Effect of Structurally Similar Additives on Crystal Habit of Organic Molecular Crystals at Low Supersaturation

Foreign molecules such as additives or impurities may influence crystal morphology to a significant extent by disrupting the growth mechanism and inhibiting the growth rate of certain crystal faces. The additives can be byproducts of a preceding reaction step, or they could be specially designed to obtain specific crystal morphologies for particular applications. Because of the specific chemical structure of structurally similar additives, they can incorporate or lock into the original lattice only in certain configurations; hence, only some of the crystal faces are able to recognize these additive molecules. Once attached, they interfere with the first turn of a rotating growth spiral and hence slow down the growth rate of the face. In this article, we develop a generic probabilistic scheme for quantitatively estimating imposter recognition on each crystal face using a combination of mean field theory and configurational energy minimization. On the basis of this recognition, the mechanistic effect of an impurity on the first turn of the spiral and hence the modified growth rates and modified crystal habits are computed. These concepts are generalized for all molecular crystals, including non-centrosymmetric molecules. We demonstrate the applicability of the model by correctly predicting the experimental morphologies of α-glycine with l -alanine impurity and paracetamol with p -acetoxyacetanilide impurity grown from water.