What is crystallization

Crystallization

Crystallization, the transition of a substance into the crystallized state. The K. can take place from the gaseous, liquid or solid phase. Examples of the various possibilities of a K are the solidification of a melt when it cools below the melting point, the crystallization from a supersaturated solution, the condensation from the vapor phase, the deposition from the vapor phase as a result of a chemical transport reaction, the phase transformation of solids (polymorphism) , the formation of crystalline products in solid-state reactions and the K. amorphous substances.

Regardless of the phase from which the C. occurs, it is always made up of the two sub-processes nucleation and crystal growth. Under the Nucleation one understands the emergence of a viable crystal of submicroscopic dimension, one Crystal seed. A seed crystal can e.g. B. arise in a supercooled melt, a supersaturated solution or a supersaturated vapor through the accidental meeting of crystal building blocks. Such an assembly with ten to one hundred building blocks has a very large surface area in relation to its volume and is therefore very energetic. For nucleation as an energetically unfavorable process, the Nucleation work are expended, which plays the role of an activation energy. If a critical germ size (10-5 until 10-7 cm) is exceeded, the nucleus continues to grow spontaneously into a crystal. A homogeneous nucleation takes place in a system with only one phase, i.e. with the exclusion of interfaces; when participating in several phases, e.g. B. in the form of foreign particles that act as germs, or of solid surfaces in the case of K. on vessel walls, one speaks of heterogeneous nucleation.

The second sub-process at K. consists in Crystal growth, i.e. in the enlargement of the submicroscopic crystal nucleus to a macroscopic crystal. The theory of crystal growth is based on the assumption that the energy gain when a crystal building block attaches to different points on the crystal surface is different. The energetically most favorable positions are always taken, either immediately upon impact or as a result of a surface migration through change of place processes. The building blocks find the cheapest possible accumulation in the Semi-crystalline layers at the edge of an incomplete network level. This edge represents a surface step that moves over the surface of the crystal due to the row-wise accumulation of building blocks. The growth of a crystal therefore takes place on a network level. The creation of a new network level always begins with the formation of a two-dimensional nucleus. The calculation of the nucleation work required for this, however, results in a significantly lower crystal growth than was measured experimentally. This discrepancy is due to the real structure of the crystals, through which the work of nucleation is reduced or even completely eliminated. A screw dislocation that emerges from a crystal surface as a step (crystal structural defect) is particularly effective for the accumulation of building blocks. As a result of them, a large number of semi-crystalline layers are present without the need for nucleation work. Since the step is retained during spiral growth around the point of intersection of the screw dislocation, it acts as a continuous surface nucleus (Spiral growth theory).

The K. is one of the most important processes for the extraction of pure solid substances. In the laboratory it is used as a final stage chem. Syntheses are used for the isolation and purification of the reaction products as well as for the quantitative separation and determination of solution components (gravimetry). The fractionated K. serves to separate mixtures of substances. In the chem. Industry plays the K. in the form of Bulk crystallization in the production of fertilizers, soda, sugar and others. a significant role. The possible uses mentioned are usually a K. from solution. The supersaturation required for this is caused by a change in temperature in the case of substances with a higher temperature coefficient of solubility (usually by cooling, Cooling crystallization, e.g. B. the K. of naphthalene or p-Xylene and the dewaxing of lubricating oils with solvents), by evaporation of the solvent (Evaporation crystallization, e.g. B. the crystallization of common salt, ammonium sulfate and sucrose) or achieved by adding a precipitant. Under certain conditions, such as vibration-free storage and keeping away from crystal nuclei, the saturation concentration can be exceeded noticeably without a K. setting in (Crystallization delay). If the solubility is exceeded quickly and a large number of crystal nuclei are present, a finely crystalline product is obtained. It is often of greater purity than larger crystals of the same substance, which grow slowly and easily trap mother liquor (occlusion). If a fine-grained crystal is in contact with the saturated solution for a longer period of time, it gradually becomes coarser, since the larger crystals have a lower solubility than the small crystals and grow at their expense (Ostwald ripening).

In the Electrocrystallization crystal growth occurs in connection with the electrolysis of a solution or melt. Important areas of application are the production of galvanic coatings and the extraction or refining of metals.

When carrying out the K for the production of single crystals of particularly high quality and sometimes of considerable size, special methods of crystal growth are used.