline population. Furthermore, in the nucleation stage, molecular orientation and aggregation mechanisms are intimately associated with the early selection of relative organization of growth units in the crystalline lattice. Therefore, crystal density and polymorphism are features related directly to the nucleation stage.Once nuclei are formed, they can evolve into macroscopic crystals by one of the following methods: keeping their structural properties, dissolving to give rise to more thermodynamically stable aggregates, or experiencing phase change due to internal reorganizations or to liquid-mediated transitions. At this stage, 151 Membrane-Assisted Crystallization Technology Downloaded from www.worldscientific.com by NATIONAL UNIVERSITY OF SINGAPORE on 10/09/15. For personal use only.152 Membrane-Assisted Crystallization Technology Nuclei (Might or might not dissolve)