Aerogel Without the Pressure
Wait, You Don’t Need Supercritical Drying to Make an Aerogel?
Nope! It is possible to make aerogels, including silica aerogels, without supercritical drying–in fact, using evaporative drying techniques. Not only that, you can make silica aerogel monoliths with high transparency, low density, and superinsulating abilities without supercritical drying. That said, there are a few catches.
Aerogels prepared by subcritical drying are usually limited to:
- Small maximum dimensions (a few cm or less)
- Specific geometries, usually high-aspect-ratio structures without sharp corners like discs and rods
- Maximum of 95% porosity (95% air) and frequently less
In comparison with comparable supercritically-dried materials, ambiently-dried aerogels typically exhibit:
- Less optical transparency
- Higher density
- Higher thermal conductivity (lower maximum insulating ability)
- Higher compressive strength and stiffness
- Enhanced hydrophobicity (water resistance)
For thin films and small monoliths, subcritical drying is very effective. Cabot Corp’s Nanogel® aerogel granules and fine powders are made this way.
For large structures like a 1″ cube, not so much.
Other disadvantages of subcritical drying include the need for several additional, reasonably toxic chemical substances, additional diffusion-controlled processes, and increased volumes of solvents.
Subcritical drying has the advantage of not requiring high pressures to work and can be done in a continuous fashion. Many people initially believe these benefits make aerogels made through this method inherently less expensive than supercritically-dried aerogels. But because of the additional chemicals and solvents required, subcritcal drying does not guarantee lower process cost. Supercritical drying is actually not that bad once you have the equipment and process down–it’s just high-pressure vessels and carbon dioxide, stuff industry has been using for over 100 years! And rapid supercritical extraction, developed at Lawrence Livermore National Laboratory, can be used to make large aerogel monoliths (with dimensions of several inches) in less than 4 hours.
Notably both methods are used profitably to make inexpensive aerogel insulation materials on an industrial level.
Discovery of Subcritical Drying aka The Spring-Back Method
Subcritical drying was discovered by Dr. Doug Smith, Dr. Ravindra Desphande, and Prof. C. Jeffrey Brinker at The University of New Mexico in the early 1990’s. The patent for the technique, “Preparation of high porosity xerogels by chemical surface modification”, USPTO Patent No. 5565142, was filed in April 1992 and issued in October 1996. The technique first appeared in the scientific literature in Ceramic Transactions, v31, 1993, with reference to the patent for more technical details.
You may have noticed the title of the patent refers to preparation of high porosity xerogels instead of aerogels. This is because at the time people thought of aerogels as the materials made by supercritical drying and xerogels as the materials made by evaporative drying, although today materials scientists would call the materials described in the patent aerogels since they have >50% porosity and many of the features of supercritically-dried aerogels.
Why Didn’t They Figure Subcritical Drying Out First?
Interestingly, aerogels were discovered because of Kistler’s experiments with supercritical drying. But once scientists knew that a structure like an aerogel could be made, methods for making aerogels without supercritical drying were eventually figured out. But even though today it is possible make aerogels without supercritical drying, one wonders if aerogels would have ever been discovered if they weren’t first made by supercritical drying. Would we have known to look for or create such nanostructures?
Additionally, advances in the understanding of aerogels, surface science, and nanotechnology in the 1980’s made possible the methods used for subcritical drying–more than 50 years after supercritical drying of aerogels was invented.
The Problem: Capillary Stress
When liquid is evaporated from a gel, capillary forces induced by the liquid as it evaportes from the nanopores of the gel exert stress on the struts of the gel’s solid framework and, as the liquid leaves, the framework collapses inwards in response. As the liquid continues to evaporate, the framework continues to collapse in on itself. In a silica gel, the framework of the gel is initially lined with surface hydroxyl groups that terminate the silicon-oxygen-silicon network making up the gel’s solid framework. Most metal oxide and organic gels also have hydroxyl groups or similar polar (sticky) groups lining their solid frameworks.
So these hydroxyl (or similar) groups love to stick to each other if given the chance. In fact, hydroxyl groups are the reason why ScotchTM tape is sticky. These surface groups stick to each other by hydrogen bonding, which originates from the hydroxyl group being polar. Thus as the framework collapses inwards on itself due to evaporation, the struts of the framework stick to each other. Think of it like when you take the backside off of a band-aid and the sticky part of the band-aid accidentally folds over onto itself and never unsticks ever again, forcing you to throw it out and get a new band-aid. Like that, but instead of a band-aid it’s hydroxyl-coated nanostruts of a gel. As evaporation proceeds and the gel collapses in and sticks to itself, the result is a dense, solid material results which we call xerogel-basically a glass with less than about 10% porosity.
1. Make the Gel’s Solid Framework Less Sticky
If instead you could replace these sticky hydroxyl groups lining the solid skeletal framework of the gel with something non-polar, then:
- The liquid in the gel’s framework wouldn’t be able to stick to the framework as well. This means that as the liquid is evaporated from the pores of the gel, it wouldn’t be able to exert as much force to draw the framework in on itself and so the gel would shrink less.
- The struts wouldn’t be able stick to each other as it did collapse.
- The collapse of the network that did result could potentially be reversed, at least partially.
2. Make the Gel’s Pore Fluid Less Sticky
Alternatively, if you replaced the fluid in the pores of the gel with something that has a low surface tension, the liquid in the pores of the gel wouldn’t be able to stick the framework of the gel as well as. To do this you would exchange the pore fluid of the gel with a solvent such as:
- Liquid CO2
3. Do Both
Combining both approaches, capillary collapse can be even further reduced, simultaneously eliminating internal stiction due to elimination of surface hydroxyls (or enough of them, anyway) and reducing the surface tension of the liquid in the pores.
Overall the goal is to balance the stiffness of the gel with the capillary stress that will result from evaporation.
The General Process
This is how subcritical drying is typically done.
- A gel is made using a standard sol-gel process like those in the Make section of Aerogel.org
- The gel is purified with an organic solvent such as alcohol or acetone as it would be in preparation for supercritical drying
- The pore fluid in the gel is exchanged with an aprotic solvent such as pentane, hexane, or toluene (generally the aprotic solvent used is also a low-surface-tension solvent as well that can be evaporated later)
- The gel is chemically modified to replace its polar surface groups with non-polar groups by diffusing a solution of aprotic solvent and waterproofing agent into its pores
- The gel is purified by exchanging into pure aprotic solvent
- The gel is optionally exchanged into a different low-surface tension solvent if the current pore fluid is not suitable for evaporative drying
- The liquid in the gel is gently evaporated, causing the gel to partially collapse
- The gel springs back once the liquid has finished evaporating from it pores (on its own or sometimes under the assistance of gentle heating or vacuum)
Reading step 7, you may be able to appreciate why monoliths made by this technique have shape and size limitations–when the gel collapses, although only partially and temporarily, sharp corners and other stress concentrators can become aggravated and cracks can form. Additionally, a complex three-dimensional stress state can arise in a thick parts as they collapse, since the outer edges dry exponentially faster than the inner volume. This stress state can cause the gel to crack as it dries, limiting crack-free monoliths to small dimensions.
Replacing Pore Fluid in Preparation for Replacing Sticky Surface Groups
After a gel has been formed and purified with alcohol or other organic solvent, the gel is exchanged into an aprotic solvent of choice that is usually also a low-surface-tension solvent. You would want to first exchange the gel into a solution of half of whatever solvent is in the pores of the gel and half of whatever solvent you want to exchange into to minimize shrinking and cracking before placing the gel into pure aprotic solvent. Shrinking and cracking can easily occur at this step since polar organics, with relatively high surface tension, and aprotic low-surface-tension organics frequently exhibit a dramatically negative ΔVmixing–that is, when you mix them together the resulting mixture contracts. Using half-and-half mixture helps to minimize this effect and ease the gel into contracting.
Replacing Sticky Surface Groups
Replacing surface hydroxyls and similar polar groups is usually done after forming a gel by performing solvent exchanges into a suitable aprotic solvent such as acetone or acetonitrile, then soaking the gel in a solution of a waterproofing agent such as trimethylchlorosilane, dimethyldichlorosilane, or hexamethyldisilazane. These compounds will then diffuse into the pores of the gel and spontaneously react with the hydroxyl groups on the surface, replacing them with big, bulky non-polar groups such as trimethylsilyl or dimethylsilyl groups, depending on what waterproofing agent you use. Excess reagents and byproducts are removed from the gel by subsequent solvent exchanges of the gel into pure aprotic solvent (that is, something that doesn’t have an -OH group on it like water or alcohol do).
Read the article on silica aerogels and look for the animation at the end under “Waterproofing Aerogels” to see how this works with hexamethyldisilazane. It’s the same process used to waterproof aerogels as well!
Alternatively, you could make the gel with a silicon alkoxide such as methyltrimethoxysilane (MTMS) that introduces hydrophobic surface groups to start with, eliminating the need for post-gelation modification of the gel. The result aerogels are generally white and not transparent but remarkably elastic–marshmallow-like–and can thus be dried subcritically. X-aerogels, which are aerogels that have a conformal polymer skin applied over their skeletal surface, are frequently both hydrophobic and incredibly stiff and so they can also be subcritically dried. See the article on Strong and Flexible Aerogels for more information.
Once the chemically-modified gels have been purified and exchanged into a low-surface-tension solvent, the solvent can be evaporated. The solvent should be evaporated slowly, for example, by leaving the gel in a jar with a bit of pentane and then putting the lid on but not screwing it on tightly. Shrinkage that results may partially reverse itself after the gel is dry or can be assisted by heating the resulting dry material and/or placing under vacuum (this is called the “spring-back method”).
Now that you’ve learned about how subcritical drying of aerogels works, try making them for yourself!