Silica Aerogel (TEOS, Base-Catalyzed)
Editor’s Note: This is an adaptation of the silica aerogel procedure from the Lawrence Berkeley National Laboratory site about aerogels, which for a long time was the only procedure for making aerogels publicly available. That procedure, we’re sorry to say, does not work. Maybe you’ve tried it. If you have, you’ll have noticed that the solution stays separated as two layers and a gel never forms. That’s because there’s not enough alcohol. Maybe it was a typo. If you have trouble with the procedure below , please leave a comment!
Materials
- Tetraethoxysilane (tetraethyl orthosilicate), Si(OC2H5)4
- Absolute (200-proof) ethanol
- Deionized water
- Ammonium hydroxide, 28-30 wt % in water
- Ammonium fluoride, NH4F
Optional
- Acetone
Gel Preparation
An Excel calculator for determining amounts of chemicals required by target volume (mL) or mass (g) is available.
- Weigh 1.852 g NH4F and add it to 100 mL of water. Add 20.50 g (22.78 mL) ammonium hydroxide solution. Store this in a bottle so you can reuse it later. This is the “ammonium fluoride/ammonium hydroxide stock solution”. If you already have stock solution prepared you can skip down to step 2.
- Mix 4.7 g (5.0 mL) TEOS and 8.68 g (11.0 mL) ethanol in a beaker. This is the “alkoxide solution”.
- Mix 7.0 g (7.0 mL) water and 8.68 g (11.0 mL) ethanol in another beaker. Add 0.364 g (0.371 mL, ~8-10 drops from a disposable pipette) of ammonium fluoride/ammonium hydroxide stock solution. This mixture is the “catalyst solution”.
- Pour the catalyst solution into the alkoxide solution and stir. This is the “sol”.
- Pour the sol into molds and allow gel to form. Gel time is approximately 8-15 min.
What Everything Does
TEOS is the source of the silica. Water is what hydrolyzes the TEOS so that it can polymerize. Ethanol is a co-solvent that is miscible with both TEOS and water to get both into the same phase so they can react. Ammonium hydroxide is a basic (alkaline) catalyst that helps to make the reactions go faster. Fluoride ion is a catalyst that helps hydrolysis happen more quickly.
What Doesn’t Work
- Not using ammonium fluoride. It actually makes a big difference with TEOS. Although fluoride also makes reactions with TMOS go faster, TMOS will work fine with just a basic catalyst without fluoride.
- Using denatured alcohol that contains anything other than methanol or isopropanol as a denaturant instead of absolute ethanol. Some hardware store alcohol works, some doesn’t.
- Using sodium hydroxide (NaOH) instead of ammonium hydroxide in equal molar concentration. NaOH is a strong base so if you use it you’ll need to use a lower molar concentration of it than for ammonium hydroxide.
Variables You Can Play With
- Try adjusting the amount of solvent used to adjust the density of the resulting aerogel.
- Try adjusting the amount of the catalysts in the stock solution or the amount of stock solution you add. This will change the gel time and possibly the clarity of the gel (more catalyst means faster gel time but possibly lower transparency).
- You can substitute sodium hydroxide, sodium carbonate, or potassium carbonate for ammonium hydroxide but you will have to experiment with the amount.
- You can substitute sodium fluoride for ammonium fluoride in equal molar concentration, although your gel time may be affected since you lose the buffering effect of the extra ammonium ions.
Gel Processing Conditions
- Once the gel has set, place it under ethanol and allow the gel to age for at least 24 h.
- Exchange into 200-proof ethanol or acetone at least four times over the course of several days to a week.
- Supercritically dry. A suggested procedure would be to heat the CO2 through its critical point (31.1°C and 72.9 bars) to ~45°C while maintaining a pressure of ~100 bars. Depressurize at a rate of ~7 bar h-1.
What You Should Get
A transparent silica aerogel with a blue cast from Rayleigh scattering that appears yellowish when viewed in front of a light source from Mie scattering.
- Density 0.040 g cm-3
- Surface area 700 m2 g-1
Useful Information
Tetraethoxysilane (tetraethyl orthosilicate):
- Molecular weight 208.33 g mol-1
- Density 0.933 g mL-1
- Smells a little bit like spearmint
- Sigma-Aldrich part number 131903
Ethanol:
- Molecular weight 46.07 g mol-1
- Density 0.789 g mL-1
- Sigma-Aldrich part number 459836 or 459844, or get Everclear from a liquor store
Ammonium fluoride:
- Molecular weight 37.04 g mol-1
- Form is a fluffy, lightweight solid
- Sigma-Aldrich part number 216011
Ammonium hydroxide:
- Concentration is 28-30 wt % in water typically
- Molecular weight of NH4OH is 35.05 g mol-1, but this is not the molecular weight of the solution
- Density 0.9 g mL-1
- Form is a pungent liquid that smells like cleaning ammonia, use in a vent hood
- Sigma-Aldrich part number 221228
July 16th, 2010 at 6:14 pm
I have been searching for hours and haven’t found a single company that will ship Tetraethyl orthosilicate to a residential address. Where can you get this without ordering through an employer account?
July 16th, 2010 at 4:00 pm
Will I be able to add Europium Doped Alkaline Earth Metal Aluminate Glow Powder without an adverse reaction?
June 22nd, 2010 at 8:55 pm
I’m interested in making some aerogel (TEOS) with my two high school age kids. The information on this site is invaluable! Still researching controlling fumes (subcritical drying). Meanwhile, I made a couple of modifications to the spreadsheet which you are welcome to distribute. The percentages where off a little bit (missing about 4 to 5 % when you run a totals column so I made the necessary adjustments. I found a third column interesting to view (now have vol. for desired vol., weight for desired vol. and weight for desired weight). Ran subtotals and totals on all columns as well. Finally I calculated the dried aerogel mass (2 different density estimates). Send me an email if you’d like me to send it to you.
June 17th, 2010 at 4:27 pm
Does anybody know where the oxygens in the resulting silica come from? In other words, do they come from the water, the TEOS, or somewhere else?
Please email to jimdbell@q.com (MIT Chemistry Class of 1980).
May 7th, 2010 at 11:49 am
How toxic are the ingredients and how toxic are the resulting mixtures? I am thinking of trying this recipe in my garage and driveway and would like advice on how dangerous or toxic this activity might be. What personal protection precautions and materials (eye goggles, N95 nose mask, gloves, etc.) should I use? Thanks, Blubba
February 3rd, 2010 at 12:47 pm
The clarity of this batch is important information for a base or standard for the other recipies. Aerogel seems to be a interesting option. In Make -> apreseating your work -> characterising aerogels -> Transparency
I am interested in the inferred and wavelenths that add to heat production.
Aerogels could be used for a insulating coating like evacuated tube vacume tube solar collectors. I wonder how a aerogel coated solar heat collector would compare to a vacume tube one?? So you can see how transparency is important.
By comparing the brightness of the beam with and without the sample present, the the transparency of the sample as a function of wavelength can be measured. The results can then be plotted as a graph of % transmittance or % absorbance vs. wavelength.
January 31st, 2010 at 7:14 pm
Hi Caroline,
Well it works for us! We tried it out multiple times with multiple variations to make sure that it would work before we posted it. Also, I believe Jing (see Connect > Questions and Answers) has successfully used it and found it worked well!
January 30th, 2010 at 2:35 pm
Hi,
I want to make silica aerogels for a chemistry project at Sherbrooke’s University, does anyone know if this recipe works? I don’t really mind about the final quality of the aerogels, but I just want to know if I’ll get something or not.
Thanks!
January 27th, 2010 at 2:46 pm
Hi MAtt,
The amount of alcohol listed here is a sufficient amount. The Lawrence Berkeley recipe originally called for the equivalent of 7.5 mL of ethanol, where we suggest 22 mL.
January 27th, 2010 at 2:44 pm
Hi Athmuri,
Depending on how large of a volume of chemicals you buy, they chemicals will run you $100-$200 but can potentially make a large volume of materials. Regarding costs of equipment, see Make > Getting Started and Make > Build a Supercritical Dryer for more information.
January 27th, 2010 at 2:42 pm
Hi JJJJJ,
Sorry, perhaps the calculator is not clear. The calculator helps you to calculate the amount (either volume or mass) of liquid solution desired. So if you prepare 100 mL of solution, you will get about 100 mL of gel and, after supercritically drying, the resulting aerogel would weigh about 4 g.
Hope this helps!
January 25th, 2010 at 11:05 pm
There seems to be a glitch or problem with the Excel calculator. The site list the density of the substance as 0.04 grams/cm^3. Since cm^3 is equivalent to mL, then the density is 0.04 g/mL. On the calculator, there are spots for both end volume and end mass for determining chemical amounts. So, for example, an end sample of 4 grams should have a volume of 100 mL, if this density is correct. However, putting 100 in for the end volume and 4 in for the end mass yield different chemical amounts, with the mass-determined amounts being about 21 times less than the volume-determined amounts. Is this some glitch, or am I just not using the calculator correctly? Thank you in advance for your reply.
January 21st, 2010 at 6:01 pm
hi,
im working on developing a silica aerogel. Could you tell me what would be the approximate cost of the equipment and chemicals required for making it?
January 17th, 2010 at 7:06 pm
In reference to the editors note at the top, how much alchohol is really needed for this recipe?