Physical Adsorption or Physisorption

Physisorption occurs when gas (or other adsorbate) molecules accumulate at a solid surface purely because of weak van der Waals forces, with no chemical bond forming between adsorbate and adsorbent. These van der Waals forces are of the same physical kind that cause a gas to condense into a liquid, so it should be no surprise that physisorption releases heat of broadly the same order of magnitude as the heat of condensation of the adsorbate. Because the underlying force is weak, physisorption itself is weak: at high pressures the adsorbed gas can build up into several stacked molecular layers rather than stopping at one, and the extent of adsorption is largest at low temperatures (since thermal agitation at higher temperatures tends to knock weakly-held molecules back off the surface). Equilibrium between the gas phase and the adsorbed layer is reached quickly, and because the holding forces are weak, physis …