CRYSTALLIZATION PROCESS
By
Asst. Lect. Zahraa Nadhum Abd
•Crystallization is the process of atoms or molecules arranging into a well-defined, rigid crystal lattice in order to minimize their energetic state. The smallest entity of crystal lattice is called a unit cell, which can accept atoms or molecules to grow a macroscopic crystal ,The crystallization process consists of two major events:
•Nucleation -Molecules gather together in clusters in a defined manner. Clusters need to be stable under current experimental conditions to reach the “critical cluster size” or they will redissolve . It is this point in the crystallization process that defines the crystal structure.
•Crystal Growth -Nuclei that have successfully achieved the “critical cluster size” begin to increase in size. Crystal growth is a dynamic process, with atoms precipitating from solution and becoming redissolved,Supersaturation and supercooling are two of the most common driving forces behind crystal formation.
•Many compounds have the ability to crystallize with some having different crystal structures, a phenomenon called polymorphism. Certain polymorphs may be metastable, meaning that although it is not in thermodynamic equilibrium, it is kinetically stable and requires some input of energy to initiate a transformation to the equilibrium phase. Each polymorph is in fact a different thermodynamic solid state and crystal polymorphs of the same compound exhibit different physical properties, such as dissolution rate, shape (angles between facets and facet growth rates), melting point, etc. For this reason, polymorphism is of major importance in industrial manufacture of crystalline products. Additionally, crystal phases can sometimes be interconverted by varying factors such as temperature.