CRYSTALLIZATION: PROCESS, TYPES, EXAMPLES, SEPARATION - CHEMISTRY - 2024

The crystallization is a physical process in which naturally or artificially a crystalline solid, ie ordered structure is formed from a liquid or gaseous medium. It differs from precipitation in the fact that the latter develops without rigorous control of the process parameters, and also that it can produce amorphous and gelatinous solids.

The objective of crystallization, as its name simply and plainly indicates, is to generate crystals. These are not only characterized by being orderly, but also by being pure solids. Therefore, in the synthesis of solid compounds, it is sought to obtain products of high purity, crystals that are as pure as possible.

Crystallization of a hypothetical purple colored solute in aqueous solution. Source: Gabriel Bolívar.

The image above shows a generalized and hypothetical crystallization of a purple solute in aqueous solution.

Note that the red bar acts as a thermometer. When the temperature is high, the solution contains the dissolved solute, which remains soluble under these conditions. However, as the temperature gradually drops, the first purple crystals begin to appear.

As the temperature continues to decrease, the crystals will grow in size to form robust purple hexagons. The change in color of the solution is indicative that the solute went from being dissolved to being incorporated into the growing crystals. The slower the crystallization, the purer the crystalline solid obtained.

There are other variables to take into account during this process: how much solute is dissolved in the determined solvent, at what temperature the solution must be heated, how long the cooling must last, how necessary it is to resort or not to sonic agitation, among others. aspects.

The crystallization process, more than a complex phenomenon that involves molecular and thermodynamic dynamics, is an art that demands constant learning, trials and errors, until it is perfected within the laboratory or in the industry.

Crystallization process

Crystallization essentially consists of two processes: nucleation and crystal growth.

Both stages always take place during crystallization, but when the first occurs rapidly, the second will hardly have time to develop. Meanwhile, if the nucleation is slow, the crystals will have more time to grow, and therefore, they will tend to be larger. The latter is the situation assumed in the image with the purple hexagons.

Nucleation

Crystals were originally said to be solids with ordered structures. From the solution where the solute is dispersed in disorder, its particles must come close enough so that their interactions, be they ionic or Van der Walls type, allow the settlement of the first group of solute particles: a cluster.

This cluster can dissolve and re-form as many times as necessary until it is stable and crystalline. The first nucleus is then said to have appeared. If the nucleus appears out of nowhere, that is, from the very homogeneity of the medium during its cooling, it will be a homogeneous nucleation.

On the other hand, if said nucleus takes place thanks to the surface provided by another insoluble solid particle, or to the imperfections of the container, then we will have a heterogeneous nucleation. The latter is the most widely used and known, especially when a tiny crystal, previously obtained, of the species that we want to crystallize is added to the solution.

Crystals can never form out of thin air without nucleation first.

Crystal growth

There is still a lot of dissolved solute in solution, but the concentration of solute in these nuclei is higher than in their surroundings. The nuclei act as supports for more solute particles to lodge and "fit" between their growing structures. In this way, their geometries are maintained and gradually grow.

For example, the first nuclei in the image are purple hexagons; this is your geometry. As solute particles are incorporated, the nuclei grow into robust hexagonal crystals, which will continue to grow further if the solution is immersed in an ice bath.

Types of crystallization

What has been explained so far consists of the crystallization by cooling the solvent.

Solvent removal crystallization

Other types of crystallization are based on the removal of solvent by evaporation, for which it is not necessary to use so much volume of it; that is, it is enough to just saturate it with solute and heat it to supersaturate it and then, a little more, then leave it at rest, so that the solute finally crystallizes.

Solvent addition crystallization

Likewise, we have the crystallization caused by the addition of a solvent to the mixture in which the solute is insoluble (antisolvent). Therefore, nucleation will be favored as there are mobile and liquid regions where the solute particles will be more concentrated than in those where it is very soluble.

Crystallization by sonification

On the other hand, there is crystallization by sonification, where ultrasound generates and breaks small bubbles that again promote nucleation, at the same time that it helps to distribute crystal sizes more evenly.

And finally, there is crystallization from vapor deposition on cold surfaces; that is to say, the inverse phenomenon to the sublimation of solids.

Crystallization separation method

Crystallization is an indispensable technique when obtaining solids and purifying them. It is very recurrent in the synthesis of organic compounds, and represents one of the last stages to guarantee the purity and quality of the product.

Example of dye

Suppose, for example, that crystals of a dye are obtained and that they have already been filtered. Because this dye was initially obtained by precipitation in a synthesis, its solid shows the appearance of being amorphous, since it has many impurities absorbed and trapped between its molecular crystals.

Therefore, it is decided to heat a solvent where the dye is slightly soluble, so that when it is added it dissolves relatively easily. Once dissolved after adding a little more solvent, the solution is separated from the heat source and left to rest. As the temperature drops, nucleation takes place.

Thus, the dye crystals will form and appear more defined (not necessarily crystalline to the eye). It is at this precise moment that the container (usually an Erlenmeyer flask or a beaker) is immersed in an ice bath. The cold of this bath ends up favoring the growth of the crystals above the nucleation.

The crystals of the dye are then vacuum filtered, washed with a solvent in which it is insoluble, and left to dry in a watch glass.

Crystallization temperature

The temperature at which crystallization occurs depends on how insoluble the solute is in the solvent medium. Likewise, it depends on the boiling point of the solvent, because if the solute has not yet dissolved at the boiling temperature, it is because another more appropriate solvent must be used.

For example, solids that can crystallize in aqueous media will do so as the water decreases its temperature (that is, from 100 to 50 ºC), or while it evaporates. If crystallization occurs by evaporation, then it is said to take place at room temperature.

On the other hand, the crystallization of metals or of some ionic solids takes place at very high temperatures, since their melting points are very high, and the molten liquid is incandescent, even when it is cooled enough to nucleate its particles and grow your crystals.

Crystallization rate

In principle there are two direct ways of controlling the rate of crystallization of a solid: by the degree of supersaturation (or supersaturation), or by sudden changes in temperature.

Degree of supersaturation

The degree of supersaturation means how much excess solute is forced to dissolve by application of heat. Therefore, the more supersaturated the solution, the faster the nucleation process, since there is a greater probability that nuclei will form.

Although crystallization is accelerated in this way, the crystals obtained will be smaller compared to those obtained with a lower degree of supersaturation; that is, when their growth is favored and not nucleation.

Temperature changes

If the temperature is lowered sharply, the nuclei will hardly have time to grow and, not only that, but they will also retain higher levels of impurities. The result is that although crystallization occurs more quickly than slow cooling, the quality, size and purity of the crystals end up being lower.

Rapid crystallization due to a sudden drop in temperature. Source: Gabriel Bolívar.

The image above serves to contrast the first. The yellow dots represent the impurities, which due to the abrupt growth of the nuclei are trapped inside them.

These impurities make it difficult to incorporate more purple hexagons, resulting in a lot of small, impure crystals in the end, rather than large, pure ones.

Applications

The crystallization of ice cream is one of the most important aspects during its industrial or artisan production. Source: Pixabay.

Crystallization, as well as recrystallization, is vital to obtaining high-quality, pure solids. For the pharmaceutical industry this is especially true because their products must be as pure as possible, just like the preservatives used in the food industry.

Furthermore, nanotechnology is highly dependent on this process so that they can synthesize nanoparticles or nanocrystals, rather than robust crystal solids.

One of the everyday examples in which crystallization has a great participation is in the production of ice cream. If you are not careful with water, it crystallizes in a separate phase (ice) from its lipid content, thus affecting its texture and flavor; In other words, it will be more like a shaved ice cream or ice cream.

Therefore, the ice crystals should be as small as possible, so that the ice cream is smooth to the taste and touch. When these ice crystals are a bit large, they can be detected in light because they give the ice cream a frosted surface.

Examples of crystallization

Finally, mention will be made of some common examples of crystallization, both natural and artificial:

Snowflakes

Snowflakes are formed through a natural crystallization process. It is known that each snow crystal is unique. This is due to the conditions that occur during the second phase of crystallization (growth).

The different geometric shapes that snow crystals present are due to the conditions that they must face during crystal growth.

Salt

Salt is the most common example of crystallization. This can be formed both naturally (such as sea salt) and artificially (as is the case with table salt).

Sugar

After salt, sugar is one of the most common crystals. It is formed through a series of complex industrial processes in which sugarcane juice is taken and subjected to an artificial crystallization process.

Diamond

Diamond is a gemstone that is formed from the crystallization of pure carbon. This is the hardest material known on the planet. Its formation can be natural, as is the case with diamonds found in mining deposits, or synthetic.

Ruby

Ruby is a reddish crystal that is formed from the crystallization of aluminum oxide (coridon).

Stalagmites

Stalagmites are structures that can be found in caves, specifically in soils (growing pointed upwards). They are composed of calcium compounds and are formed from the crystallization of calcium salts found in the water that falls from the ceilings of caves.

Stalactites

Stalactites, like stalagmites, are made of calcium and are found in caves. They differ from the latter because they hang from the ceilings. They are formed by the crystallization of the calcium salts present in the water that infiltrates the caves.

Quartz

Quartz is a gem that is formed from the crystallization of silicic anhydride. It is one of the most abundant minerals in rocks and its color is variable.

Peridot

Also called olivine, this gemstone is formed thanks to the crystallization of iron and magnesium. It is greenish in color and is usually diamond-shaped.

Silicates

Silicates are materials created by the crystallization of silica and other elements (iron, aluminum, calcium, magnesium). They are present in all rocks.

Candies

Candies are made with sugar crystals, so it can be said that two crystallization processes intervene: the first for the formation of sugar and the second for the formation of molasses.

Creamy ice-cream

Creamy ice cream contains a series of crystals that give it the final smooth texture. Among the crystals that creamy ice cream contains, lipid crystals (formed from fat) and ice crystals stand out. It should be noted that some ice creams also contain lactose crystals.

In this sense, ice cream is obtained through various artificial crystallization processes (one for lipids, one for ice and one for lactose).

Others

-Preparation of sugar crystals around a thread or rope and a supersaturated sweet solution

-Formation of sugar crystals from honeys deposited at the bottom of their jars

-The growth of kidney stones, which consist of the essence of calcium oxalate crystals

-The crystallization of minerals, including gems and diamonds, over the years, whose shapes and edges are a reflection of their orderly internal structures

-Deposition of hot metal vapors on cold bars as supports for the growth of their crystals.

References

1. Day, R., & Underwood, A. (1989). Quantitative Analytical Chemistry. (fifth ed.). PEARSON Prentice Hall.
2. Wikipedia. (2019). Crystallization. Recovered from: en.wikipedia.org
3. Helmenstine, Anne Marie, Ph.D. (May 23, 2019). Crystallization Definition. Recovered from: thoughtco.com
4. University of Colorado. (sf). Crystallization. Organic Chemistry. Recovered from: orgchemboulder.com
5. Syrris. (2019). What is crystallization? Recovered from: syrris.com