REVERSE SUBLIMATION: CONCEPT AND EXAMPLES - CHEMISTRY - 2025

The reverse sublimation is a thermodynamic process in which a change occurs from state exothermic gas to a solid without first becoming a liquid. It is also known by the names of regressive sublimation, desublimation, or deposition; the latter is the most used in school and encyclopedic texts.

Reverse sublimation is said to be an exothermic process because gaseous particles (atoms or molecules) must lose energy by releasing heat into the environment; in such a way as to cool enough to form crystals, solidify, or freeze on a surface.

Reverse sublimation occurs wherever there is a sufficiently cold surface for crystals to be deposited on it directly from a gas phase. Source: Pixabay.

The word 'deposition' (and not 'deposition') means that the particle is deposited from a gaseous phase without wetting the receiving surface. That is why inverse sublimation phenomena are often found on icy objects; such as with the frost deposited on the leaves or winter landscapes.

Such deposition is often detected by a thin layer of crystals; although it can also be made of an apparent powder or clay. By controlling this process, new multilayer materials can be designed, where each layer consists of a specific solid deposited by chemical or physical processes.

Reverse sublimation concept

Reverse sublimation, as its name alone reveals, is the opposite phenomenon to sublimation: it starts not from a solid that evaporates, but from a gas that solidifies or freezes.

If you reason molecularly, it will look amazing that a gas is able to cool down to the point where it doesn't even condense in the first place; that is to say, that it passes to the liquid state.

The role of the surface

A gas, highly disordered and diffuse, suddenly manages to rearrange its particles and establish itself as a solid (whatever its appearance).

By itself this will be kinetic and thermodynamically difficult, since it needs a support that receives the gas particles and concentrates them so that they interact with each other while losing energy; that is, while they cool down. This is where the surface exposed to the gas participates: serving as a support and heat exchanger.

Gas particles exchange heat with the colder or icy surface, so they slow down and little by little the first crystalline nuclei are formed. On these nuclei, colder than the surrounding gas, other particles begin to be deposited, which are incorporated into their structure.

The end result of this process is that a layer of crystals or solid ends up forming on the surface.

Terms

In order for reverse sublimation to happen, either of these two conditions must ordinarily exist: the surface in contact with the gas must have a temperature below its freezing point; or the gas has to be supercooled, in such a way that as soon as it touches the surface it is deposited when it disturbs its goal stability.

On the other hand, deposition can also occur when the gas is hot. If the surface is cool enough, the high temperature of the gas will transfer to it suddenly and cause its particles to adapt to the structure of the surface.

In fact, there are methods where the surface does not even have to be cold, since it participates directly in a reaction with the gaseous particles that end up covalently (or metallic) deposited on it.

In the technology industry a methodology that works from this principle is widely used and is called chemical vapor deposition by combustion.

Examples of reverse sublimation

Beer dressed as a bride

When a beer is so cold that the glass of its bottle is covered in white when it is taken out of the refrigerator, it is said that it is dressed as a bride.

The beer bottle provides the necessary surface area for the molecules of water vapor, H ₂ O, to collide and lose energy rapidly. If the glass is black, you will notice how it becomes white out of nowhere, and you can tear it off with your fingernail to write messages or draw pictures on it.

Sometimes the deposition of humidity from the environment is such that the beer appears covered with white frost; but the effect does not last long, because as the minutes go by it condenses and moistens the hand of those who hold it and drink it.

Frost

Similar to what happens on the walls of a beer, frost is deposited on the inside walls of some refrigerators. Likewise, these layers of ice crystals are observed in nature at ground level; it does not fall from the sky unlike snow.

The supercooled water vapor collides with the surface of leaves, trees, grass, etc., and ends up giving them heat, in order to cool down and be able to settle on them, and manifest in their characteristic and radiant crystalline patterns.

Physical deposition

Until now, there has been talk of water; But what about other substances or compounds? If there are gaseous gold particles in a chamber, for example, and a cold and resistant object is introduced, then a layer of gold will be deposited on it. The same would happen with other metals or compounds, as long as they do not require an increase in pressure or a vacuum.

What has just been described is about a method called physical deposition, and it is used in the materials industry to create metallic coatings on specific parts. Now, the problem lies in how to obtain the gaseous gold atoms without high energy consumption, since very high temperatures are required.

It is there where the vacuum comes in, to facilitate the passage from solid to gas (sublimation), as well as the use of electron beams.

Soot on chimney walls is often cited as an example of physical deposition; although, the very fine carbon particles, already in solid state, and suspended in the smoke, simply settle down without undergoing a change of state. This leads to blackening of the walls.

Chemical deposition

If there is a chemical reaction between the gas and the surface, then it is a chemical deposition. This technique is common in the synthesis of semiconductors, in the coating of polymers by bactericidal and photocatalytic layers of TiO ₂, or to provide a mechanical protection material by coating them with ZrO ₂.

Thanks to chemical deposition, it is possible to have surfaces of diamonds, tungsten, tellurides, nitrides, carbides, silicon, graphenes, carbon nanotubes, etc.

Compounds that have the M atom to be deposited, and that are also susceptible to thermal decomposition, can yield M to the surface structure so that it becomes permanently attached.

That is why organometallic reagents are usually used, which when decomposed give up the metal atoms without the need to obtain it directly from it; that is, it would not be necessary to use metallic gold, but rather a gold complex to create the desired gold “plating”.

Note how the initial concept of inverse sublimation or deposition ends up evolving according to technological applications.

References

1. Whitten, Davis, Peck & Stanley. (2008). Chemistry (8th ed.). CENGAGE Learning.
2. Maria Estela Raffino. (November 12, 2019). Reverse sublimation. Recovered from: concept.de
3. Wikipedia. (2019). Deposition (phase transition). Recovered from: en.wikipedia.org
4. Helmenstine, Anne Marie, Ph.D. (January 13, 2019). Definition of Deposition in Chemistry. Recovered from: thoughtco.com
5. Malesky, Mallory. (December 06, 2019). The Difference Between Deposition & Sublimation. sciencing.com. Recovered from: sciencing.com
6. Encyclopedia of Examples (2019). Deposition Recovered from: examples.co