Today I would like to tell you about the lightest material on planet Earth – aerogel. As a matter of fact, there exist quite a lot of kinds of aerogel. Some of them consist of metal oxides, for instance aluminium oxides. Others can consist of carbon tubes and graphenes. By the way such an aerographene is a few times lighter than air. However, I can not manage to get this rare material that is why today I am going to show you classic silicon dioxide-based silica aerogel. As you can see this material is very fragile and it crumbled when it was transported in the package.
source/image: Thoisoi2 – Chemical Experiments!
When aerogel crumbles are put on paper sheet, it can be seen that it looks translucent and very light and it is almost invisible against a light background. Aerogel’s density is extremely small about one tenth of a gram per milliliter. The density of aerogels of higher quality can even be compared to that of air. It’s not surprising because aerogel mostly consists of air. To illustrate what aerogel’s structure is like, we can compare it to that of any other gel-like substance. For instance water ball or shampoo.
Now try to imagine that all water in such a material has been substituted with air. Roughly speaking, this is what aerogel looks like from inside – small porous material filled with air or another gas. It is quite easy to make aerogel. First, regular silica gel has to be made for instance by mixing together tetraethyl orthosilicate and ammonia solutions in methanol medium. Later such a gel is washed with methanol in order to wash away all traces of water. After that it is sent to a supercritical extractor. Let the complex names not scare you because everything works as simple as a bicycle.
Carbon dioxide is pumped under pressure into such an extractor after that the case of of the extractor starts to heat up. When the temperature rises, the liquid carbon dioxide passes to the supercritical state that is becomes something something between liquid and gas. In such a state it can easily wash away methanol from gel substituting it with gas. This is how aerogel forms. If silica gel is just left to dry up in air, this gel will just shrink, turning into a piece of silicon dioxide because of methanol surface tension.