100 Nano-Stories: My Aerogel Is Destroyed!
Episode #13: Why LTSCD/Hydrophilic Aerogels Are Trash… Kinda.
Wait, Carlos, I thought your series is supposed to be inspiring. And now you call hydrophilic- ish aerogel garbage?
Uhhh.. reader, —
Also, that is a hydrophobic aerogel, not a hydrophilic aerogel. I think you might want to get your facts straight, Carlos.
Glad to know you caught that, reader. The reason why I say that hydrophilic aerogel is “garbage” is because hydrophilic aerogels adsorb liquids. If your hydrophilic aerogel were to come into contact with water or any other liquid, the silica surface network would collapse, and your aerogel would turn from the classic blue color to a shade of white.
So then what makes hydrophobic aerogels so special then, Carlos?
Hydrophobic aerogels repel water. Their gel pores and the silica surface framework can be naturally prepared or altered to repel the water and maintain its silica framework remains intact.
Finally, hydrophilic silica aerogel is made through LTSCD Drying, and hydrophobic silica aerogel is made through HTSCD Drying!
Oh, no! But how can we fix that? You know, how about you talk to us about the gel pores of the hydrophilic aerogel today?
Sure! Glad you asked! Now let’s get into it!
Wait, Carlos! I don’t understand the technical terms, so can you link your article that talks about this, or add some vocabulary terms?
LTSCD → Hydroxyl Groups
Well, this is interesting Carlos! What do you mean by “Hydroxyl Groups”?
When we allow aerogels to go through LTSCD Drying, otherwise known as CO2 exchange, the aerogel will create hydroxyl groups. These hydroxyl groups will not react with the solvent of the silica gel, and the hydroxyl groups end up on the surface of the silica network.
But how do these hydroxyl groups look like, Carlos?
Dear God! What is this supposed to be?
The OH group is what makes the surface of the silica aerogel hydrophilic, but there is more than one way to make silica aerogel hydrophilic.
Some examples can include the process of hydrolysis & polycondensation. The gel will become a mixture of water, a catalyst, and other solvents that will make up the gel. But sometimes, fluid (mostly water) can get trapped in the nanopores of the surface of the silica gel network, which is very difficult to remove from the pores themselves.
So, why not just dry the gel? Not evaporation, I meant through LTSCD Drying.
That’s the problem, reader. If we dry the aerogel via LTSCD Drying with the pores trapped, we end up with complications such as cracking the gel under the heat & pressure being put on the silica gel. But if we neglect to remove deposits from the surface silica network, (i.e. water) the gel structure can weaken and some of it affects the properties of the surface of the gel.
But can we go back to the photo? Sorry to interrupt you, Carlos, but I’m still interested in what makes aerogels hydrophilic?
It is the silanol polar groups SI-OH that render the aerogel hydrophilic. Polar means that the positively & negatively charged forces in the water molecule are unevenly spread out, so positive or negative charges are formed on the molecule.
But water (H2O) is also polar. Hydrogen is positively charged, and oxygen is negatively charged. The reason is that hydrogen atoms do not have a strong attraction with their electrons, while oxygen has such a strong bond to the electrons, it’s almost as if the oxygen atom is hogging the electrons for itself! So if our Si-OH & H2O molecules are polar, they will mix and create hydrophilic in the aerogel, and attract water molecules, which promotes the absorption of water.
So then, how does the hydrophilic aerogel look like after it gets wet?
Jeez! What happened to all the blue color in the gel, Carlos?
That’s damage that we can’t fix, unfortunately. But don’t be disappointed. There are always solutions to this problem! We can talk about that in the next article!
Hmmm… but how do hydrophobic aerogels look like?
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.
LTSCD - Low-Temperature Supercritical Drying (a.k.a. COLD Drying)
HTSCD - High-Temperature Supercritical Drying (a.k.a. HOT Drying)
Polycondensation - Condensed molecules during the bonding process, higher molecular weight.
Solute - A substance (either solid or liquid or gas) that is dissolved in a solvent.
Solvent - The liquid in which a solute is dissolved to form a solution.
Supercritical Drying - Liquid in a gel/substance is being transformed into a gas with the absence of the surface tension and the capillary stress of the gel. It’s what allows for a silica gel to transform into an aerogel.
Hydroxyl - a molecule containing one oxygen atom bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain hydroxyl groups.
Polar (Group) - The positively & negatively charged forces in the water molecule are unevenly spread out, so positive or negative charges are formed on the molecule.
Electronegative - The tendency of an atom to attract a bonding pair of electrons is very high, and oxygen’s electronegativity is now of the highest on The Periodic Table Of Elements.
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