Aerogel: Alginate Aerogels
Hello everyone! My intentions for writing these articles are:
- Explain technical knowledge about aerogels in simple terms (to the public)
- Store information and habits for my future self and others (in <7 minutes)
Coolio? Sweet. Enjoy the series :-)
Aerogels: The Basics 📃
Aerogels are porous materials. The material has holes that allow liquid/gas to pass through. The holes are very small in aerogels. How small? On the nanometer scale.
Can’t picture how big a nanometer is? Think of The Sun as one meter. And The Earth would be one nanometer. THAT would be one nanometer.
The holes in aerogels are not all the same. There are 3 different types:
- Micropores - Under 2 nanometers in diameter/size
- Mesopores - Between 2 - 50 nanometers in diameter/size
- Macropores - Over 50 nanometers in diameter/size
Because these super small pores allow liquid/gas to pass through, aerogels are known as open-porous materials. All the pores are interconnected and form “an underground railroad” of tunnels that allow liquid/gas to pass through.
The Sol-Gel Process 🌡️
The Sol-Gel is a combination of a solution-based gel that will be dried and converted into an aerogel. In simple terms:
The Sol: A solution of many chemicals. Solid colloidal particles with sizes between 1–100 nanometers float around in a liquid. (This liquid is the eventual aerogel.)
The Gel: A gel that contains a 3-dimensional network of pores that contain a liquid (most likely) inside of the pores. (An example of a gel is gelatin.)
The main liquid used for aerogel is ultra-purified water. But the material is called a hydrogel if it still has water. One must remove the liquid inside of the material and replace the liquid with gas without destroying the “tunnels” of pores. Then we have an aerogel.
Alginate Aerogel Perlas ⚛️
Alginate is a broad term for a large group of polymers called polysaccharides. Polysaccharides are a bunch of sugar molecules bonded together. Alginate is one of them.
You can find alginate in brown algae/seaweed. It is the same kind of seaweed that you find on the beach. Alginate makes up ~40% of the dry mass of that beach seaweed. This is why it is tough to break and even harder to pull out of the water…
But, Carlos, what makes it so strong and so special?
Because it is anionic… or negatively charged.
There are two main chemical groups/blocks to pay attention to:
- The Mannuronic Acids (M-Acid)
- The Guluronic Acids (G-Acid)
G-acid blocks are rigid and are more suitable to create stable aerogels. (of whatever size!) The preference is to have longer G-acid blocks during the development of the aerogel, but both are required, as M-acid blocks allow for flexibility.
The cool part about the structure of alginate is their metal-adsorbing hydroxyl and carboxyl groups!
Hydroxyl → OH (Blue)
Carboxyl → COOH (Green)
These groups tend to be negatively charged. Heavy metal ions (ex: copper, lead, nickel, mercury, etc.) tend to be positively charged.
Like magnets: Opposites attract.
Concept Of The Week: Viscosity ✏️
For aerogels: Gelation describes the transformation from a liquid state into a semi-solid state. But viscosity is a critical component in allowing the gel to solidify…
Viscosity is the resistance (of a fluid) against moving by gravity.
For aerogels, the fluid inside of a container will contain a force that does not want it to leave the container it's in. This is the viscosity.
The larger the particles inside of the aerogel fluid, one must apply more force (dynamic viscosity) to push it out of the container.
But Carlos, how does one know if the gel sample has solidified enough if you cannot touch the gel? Otherwise, you destroy the network of pores and the material itself!?
Use tilting as a simple measure of gelation time.
The simplest approach to determine the time gelation is to take a gel liquid and tilt it.
No gelation has occurred if the solution moves around. (photo on the right)
Gelation (hydrogel) occurs if the solution stays in place. (photo on the left)
Gelation time and viscosity are all determined by a key factor: aerogel size.
The gelation time may take hours or days if the desired aerogel is the size of a computer. But the gelation time can take minutes if the desired aerogel are in millimeters…
Predicting Gelation Time ⏱️
Gelation occurs when a large cluster of particles stops moving and forms large networks of particles that touch one end of the container to the other.
Here is how you can roughly determine the gelation time of gels (simplified):
- The larger the sample volume, the longer the gelation time.
- The larger the viscosity, the longer the gelation time.
Why? Because the viscosity will determine the dependence on the temperature of the gelled solution.
But the MVP Trait is:
The smaller the sample volume, the shorter the gelation time.
Freeze-Drying Aerogel Perlas! ❄️
Freeze-drying aerogels are both cost-effective and environmentally friendly for the aerogel perlas!
It is faster to use the freeze dryer because there is no pressure in the pores when the liquid in the gel is in a frozen state. (solid-gas interface)
There is no other fluid for it to push against. So you can evaporate the contents of the gel without destroying the aerogel itself! #guiltfree
But take this into precaution:
Small particles of the wet gel (perlas) are preferable and will lead to faster cooling AND there has to be a huge temperature difference between the gel and the cooling liquid. [that will be used]
Quick remediation is liquid nitrogen. This bad boy can be cooled up to -196°C!
Freeze-drying is a killer method of creating the aerogel beads. You must control the freezing process to not destroy the material from the super-low freezing temperatures!
Perform freeze-drying from a specific direction (left to right, up to down) with a well-defined shape of our target perla size, and well-defined thermal boundaries.
© 2023 by Carlos Manuel Jarquín Sánchez. All Rights Reserved.