Growth Morphology of a-Glycine Crystals in Solutions: An Extended Interface Structure Analysis

Abstract

Understanding the molecular mechanisms of crystal growth is an essential step towards controlling crystal growth, morphology and shape, which are of prime interest and importance in chemical and pharmaceutical industries. Since crystals interact with their surroundings predominantly through their surfaces, the shape of a crystal influences their behavior and chemical and physical properties. Although the environment in which the crystals are grown has a strong influence on the crystal habits, the role played by solvent/crystal interface in crystal growth is not completely resolved. This research seeks to provide a systematic method for studying and incorporating the effects of solvents on crystal morphology, and to probe the effects of solvents as well as mixtures of solvents on the morphology of organic crystals grown in solutions. We choose glycine for examination as it has a simple molecular structure, is a basic building block for proteins and has prochiral property and since glycine crystallization has been studied extensively experimentally. The methodology we have chosen is an extended interface structure analysis (EISA) that includes the full orientational characterization of the interfacial molecules. The morphologies of glycine crystals in aqueous solutions and in methanol/water mixture are then predicted using a three-stage procedure. Our method is able to predict the growth morphology consistent with experimental observations; e.g., a bipyramidical crystal shape with a less well-developed (010) surface in aqueous solutions and plate-like with well-developed (010) surface in a 1:1 methanol/water (by volume), are predicted and are supported by available experimental observations. The proposed method presents a sufficiently rigorous and systematic molecular approach to examine and predict the effects of solvent environments on crystal shape and morphology for the first time. The approach presented thus paves the way for exploring the effects of other solvents and impurities on the kinetics and the morphology of crystal growth.