100 Nano-Stories: Methoxylation!

Episode #46: Hydrophilic Aerogel → Hydrophobic Aerogel (Part 1)!

Carlos Manuel Jarquín Sánchez
5 min readMar 15, 2021

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Preface! ✨

It’s your favorite material science & nanotechnology enthusiast! A couple of days ago, I explained the difference between a hydrophilic aerogel and a hydrophobic aerogel! From terms such as polar and nonpolar to electronegativity and dipole moments!

This article is a must-read to proceed to the following article! The article is only 7 minutes long!

The essential points in the previous article are as follows:

  • Essentially, any molecule that is composed of only oxygen and hydrogen (water) will have polar bonds. Therefore, the molecule will be polar.
  • Essentially, any molecule that is composed of only carbon or hydrogen will have non-polar bonds. Therefore, the molecule/organic compound will be non-polar.
  • Polar Molecules → Hydrophilic Aerogel.
  • Non-polar Molecules → Hydrophobic Aerogel.
  • Hydrophilic Aerogels will absorb water and destroy the aerogel.
  • Hydrophobic Aerogels will repel water and bounce away from the aerogel.

But now that we know what we need to know, let’s move on to how to convert a hydrophilic aerogel to a hydrophobic aerogel!

Methoxylation! 💡

The Process! 🔑

Methoxylation or vapor-phase treatment is the process of heating the initially hydrophilic aerogel with methanol vapor. This methanol vapor converts the Si-OH groups on the surface of the aerogel to Si–OCH3 groups. This renders the aerogel hydrophobic and repels water from being absorbed into the aerogel.

But Carlos, what is Methanol?

Methanol → H3C-OH.

Methanol vapor is our hydrophobe. This will be our reactive chemical that can rearrange the polar groups of the hydrophilic aerogel. The overall surface of the aerogel will become non-polar, a.k.a hydrophobic.

The Explanation! 🔑

Methanol is composed of only carbon and hydrogen atoms. The reason why is because of their electronegativity, or EN.(the chance that a pair of elections will be attached to a certain atom). If the EN of a molecule is greater or equal to 0.5, you have polar bonds. If the EN of a molecule is less than 0.5, you have non-polar bonds.

But Carlos, Si-OCH3 has hydrogen in it? So explain that! 😠

In simple terms, any molecule that has hydrogen bonding is polar.

Hydrogen bonding → OH bonds.

The original surface of the aerogel is covered with Si-OH bonds. And the OH bond is what makes aerogel hydrophilic. But if we can separate the hydrogen bonding and add the methanol vapor, we can change the chemical properties and render the aerogel hydrophilic because there are no longer any hydrogen bonding/hydroxyl groups (Si-OCH3).

But Carlos, where did the “Si” in Si-OCH3 come from? Explain that! 😠

Why so angry, reader? 😅

But anyway, let’s make it simple to understand.

Si → Silicon.

The Silicon came from the precursor that was made to create the gel in the first place. A precursor is a compound that participates in a chemical reaction that will produce another compound. In this case, the compound we want to produce the sol-gel that will become our hydrophilic aerogel.

The precursor that is used for this methanol vapor process is tetramethoxysilane (TMOS).

If the name tetramethoxysilane confuses you, or you wonder why it has that funky name, I’ll break it down for you, reader.

Tetra → Four. The “Tetra” is used because of the four methane groups attached to the oxygen atoms.

Meth → Methanol. Methanol is one of the main compounds used to render the aerogel from hydrophilic to hydrophobic. No jokes here, reader. 😂

Oxy → Oxygen. Oxygen is attached to the methane groups, which are single-bonded to the silicon atom.

Sil → Silicon. Silicon is the main substance in the aerogel, otherwise known as silica aerogels.

Ane → single bonds. The entire molecule (TMOS) is an alkane. Alkanes are single bonds that occur between carbon atoms in a molecule. Visually, single bonds mean that there is only one line connecting the atoms in a molecule.

Here is a single-bond (alkane) to demonstrate the visual example.

Methane Example (Single-Bonds)!

Finally, for the surface of the gel to go from hydrophilic to hydrophobic, we must allow the methanol vapor to go past its critical temperature & pressure.

For Methanol, the critical temperature is around 239.45 °C or 463.01 °F. The critical pressure is 81.035 Bar Pressure or 8103500 Pascals.

Once we pass this critical pressure and temperature, the methanol vapor can cover the surface and make OH groups disappear from the hydrophilic aerogel! We end up with Si-OCH3 Groups and a hydrophobic aerogel!

Closing Thoughts! 💭

That is the first process of making a hydrophobic aerogel! Adding an organic group that renders non-polar bonds will change the surface of the aerogel into one that perfectly repels water!

See you soon for the second process of converting a hydrophilic aerogel into a hydrophobic aerogel! 👋🏽

Vocabulary! 📓

Hydrophilic Aerogel → Polar molecules are attached to the surface of the aerogel. Hydrophilic Aerogels will absorb water.

Hydrophobic Aerogel → Non-Polar molecules are attached to the surface of the aerogel. Hydrophobic Aerogels will repel water.

Polar molecules → The electrical charges of the molecule are not evenly distributed. The OH group is a polar molecule commonly found on the surface of hydrophilic aerogels. Polar molecules are hydrophilic.

Non-Polar molecules → The electrical charges of the molecule are evenly distributed. No positive or negative charges are formed in the molecule. All alkanes, alkenes, and alkynes are non-polar molecules because they are composed of carbon and hydrogen.

Electronegativity → The chance that a pair of elections will be attached to a certain atom. If the difference between the electronegativity is greater or equal to 0.5, the molecule is polar. If the difference in electronegativity in the molecule is less than 0.5, the molecule is non-polar.

Alkanes → Single Bonds that occur between carbon atoms in a molecule in organic chemistry.

Methanol → H3C-OH.

H → Hydrogen.

C → Carbon.

O → Oxygen.

Si → Silicon.

Hydrogen Bonding → OH bonds.

Precursor → A (chemical) compound that participates in a chemical reaction that will produce another (chemical) compound.

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

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