Crystallization
From Wikipedia, the free encyclopedia
- For other senses of this word, see crystallization (disambiguation).
Crystallization is the (natural or artificial) process of formation of solid crystals from a homogeneous solution. Crystallization is also a chemical solid-liquid separation technique.
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[edit] Process
The crystallization process consists of two major events, nucleation and crystal growth.
Nucleation is the step where the solute molecules dispersed in the solvent start to gather to create clusters in the nanometer scale (elevating solute concentration in a small region) as to become stable under the current operating conditions. These stable clusters constitute the nuclei. However when the clusters are not stable, they redissolve. Therefore, the clusters need to reach a critical size in order to become stable nuclei. Such critical size is dictated by the operating conditions (temperature, supersaturation, irregularities, etc.). It is at the stage of nucleation that the atoms arrange in a defined and periodic manner that defines the crystal structure — note that "crystal structure" is a special term that refers to the internal arrangement of the atoms, but NOT the physical external macroscopic properties of the crystal, size and shape.
The crystal growth is the subsequent growth of the nuclei that succeed in achieving the critical cluster size. Subsequently, nucleation and growth continue to occur simultaneously while the supersaturation exists. Supersaturation is the driving force of the crystallization, hence the rate of nucleation and growth is driven by the existing supersaturation in the solution. Depending upon the conditions, either nucleation or growth may be predominant over the other, and as a result, crystals with different sizes and shapes are obtained (Control of crystal size and shape constitutes one of the main challenges in industrial manufacturing, such as for pharmaceuticals). Once the supersaturation is exhausted, the solid-liquid system reaches the equilibrium and the crystallization is completed, unless the operating conditions are modified from equilibrium as to supersaturate the solution again.
[edit] Crystallization in nature
There are many examples of natural process that involve crystallization.
Geological time scale process examples include:
- Natural (mineral) crystal formation (see also gemstone);
- Stalactite/stalagmite, rings formation.
Usual time scale process examples include:
- Snow flakes formation (see also Koch snowflake);
- Honey crystallization (nearly all types of honey crystallize).
[edit] Artificial methods
For crystallization to occur the solution must be supersaturated. This means that the solution has to contain more solute entities (molecules or ions) dissolved than it would contain under the equilibrium (saturated solution). This can be achieved by various methods, with 1) solution cooling, 2) addition of a second solvent to reduce the solubility of the solute (technique known as anti-solvent or drown-out), 3) chemical reaction and 4) change in pH being the most common methods used in industrial practice. Other methods, such as solvent evaporation, can also be used.
[edit] Applications
There are two major groups of applications for the artificial crystallization process: crystal production and purification.
[edit] Crystal production
From a material industry perspective:
- Macroscopic crystal production, for supply the demand of natural-like crystals with methods that "accelerate time-scale" for massive production and/or perfection:
- Tiny size crystal and thin film production: using methods for powder and controlled (nanotechnology fruits) forms.
Massive production examples:
- "Powder salt for food" industry;
- Silicon crystal wafer production.
[edit] Purification
- See also: Recrystallization
Well formed crystals are very pure because each molecule or ion must fit perfectly into the lattice as it leaves the solution. Impurities do not fit as well, and thus remain in solution preferentially. As a result, crystallization is a method for purifying mixtures.
Depending on the nature of the crystal system, crystals consist of only one enantiomer. The spontaneous resolution of racemic tartaric acid into enantiomeric crystals allowed Louis Pasteur to discover chirality.
[edit] External links
[edit] See also
- Chiral synthesis
- Crystal
- Crystal habit
- Crystal structure
- Crystallite
- Crystallization (Engineering Aspects)
- Fractional crystallization
- Gemstone
- Igneous differentiation
- Quasicrystal
- Seed crystal
- Single crystalbg:Кристализация
cs:Krystalizace de:Kristallisation es:Cristalización fr:Cristallisation it:Cristallizzazione nl:Kristallisatie ja:結晶化 pl:Krystalizacja ru:Кристаллизация simple:Crystallization processes fi:Kiteyttäminen (kemia) zh:结晶
