(252f) A New Model for Capturing the Effect of Supersaturation On Crystal Shape

Shape can impact the downstream processing efficiency and end-use efficacy of crystalline products. It is known that some crystalline materials exhibit notably different shapes when grown from solutions with only small differences in supersaturation (e.g., paracetamol, which changes its shape from a rod to a nearly equant shape [1] over a range of supersaturations from 10 ? 15%). In light of these observations, we have developed a predictive model that captures the effects of supersaturation on the steady-state shape of a growing crystal. This model is mechanistic in nature and requires only the crystal structure and solubility parameter data as inputs.

Previous predictive models for crystal shape have assumed that all F faces (faces with 2 or more periodic bond chains, further defined in Lovette et al. [2]) grow by a spiral mechanism limiting their application to growth at low supersaturation. Our new model captures the effects of supersaturation on growth shape by accounting for growth by both spiral and two-dimensional (2D) nucleation mechanisms; evaluating the applicability of each on a face-by-face basis over a given range of supersaturation. For each face the growth model developed by Snyder and Doherty [3] was applied over the supersaturation range where spiral growth was determined to be the dominant mechanism. At higher supersaturations, a new model for 2D nucleation, developed in agreement with the bond structure of the face, was applied. The implementation of this composite growth model is tested through a case study of naphthalene grown in ethanol, for which the model yields predictions that are in good agreement with the experimentally obtained shapes.

References

[1] Ristic, R.; Finnie, S.; Sheen, D. & Sherwood, J. Macro- and Micromorphology of Monoclinic Paracetamol Grown from Pure Aqueous Solution, J. Phys. Chem. B, 2001, 105, 9057.

[2] Lovette, M. A.; Browning, A. R.; Griffin, D. W.; Sizemore, J. P.; Snyder, R. C. & Doherty, M. F. Crystal Shape Engineering, IECR, 2008, 47, 9812.

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