Molecular theory of oiling-out during crystallization

Oiling-out, an undesirable liquid-liquid phase separation (LLPS) frequently encountered in pharmaceutical crystallization, poses significant challenges by reducing purity, yield, and process robustness. While macroscopic thermodynamics describes the phase boundaries of oiling-out, the molecular origins of this phenomenon remain poorly understood, with no clear mechanistic framework to explain how solution chemistry triggers phase separation. In this work, we employ atomistic molecular dynamics (MD) simulations to elucidate the molecular mechanism of crystallization during oiling-out in the β-alanine-water-isopropanol (IPA) system. Analysis of radial and spatial distribution functions reveals that LLPS is not driven by the direct replacement of solvent by antisolvent, but by the antisolvent-induced disruption of the hydration network. This breakdown creates solvation voids, regions of incomplete solvation where IPA fails to compensate for the loss of water, resulting in a partially desolvated state that promotes solute clustering. Using a double-well potential analysis, we construct energy landscapes that quantify the activation barriers for crystallization. We identify two distinct kinetic bottlenecks: the enthalpic cost of desolvation and the energetic penalty of structural reorganization. Our results demonstrate that conditions leading to oiling-out drastically lower both barriers compared to homogeneous solutions, reducing the desolvation penalty significantly. These findings confirm that the solute-rich droplets formed during oiling-out serve as kinetically favored precursors, supporting the two-step nucleation theory and providing a quantitative molecular framework for controlling crystallization outcomes in complex solvent mixtures.