Crystallization

Crystallization is a physical process by which a solid, the crystal, is formed from a gas, a liquid or a solution, in which the ions, atoms or molecules are highly organized, by establishing links forming a crystalline network. Crystallization is used quite often in chemistry to purify a solid substance.

Crystallization occurs in two main steps. The first is nucleation, the appearance of a crystalline phase from a supercooled liquid or from a supersaturated solvent. The second step is known as crystal growth, which is the increase in the size of the particles and leads to a crystalline state. An important feature of this step is that loose particles form layers on the crystal surface and lodge in open inconsistencies such as pores, cracks, etc.

Most minerals and organic molecules crystallize easily, and the resulting crystals are generally of good quality, that is, without visible defects. However, the largest biochemical particles, such as proteins, are often difficult to crystallize. The ease with which molecules will strongly crystallize depends on the strength of atomic forces (in the case of mineral substances), intermolecular forces (organic and biochemical substances) or intramolecular forces (biochemical substances).

Crystallization is also a solid-liquid chemical separation technique, in which the mass transfer of a solute from the liquid solution to a pure solid crystalline phase occurs. In chemical engineering, crystallization occurs in a crystallizer. Crystallization is therefore related to precipitation, although the result is not amorphous or disordered, but a crystal.

Cooling of a concentrated solution

If a concentrated solution is prepared at high temperatures and it is cooled, a supersaturated solution is formed, which is the one that has, momentarily, more solute dissolved than that admissible by the solution at that temperature under equilibrium conditions. Subsequently, the solution can be made to crystallize by controlled cooling. This is done so that the crystals have a medium size, since if the crystals are very small the impurities are deposited on the surface of the entire mass, and if the crystals are very large the impurities are trapped within the crystalline lattices. Essentially the main compound crystallizes, and those that are enriched with the impurities present in the initial mixture do not reach their solubility limit.

For this method of purification to be used, there must be a significant variation in solubility with temperature, which is not always the case. Sea salt (NaCl), for example, has this effect.

Solvent change

Preparing a concentrated solution of a substance in a good solvent and adding a solvent, but one that is miscible with the first, the main part of the dissolved solid begins to precipitate, and the mother liquor becomes relatively rich in impurities. For example, benzoic acid can be separated from a solution of it in acetone by adding water.

Solvent evaporation

In an analogous way, evaporating the solvent from a solution can make the solids that were dissolved begin to crystallize when the limits of their solubility are reached. This method has been used for millennia in the manufacture of salt from brine or seawater, etc.

Sublimation

In some compounds the vapor pressure of a solid can become high enough to evaporate notable amounts of this compound without reaching its melting point (sublimation). The vapors formed condense in colder areas, for example in the form of a "cold finger", usually passing directly from the gaseous state to the solid (regressive sublimation), thus separating themselves from possible impurities. Following this procedure, pure solids of substances that easily sublimate such as caffeine, elemental sulfur, salicylic acid, iodine, etc. can be obtained.

Selective cooling of a molten solid

To purify a crystalline solid it can be melted. Firstly, the pure solid crystallizes from the liquid obtained, enriching the liquid phase with the impurities present in the original solid. For example, this is the method used to obtain ultra-pure silicon for the manufacture of substrates or wafers in the semiconductor industry. The solid material (unpurified silicon obtained previously in an electric induction furnace) is given a cylindrical shape. A zone fusion is then carried out on the cylinder. It begins by melting a strip or section of the cylinder at one end and said area is moved along it until it reaches the other end. As the impurities are soluble in the melt, they separate from the solid and drag themselves towards the other end. This zone melting process can be done several times to ensure that the degree of purity is as desired. Finally, the end where the impurities have accumulated is cut off and separated from the rest. The advantage of this process is that by properly controlling the temperature and the speed at which the molten strip moves through the cylindrical part, a material can be obtained that is a single crystal of silicon that presents the faces of the crystalline lattice oriented in the right direction. desired way.

Crystal growth

Lissoima monocristal for X-ray diffraction study.

Other techniques have been developed to obtain large crystals of poorly soluble products. For example, two starting compounds can be made to diffuse into a gelatinous matrix. Thus the compound is formed slowly giving rise to larger crystals. In general, however, the slower the crystallization process, the better the result tends to be with respect to the cleanliness of the starting products and the larger the crystals formed.

The shape and size of the crystals can be influenced separately by conditions such as the solvent or the concentration of the compounds, adding traces of other components such as proteins (this is the way in which mollusks, diatoms, corals, etc., manage to deposit their shells or skeletons of calcite or quartz in the desired shape).

The most widely accepted theory for this phenomenon is that crystalline growth occurs by forming monomolecular layers around the seed of crystallization or an initial crystallite. New molecules preferentially adhere to the face where their adhesion releases more energy. The energetic differences are usually small and can be modified by the presence of said impurities or by changing the crystallization conditions.

In a multitude of applications it may be necessary to obtain crystals with a certain shape and/or size such as: determination of the chemical structure by means of X-ray diffraction, nanotechnology, obtaining especially sensitive films made up of salt crystals silver plates oriented perpendicular to the light of incidence, the preparation of the active principles of the drugs, etc.

Recrystallization

The crystallization process is repeated in a solution in which said process had already been carried out. The remaining waters still contain dissolved solute that can crystallize. For a faster crystallization process, apply a crystallization core.