Levich Institute Seminar Announcement, 03/22/2011
Steinman Hall, Room #312
(Chemical Engineering Conference Room)
Professor Michael Doherty
University of California, Santa Barbara
Department of Chemical Engineering
"A Stochastic Model for Morphology Evolution of Crystals in the Presence of Structurally Similar Additives"
Crystal growth is a surface-controlled phenomenon in which solute molecules are incorporated stereo-specifically into surface lattice sites in order to yield the bulk long range order that characterizes crystalline materials. Such surface processes are naturally highly susceptible to the presence of small concentrations of other surface active molecules. These may be deliberately added to a crystallization process or may be inherent as reaction by-products - either way their activity is based on their stereo-chemical similarity to the desired solute and they are known to play havoc with the crystallization of organic crystalline materials. For fifty years, crystal growth models of this effect have assumed that adsorbed immobile impurities decrease the perpendicular growth rate of a crystal face by reducing the velocity (rate of solute uptake) at an edge; specifically, the immobile impurities partition the edge into a collection of segments and the growth of those segments whose length is less than or equal to some critical length is arrested, thus decreasing the edge velocity. This is the step-pinning mechanism of Cabrera and Vermilyea. However, we show that under dilute imposter conditions this is not expected, and we demonstrate that the rate of solute uptake at an edge is, on average, unchanged. Rather, we argue the distances travelled by edges during the first turn of a growth spiral on a crystal face are increased, thereby decreasing the density of steps across the face and reducing the perpendicular growth rate of the crystal face. Our new model is able to successfully predict the growth rate reduction of specific faces of alpha-glycine crystals grown from aqueous solution in the presence of L-alanine impurity. The model will be described along with prospects for future validation and improvement to crystals of API complexity.
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