100 Nano-Stories: Ageing of Silica Gel!
Episode #08: Fundamental Concepts! (Section 1)
Welcome back, reader! Now that we talked about the Sol-Gel Process of how silica gel is made, now we can talk about the process of the ageing of silica gels!
Carlos, I’m new here to your series on “100 Nano-Stories”, so can you please give me your previous article on silica gels?
Thank you, Carlos! You’re a real life-saver!
As a bonus, I will also be including vocabulary terms and extra resources at the end for anyone who got lost in the article or for anyone interested in diving more into the topic, reader!
Thanks, Carlos! But before you start talking about the ageing of silica gels, can you just briefly talk about where you ended in your last article?
A wet gel has been formed because of the mixture of a precursor (silicon alkoxide), water, a catalyst (Ethanol), and a reagent (TEOS) for the structure will finally begin to take place because of a hydrolysis reaction with water and make the compound will polycondensate because of our chemical reaction!
However, despite the wet silica gel being formed after the sol-gel process, there are still some silica alkoxide groups that have not formed with the gel. So hydrolysis and polycondensation will continue to react within the gel, but this is also a good thing, as it will strengthen the structure of the silica gel’s network of nanopores (a.k.a. skeleton of the silica gel).
However, if you want to know how strong, or the density of the silica gel is (the lower our gel density is, the gel overall has less mechanical strength and stiffness), you can use check either the precursor concentration or the overall water solution inside of our new Sol-Gel Solution!
But not all Sol-Gels end up with strong mechanical properties suitable for real-world applications because some silica particles are attached to some of the weaker siloxane bonds of the gel, causing the structure of the silica gel to eventually collapse when undergoing the drying process of silica gels. To attempt to reinforce the strength of the remaining silica particles, we and add more of the siloxane bonds via “The Ageing Process”.
Introduction To “The Ageing Process”
The reason we allow for the ageing process to happen in silica gels is for a surplus of silica monomers, which men's that these molecules will bond to identical silica molecules to form polymers, and we have our polycondensation to strengthen our silica gel structure!
But, Carlos, let’s take a step back; you’re going too fast! Isn’t there something you forgot? They can’t just go through the ageing process that quickly!
Oh, yeah! My bad!
So, before silica gels undergo the ageing process, they have to be washed in a solution of water. This can increase the strength & stiffness of the gel, thus, leading to a higher-quality gel for real-world applications! However, this is not the end of it. This solution of water does not prevent the loss of the gel’s strength and the gel’s structure, so enforcing the procedures is our best choice.
Now can you talk about the Ageing Process, Carlos?
The Ageing Process strengthens the network of silica particles by the dissolution and re-precipitation of silica particles, appending new silica monomers, and enhanced activity of polycondensation.
But Carlos, can you explain the dissolution and re-precipitation part of the ageing process?
Dissolution means that the solid silica particles have combined with the gel, to break down & form a solution with the gel. For re-precipitation, it means that the chemical reaction from the alkoxide, water, and our catalyst polycondensate to become a heavier molecule in our gel solution, but re-precipitating our solution returns our molecules into a dissolved state in our sol-gel solution.
Got it! Thanks, Carlos! Now continue! 😁
Two different mechanisms might operate during ageing that affects the structure and properties of a gel: the dissolution of small particles to larger particles and transportation of material to the neck regions of the silica gel. Often, when a silica aerogel is going through the ageing process, some of these silica groups can dissolve back into the solvent and then reprecipitate into the neck regions of the gel. When this happens, it can lead to more strength & thickness of the weak necks in the silica gel groups/network, but unfortunately, this process is time-consuming. This is known as Ostwald Ripening.
But can you save this for another time, Carlos? It’s about 5 a.m. here, and this is when teenagers like me are supposed to go to sleep… 😅
Well, first of all, why do you go to sleep at 5 a.m.?
And secondly, sure! Even I could use a break from writing! See you soon, reader! 👋🏽
Solubility - The maximum amount of solute that can dissolve in a known quantity of solvent at a certain temperature is its solubility.
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.
Reprecipitation - After returning a precipitate to a dissolved state.
Precipitation - The process of converting a chemical substance into a solid from a solution, mostly a liquid solution.
Dissolution - The act of dissolving; to become or caused to become included into a liquid to form a solution.
Monomer - A small molecule that reacts with a similar molecule to form a larger molecule; bonds with similar molecules to form a polymer.
Polymer - A large, chain-like molecule made up of monomers; macromolecules.
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