Sublimation

What is Sublimation in Chemistry?

Before we go into the subject of sublimation, let us try to understand a few terminologies that will be relevant to this topic. In day to day life, we see substances occurring in many states, called states of matter. We see solids, liquids, and gases. Each of these is a phase. When you say solid, it is a substance in which the particles which constitute it, are arranged in specific patterns. This arrangement provides a certain property to it. When you say liquids, they are free to flow and don't have a definite shape. The molecules are more loosely arranged in liquids as compared to solids. A gaseous state is one where there is no definite volume or shape. The molecules are much more loosely arranged than in liquids.

Few regular examples of these are:
Examples of Solid: Brick, Wood etc
Examples of Liquid: Water, coffee etc
Examples of gas: Water vapor, Hydrogen, oxygen etc:

Terminology:

We all know that substances undergo a change of phase when they are subjected to certain conditions. For example, ice melts to form water, water freezes to form ice, and when water is boiled it becomes water vapor. Hence solid becomes liquid, the liquid becomes gas, etc: Generally, solids pass through the liquid phase and then into the gas phase. However, there are certain situations where a solid enters the gas phase directly without going through the liquid phase. Such a process is called 
Sublimation. Sublimation is defined as the change or transition from the solid phase into the gas phase without entering the liquid phase. The opposite process of this where the gas goes directly to the solid phase is called de-sublimation or deposition. In the reverse process, energy is given out. In cold temperatures, water vapor undergoes a change and forms a thin layer of solid ice on leaves and grass. This is called deposition.

The solid substance that undergoes the change to form gas is called Sublime.

The solid obtained by cooling of the vapors is called as sublimate.

Now we bring in another term here, “Endothermic”. A process which absorbs thermal or heat energy while undergoing a change is called Endothermic. Water absorbs heat to become water vapor. Another term now is “Triple Point”. The triple point is defined as the temperature and pressure of a substance, at which its solid, liquid and gaseous phase coexist in complete equilibrium. Now to get back to sublimation, the change from the solid phase to the gas phase occurs at temperature and pressure which are below the triple point.

It is very important to note that the term sublimation is applicable only when a purely physical change of state happens. A chemical reaction resulting in the formation of gas from solid is not sublimation. Ex: We all have seen candles burning. When a candle burns, the paraffin in the candle burns and it is vaporized. This vapor reacts with oxygen in the atmosphere and forms carbon dioxide. This is NOT sublimation because this happens because of a chemical reaction.

A few examples of sublimation:

  • 1. Naphthalene balls used in mothballs readily sublimates at room temperature and pressure.

  • 2. Dry ice which is solid carbon dioxide readily sublimates at room temperature and pressure.

  • Another term that is relevant to understanding sublimation is "Vapour Pressure". The pressure is exerted by the vapor in thermodynamic equilibrium with its solid or liquid phase at a temperature in a closed system. In simple terms, it relates to the ease with which particles can escape from the solid or liquid phase into the gas phase. A substance which has a high vapor pressure at normal temperatures is called Volatile.

    The process:

    When heat energy is absorbed by some of the molecules, they acquire much higher energy than their neighbors and thereby overcome the forces of attraction and escape into the vapor phase. Since this requires additional energy, it is called as an 
    endothermic reaction. The heat or energy required to change the state from solid to gas is called the enthalpy of sublimation and this is unique to each substance. Generally, it is expressed as KJ/mol or even KJ/kg. 

    Relatively, very few solids are capable of sublimation. Hence this process of sublimation can be used as an excellent purification method. When a solid is contaminated with non-volatile impurities then this is a very good method for separation and purification. The contaminated or impure solid is heated in a vessel while keeping it in contact with a cold surface. On heating, the volatile solid sublimes and attaches to the cold surface above it, while the impurities remain below. This is a very eco-friendly process because this does not use any solvents, or no waste is generated. The limitation is that it is not very efficient in separating volatile solids from one another.

    In order to effect sublimation, many criteria need to be met. First, the sample should be kept at a sufficient temperature to maintain high vapor pressure, because if it goes below that temperature the material could decompose. Second, a surface must be available on which the sublimed vapor can condense or solidify.

    How is Sublimation different from Evaporation?

    In sublimation, the transition is from the solid phase to the gas phase, whereas in evaporation it is the change from Liquid phase to the gas phase.

    Applications:

    Is the process of sublimation useful to us in day to day living? Do we see this working? The answer is Yes, We do. For instance, consider the case of air fresheners which we keep in bathrooms or cupboards. When the air fresheners are heated in a hot water bath you can see the transition from solid to the gas phase. Appropriate precautions should be taken when heating such substances. Similarly, the case of Naphthalene balls or also called as mothballs. They sublime and it helps keep moth and other insects away. Camphor is another substance which sublimes to give a very pleasant smell. There are specialized printers that use this process. In the process of printing, the ink turns from solid to gas and then again into the solid phase. These techniques are used in fabric printing also in textile industry.

    Another important use of sublimation is in the frozen food industry. This is called freeze-drying. The frozen water in the material will sublimate from the solid phase to the gas phase when you reduce the surrounding pressure. There is no heat involved in the removal of the water, so this is different from evaporation. Since it uses very low temperatures, it results in a high-quality product. The shape of the product is also retained and once the product is rehydrated it has excellent quality. This technique is used when the foodstuff is to be preserved for a longer period of time or when astronauts go out to outer space, NASA could provide quality food using this technique.

    Pharmaceutical companies also use this technique. Once the water is removed from the material and is stored in a vessel, the material can be easily stored, and shipped to other locations. In the destination, it can be reconstituted to its original form. Examples of such products are the Measles virus vaccine, Typhoid Vaccine among others. It is also used for manufacturing the raw material for the pharmaceutical products.

    Dye sublimation printers are now replacing inkjet printers. The prints get dried as soon as they exit the printer and are ready to use. Such printers use minimal moving parts; so maintenance is easier. In the textile industry, dye sublimation process is used to printing in synthetic fabrics like polyester. Even in T-Shirts, banners, flags etc: this technique is used. The principal advantage in the textile industry is the colors in printing are extremely brilliant due to the bonding of the dye to the synthetic fibers.
    With the sublimation process, many decorative works in printing are also being done. Also, wash resistant, scratch proof images can be sublimated with the current technology that is available. In areas of awards and recognition, sublimation allows manufacturing of full-color plaques, signs, name badges etc: A lot of products like coffee mugs, pens, and bags can be printed with dye sublimation at very low cost.

    Dye sublimation is fairly eco-friendly and safe for the worker process. There is absolutely no waste produced in this process. When compared to the process of screen printing in garment and apparels, this process does not result in any wastewater discharge. There is a potential for hazard during the heating process of the inks which can result in fumes being emitted.
    Finally, the efficiency of a sublimation process depends on the vapor pressure of the solid that is to be purified and also of the impurities that are to be removed.

    Thus, so far we have had a brief introduction to the topic of sublimation, understanding of relevant terms, the common examples of sublimation as we see in day to day life, the industrial and pharmaceutical uses and some benefits to the environment compared to another conventional process.