Experimental Evaluation of Contact Secondary Nucleation Mechanisms

Contact secondary nucleation has vital importance in industrial crystallizers for reactions and purification, and it recently has been linked to contributing to biological homochirality emerged at an abiotic evolutionary stage. Despite years of studies, the mechanism of contact secondary nucleation has not been resolved whether contact secondary nuclei originate from parent crystals via microattrition or from semiordered solute clusters at the interface of parent crystals. This study takes advantage of the unique thermodynamic and kinetic properties of the glycine system that is capable of differentiating the origin of contact secondary nuclei based on the polymorph of secondary nuclei obtained. It is demonstrated using two different experimental designs that contact secondary nuclei could originate both from the semiordered solute molecules at the interface layer of existing crystals and from parent crystals themselves via the mechanism of microattrition depending on the magnitude of the contact force. When the contact force is relatively small (<2 N for γ-glycine), it is only sufficient to disturb the semiordered solute molecules at the interface layer, which could generate secondary nuclei; when the contact force exceeds a certain threshold (>2 N for γ-glycine), it not only disturbs the interface layer but also causes mechanical damages to the parent crystals.