Modelling Impurity-Mediated Crystal Growth and Recent Deveopments in ADDICT3

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Author Last Name: 
Michael Doherty, Tobias Mazal, and Yongsheng Zhao
Report Type: 
Research Area: 
Particle Formation
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Publication Month: 
United States

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 speci c crystal habits for optimal bioperformance [1]. Crystal engineering is also of
importance for developing catalysts with tailored surfaces to maximize active sites [2]. Furthermore, tuning
crystallization is desirable for varying electrical and optical properties in the eld of electronic materials
such as OLEDs [3] and for altering the impact sensitivity of energetic materials such as RDX and HMX [4].
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 (simple cubic single molecules with
equal surfaces). 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.
The goal of this project is to investigate the e ect of impurities or `imposter molecules' on the crystal growth
process and to develop theoretical models for the mechanisms by which impurities in uence crystal growth
and hence a ect crystal morphology and size.
Impurities a ect growth kinetics at the scale of kink attachment and detachment events, which are too
ne to examine experimentally in real time. Thus, we use simulations to study the proposed mechanisms
for growth inhibition. We employ Kinetic Monte Carlo (KMC) methods to simulate the time evolution of
centrosymmetric organic crystal growth. Rare event rates are determined as functions of energetic barriers
for desolvation and attachment/detachment works. Various mechanisms have been proposed to explain
the growth-inhibiting e ect of impurities, such as step pinning and spiral pinning. These mechanisms are
considered in the context of KMC simulations for model development, and compared to experimental values
in the literature for validation. Once we have established e ective models, we will look to incorporate them
into ADDICT (Advanced Design and Development of Industrial Crystallization Technology), an engineering
tool that predicts relative growth rates and crystal morphology of solution-grown faceted crystals [5].