PROGRESSIVE SUBLIMATION: CONCEPT, PROCESS AND EXAMPLES - CHEMISTRY - 2025

The progressive sublimation is a thermodynamic process in which an endothermic change occurs directly state from a solid to a gas, without prior formation of the liquid. The behavior of the solid under normal conditions is to heat up and melt; that is, to merge. Meanwhile, in sublimation the solid begins to smoke directly, without the previous appearance of drops indicative of its melting.

What is described in the paragraph above is represented in the image above. Suppose a solid orange mixture (left), which begins to heat up. The mixture consists of two components or solids: one yellow and the other red, the combination of which produces the orange color.

Example of sublimation of a hypothetical orange solid. Source: Gabriel Bolívar.

The red solid sublimates, since a liquid does not form from it but ends up deposited (red triangles) at the base of the upper container; the one that contains ice cubes, and therefore offers a cold surface. Meanwhile, the yellow solid remains unchanged by heat (yellow rectangle).

The red triangles or crystals are deposited thanks to the cold surface of the receiving container (right), which absorbs their temperature; And even if it's not showing, the size of your ice cubes should decrease due to heat absorption. The yellow solid is not sublimable, and if you keep heating it sooner or later it will melt.

Progressive sublimation concept

Process

It has already been said that sublimation is an endothermic state change, because for it to occur there must be heat absorption. If the solid absorbs heat its energy will increase, so its particles will also vibrate at higher frequencies.

When these vibrations become very strong, they end up affecting intermolecular interactions (not covalent bonds); and consequently, sooner or later the particles will move further away from each other, until they manage to flow and move more freely through the regions of space.

In some solids, the vibrations are so strong that some particles “shoot” out of the structure instead of agglomerating in moving clusters that define a droplet. These particles escape and integrate the first "bubble", which would rather come to form the first vapors of the sublimated solid.

We speak then not of a melting point, but of a sublimation point. Although both are dependent on the pressure prevailing on the solid, the sublimation point is more so; therefore, its temperature varies remarkably with changes in pressure (as does the boiling point).

From solid structure to gaseous disorder

In sublimation it is also said that there is an increase in the entropy of the system. The energetic states of the particles go from being limited by their fixed positions in the solid structure, to homogenizing in their capricious and chaotic directions in the gaseous, more uniform state, where they finally acquire an average kinetic energy.

Phase diagram and triple point

The sublimation point depends on the pressure; Because otherwise, the solid particles would absorb heat not to shoot out into space outside the solid, but to form droplets. It would not sublimate, but would melt or melt, as is the most usual.

The greater the external pressure, the less likely sublimation is, as the solid is forced to melt.

But which solids are sublimable and which are not? The answer lies in your P vs T phase diagrams, like the one shown below:

Phase diagram for a hypothetical substance. Source: Gabriel Bolívar.

We must first look at the triple point and go through the lower section: the one that separates the solid and gaseous states. Note that in the region of the solid, there must be a drop in pressure for sublimation to occur (not necessarily at 1 atm, our atmospheric pressure). At 1 atm, the hypothetical substance will sublimate to a temperature Ts expressed in K.

The longer and horizontal the section or curve below the triple point, the greater the capacity of the solid to sublimate at different temperatures; but if it is well below 1 atm, then high vacuums will be needed to achieve sublimation, such that pressures are lowered (0.0001 atm, for example).

Terms

If the triple point is thousands of times lower than atmospheric pressure, the solid will never sublimate even with ultra-vacuum (not to mention its susceptibility to decomposition by the action of heat).

If this is not the case, the sublimations are carried out by heating moderately, and subjecting the solid to a vacuum so that its particles escape more easily, without the need for them to absorb so much heat.

Sublimation becomes very important when dealing especially with solids with a high vapor pressure; that is, the pressure inside, a reflection of the efficiency of their interactions. The higher its vapor pressure, the more fragrant it is, and the more sublimable it is.

Examples

Solids purification

The image of the orange solid and its sublimable reddish component is an example of what sublimation represents as it relates to the purification of solids. Red triangles can be re-sublimated as needed until high purity is guaranteed.

This technique is used mostly with fragrant solids. For example: camphor, caffeine, benzoin, and menthol.

Among other solids that can be sublimation we have: iodine, ice (at high altitudes), theobromine (from chocolate), saccharin, morphine and other drugs, nitrogenous bases and anthracene.

Crystal synthesis

Returning to the red triangles, sublimation offers an alternative to conventional crystallization; Crystals will no longer be synthesized from a solution, but through the most controlled possible deposition of vapors on a cold surface, where there may conveniently be crystalline seeds to favor a specific morphology.

Say, if you have red squares, the crystal growth will keep this geometry and they shouldn't become triangular. The red squares will gradually grow as the sublimation takes place. However, it is an operationally and molecularly complex complex, in which many variables are involved.

Examples of crystals synthesized via sublimation are: silicon carbide (SiC), graphite, arsenic, selenium, phosphorus, aluminum nitride (AlN), cadmium sulfide (CdS), zinc selenide (ZnSe), mercury iodide (HgI ₂), graphene, among others.

Note that these are really two intertwined phenomena: progressive sublimation and deposition (or inverse sublimation); the vapor migrates from the solid to cooler regions or surfaces, finally settling in the form of crystals.

References

1. Whitten, Davis, Peck & Stanley. (2008). Chemistry (8th ed.). CENGAGE Learning.
2. Wikipedia. (2019). Sublimation (phase transition). Recovered from: en.wikipedia.org
3. Jones, Andrew Zimmerman. (January 27, 2019). Sublimation. Recovered from: thoughtco.com
4. Sheila Morrissey. (2019). What is Sublimation in Chemistry? - Definition, Process & Examples. Study. Recovered from: study.com
5. Elsevier BV (2019). Sublimation Method. ScienceDirect. Recovered from: sciencedirect.com