100 Nano-Stories: pH Value (Part 2)!
Episode #95: Hydrolysis x Polycondensation In Silica Aerogels!
Preface! ✨
It’s your favorite material science & nanotechnology enthusiast! Today, we will cover the pH gelation time results for a silica aerogel (sol-gel chemistry)!
Here is the article below that covers the fundamentals of today’s topic:
TL;DR → pH Value (Part 1)! 🔑
pH is the scale where we can measure the power/strength of acids and bases. The pH of water is 7, so water is neutral, anything lower than 7 has a LOT of hydrogen ions, and anything above than 7 has a LOT of hydroxyl ions.
Hydrolysis is the chemical breakdown of a compound/molecule due to a water reaction with the compound.
Polycondensation is the contraction/compression of smaller molecules that get closer to each other (condense) to form a compound with high molecular weight (aka increased fluid friction/viscosity). Water is released from the condensation of these smaller molecules.
Viscosity is the fluid’s resistance to flow. This is either because of the intermolecular forces or the polymers cross-linked with each other to prevent molecular flow.
Authors Note:
This was an overview of the previous article. If you are left confused or want to know why the mechanisms work behind solid structures, I highly recommend reading the article above for detailed information! 🙏🏽
TL;DR complete! Let’s jump into today’s topic! 😄
What about the pH Gellation Time For Aerogels? 💡
Requirements + Importance! 🔑
In the sol-gel process, the pH value directly relates to the values of hydrolysis and the (poly)condensation of tetra alkoxysilanes (tetra methoxy silane (TMOS) or tetra ethoxy silane (TEOS).
For hydrolysis to occur, the sol (solution) has to occur in an acidic state, which means the pH levels have to be between 1–6. The reason that the acidic state is preferred for hydrolysis is because of electrical properties in the pH states and the electronegativity in polar molecules. But because you have to change all the molecules in TEOS or TMOS to have water as a byproduct, condensation will take a long time to occur. So when the conditions are acidic, condensation will take a while, and condensation is known as the rate-determining step.
Condensation → A reaction in which two molecules combine to form a larger molecule, producing a small molecule such as H2O as a byproduct.
The main reason why the sol (solution) and the silicon atoms prefer to be in an acidic state is because of the electrical properties of the acidic substance (hydrogen ions are actions, which means that hydrogen lost an electron, becoming positively charged).
For condensation to occur, you have to be at a pH of 8–14, but this time, hydrolysis is the rate-determining step because hydrolysis comes first before condensation, and in some cases, hydrolysis can take a while depending on the precursor used (TMOS, TEOS, etc.)
Since condensation is the main reaction, hydrolysis will become engulfed in the reaction, since hydrolysis is just a chemical reacting with water. The condensation is the final product: A new chemical with water molecules as a byproduct/result of the reaction.
Results! 🔑
For polycondensation of orthosilicic acid (H₄SiO₄) specifically, the pH value has to be 6 for the solution to become a gel (similar to edible gelatin). pH 6 is the best pH level for proper gel time since the best aerogels have high porosity (emptiness of a material), which also leads to low density and a high surface area. The lowest density aerogels are around 95–99.8% air, and the high surface area can be around 861 m²/g in experiments with hydrochloric acid (H-Cl) as long as the pH level is 1.
Closing Thoughts! 💭
With the results, we can now explain all of the mechanisms (condensation, polycondensation, hydrolysis, and pH levels) and how it works.
Summary:
In the sol-gel process, the pH value directly relates to the values of hydrolysis and the (poly)condensation of tetra alkoxysilanes.
For hydrolysis to occur, the sol (solution) has to occur in an acidic state, which means the pH levels have to be between 1–6. Condensation is the rate-determining step because condensation needs to “expel” water from the reaction to create the OH Groups.
For condensation to occur, you have to be at a pH of 8–14, but this time, hydrolysis is the rate-determining step because hydrolysis comes first before condensation (before condensation can continue “expelling water”). Hydrolysis will become engulfed in the reaction since hydrolysis is just a chemical reacting with water.
See you next week to finalize the knowledge between sp² and sp³ hybridized orbitals for graphene and carbon aerogels! ✌🏽
Vocabulary! 📓
Condensation → A reaction in which two molecules combine to form a larger molecule, producing a small molecule such as H2O as a byproduct.
Rate-Determining Step → The slowest step (most time-consuming) of a chemical reaction determines the speed for the next part of the reaction.
pH → The scale where we can measure the power/strength of acids and bases.
Acids → Tend to have a LOT of hydrogen ions (H+/H3O+)
Bases/Alkalines → Tend to have a LOT of hydroxyl ions (OH-)
Hydrolysis → The chemical breakdown of a compound/molecule due to a water reaction with the compound.
Polycondensation → The contraction/compression of smaller molecules that get closer to each other (condense) to form a compound with high molecular weight (aka increased fluid friction/viscosity). Water is released from the condensation of these smaller molecules.
Bonus Resources! 💻
Previous “100 Nano-Stories!” 🔖
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