(140d) Conditional And Absolute Formation Of Crystals With Needle Morhpology

Crystals which possess needle morphologies (aspect ratios >50:1:1) are typically undesirable from a solids processing perspective. They tend to have poor flow and separation characteristics, and often have very low bulk density (a volume fraction of ~5% solids gives these systems the consistency of cotton candy). These characteristics present great difficulties to process engineers and scientists, especially when the formation of needles is not well-anticipated. Examples of needles include crystals of beta-glycine, aspartame, and caffeine. Crystals with aspect ratios ranging from 10-30:1:1 are better termed ?rods? and tend to have better processing characteristics.

Experience suggests that there are molecular systems whose crystals of a particular polymorph possess needle morphology regardless of the solution environment [1]; in contrast, a particular polymorph of a different molecule can crystallize as needles in particular solvents while having more equant shapes in others. We propose to classify the former systems as being absolute needles, while the latter systems as being conditional needles. In this presentation, we will describe methods for testing whether a system is ?absolute? or ?conditional? based on the polymorph's intrinsic structure and growth behavior as a function of supersaturation.

These tests can then be translated into a decision hierarchy for crystal engineers. The benefit of such a hierarchy is clear: if a new molecule and polymorph in the production pipeline, having been observed to exhibit needle morphology, is deemed to be absolute, then any attempts to modify the morphology by means of solvent selection are rendered useless and other strategies must be pursued for process development.

References

[1] Hatada, M., Jancarik, J., Graves, B., and Kim, S. Crystal Structure of Aspartame, a Peptide Sweetener. J. Am. Chem. Soc. 107, 4297?4282 (1985).