Oiling out in crystallization is a process in which supersaturation causes the initially dissolved compound to separate from solution by creating a secondary liquid phase (emulsion) (image 1a) instead of a solid, crystalline phase (suspension) (image 1c). Oiling out mostly occurs when the integration of solute molecules into the crystal lattice is kinetically hindered, delayed or the system experiences very high supersaturation.
Solute molecules in the product-rich oiling out droplets can arrange themselves randomly and show much higher mobility than in a ridged crystal lattice. Often, this makes oiling out droplets a good solvent for unwanted impurities. Oiling out droplets are usually unstable and can transform (image 1b) to amorphous material over time or spontaneously solidify into crystalline solid.
Oiling out is often not easily detected and can be missed, accounting for many process challenges during crystallization development and manufacturing.