Polymorph selective crystallization using organized organic assemblies as templates

The size, crystalline phase (polymorphism) and morphology of particles greatly affect the chemical and physical properties of particulate materials. In many applications, such as agrochemical, pharmaceutical and material industries, it is very important to obtain a desired crystalline phase which may exist as several polymorphs. In this research, we have used organized organic assemblies as templates to control the polymorphism of crystals. Compared to the traditional crystallization methods, this approach utilizes the potential of controlling the molecular recognition events which may exist between the incipient nuclei and organized organic assemblies. By using Langmuir monolayers at the air/liquid interfaces and Self-Assembled Monolayers (SAMs) at the solid/liquid interfaces as templates, we have successfully controlled the polymorphism of CaCO ₃ crystals. Vaterite crystals, which are the thermodynamically unfavored from homogeneous crystallization, have been nucleated by carefully choosing the functional groups of organic assemblies.; Several Langmuir monolayers and SAMs with various properties have been investigated as templates for polymorph selective crystallization of CaCO ₃. Our experimental results have clearly shown that the electrostatic interaction and stereochemical correspondence between incipient nuclei and organized organic assemblies are the predominant factors in determining the specific nucleation face and the polymorphism of the crystals, while exact geometric matching is not necessary for template directed nucleation of calcium carbonate.; In situ ATR-FTIR was used to investigate the interactions between the templates and the moieties of crystals. The calcium ion binding to the stearic acid headgroups was evident from the increase of carboxylate asymmetric stretching band intensity. Atomic force microscopy (AFM) was used to investigate the morphology development and crystal growth of nuclei on the solid surface at the early stage of template crystallization. The results of AFM showed that nucleation only happened at the very beginning of the process. The frequency of nuclei on the functionalized surfaces is extremely high at the early stage of nucleation. However, only a very small portion of these nuclei (one out of 0.96 × 105) survived and grew into micron-sized particle during crystal growth process.