Crystallization of polar organic materials from solution

The ability of a chemical compound to crystallize in a variety of shapes has sparked the imagination of scientists for many years. Particularly since the external shape of a crystal is associated with some of the most important characteristics of the crystalline product, including chemical stability, solubility, processing characteristics and more. Models for predicting crystal shape use geometrical rules and energy calculations of the interactions between the building blocks of the crystal, but often ignore the environment in which the crystal grows. Systems that contain polar molecules tend to be influenced by the environment, since polar molecules interact with polar solvents, and this interaction influences the final shape. Three of the most prominent models for predicting crystal shape: the Bravais-Friedel-Donnay-Harker (BFDH) model, the equilibrium model, and the attachment energy model, have been explored in this thesis. Two major kinetic theories: two dimensional nucleation and screw dislocation have been explored as well. The latter, also known as the BCF growth mechanism, serves as a basis for a proposed model to predict the shape of polar organic crystals grown from solution. The model accounts for the solute-solvent interactions at the interface by using face specific and solvent specific properties. Polar solute-polar solvent interactions were incorporated using a method developed by Fowkes. A description of the size and the nature of the intermolecular interactions along the different growth directions is a prerequisite for application of the model. Using this model, the shapes of urea grown from polar solvents was successfully predicted. In an effort to extend the functionality of this model, amino acids were selected as model systems. The selection of a reliable force field, the description of charge distribution and the nature of the growth unit were found to be primary factors in modeling the interactions between highly polar organic molecules. The method proposed to determine these factors involves quantitative measures, as the comparison of the calculated lattice energy to the sublimation enthalpy, and heuristic rules which are based on experience. The shapes of α-glycie and L-alanine grown from aqueous solution were successfully predicted and compared with experimental results

Controlling and predicting crystal shapes: The case of urea

Understanding crystal growth from solution is crucial to control the evolution of crystal morphologies. Experiments, molecular simulations, and theory were combined to examine the morphology of urea crystals grown in different solutions. To get a rational representation of all the possible habits a shape diagram (see picture) is introduced in which the habit dependence on the relative growth rates is illustrated