100 Nano-Stories: The Sol-Gel Process & Transition!
Episode #07: Production of Silica Gels!
Preface
We’re back with another article, reader! I talked about the optical properties of silica aerogel, and I did promise you to talk about all the types of aerogel that are being produced, but I have to explain to you the elementary concepts of aerogel before I explain any other concepts to you, reader!
Carlos, I’m new here to your series on “100 Nano-Stories”, so can you please give me your previous article on silica aerogel?
Thank you, Carlos! You’re a real life-saver!
As a bonus, I will also be including vocabulary terms at the end for anyone who got lost in the article, reader!
Thank you, Carlos, but before we start, can you answer me this question real quick?…
What is Sol-Gel?
Sol-Gel is the process where solid materials from solid nanoparticles from small molecules. This means that these solid nanoparticles form together to create a network of nanoparticles that bundle up in a liquid (or in this case, a gel!).
So where does the “gel” come from “Sol-Gel”?
All gels come from the “Sol Process”. Don’t worry the name is not that important right now, let’s look at the overview of the Sol-Gel Process!
The first step to start the Sol-Gel Process is Sol Preparation. Sol Preparation will result in a colloidal suspension.
Carlos, what is a colloidal suspension?
Let’s break this down very simply. A colloid is a heterogeneous non-crystalline structure with nanoparticles from one single substance that is mixed with a second substance. The size of these particles range from 1–1000 nanometers in diameter, and can’t be separated through filtration.
A suspension is also a heterogeneous solution with particles that range from more than 1000 nanometers in diameter, and these materials settle at the bottom of the mixture.
So a colloidal suspension is essentially a mixture of colloid and suspension nanoparticles that range from 1 nanometer (10^-9 meters) to hundreds of nanometers (10^-6 meters). Some examples of colloidal suspension are blood or muddy water!
But Carlos, can you define a colloid and a suspension in simpler terms?
Sure! We can define something as a colloid when the nanoparticles do not settle at the bottom of the mixture. We define something as a suspension when the nanoparticles are big enough to finally sink in the mixture.
Got it, Carlos! Now, what are the terms for “heterogenous” and “non-crystalline” supposed to mean? Why do they matter?
For those who don’t know the terms “heterogeneous” or “non-crystalline”, heterogeneous means that the composition of a mixture varies from one specific location to another; it’s not uniform. Think of the chocolate chip cookies that you ate for New Years Day: the cookie is not in the total uniform of composition, there may be more chocolate chips on one side of the cookie and not a lot of chocolate chips on the other side of the cookie.
For a non-crystalline (also known as amorphous) structure, it essentially means that the chemical composition is the arrangement of the nanoparticles in the sol is not in a box-shaped form/crystal structure.
Sick! Got it now, Carlos! Proceed! 😁
So, certain sol-gel materials can come from silica oxides/metal oxides (Classic Silica Aerogel, Iron Oxide Aerogel), organic materials (Resorcinol-Formaldehyde Polymer Aerogels), and carbon materials (Graphene Aerogel).
But for this article, let’s stay with Silica Oxides or Classic Silica Aerogel Sol-Gel Processing!
Sure, Carlos! I thought we were going on forever with the list of aerogels!
Silica Aerogel Precursors
To understand how we start with sol-gel to aerogel, it would be in our best interest to look at how the process works:
Hmmm… what is a precursor, Carlos?
A precursor is a compound that participates in a chemical reaction that will produce another compound. In the production of a sol, we combine the precursor with a solvent, and additionally, we will add in a catalyst as well. A catalyst is a substance that can increase the rate of a chemical change without the substance undergoing any permanent chemical change. One example of a catalyst is an enzyme: it increases chemical changes in your body without the enzyme itself undergoing a chemical change!
Interesting… but can you go back to Silica Gel Precursors?
For Silica Gel Precursors, scientists most commonly use silicon alkoxides. However, researchers spent time testing certain precursors to see which different physical properties of the aerogel were being obtained from the precursor in the silicon alkoxide (nanopore size, distribution, surface area, etc.). In the end, the chemical Tetramethyl Orthosilicate (TMOS) generated a gel of higher surface area (surface area of half a football field), more uniform (homogenous) pores, and lower thermal conductivity (less heat passes through the silica aerogel) than other precursors such as Methyltrimethoxysilane (MTES)!
Damn! But isn’t that expensive?
How did you know, reader!? TMOS is a very efficient precursor in the production of aerogel, but less cost-effective than a different precursor: Tetraethylorthosilicate (TEOS). Despite not producing the same properties as using TMOS as the precursor for silicon alkoxide, it was cheaper, and it was a less harmful reagent and a reagent means that it is a substance for use in chemical analysis.
So does that mean that TEOS is for aerogel research, and TMOS is for aerogel production?
Now that we know about our precursors and silicon alkoxide, let’s briefly talk about our reactants and our catalysts. In the production of silica aerogel, water will be our reactant to help the silicon alkoxide molecules bond/join together, combined with a solvent (most commonly methanol). After this, we introduce a catalyst (such as ammonium hydroxide or ammonium fluoride) to aid the underlying chemical reactions to go fast enough to be practically useful for specific applications of aerogel.
Can you draw for me how the process might look like, Carlos?
So, reader, structures are formed throughout the process, but the reason we add the catalyst is that silicon alkoxide is non-polar, which 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.
This is why we need to add EtOH (Ethanol). It can get the alkoxide, water, and our catalyst to become a homogeneous mixture (uniform structure), and now, the silica aerogel begins to form a proper structure!
The structure will finally begin to take place because of a hydrolysis reaction with water and make the compound polycondensate ( higher molecular weight) because of our chemical reaction!
And this is where we reach the Silica Cross-Links to make the overall structure of Silica Aerogel, Carlos?
Almost! What it really means is the conclusion of the Sol-Gel Process & Transition to the Ageing of the Aerogel!
Oh! Will that be for tomorrow, Carlos?
Maybe, reader, maybe… 😉
Vocabulary 📓
Sol Preparation - a chemical process resulting in a colloidal suspension.
Colloid - a heterogeneous non-crystalline structure with nanoparticles from one single substance that is mixed with a second substance.
Suspension - a heterogeneous solution with particles that settle at the bottom of a mixture.
Colloidal suspension - a mixture of colloid and suspension nanoparticles that range from 1 nanometer (10^-9 meters) to hundreds of nanometers (10^-6 meters).
Heterogeneous - the composition is not in uniform with the entirety of a mixture/structure in all locations.
Non-Crystalline (Amorphous) - the chemical composition is the arrangement is not in a crystal structure.
Precursor - a compound that participates in a chemical reaction that will produce another compound.
Catalyst - a substance that can increase the rate of a chemical change without the substance undergoing any permanent chemical change.
Reagent - a substance for use in chemical analysis.
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.
Homogenous - a mixture/structure where the composition is uniform in every location of the mixture/structure.
Polycondensation - condensed molecules during the bonding process, higher molecular weight.
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