gas adsorption

• gas molecules sticking to a surface (adsorption), vs. absorption: uptake into the bulk
• scales with surface area → porous adsorbents (activated carbon: ~10³ m² per gram)
• physisorption: weak van der Waals forces, easily reversed
• chemisorption: chemical bond with the surface (~10× stronger), selective, often irreversible
• adsorption isotherm: amount adsorbed vs. pressure, at fixed temperature
• Langmuir isotherm (identical independent sites, one molecule each): $\theta =\frac{KP}{1+KP}$, with $\theta$ the fraction of sites occupied, $P$ the gas pressure, $K$ an equilibrium constant
• guest molecules filling clathrate cages: Langmuir-type adsorption, in 3D