CRYSTALLIZATION BY: TAHSEEN ISMAIL

Crystallization Definition: “Crystallization is the (natural or artificial) process of formation of solid crystals precipitating from a solution, melt or more rarely deposited directly from a gas.” “Crystallization is also a chemical solid-liquid separation technique, in which mass transfer of a solute from the liquid solution to a pure solid crystalline phase occurs.”

Crystal: “A crystal or crystalline solid is a solid material, whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions.” “The scientific study of crystals and crystal formation is crystallography.” “The process of crystal formation via mechanisms of crystal growth is called crystallization or solidification.”

TYPES OF CRYSTALS: a) Based on physical & chemical properties. 1) Covalent crystals 2) Metallic crystals 3) Ionic crystals 4) Molecular crystals

Covalent Crystals: “This is a crystal which has real chemical covalent between all of the atoms in the crystal.” A single crystal of a covalent crystals is really just one big molecule. Covalent crystals can have extremely high melting points. Example: Diamond or Zinc sulphide.

Metallic Crystals: “Individual metal atoms occur on lattice sites while the outer electrons from these atoms are able to flow freely around the lattice.” Metallic crystals normally have high melting points and densities.

Ionic Crystals: “A crystal in which the individual atoms don't have covalent bonds between them, but are held together by electrostatic forces.” Ionic crystals are hard and have relatively high melting points. Example: Sodium chloride (NaCl).

Molecular Crystals: “A crystal in which there are recognizable molecules in the structure and the crystal is held together by non-covalent interactions like van der Waals forces or hydrogen bonding”. They are soft and have lower melting points Example: Sugar

b) Based on crystal system. 1) Triclinic 2) Monoclinic 3) Orthorhombic 4) Tetragonal 5) Trigonal 6) Hexagonal 7) Cubic

Crystalline phases: 1) Polymorphism is the ability of a solid to exist in more than one crystal form. For example, water ice is ordinarily found in the hexagonal form Ice, but can also exist as the cubic Ice, the rhombohedral ice. 2) Amorphous phases are also possible with the same molecule, such as amorphous ice. In this case, the phenomenon is known as polyamorphism.

Mechanism: It involves 2 steps. 1) Nucleation (creation of crystalline nuclei) 2) Crystal growth (nuclei grow in to crystals)

NUCLEATION: It is the step where the solute molecule dispersed in solvent, start to gather into clusters, elevating solute conc. in small region. It may arise spontaneously or by introduction of minute crystal of dissolved substance in the solution.

Seeding: “Addition of same solute in that solution creates crystal nuclei this process is known as seeding.” Theory of Miers: It postulates that a definite relationship exists between the conc. & temp. at which crystals will spontaneously form in an initially un seeded soln.

This form of relationship is a super solubility curve roughly parallel to and above the normal solubility curve. Such a curve may be imagined as solubility for exceedingly fine particles hence have solubility greater than large crystals. Solubility curve (diagram). Metastable zone: “There will no appreciable spontaneous nucleation. But above solubility curve there is copious spontaneous nucleation.”

‘E’ spontaneous crystalline solution its concentration falls to that given by normal solubility curve at prevailing temp. i.e, ‘G’. Solution represented at point ‘F’ in meta stable zone will be unchanged unless seeds are added and when nucleation occurs the conc. will again drop to ‘G’.

CRYSTAL GROWTH: Crystal growth is the subsequent growth of nuclei that succeed in achieving the critical cluster size. nucleation and growth occurs simultaneous while super saturation exist.

Super saturation: “It is the driving force of crystallization hence rate of nucleation and growth is driven by the existing super saturation in the solution.”

Size of crystals: Depends on conditions of crystallization 1) very small size: By rapid cooling with frequent stirring of soln., almost saturation at their bp usually avoided. 2) Median size: Soln. saturated about 60 -80 c. 3) Large size: BY evaporation the solvent may be facilitated by seeding.

Mother liquor: Liquid remaining after crop of crystals. Again used till whole crop of crystal is obtained.

Applications of crystallization: 1) Crystal production: Macroscopic crystal, tiny size crystal. 2) Purification: Use to obtain very pure substances and to verify their purity.

thatz all