Aerogels

Can you think of any solid that is composed of only 99.8% of air?

Well, this strain credulity but these solids exist and are known as aerogels.

Well, I actually learned about aerogels out of boredom and deemed it judicious to push my curiosity a step further.
Basically what you need to know about aerogels is that they are the world's lightest solid owing to their porous structure and very low density.
The most remarkable aspect of aerogels is that it is composed of only 99.8% of air.
The picture on the right shows a silica aerogel.


Aerogels are mostly made from silica gels but can be made from other gels such as gelatin.
To get a better picture of what are aerogels lets consider jello.
Aerogels can be compared to jello in which all of the solvent(usually water) is removed from the jello leaving the solid structure of the jello untouched. The solid structure is aerogel.

But how can this be done???
Well, jello has a solid framework, where the cavities are filled with a solvent(normally water) and it has more liquid than solid. It is made of gelatin which is itself made of long chains of amino acids and the water molecule present forms a weak hydrogen bond with the amino acid(the electronegative oxygen on water molecule with the hydrogen from the amino acid). The jello structure is shown in the picture in the right.

To remove the water from the jello, we may try to evaporate the water directly by heating but this cannot be done.
The water molecules gain kinetic energy, become more energetic, break the weak hydrogen bonds and leave the jello. By doing so, the jello solid structure shrinks and hence we cannot form the aerogel. (remember that the aerogel is the solid framework of the jello which is untouched).After evaporation, the shrunk structure is shown on the right.


But how can this be done?
The answer remained a mystery for long when in 1929, an American scientist, Dr.Samuel Stephens Kistler crossed the boundary and finally synthesized aerogels.
Before indulging in the methods he used, we need to gather some preponderant points.
Critical temperature
Basically, it is the temperature above which a gas cannot be converted a liquid by compression only. If compression takes place at the critical temperature, there is no surface separating the gas and the liquid and each end of the horizontal part have merged into a single point known as the critical point of the gas.
Above the critical point, at a certain pressure and temperature, there exists a state of matter which we call a supercritical fluid. This phase can be described both as a gas and a liquid but with no boundary separating both. We can say that the supercritical fluid is a very dense gas whose density may be compared to that of a liquid.
To better visualize this concept, let us consider a liquid in a closed vessel that can withstand huge pressures. If the liquid is heated, the pressure of the vapor will increase proportionally to the amount of heat supplied as more molecules will enter the gas phase. Eventually, the molecules in the gas will become so close that it can be comparable to a liquid.
On the other hand, the liquid is being heated and gains kinetic energy, forcing the liquid molecule to break apart. The result of both the liquid and gas gives a single phase known as the supercritical fluid.

After a good overview of the above-mentioned things, lets now take a look at how Dr.Kistler successfully managed to synthesize an aerogel.
Like it is already mentioned, aerogels are prepared from gels, either from silica gels or other gels such as gelatin.
There involve two main processes involved in the preparation of an aerogel:

• Solvent exchange

During this process, the water solvent in the gel is replaced by alcohol(methanol), by simply dipping the gel in the alcohol. The gel is placed in methanol baths repeatedly over several days, allowing the methanol to diffuse in the gel and all the water to diffuse out, and replacement by methanol helps to keep the solid network. (shown below)Moreover, the solvent exchange is very important as it allows the second step to take place, which is supercritical drying,

• Supercritical drying

Supercritical drying, also known as critical point drying, is a process to remove a liquid from a substance in a precise and controlled way.
The gel is placed in a supercritical chamber where liquid carbon dioxide is added. The liquid CO2 replaces the methanol which becomes an impurity and the system is raised to a certain temperature and pressure above the critical point of CO2 to convert it to supercritical(single phase of semi-liquid and semi-gas which has no separating boundary). The CO2 can then be easily vented and we are left with the aerogel where the solid structure of the gel is unchanged.

Quite easy and fascinating right?
But just imagine back in time how did Dr.Kistler manage to devise these steps by himself.....



Application of aerogels
The range of application is almost illimitable, being a good thermal and acoustical(sound) insulator
Thermal insulation is explained by the fact that aerogels are made up of mostly air which is a poor thermal conductor. Another factor affecting thermal conductivity is the Knudsen Effect. This effect is the decrease in thermal conductivity due to the restriction of the movement of gas molecules due to the porous and cave structure of the aerogel.
Being good thermal insulators, they have been used to make clothes that can withstand extreme temperatures. They are also used in electronics, optics, chemistry and a plethora of other fields.

NASA has put aerogels to more exotic use, notably to catch dust particles from comets and as a thermal insulator for the Mars Rovers(which died on Feb 13, 2019, after 15 years of operation)
Reinforced with polymers, the aerogels have been used to capture high-velocity cometary samples and interstellar dust. The picture on the right shows a racket-shaped dust collector with small rectangular pieces of aerogels.


 If these dust, having a speed of around 6000kmps, were to be high speed captured, the shock would have altered their shape and chemical composition or even vaporize them. Aerogels, due to their high porous structures are able to act as good kinetic energy absorbers by decreasing the kinetic energy of the colliding particle until a stop(the particles break apart the network of the solid framework to lose
kinetic energy). This is shown by the picture on the right.


Well, this is the end of my first blog and I hope it was informative as well as captivating and for further reading, see the links below and don't forget to subscribe for more posts.
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research gate
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NASA
Atkins-Physical Chemistry 8th edition.