Silica Aerogel (TMOS, Base-Catalyzed)
Editor’s Note: This recipe was contributed by Michael Grogan at the University of Bath, UK. It is designed to make high-quality optically-transparent silica aerogels for optical analysis. This is the same recipe presented in Episode 5 of The Critical Point (How to Make Silica Aerogels: Part 1). Thank you, Michael!
Materials
- Tetramethoxysilane (tetramethyl orthosilicate), Si(OCH3)4
- Methanol
- Deionized water or, better yet, analytical reagent (AR) grade water
- Ammonium hydroxide, 28-30 wt % in water
Optional
- Ethanol
- Acetone
Warning!
- Tetramethoxysilane (TMOS) is hazardous to your lungs and eyes. In your lungs its vapors can hydrolyze and form silica. In your eyes it can methylate and denature the collagen in your retina and cause blindness. But never fear! A pair of safety goggles (not glasses) and adequate ventilation, such as a chemical fume hood, will protect you.
- Methanol also causes blindness so don’t get it in your eyes. Again a pair of safety goggles will protect you. And by all means, methanol is not drinking alcohol (drinking alcohol is ethanol) so don’t get any stupid ideas. It’s a chemical.
- Concentrated ammonium hydroxide is caustic (a corrosive base), has a pungent odor, and can sting your eyes. Use with adequate ventilation.
Gel Preparation
- Add 4.86 g (5.40 mL) of concentrated ammonium hydroxide to 1000 mL of water. Store this in a bottle so you can reuse it later. This is the “ammonium hydroxide stock solution”. If you already have stock solution prepared you can skip down to step 2.
- Mix 10.2 g (10.0 mL) TMOS and 7.92 g (10.0 mL) methanol in a beaker. This is the “alkoxide solution” or “solution A”.
- Mix 5.0 g (5.0 mL) of ammonium hydroxide stock solution (what you prepared in step 1) with 7.92 g (10.0 mL) methanol in another beaker. This mixture is the “catalyst solution” or “solution B”.
- Pour the catalyst solution (solution B) into the alkoxide solution (solution A) 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
TMOS is the source of the silica. Water is what hydrolyzes the TMOS so that it can polymerize. Methanol is a co-solvent that is miscible with both TMOS 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.
TMOS is easier to get to work with than TEOS, used in the recipe Silica Aerogel (TEOS, Base Catalyzed). If you’ve had trouble getting TEOS to work for your science, try TMOS. It doesn’t mess around.
What Doesn’t Work
- 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
- You can scale the volumes of the chemicals in steps 2 and 3 to make more or less volume of gel.
- 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.
Gel Processing Conditions
- Once the gel has set, place it under methanol and allow the gel to age for at least 24 h.
- Exchange into methanol, 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 0.090 g cm-3
- Surface area 700 m2 g-1
Silica aerogel you can make (image courtesy Michael Grogan, University of Bath, UK)
Useful Information
Tetramethoxysilane (tetramethyl orthosilicate):
- Molecular weight 152.22 g mol-1
- Density 1.023 g mL-1
- Smells a little bit like spearmint
- Sigma-Aldrich part number 218472
Methanol:
- Molecular weight 32.04 g mol-1
- Density 0.792 g mL-1
- Sigma-Aldrich part number 322415
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
What are the dimensions of the aerogel in the picture from University of Bath and how long did it take them to dry it
what are the dimensions of the aerogel in the picture from the university of bath and how long did it take for them to dry?
Hi,
If the transparency doesn’t mean anything is there then a cheaper way to make silica aerogel using another silicate compound? (I’m actually only interested in the low weight and high insulation)
Thanks
Ole
Hi Ole,
Certainly–sodium silicate (or “waterglass”) is the next best approach for making silica gels with low density, good insulating properties, but with less transparency. Try looking into that! We’re working to post a good sodium silicate recipe soon.
Josh,
Q: In the section “Variables You Can Play With”,
What of these variables have people had sucess with making the gell clear and stay together?
You can adjust all of these parameters and keep the resulting aerogels clear. The amount of Rayleigh scattering that results may change depending on the purity and concentration of your reagents and which parameter you wish to adjust so you may need to adjust multiple parameters. The full scope of the parametric space is too much to go into in a comment field so I recommend looking into the scientific literature or just trying some procedures yourself. These are very transparent over the range of wavelengths to which most solar cells are sensitive.
Try this procedure, see how it works for you, do it wrong, then do it better. The results will more than likely be good enough for what you want to do… you don’t have to have everything perfect to start!
Solvant adjusts density
Yes… amount of non-alkoxide liquid adjusts density. Since the solvent does not contribute to hydrolysis significantly this is the liquid to adjust.
catalist or stock solution ammount adjusts gel time and possibly the clarity?
Yes, that is correct. To get objectively reproducible results you need high purity reagents but you can still get excellent results with ordinary reagents, it just takes some fine tuning.
When it says more analisis means faster gel time but possibly lower transparency? that is saying both of thowse pieces of information are unknown.
I think you mean catalysis, not analysis, and no, it’s not that these things are unknown, it’s just complicated and circumstantial. If you heavily overcatalyze your gel it will cloud up due to formation of white silica precipitate.
Clarity is important for insulation and solar collectors. So more solvant would adjust the density but not the clarity?
What does clarity mean to you? You must consider optical transmittance for a particular wavelength, and factor in that for aerogels the short wavelengths will tend to scatter, but as you can see from the image provided by Michael Grogan, the transmittance at all wavelengths is excellent for aerogels made using this procedure. I will see if there is a spectrum available for this formulation but otherwise just look in the literature for comparable materials if you can.
Is their any way to filter these things through osmosis or difusion barriers to get good quality from cheaper chemicals?
If you are referring to using sodium silicate, you’ll never beat TMOS in terms of clarity. This procedure is pretty good. The only better procedure is the two-step gelation method of Tillotson, which we will post in the future some time.
How do the different chemcial substitutions effect clarity at different light spectrums and what is the light spectrum clarity for the original formula and how does it compare with this one?
That is a broad parametric study you’ll have to do yourself!
In the section “Variables You Can Play With”
What of these variables have people had sucess with making the gell clear and stay together?
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.
Solvant adjusts density
catalist or stock solution ammount adjusts gel time and possibly the clarity?
When it says more analisis means faster gel time but possibly lower transparency? that is saying both of thowse pieces of information are unknown.
Clarity is important for insulation and solar collectors. So more solvant would adjust the density but not the clarity?
Is their any way to filter these things through osmosis or difusion barriers to get good quality from cheaper chemicals?
How do the different chemcial substitutions effect clarity at different light spectrums and what is the light spectrum clarity for the original formula and how does it compare with this one?