100 Nano-Stories: Adsorption!

Preface! ✨

It’s your favorite material science & nanotechnology enthusiast! It’s time to begin the last rabbit hole in this series: Sol-Gel Chemistry and Surface Chemistry of Aerogels!

For those who are wondering about the physics/properties of aerogel, you can find more about those topics below:

Today, we will discuss a concept in surface chemistry called adsorption. This concept will allow us to prepare our minds for what we will learn about in the sol-gel chemistry of aerogels.

There is no article to prepare your mind for this brain dump! Let’s dive right into today’s topic! 😄

What’s Adsorption? 💡

Definitions! 🔑

For starters, some of you may confuse the word adsorption with absorption.

However, these words have different meanings.

Absorption means that particles/molecules are moving into another substance. In some way, they are being sucked up by the material and are being spread out widely.

Adsorption means that the particles/molecules of something will collectively add up on the surface of a material. In some way, you could say the particles are “resting” on the material.

In adsorption, the particles (or the purple dots) are what’s known as the adsorbate.

The medium/material (the yellow circle) is what’s known as the adsorbent.

Positive & Negative! 🔑

In certain gels, we may find out that there are two kinds of adsorption:

  • Positive Adsorption
  • Negative Adsorption

Positive Adsorption occurs when the amount of adsorbate (particles) occurs more at the surface of the material than the inside (bulk) of the material.

Negative Adsorption occurs when the amount of adsorbate (particles) occurs less at the surface of the material than the inside (bulk) of the material.

Negative Adsorption vs. Positive Adsorption

What About Adsorption In Silica Aerogels?

In silica aerogels, the bonds at the surface/backbone of the aerogel are hydroxyl groups (OH) Groups.

O → Oxygen

H → Hydrogen

When the aerogel comes into contact with water, the aerogel undergoes physical adsorption. Physical Adsorption occurs when the adsorbates stay on the surface of the adsorbent via physical forces (London Dispersion Forces, Van der Waal forces, hydrogen bonds, etc.)

Physical adsorption is reversible, which perfectly allows aerogel to change its surface chemistry from hydroxyl groups to trimethyl silane groups. This means that we can change the aerogel from adsorbing water to repel the water.

The other reason why silica aerogel is such a good adsorbent is because of its surface area to volume ratio.

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

  • 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. 💡

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! 🚿

That means there is a lot of surface area for adsorbates to stick on when coming into contact with silica aerogel!

This is another property of silica gels, which is why when removing water from something, the water “rests” on the surface of the aerogel pores instead of entering the middle (the bulk of the aerogel.)

Closing Thoughts! 💭

This concludes the BRIEF introduction to the fundamental definitions and properties of adsorption for silica gels & silica aerogels!

If there is anything that I missed or left you confused, please let me know, and I will attempt to help you understand the topic!

See you in a few days to introduce Sol-Gel Chemistry! 👋🏽

Vocabulary! 📓

Absorption Particles/molecules are moving into another substance. In some way, they are being sucked up by the material and are being spread out widely.

Adsorption Particles/molecules of something will collectively add up on the surface of a material. In some way, you could say the particles are “resting” on the material.

Physical AdsorptionThe adsorbates stay on the surface of the adsorbent via physical forces (London Dispersion Forces, Van der Waal forces, hydrogen bonds, etc.)

Positive AdsorptionThe amount of adsorbate (particles) occurs more at the surface of the material than the inside (bulk) of the material.

Negative AdsorptionThe amount of adsorbate (particles) occurs less at the surface of the material than the inside (bulk) of the material.

AdsorbateThe particles that stick or “rest” on top of the material (aerogel).

AdsorbentThe material that the particles stick to or “rest”.

Surface AreaA measure of the total area that the surface of a 3-dimensional object covers/occupies.

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

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