Adsorption

Certain solid surfaces have a natural tendency to grab and hold on to molecules of whatever phase they are exposed to. Crucially, those molecules stay stuck at the surface — they do not sink into the interior of the material.

• Adsorption is the accumulation of a molecular species at the surface of a solid or liquid rather than in its bulk.
• The species that piles up on the surface is the adsorbate.
• The material whose surface does the collecting is the adsorbent.

Because adsorption happens only at the surface, materials with a large surface area per unit mass make the best adsorbents. This is why finely divided or porous solids — charcoal, silica gel, alumina gel, clay, finely divided metals and colloids — are excellent adsorbents.

Adsorption in action

• Enclose a gas such as $O_2$, $H_2$, $CO$, $Cl_2$, $NH_3$ or $SO_2$ with powdered charcoal in a closed vessel and the gas pressure falls, because the molecules gather on the charcoal surface.
• Shaking a coloured dye solution (e.g. methylene blue) with animal charcoal leaves the filtrate colourless — the dye molecules are held on the charcoal.
• Raw sugar solution passed over animal charcoal loses its colour as the coloured impurities are adsorbed.
• Air dries out near silica gel because water molecules are adsorbed on it.

The reverse process — removing an already-adsorbed substance from the surface — is called desorption.