(20b) Spiral Growth Model for Organic Crystals of Real Complexity: How to Account for Unstable Spiral Edges

Crystal morphology influences the end-use efficacy of solid products as well as the downstream performance of the entire manufacturing and formulation process. The desired crystal size and shape distributions is a function of the final application, hence a general methodology for the prediction and improvement of crystal morphology is one of the major goals in the field of crystallization. The classical Burton-Cabrera-Frank (BCF) spiral growth model fails to work satisfactorily for many non-centrosymmetric organic molecules such as APIs, nonlinear optical compounds, etc., due to the inherent assumption of the Kossel crystal structure in the classical model. These complex molecules warrant a more rigorous treatment for successful prediction of crystal shapes. We develop a general mechanistic spiral growth model that can be used to predict crystal morphology of any organic molecule, centrosymmetric as well as non-centrosymmetric.

This work discusses in detail the stable and unstable periodic bond chain (PBC) edges that arise due to complex asymmetric bonding patterns. Modified step velocity expressions are derived that account for unstable edges. The entire model is successfully applied to industrially relevant systems like acetaminophen and lovastatin and the results are discussed with experimental data from the literature and from our laboratory.

References

Kuvadia, Z and Doherty M. F. 2011. Spiral Growth Model for Faceted Crystals of Non-Centrosymmetric Organic Molecules Grown from solution.  Crystal Growth and Design

Burton,W.K., Cabrera, N. & Frank, F.C. 1951. The growth of crystals and the equilibrium structure of their surfaces. Phil. Trans. R. Soc. A 243, 299-358

Snyder, R. C. & Doherty, M. F. 2009. Predicting crystal growth by spiral motion. Proceedings of the Royal Society A: Mathematical, Physical and Engineering Science 465, 1145-1171