Question 14 Flashcards
q14a)
You have a bottle full with a titania nanopowder. In order to compare two types of porous materials with each other, you want to prepare an aerogel and a foam from this titania nanopowders.
How would you do that experimentally?
Aerogel:
- redisperse titania powder in ethanol with Trizma
- Add water → Trizma is detached at 001 crystal facets (there are the -OH groups from the water)
→ gelation (condensation of the OH groups) by oriented attachment (if not too much Trizma → 3D structure bc not all possible crystal facets have Trizma - some have -OH)
- Supercritical drying with CO2: replace water with ethanol / acetone because it is soluble in supercritical CO2 (and water not), supercritical: to prevent breakage of pores through capillary effects → ethanol dissolved in CO2→ aerogel
Foam (particle stabilized foam):
- redisperse titania powder in water
- attach surfactant with hydrophobic tail (not too long e.g. valeric acid) and hydrophilic head → titania particles are partially hydrophobic
- particle needs to be partially hydrophobic bc it should go to the surface (not too hydrophilic) but should not agglomerate (not too hydrophobic)
- put in water and mix a lot to bring air bubbles in
- as particle are partially hydrophobic: they go to the air/liquid interface
- drying, sintering to fix the structure
q14b)
Which properties are different between these two materials and how would you experimentally prove that?
Density/porosity:
- aerogel: 99.9% air
- foam has ~98% air
→ aerogel has a lower density and has higher porosity (smaller pores and more)
→ aerogel has a higher specific surface area (more porosity)
→ prove by measuring mass -> calculate density and porosity
→ measure pore size: mercury intrusion porosimetry or gas adsorption (or visual by SEM or TEM)
Mechanical properties:
- aerogel: very fragile and brittle
- foam: more robust
→ measuring by with strength tests (compression)
Thermal conductivity:
- aerogel: very low thermal conductivity (see video with bunsen burner → cannot feel heat)
- foam: higher thermal conductivity
→ measuring by: DSC
Optical properties:
- aerogel: whitish color, little transparent
- foam: less transparent
→ measuring by: UV-Vis spectroscopy
