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Title: Effects of impurities on crystal growth processes
Keywords: crystal habit, polymorphism, impurities, additives, molecular modeling, AFM
Issue Date: 9-Jun-2008
Citation: SENDHIL KUMAR POORNACHARY (2008-06-09). Effects of impurities on crystal growth processes. ScholarBank@NUS Repository.
Abstract: In industrial crystallization systems, impurities that have many of the structural and chemical characteristics of the solute molecule are often present in small quantity in solutions from upstream processes. These impurities are found to have serious and unpredictable impact on crystal attributes such as habit, polymorphs, size, and purity, thus leading to inconsistent batch-to-batch performance. Hence, the objective of this work is to understand the mechanisms underlying the role of impurities on crystal growth process, and consequently, develop a systematic approach to predict impurity effects on crystal habit and polymorphism. In this study, glycine, a simple amino acid, was used as the primary solute. The higher homologous amino acids were added in trace amounts to glycine solutions in order to simulate the presence of impurities. It was found that the chosen impurities (aspartic and glutamic acids) resulted in habit modification along the b-axis and c-axis of alpha-glycine crystals depending on the solution pH conditions. On the basis of the fact that these two amino acids exist in two charged states (zwitterions and anions) and building on the b stereoselectivityb mechanism, it was surmised that the zwitterions interact with the (010) facets and the anions with the (011) facets of alpha-glycine crystal. Consequently, the adsorbed impurity molecules inhibit crystal growth by disrupting the incorporation of solute molecules onto the surface. Towards rationalizing these observations, molecular modeling techniques were used to quantify the interaction of impurity species at the crystal surfaces using atomistic potential energy calculations. Supporting this, in situ observation using an atomic force microscope provided a molecular-scale picture of the physical processes taking place during crystal growth in an impurity-doped solution environment. Furthermore, a systematic approach was proposed to select amino acid impurities that can operate as stereospecific nucleation inhibitors, and in doing so affect the polymorph formation of glycine crystals.
Appears in Collections:Ph.D Theses (Open)

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