A New Model for the Effect of Molecular Imposters on the Shape of Faceted Molecular Crystals

In a solution crystallization process, dissolved molecules that are structurally related to the solute (here termed “imposters”) can exert effects on the shape of the solute crystal. We propose a new model for the effect of imposters based on a mechanistic description of the way a molecular imposter affects a growing crystal surface. Crystals growing at relatively small supersaturations are known to grow by a flow of steps across crystal faces manifested as spirals originating from screw dislocations. We argue that, when the imposter is present on the surface at low concentrations, it has no net effect on these spirals except in the region where the first turn of a spiral occurs. The model is applied to α-glycine growing from water in the presence of l-alanine, and our shape prediction compares favorably with available experimental data.