(318d) Modeling Impurity-Mediated Crystal Growth

Crystallization is commonly used in industrial processes to convert solute molecules dissolved in solvent to a structured solid state. Pharmaceutical companies often crystallize APIs in the form of organic molecules to selectively formulate specific crystal habits for optimal bioperformance. Crystal engineering is also of importance for developing catalysts with tailored surfaces to maximize active sites. Furthermore, tuning crystallization is desirable for varying electrical and optical properties in the field of electronic materials and for altering the impact sensitivity of energetic materials. Given the ubiquity of crystal growth in industrial processes, there is substantial demand for predictive and mechanistic modelling of crystallization. Crystallization of organic molecules is well understood for ideal systems (i.e., Kossel crystals with a single centrosymmetric growth unit). There is interest in studying crystal systems in which non-idealities are introduced, as these are more representative of realistic conditions. One such non-ideality involves the presence of impurities or ‘imposter molecules.’ There are numerous notable examples of crystal growth systems in which the presence of impurities significantly influences crystallization processes. Examples include “antifreeze” proteins and antimalarial medications. While these systems are prevalent and of importance in many fields, there is currently no comprehensive model that captures the full effect of impurities on crystal growth processes.

The goal of this research is to investigate the effect of impurities on the crystal growth process and to develop theoretical models for the mechanisms by which impurities influence crystal growth and hence affect crystal morphology and size. Impurities affect growth kinetics at the scale of kink attachment and detachment events, which are too fine to examine experimentally in real time. Thus, we use simulations to study the proposed mechanisms for growth inhibition. We employ Kinetic Monte Carlo (KMC) to simulate the time evolution of centrosymmetric organic crystal growth. Various mechanisms have been proposed to explain the growth-inhibiting effect of impurities, such as Cabrera and Vermilyea’s step-pinning model and Doherty and Sizemore’s spiral-pinning models. Model equations have been developed in accordance with these mechanisms to quantify the effect of inhibition on the crystal face. We use KMC simulations to examine the limits of these equations under different growth regimes and to elucidate the form of a more general model. The simulations serve to quantify the degree of growth inhibition of a simple crystal growth system populated with impurities. The results suggest a monotonic decrease of the step velocity with increasing impurity concentration, though the form of this dependence is based on the nature of the impurity distribution. Furthermore, the simulations suggest a critical impurity coverage at which crystal growth is halted completely.