Aerogel Drying

Using supercritical fluids to dry aerogels

Aerogels are highly porous materials with large internal surface area and large pore volumes. Their densities are as low as 3 kg/m3 and have porosities as high as 99.9%. This makes them excellent thermal insulators. In fact, aerogels are listed in the Guinness Book of World Records for the being the best insulators and the lowest-density solids.

In addition to their thermal insulating capabilities, aerogels have structural strength and impressive load-bearing ability, exceptional absorptive properties, and acoustic insulating capabilities. A short list of specific applications:

• Thermal insulation to windows and skylights
• Chemical absorber for cleaning up spills
• Thickening agents in paints and cosmetics
• Commercial manufacture of aerogel "blankets"
• NASA used aerogels to trap space dust particles aboard Stardust spacecraft
• NASA also used aerogel for thermal insulation of the Mars Rover space suits
• US Navy is evaluating aerogel undergarments as passive thermal protection for divers
• Use as a drug delivery system due to its biocompatibility. (Due to its high surface area and porous structure, drugs can be adsorbed from supercritical CO₂)

Although there are other types of aerogels, such as carbon and alumina, silica aerogels are the most common. They are made with a liquid alcohol like ethanol which is mixed with a silicon alkoxide precursor to form a silicon dioxide sol gel (silica gel). However, removing the aerogels from the solvent bath for common use can be problematic.

Since the structure is so fine, normal drying at atmosphere collapses the network rendering to dust. This is caused by normal capillary pressure at the liquid/vapor interface on the inside of the pore. The energy of vaporization is greater than the wall strength of the pore.

Because a supercritical fluid has no surface tension, it can be used to dry the aerogel without consequence. The end result removes all liquid from the gel and replaces it with gas, without allowing the gel structure to collapse or lose volume.

There are two methods for drying aerogels:

• Low temperatures
  • Liquid CO₂ displacement of an organic solvent
  • SC-CO₂ extraction of an organic solvent
  • SC-CO₂ venting
• High temperature
  • Conversion of a liquid organic solvent to the supercritical state with subsequent venting

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Read about the other ways SCF is being used. Supercritical Fluids will change the way you work!

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Virtual Video Warehouse What is Supercritical Fluid Extraction: A look at this green technology that uses SCF How to pack a vessel: Learn how to effectively pack a vessel for any of the Spe-edSFE instruments in our line. The Spe-ed SFE Prime from Applied Separations: Learn about this low cost instrument, created especially for the educational market. Teaching Environmentally Benign Extraction Technologies with a Cost Effective, User Friendly Instrument: This paper, presented at ACS 2008, describes the fundamentals of the supercritical fluid extraction process and the implementation of this environmentally friendly, green chemistry technique, using the low cost, student-friendly Spe-ed SFE Prime. The "Green" Extraction of Oils from Natural Products Using Supercritical Fluids: Learn how to use SCF for natural product extractions from this poster, presented at ACS 2008. SCF Product Line from Applied Separations: Check out the extensive line of Spe-edSFE instruments available from Applied Separations.    More on ASInteractive...

What is SCF? Carbon dioxide is in its supercritical fluid state when both the temperature and pressure equal or exceed the critical point of 31°C and 73 atm (see diagram). In its supercritical state, CO2 has both gas-like and liquid-like qualities, and it is this dual characteristic of supercritical fluids that provides the ideal conditions for extracting compounds with a high degree of recovery in a short period of time. By controlling or regulating pressure and temperature, the density, or solvent strength, of supercritical fluids can be altered to simulate organic solvents ranging from chloroform to methylene chloride to hexane. This dissolving power can be applied to purify, extract, fractionate, infuse, and recrystallize a wide array of materials. Because CO2 is non-polar, a polar organic co-solvent (or modifier) can be added to the supercritical fluid for processing polar compounds. By controlling the level of pressure/ temperature/ modifier, supercritical CO2 can dissolve a broad range of compounds, both polar and non-polar.