100 Nano-Stories: Catalysts + Hydrolysis!
Episode #87: Gelation Of Silica Aerogels!
Preface! ✨
It’s your favorite material science & nanotechnology enthusiast! Today, we will continue on the mechanisms to create the gelation of silica aerogels!
Here is the article that introduces Sol-Gel Chemistry for Aerogel Technologies:
TL;DR → Sol-Gel Chemistry! 🔑
- Sol-Gel Chemistry means a solution of either a metal or carbon-based precursor with ethers to form a polymer. this polymer will create a vast network of polymers bonded to each other, creating a gel!
- The main chemicals to start with the formation of the hydrophilic gel (aka precursors) are tetramethoxysilane (TMOS) and tetraethoxysilane (TEOS).
- The main chemicals to start the formation of hydrophobic gel are methyltriethoxysilane (MTES).
- The Silica Atom has a partial positive charge (δ+) ≈ 0.32 in TEOS. This means that the silicon atom (Si) tends to lose its Valence Electrons (VE) in its outer shell to the oxygen atoms attached to the silicon atom.
- To mitigate the loss in negative charge, we can fix it via hydrolysis and catalysts!
Now it’s time to begin the main dive into hydrolysis and catalysts!
Catalysts + Hydrolysis! 💡
Hydrolysis! 🔑
Hydrolysis is the chemical breakdown of a compound/molecule due to a water reaction with the compound.
In this process, the process of hydrolysis allows the silica groups to turn into certain alkoxides like TMOS, TEOS, and MTES. The alkoxides will look like this: M(OR)x
M → A Metal (Silica[which is technically a metalloid])
O → Oxygen Atom
R → Alkyl Groups/Substituents. (CH₃, C₂H₅, etc. . .)
x → The number of atoms & groups of oxygen and alkyl groups in the compound.
Let’s use TEOS as our main precursor that will undergo hydrolysis. Our silica alkoxide will be Si(OC₂H₅)₄
Now that we have the precursor, let's find out what happens when we undergo hydrolysis!
The chemical (TEOS) will lose its methyl groups and have them replaced with hydroxyl groups (OH groups). This will result in the aerogel being hydrophilic (adsorbs water) and being polar. However, it depends on how many water molecules we add because we can only add up to 4 water molecules per TEOS Compound.
The more water molecules you add, the more methyl (CH3) groups are replaced with hydroxyl groups (OH).
But this is just 1 TEOS Compound. Now picture this for the entire gel! Millions of these TEOS molecules must undergo hydrolysis! Not an easy task! 😅
However, there are 2 ways we can speed up hydrolysis in an aerogel:
- The OH groups are the nucleophile. A nucleophile is a chemical/reactant that wants to give a pair of electrons to create a covalent bond. The TEOS Compound is kind of like the electrophile, which wants the pair of electrons to create covalent bonds.
The hydroxyl groups are going to be substitute with the methyl groups. But the more carbon and hydrogen atoms are in a compound, the time for the substitution of the nucleophile and the methyl group will get longer.
Basically, TMOS {Si(OCH₃)₄}, takes less time to undergo hydrolysis than TEOS {Si(OC₂H₅)₄} because there are fewer Methylene (CH2) groups in the alkoxide precursor.
“Me” (Methyl) has 0 Methylene groups.
“Et” (Ethyl) has 4 Methylene groups.
These Methylene groups make it harder for the Hydroxyl Groups to substitute the Methyl Group, (because of the length of the alkyl groups). This means that the process is also very slow, and gelation time increases as the alkyl groups increase (or the methylene groups increase).
The other way to change all the methyl groups to hydroxyl groups faster via hydrolysis is to add a catalyst! 😄
Catalysts! 🔑
We just explained that most gels take time to react to hydrolysis because of the extra Methylene groups attached to the aerogel alkoxide precursors!
Now, this when the catalyst comes in handy!
A Catalyst is a chemical that speeds up a chemical reaction without being affected, and it lowers the activation energy of the reactants so the reaction between two compounds can occur and form the new compound.
But in the case of hydrolysis, only acidic catalysts work to increase the rate of hydrolysis. Acidic catalysts have a pH of less than 7.
But why should we use acidic catalysts? 💭
With the acidic catalyst, we can transfer a proton to an atom, molecule, or ion.
In this case, the acidic catalyst will be transferring the proton to the negatively charged ligand (mostly the CH₃, since that is the group that will leave the TMOS, TEOS compound).
This addition of the proton can make the ligand more efficient in leaving the gel (OH-CH₃).
For TEOS, the catalysts that will work to increase hydrolysis are the ones with a pH lower than 7, but certain catalysts will allow the gel to decrease gelation time.
Closing Thoughts! 💭
We have completed our introduction to hydrolysis and catalysts! I will talk a bit more about the details behind the mechanisms in future articles, but if there is anything that I failed to explain clearly, please let me know, as I will do my best!
See you soon on more Organic Chemistry & Coordinate Bonding! ✌🏽
Vocabulary! 📓
Polymers → A chain of many repeating/identical large molecules that make up a material.
Precursors → Chemical compounds that participate in a chemical reaction to produce a new compound/molecule.
Catalyst → A chemical that speeds up a chemical reaction without being affected; and it lowers the activation energy of the reactants so the reaction between two compounds can occur and form the new compound.
Hydrolysis → The chemical breakdown of a compound/molecule due to a water reaction with the compound.
Nucleophile → A chemical/reactant that wants to give a pair of electrons to create a covalent bond.
Electrophile → A chemical/reactant that wants the pair of electrons to create covalent bonds.
Protonate → The transfer of a proton to an atom, molecule, or ion.
Ligand → An ion or molecule attached to a metal atom by coordinate bonding.
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