Hydrophobic and Subcritically-Dried Silica Aerogel

Materials

• An already-prepared silica gel
• 200-proof ethanol or acetone
• Hexane
• Trimethylchlorosilane (TMCS)
• Chemical-resistant wide-mouth jar or bottle
• Oven or hotplate
• Chemical fume hood

Optional

• Pentane
• Hexamethyldisilazane (HMDS)

Warning!

• Trimethylchlorosilane is hazardous to your lungs and eyes. But never fear! A pair of safety goggles and adequate ventilation, such as a chemical fume hood, will protect you.
• The solvents and chemicals used in this process are nasty and you will be heating them at one point. Only do this recipe in a chemical fume hood.
• Make sure the chemical-resistant container you get is compatible with hexane and acetone. If you are using a plastic container, make sure you either test it in advance by exposing it to these chemicals for a few days or check for materials compatibility in a database. See Information About Chemicals under the Explore section for a link to the Cole-Parmer chemical compatability database.
• Make sure your chemical-resistant container also doesn’t melt below 100ºC.

Gel Pre-Processing Conditions

1. Prepare a silica gel using any method and purify it through solvent exchange with either ethanol or acetone as if you were going to supercritically dry it. Exchange the gel at least four times over the course of several days to a week, depending on the shape and size of the gel.
2. Prepare a solution of 25 vol % hexane, 75 vol % ethanol or acetone (either is acceptable). The volume should be at least 5 times the volume of the gel(s) you are processing. For example, to process five 4-mL monoliths, you could prepare a solution of 25 mL hexane and 75 mL acetone. This is your “25-75 mixture”.
3. Similarly prepare a solution of 50 vol % hexane, 50 vol % ethanol or acetone. This is your “50-50 mixture”.
4. Similarly prepare a solution of 75 vol % hexane, 25 vol % ethanol or acetone. This is your “75-25 mixture”
5. Exchange the gel into the 25-75 mixture and allow it to soak for 12-48 hours, depending on the size and shape of the gel. Subsequently exchange your gel into the 50-50 and 75-25 mixtures.
6. Exchange the gel into pure hexane three times over the course of 1-3 days.
7. Prepare a solution of 6 wt % trimethylchlorosilane in hexane. Prepare enough so that you will have enough to exchange at least 5-10 times the volume of the gel(s) you are processing 3 times (a total of 15 times the volume of gel). For example, add 18 g trimethylchlorosilane to 282 g hexane. This is your “TMCS solution”.
8. Place the gel in sealed chemical-resistant wide-mouth container. Add a volume of TMCS solution 5-10 times the volume of the gel(s). Seal container. If you are using a glass jar you may want to lubricate the lid with silicone vacuum grease or another suitable lubricant, as the following steps may cause the lid to get stuck.
9. Heat container with gel and TMCS solution to 60ºC for 12-24 h. Make sure the container is sealed.
10. Allow the container to cool to room temperature. Do not open the container while it is hot. Exchange the TMCS solution with fresh TMCS solution.
11. Repeat steps 9-10.
12. Repeat step 9 again.
13. Allow the container to cool to room temperature. Replace the TMCS solution with fresh hexane.
14. Allow the gel to soak in hexane for 12-24 h. Exchange the gel into fresh hexane 2 more times over the course of 1-3 days.

What Everything Does

TMCS reacts with hydroxyl groups that line the solid framework of the silica gel to replace them with non-polar trimethylsilyl groups. Replacing just 20-30% of these groups will render the gel hydrophobic (waterproof).

The gradient mixtures of hexane and acetone or ethanol help to prevent the gel from shrinking and/or cracking due to contraction when the polar organic pore fluid in the precursor gel mixes with hexane.

The heating helps the trimethylchlorosilane to react with the hydroxyl groups more rapidly.

What Doesn’t Work

• Using methanol instead of ethanol or acetone. Methanol is not substantially miscible with hexane.
• Putting the precursor gel straight into hexane. You will probably get shrinking and cracking.
• Using water, methanol, ethanol, or isopropanol instead of hexane. These will react with the trimethylchlorosilane before it has the chance to contact the solid framework of the gel.

Variables You Can Play With

• Adjust the number of solvent exchanges and length of solvent exchanges based on what you find is necessary.  For example, for powders, granules, or small monoliths, you may only need one exchange into TMCS/hexane solution.
• You may only need one gradient solution to get good results depending on the shape and size of your gels.
• Try replacing trimethylchlorosilane with hexamethyldisilazane. It’s a little less nasty.
• Try replacing hexane with pentane or toluene.
• Try not heating the gels when they are soaking in TMCS/hexane solution or equivalent. You may find heating is unnecessary.
• Try using a metal oxide gel instead of a silica gel.

Gel Drying

1. 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.

Alternatively,

1. Subcritically dry. A suggested procedure would be to place the gel in a jar with a little bit of hexane and to screw the lid on part way. Allow the gel to dry out over the course of 1-2 days. If the gel has collapsed more than you would like, you can try heating the aerogel gently with a hot plate at 40-140ºC for 24 h or placing it under a light vacuum with or without gentle heating to get it to “spring back”.

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 and is impervious to water. Aerogel should float perfectly on top of a beaker of water and rebound fully when dunked. Water droplets should bead off.

Useful Information

Trimethylchlorosilane:

• Molecular weight 108.64 g mol^-1
• Density 0.856 g mL^-1
• Sigma-Aldrich part number 95541 or C72854

Hexane:

• Molecular weight 86.18 g mol^-1
• Density 0.672 g mL^-1
• Sigma-Aldrich part number 178918

Acetone:

• Molecular weight 58.08 g mol^-1
• Density 0.791 g mL^-1

Hexamethyldisilazane:

• Molecular weight 161.39 g mol^-1
• Density 0.774 g mL^-1
• Sigma-Aldrich part number 52619

References

1. Ravindra Deshpande, Douglas M. Smith, C. Jeffrey Brinker “Preparation of High Porosity Xerogels by Chemical Surface Modification”, USPTO Patent No. 5565142, 1996.