ABSTRACT

The study and engineering of crystal faces have attracted immense interest among artists and the crystal grower community since at least the Bronze Age [1]. Although signižcant efforts have been made over the last few decades to precisely predict the growth morphology of crystals, it still remains a challenging task to this date. Crystal growth morphology has diverse applications ranging from drug design [2] to explosives [3] and inverse gas chromatography data [4]. Therefore, knowledge of crystal growth habits and their morphological properties is important in understanding and exploiting many of their physicochemical properties. In this regard, the in§uence of additives on its crystal habit has received considerable attention. It has been found that the nucleation, growth, and morphology of crystals can be signižcantly altered by the presence of low concentrations of impurities such as reaction byproducts, impurities present in the reactants, and additives that are purposely added to

8.1 Introduction .................................................................................................. 109 8.2 Computational and Experimental Methods .................................................. 111

8.2.1 Computational Procedure ................................................................. 111 8.2.2 Experimental Procedure ................................................................... 112

8.2.2.1 Batch Crystallization ......................................................... 112 8.2.2.2 Instrumentation .................................................................. 113 8.2.2.3 Optical Microscopy............................................................ 113

8.3 Results and Discussion ................................................................................. 113 8.4 Conclusion .................................................................................................... 117 Acknowledgments .................................................................................................. 117 References .............................................................................................................. 117

alter the crystallization process [5,6]. Additives can reduce crystal growth rate and alter morphology by binding to crystal faces and interfering with propagation steps [7,8]. Rome de l’Isle showed that octahedrons are formed instead of normal cubes if rocksalt is grown in the presence of urine [9]. Many authors have since reported the cube-octahedron shape transition for various experimental conditions. Some early work reported that octahedron can also be obtained from pure water solution [10-12]. Radenović et al. have experimentally studied the habit change of sodium chloride from cubic to octahedron in the presence of smaller amides [13,14]. The observed results have been rationalized based on charge distributions and strong interactions between the more exposed carbonyl oxygen of amides such as formamide and urea with the sodium ions, which stabilize the {111} surface of sodium chloride and lead to the octahedron morphology of sodium chloride [13,14]. In earlier observations, Speidel and Bunn have also considered that the additive strongly interacts with certain crystal faces to in§uence the morphology of the crystal [15-17]. Bunn explained that the habit modižcation of NaCl by urea in aqueous solutions is due to adsorption of the impurity on certain crystal faces during crystal growth [17]. There were many proposals that have been reported to explain the habit of sodium chloride; however, studies toward the interactions at the molecular level are limited. We have recently performed a detailed computational analysis of urea interaction with the surfaces of sodium chloride [18]. The calculated results suggest that the interactions of additives with certain crystal faces are one of the important factors toward the change in morphology of alkali halides. A number of studies have been performed toward the habit of sodium chloride crystals in the presence of impurities; however, the higher homologue KCl has received little attention. Recently, there have been some efforts to crystallize KCl in carbon nanotubes [19]. The KCl crystals were grown in cubic form with predominantly stable {100} faces. It is mentioned in one of the reports that urea can act as an additive for KCl [6,20]. However, we have not come across any detailed experimental study of the effect of urea on KCl crystals. Our effort to understand the growth of alkali halide crystals prompted us to examine the effect of impurities on KCl crystals. Therefore, we have undertaken the computational approach to study the in§uence of urea on the morphology of KCl crystals followed by the experimental observations. The experimental results would be useful to examine the predictive nature of these computational analyses.