Thermodynamic and Kinetic Levers in Cooling Crystallization for Nanoscale APIs

Abstract

Poor solubility limits the bioavailability of Active Pharmaceutical Ingredients (APIs), motivating research on nanocrystal formations with enhanced dissolution rate. Cooling crystallization is widely used in bulk API production, though its application to nanocrystal design is limited. Classical Nucleation Theory has shown that high supersaturation is required to produce nanoscale crystalline structures. However, such a labile zone operation may induce poor reproducibility, broad Particle Size Distribution (PSD), and crystal defects. This review examines the three major process levers — cooling rate, seeding, and agitation — that govern the balance between nucleation and crystal growth. While effective for bulk API production, they cannot be used alone to deliver reproducible PSD. Hybrid intensification techniques, such as antisolvent-cooling crystallization, are employed to create a localized supersaturation burst for controlled nucleation. Computational tools, including Process Analytical Techniques (PATs) with population balance models (PBMs), extend the use of cooling crystallization to predict PSD under dynamic conditions. At the same time, molecular simulations clarify possible non-classical nucleation pathways. Thus, understanding of cooling crystallization provides a theoretical basis, but requires hybrid intensification strategies and modeling techniques for nanoscale API production.