100 Nano-Stories: You Fixed My Aerogel!
Episode #14: Hydrophilic Aerogel → Hydrophobic Aerogel!
Preface
Okay, Carlos, what’s the secret to make the aerogel repel water and not look like this:
I will in a second, but reader, there may be new readers to the series of “100 Nano-Stories”. Don’t intimate them or scare them yet. Let them catch up with us!
Makes sense, Carlos. But how? Do you have anything to help them catch up with us?
How do you always come prepared to help everyone, Carlos?
Let’s just say I put myself in the reader’s shoes, reader.
But briefly, we discussed the difference between hydrophilic and hydrophobic silica aerogels and their properties. Secondly, we discussed why COLD Drying is ineffective to produce hydrophobic aerogels, and why H2O and Si-OH create a huge polar group that adsorbs the liquid into the surface of the silica network and turns the aerogel from blue to gray/white color.
If you don’t know what I am saying, I got you covered with vocabulary terms at the end of this article! I know what my readers want! 😉
Let’s Repel Water!
Sounds awkward, but that is the point of the hydrophobic aerogel! If you remember from our previous article, Si-OH & H2O are both polar, and will adsorb the water, only for the gel to collapse.
To counteract the hydrophilic problem, we-
Wait, Carlos, don’t you mean this? 😉
That’s one way to put it, reader! If that helps you understand the content, that’s good for me! But anyway, to counteract the hydrophilic problem, what we need is a reagent or a silylating agent that can “uno-reverse-card” the problem with our silica surface network.
Woah! Sounds intriguing! But is there only one option, or is there more than of these reagents or silylating agent thingies?
How about we start with silyation? Silyation is the process where we replace hydrogen in the Si-OH groups and replace it with a hydrophobe, mostly known as CH3-Si or SiR3. But why would they use these types of chemicals? It’s because these chemicals are non-polar. Non-Polar means that the positively & negatively charged forces in the water molecule are evenly spread out, so no positive or negative charge is formed on the molecule.
So does that mean it cancels the polar group?
Almost there, reader! Almost. What really happens is a chemical reaction occurs that rearranges the structure of the surface silica network that can repel water. To demonstrate, let’s use SiR3. This is what happens when we let the OH (Hydroxyl) Group combine & react with SiR3.
We introduced SiR3 to replace hydrogen and the OH group altogether! Remember, the OH group is what allowed for water to be adsorbed into the aerogel, but now that we removed H, the water will bounce off the aerogel! Hooray! Now we can have an aerogel that looks like this:
But Carlos, I still have two questions! What is the name of these chemicals, and can we do another article on this? This is so interesting!
Oh yeah! I almost forgot to describe the chemicals! Thanks for reminding me, otherwise I would have “repelled you” from reading more!😉
Okay, okay, moving on. Tough crowd, I see.
O stands for Oxygen.
H stands for Hydrogen.
Si stands for Silicon.
R3 stands for a hydroxyl group (OH).
And for your second question, I think it might be better to look at these terms once again. There will be confusion if I don’t explain these terms in further depth. So for now, this will be the end of the article.
In the meantime, I will be going to get a glass of water! I’m parched! 💧
Vocabulary 📓
O → Oxygen
H → Hydrogen
Si → Silicon
R3 → A Hydroxyl Group (OH)
H2O → Water
Non-Polar → The positively & negatively charged forces in the water molecule are evenly spread out, so no positive or negative charge is formed on the molecule.
Silyation → The process where we replace hydrogen in the Si-OH groups and replace it with a hydrophobe.
Reagent → A substance for use in chemical analysis.
Catalyst → A substance that can increase the rate of a chemical change without the substance undergoing any permanent chemical change.
Adsorb → The molecules are held loosely on the surface of the adsorbent (molecules of the solvent in the silica gel) and can be easily removed.
Hydrophilic → A substance/material that will tend to mix or dissolve if wetted by water.
Hydrophobic → A substance/material that will tend to repel or fail to mix with water.
Hydroxyl → A molecule containing one oxygen atom bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain hydroxyl groups.
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