Mechanism of Adsorption

Why surfaces adsorb

A particle deep inside the adsorbent is pulled equally in all directions, so its attractive forces are mutually balanced. A particle at the surface, however, has no neighbours of its own kind on the outward side, leaving it with unbalanced (residual) attractive forces. These residual forces reach out and hold adsorbate particles on the surface. Naturally, the more surface area available per unit mass (at a given temperature and pressure), the greater the extent of adsorption.

The thermodynamics of adsorption

As the surface captures adsorbate, its residual forces are satisfied, so the surface energy decreases and that energy is released as heat. Adsorption is therefore always exothermic:

At the same time, an adsorbed molecule loses freedom of movement, so the system becomes more ordered and its entropy falls:

For a spontaneous process at constant temperature and pressure the Gibbs energy must decrease:

$\Delta G = \Delta H - T\Delta S$ …