Understanding the Roles of Oiling-out on Crystallization of β‑Alanine: Unusual Behavior in Metastable Zone Width and Surface Nucleation during Growth Stage

Oiling-out crystallization of β-alanine was conducted in water–ethanol solvent mixtures, and the roles of oiling-out on the crystallization of β-alanine were investigated during the crystal nucleation and growth stages, respectively. Nucleation metastable zone width broadened with an increase of saturation temperature when oiling-out occurred, contrary to that observed in absence of oiling-out. This observed phenomenon could be explained by the solvent composition change in the nucleation phase (dense oil droplets) under different temperatures. In the crystal growth stage, the size of oil droplets could be controlled by varying agitation speeds. The resulting crystals were lumpy with bulges on specific surfaces when the agitation speed exceeded a certain level. It was supposed to be that the absorption of oil droplets on the crystal surfaces during growth stage led to the surface nucleation, and then the bulges formed consequently. Such performance was never revealed in normal single-phase crystallization, and this character of oiling-out could be taken to control crystal surfaces.