Crystallization Technology Handbook, Second Edition,
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Edited by Owen Linzmayer. Encyclopaedia of Pharmaceutical Technology, 2nd edition Edited by J. Crystallization of Polymers, 2nd ed. Growth Des. 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 , 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 relative arrangement of the atoms, not the macroscopic properties of the crystal size and shape , although those are a result of the internal crystal structure.
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.
André Luis Cavaignac
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.
Many compounds have the ability to crystallize with different crystal structures, a phenomenon called polymorphism. 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. Geological time scale process examples include:. For crystallization see also recrystallization to occur from a solution it 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.
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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 antisolvent 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.
From a material industry perspective:. Crystallization separates a product from a liquid feedstream, often in extremely pure form, by cooling the feedstream or adding precipitants which lower the solubility of the desired product so that it forms crystals. Well formed crystals are expected to be pure because each molecule or ion must fit perfectly into the lattice as it leaves the solution. Impurities would normally not fit as well in the lattice, and thus remain in solution preferentially.
Hence, molecular recognition is the principle of purification in crystallization. However, there are instances when impurities incorporate into the lattice, hence, decreasing the level of purity of the final crystal product. Also, in some cases, the solvent may incorporate into the lattice forming a solvate.
In addition, the solvent may be 'trapped' in liquid state within the crystal formed, and this phenomenon is known as "inclusion". Equipment for the main industrial processes for crystallization. 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:. 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.
Crystallization Technology Handbook
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. As mentioned above, a crystal is formed following a well-defined pattern, or structure, dictated by forces acting at the molecular level. As a consequence, during its formation process the crystal is in an environment where the solute concentration reaches a certain critical value, before changing status.
Solid formation, impossible below the solubility threshold at the given temperature and pressure conditions, may then take place at a concentration higher than the theoretical solubility level. The difference between the actual value of the solute concentration at the crystallization limit and the theoretical static solubility threshold is called supersaturation and is a fundamental factor in crystallization dynamics.
Supersaturation is the driving force for both the initial nucleation step and the following crystal growth, both of which could not occur in saturated or undersaturated conditions. It is a consequence of rapid local fluctuations on a molecular scale in a homogeneous phase that is in a state of metastable equilibrium.
Total nucleation is the sum effect of two categories of nucleation — primary and secondary.
Primary nucleation is the initial formation of a crystal where there are no other crystals present or where, if there are crystals present in the system, they do not have any influence on the process. This can occur in two conditions.
The first is homogeneous nucleation, which is nucleation that is not influenced in any way by solids. These solids include the walls of the crystallizer vessel and particles of any foreign substance. The second category, then, is heterogeneous nucleation. This occurs when solid particles of foreign substances cause an increase in the rate of nucleation that would otherwise not be seen without the existence of these foreign particles. Homogeneous nucleation rarely occurs in practice due to the high energy necessary to begin nucleation without a solid surface to catalyse the nucleation.
Primary nucleation both homogeneous and heterogeneous has been modelled with the following: . Secondary nucleation is the formation of nuclei attributable to the influence of the existing microscopic crystals in the magma. Fluid shear nucleation occurs when liquid travels across a Crystal at a high speed, sweeping away nuclei that would otherwise be incorporated into a Crystal, causing the swept-away nuclei to become new crystals.
Contact nucleation has been found to be the most effective and common method for nucleation.