100 Nano-Stories: Surface Area Of Aerogels Explained!

Preface! → ✨

It’s your favorite material science & nanotechnology enthusiast reporting back for another article! In the last article, we talked about my personal cheatsheet on identifying alkanes and naming them by just looking at them!

But do you know why that aerogel has so much surface area, despite its size? Wait, you don’t really understand what it means to have a surface area?

It’s time for another brief episode of “Aerogel Breakdown!”

What’s “Surface Area”? → 🤔

Surface Area means that it is a measure of the total area that the surface of a 3-dimensional object covers/occupies.

These 3 dimensions are:

  • Length (L)
  • Width (W)
  • Height (H)

But remember, we are talking about the surface area, not the area.

In aerogel, the surface is covered with small nanopores, and each of these nano-sized frameworks makes up the open-porous skeleton of the aerogel!

These open pores allow for molecules, air, or other particles to undergo a chemical reaction or attach to the aerogel. This can increase the surface area to about half a football field, or approximately 500–1500 m²/g.

But I don’t get it, Carlos! Give me some examples!

Example 1 → 💡

Imagine you have a piece of cardboard and a sheet of homework paper. If I told you to crumple up the cardboard and make a small ball made out of cardboard, it would take you some time.

But if you crumpled up the homework paper, you can get a ball that is WAY smaller than the “ball” you were able to make with the notebook. Why? Because of the thickness of the paper. It is so thin, that the paper can be crumbled up easily. The surface area has now decreased.

But what if I asked you to make a ball the same size as the cardboard ball, but only with that same sheet of homework paper? Well, it would take more sheets of paper to achieve the same size as your original sheet of cardboard! So you would increase the surface area because you are adding up the pieces of paper to equal the size of the cardboard ball. Therefore, you have more Surface Area crumbled up in a small space!

It’s the same reasoning with aerogels. The surface of aerogel has these small nano-pores that make up the surface area. But adding so many to the surface of the gel adds more surface area. But because these pores can bond together to create a 3-dimensional network to make the gel, we create this large surface area in the aerogel!

Woah! That’s interesting, Carlos! But aerogel can absorb a lot, especially water! So how is that possible? Is it because of surface area, or is it because of volume?

Example 2 → 💡

If I am to explain how aerogel can absorb anything, let’s explain this concept again with a sheet of paper!

With a sheet of paper, it has some surface area, but the volume is close to zero! The paper is extremely thin!

This is known as the surface area to volume ratio. Paper & aerogels have a VERY high surface area in comparison to the volume of the paper/aerogel, which is not a lot.

Here are some numbers to help you out:

  • Surface Area of an 11.5 x 8 in. sheet of Paper → 603.2246 cm²
  • The Volume of an 11.5 x 8 in. sheet of Paper → 5.8223 cm³
  • Surface Area of a 2.2 cm-sides piece of Aerogel → 7,500,000 cm²
  • The Volume of a 2.2 cm-sides piece of Aerogel → 10.648 cm³

To get the Surface Area (SA) to Volume (V) ratio, you divide the surface area of something divided by the volume. 💡

  • Paper (SA / V) Ratio → Approx. 103.61: 1
  • Aerogel (SA / V) Ratio → Approx. 704357.63: 1

Basically, the smaller an object is, the larger its SA / V ratio. A 2 x 2 cm piece of aerogel has an incredibly large SA / V ratio. Therefore, it can absorb A LOT of things, including water! 🚿

Thanks, Carlos! But what about insulation? How do we know what type of aerogel is the best for insulating something huge?

Example 3 → 💡

For this last example, let’s use a long piece of aerogel, like Pyrogel, and a leaf to demonstrate the final example.

A leaf’s objective is to turn sunlight into energy. But to do that, it needs to first face the sun (duh!).

But here’s the catch, reader. The plant wants to have the maximum surface area in comparison to the volume of the leaf (SA / V). This is why a leaf is very thin. The ratio of SA / V needs to be as high as possible to absorb the most amount of sunlight possible.

This is the same example with Pyrogel from Aspen Aerogels. Their aerogel is very long but VERY thin. There is not a lot of volume for the Pyrogel to hold on to, but there is so much surface area. The ratio of SA / V is so high, that the Pyrogel is essentially a maximum-insulating piece of material!

Closing Thoughts! → 💭

Well, that was an interesting discussion, reader! Now we understand how the surface area is used in aerogel to determine porosity, why it can absorb anything, and how surface area & volume love to be with one another!😤

See you tomorrow for another lesson on aerogel technologies & organic chemistry!

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© 2021 by Carlos Manuel Jarquin Sanchez. All Rights Reserved.

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