Adsorption is the process by which molecules (such as water vapour or gases) attach themselves to the surface of a solid material, without being absorbed into the material itself. It differs from absorption, where molecules penetrate into the bulk of the material.
How does adsorption work?
During adsorption, the surface molecules of a material attract water vapour or gas molecules, which form a thin layer on the surface. The process is reversible: when conditions change (lower humidity, higher temperature), the molecules detach again. This is called desorption.
Application
In construction, adsorption plays a role in:
- Moisture regulation — porous materials such as clay, lime and untreated wood adsorb water vapour from the air and release it later, stabilising the indoor climate
- Building drying — adsorption dryers extract moisture from the air by binding it to an adsorbent (e.g. silica gel or zeolite)
- Air purification — activated carbon filters in ventilation systems work on the basis of adsorption
- Heat storage — adsorption heat pumps use zeolite to chemically store heat and release it later
Difference from absorption
| Adsorption | Absorption | |
|---|---|---|
| Location | On the surface | Inside the material |
| Process | Surface binding | Volume uptake |
| Example | Water vapour on lime plaster | Water in a sponge |
Related terms
- Absorption
- Breathing
- Indoor climate
- Building physics
- Vapour-permeable construction
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