Welcoming Remarks

As a separation process, crystallization inherently deals with complex mixtures containing multiple compounds. Impurities present in crystallizing media often end up incorporated into the crystal lattice to some extent. However, this incorporation is not directly observable or apparent, and it is instead measured as an average powder purity that does not consider whether internal impurity gradients exist in the product. As will be discussed in this talk, impure regions within a crystal present significantly different growth and dissolution behaviors than the equivalent pure lattice regions.

Crystals of small-molecule pharmaceuticals have been grown in the presence of dyes as model impurities, and studied in parallel with well-known dyed crystals from literature. By directly visualizing the location of those impurities inside the crystal, one can study the role that the growth environment plays not just in the generation of crystals with marked composition anisotropy, but also in a changing crystal morphology. The behavior of those crystals has been studied in both undersaturated and supersaturated solutions near-equilibrium, revealing the effect that dissolved impurities as well as lattice impurities have on changing kinetics and crystal habit. Primarily, we will visually demonstrate that (i) impure regions within a crystal tend to present faster dissolution kinetics, (ii) the impurity profile in the liquid matters as a modifier of the most stable crystal habit, which may prevent impurity release altogether, and (iii) inhibition of crystal growth for faces that incorporate impurity can’t be solely explained through surface adsorption and site poisoning, but may instead be driven entirely by a change in the most stable crystal habit.