Silica gel White
Silica gel consists of 99% silicon dioxide (SiO2). Due to its high porosity, narrow-pored silica gel has a large internal surface area of up to 850 m²/g. The pore structure is not uniform and the funnel-shaped openings have a diameter of about 2 to 12.5 nm. This allows loading with water molecules in several layers (capillary condensation), which explains the very high uptake capacity of this adsorbent. Due to the extreme hydrophilicity (attraction of polar substances, such as water) and an easy regeneration of the white silica gel by temperature increases to about 150°C, it is preferably used as a desiccant.
Particularly at a relative humidity of approx. 70 %rH, silica gel shows excellent absorption capacity and can thus adsorb large amounts of water from the air. The maximum capacity is approx. 37 % of the dry weight of the fill. Silica gel white has no color indicator.
Other silica gels
Chemical properties
Chemical name: Amorphous silicon dioxide
Composition: SiO2
CAS number: 112926-00-8
| Narrow-pore | Wide-pore | Water-resistent | |
|---|---|---|---|
| Appearance and shape | Round, solid balls | Crystal granules | Round, solid balls |
| Grain size | 0,-1 mm / 1-3 mm / 2-5mm | 0,5–1 mm / 1–3 mm / 2–5 mm | 2-4mm |
| Bulk density | 0,75 – 0,85 kg/l | 0,38–0,45 kg/l | 0,60-0,70 kg/l |
| Pore volume | 0,35 – 0,45 ml/g | 0,85–1,0 ml/g | 0,35-0,45 ml/g |
| Breaking strength | > 200 N | >200 N | 70 N |
| Specific surface | 600 – 800 m²/g | 350–450 m²/g | 550 – 650 m²/g |
| Silicon dioxide (SiO2) | 99% | 99.8% | 97% |
| Trace elements | 1% | 0.2% | 3% |
| Water adsorption capacity | > 370 ml/kg | >900 ml/kg | > 370 ml/g |
Equilibrium loading
The combination of narrow-pore silica gel and water corresponds to a convex isotherm type. The isotherm exhibits a steep slope. The monomolecular occupancy of the silica gel is completed at about 30%, after which multilayer occupancy of the adsorbent surface and capillary condensation take place. At an adsorption capacity of about 35% it takes a plateau and increases only slightly.
Adsorption isotherms of narrow-pore and wide-pore silica gels
Isotherms of narrow-pore silica gel at 20°C, 40°C, 60°C
Regeneration
In practice, the most common regeneration method is temperature increase. When desorbing water vapor from silica gel, the temperature must be above 100°C. For silica gels without a color indicator, a regeneration temperature between 150°C and 175°C is recommended.
