Crystallization
From Wikipedia, the free encyclopedia
Crystallization is the (natural or artificial) process of formation of solid crystals from a uniform 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 into clusters, on the nanometer scale (elevating solute concentration in a small region), that 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, not the 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. 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 equilibrium and the crystallization is complete, unless the operating conditions are modified from equilibrium so 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.
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 crystals:
- Powder, sand and smaller sizes: using methods for powder and controlled (nanotechnology fruits) forms.
- Mass-production: on chemical industry, like salt-powder production.
- Sample production: small production of tiny crystals for material characterization. Controlled recrystallization is an important method to supply unusual crystals, that are needed to reveal the molecular structure and nuclear forces inside a typical molecule of a crystal. Many techniques, like X-ray crystallography and NMR spectroscopy, are widely used in chemistry and biochemistry to determine the structures of an immense variety of molecules, including inorganic compounds and bio-macromolecules.
- Thin film production.
- Powder, sand and smaller sizes: 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.
[edit] Thermodynamic view
The nature of a crystallization process is governed by both thermodynamic and kinetic factors, which can make it highly variable and difficult to control. Factors such as impurity level, mixing regime, vessel design, and cooling profile can have a major impact on the size, number, and shape of crystals produced.
Now put yourself in the place of a molecule within a pure and perfect crystal, being heated by an external source. At some sharply defined temperature, a bell rings, you must leave your neighbours, and the complicated architecture of the crystal collapses to that of a liquid. Textbook thermodynamics says that melting occurs because the entropy, S, gain in your system by spatial randomization of the molecules has overcome the enthalpy, H, loss due to breaking the crystal packing forces:
T(Sliquid − Ssolid) > Hliquid − Hsolid
Gliquid < Gsolid
This rule suffers no exceptions when the temperature is rising. By the same token, on cooling the melt, at the very same temperature the bell should ring again, and molecules should click back into the very same crystalline form. The entropy decrease due to the ordering of molecules within the system is overcompensated by the thermal randomization of the surroundings, due to the release of the heat of fusion; the entropy of the universe increases.
But liquids that behave in this way on cooling are the exception rather than the rule; in spite of the second principle of thermodynamics, crystallization usually occurs at lower temperatures (supercooling). This can only mean that a crystal is more easily destroyed than it is formed. Similarly, it is usually much easier to dissolve a perfect crystal in a solvent than to grow again a good crystal from the resulting solution. The nucleation and growth of a crystal are under kinetic, rather than thermodynamic, control.
[edit] References
- Glynn P.D. and Reardon E.J. (1990) "Solid-solution aqueous-solution equilibria: thermodynamic theory and representation". Amer. J. Sci. 290, 164-201.
