Crystallization

Crystallization is the process by which a solid with a highly ordered internal structure, a crystal, forms from a solution, a melt, or a gas. It is a common method for purifying chemicals and for producing materials with regular microstructure. Crystals form when constituent particles organize into a periodic lattice, typically during steps involving supersaturation, cooling, or solvent removal. Impurities can influence crystal form and quality.

Mechanism and guiding factors: Crystallization proceeds by nucleation and growth. Nucleation is the formation of a stable cluster of molecules; it may be homogeneous (in the bulk) or heterogeneous (on surfaces or impurities). Once nuclei exist, further molecules add to the lattice, producing larger crystals. The rate and outcome depend on temperature, concentration, solvent properties, agitation, and the presence of seeds or impurities, which can promote or inhibit crystallization and may cause polymorphism.

Methods and variants: Common crystallization approaches include cooling crystallization (lowering temperature to reduce solubility), evaporation crystallization (removing solvent to raise solute concentration), and antisolvent crystallization (adding a solvent in which the solute is less soluble). Crystallization from a melt is used for some polymers and inorganic salts. In some processes, gas-phase deposition or precipitation contributes to crystal formation.

Applications and considerations: Crystallization is central to purifying drugs, salts, sugars, and specialty chemicals. Crystal size distribution, shape, and purity affect handling, dissolution, and stability. Process control aims to produce uniform crystals and to minimize undesired polymorphs and impurities. Scale-up challenges include managing heat and mass transfer, mixing, and controlling nucleation to achieve consistent crystal quality.