Crystallization is natural or artificial process of formation of solid crystal by precipitating from solution, melt or deposited directly from gas. Crystallization is also a chemical solid-liquid separation technique, in which the mass transfer of solute from liquid solution to pure solid crystalline phase occurs.
The crystallization process consists of two major events, crystal growth and nucleation. Nucleation is step where solute molecules dispersed in solvent start to gather into clusters elevating solute concentration in a small region that becomes stable under the current operating conditions. These stable clusters formed constitute the nuclei. When the clusters are not stable, they redissolve and thus clusters need to reach a critical size in order to become stable nuclei. Such critical size for stabilization is dictated by the operating conditions. It is at the stage of nucleation that the atoms arrange in a defined and periodic manner that defines crystal structures which is a special term that refers to the relative arrangement of the atoms, not the macroscopic properties of the crystal that is size and shape, although those are result of internal crystal structure.
The crystal growth is subsequent growth of nuclei that succeed in achieving the critical cluster size. The Nucleation and the growth continue to occur simultaneously while supersaturation exists. The supersaturation is driving force of the crystallization; hence rate of nucleation and growth is driven by the existing supersaturation in solution. Depending upon conditions, either the nucleation or the growth may be predominant over the other, and thus the crystals with different sizes and shapes are obtained. The control of crystal size and shape constitutes one of the main challenges in industrial manufacturing, such as for the pharmaceuticals. Once supersaturation is exhausted, the solid-liquid system reaches equilibrium and crystallization is completed, unless the operating conditions are modified from the equilibrium position so as to supersaturate the solution again.
Many compounds have the ability to crystallize with different crystal structuring and the phenomenon called polymorphism. Each polymorph is in fact a different thermodynamic solid state and crystal polymorphs of same compound exhibit different physical properties, such as the dissolution rate, the shape, angles between the facets and the facet growth rates, the melting point, etc. For this reason, polymorphism is of the major importance in industrial manufacture of crystalline products.
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