Aerogels | Uses for Supercritical Fluids

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. Immersed in a solvent, usually ethanol, formation of this highly porous structure is by sol gel chemistry.

 

Removing the aerogels from the solvent bath for common use is fraught with problems. Because the structure is so fine, normal drying at atmosphere collapses the network into dust. This is from the normal capillary pressure at the liquid/vapor interface on the inside of the pore.

 

Supercritical fluids exhibit much lower surface tension both at the interface to liquids and to the interface at gases. It is this property that allows for the aerogel to be dried without destruction.

 

Aerogels are dried by one of three methods:

 

  1. High temperature conversion of a liquid organic solvent to the supercritical state with subsequent venting
  2. Liquid CO2 displacement of an organic solvent with subsequent supercritical CO2 venting
  3. Supercritical CO2 extraction of an organic solvent at low temperatures

High temperature conversion of a liquid organic solvent to the supercritical state with subsequent venting

With this method, the liquid organic solvent (eg. Ethanol) is heated above its critical temperature of 243°C. The solvent is pressurized above its critical pressure and then released to atmosphere while maintaining an elevated temperature.

Liquid CO2 displacement of an organic solvent with subsequent supercritical CO2 venting

With this more popular method, the ethanol is displaced with liquid CO2. The solution is pressurized above 73 atm and heated to above 31°C (supercritical point for CO2). The supercritical CO2 is then depressurized keeping the temperature above 31°C.

Supercritical CO2 extraction of an organic solvent at low temperature

Similar to the liquid displacement method, the ethanol is this time displaced with supercritical CO2. The supercritical CO2 is then depressurized keeping the temperature above 31°C.

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