Page 34 - Adsorption Technology & Design, Elsevier (1998)

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Fundamentals of

adsorption equilibria

The phenomenon of adsorption - the accumulation of concentration at a
surface - is essentially an attraction of adsorbate molecules (a gaseous or
liquid component) to an adsorbent surface (a porous solid). Interaction
between adsorbate and adsorbent consists of molecular forces embracing
permanent dipole, induced dipole and quadrupole electrostatic effects,
otherwise known as van der Waal's forces. The preferential concentration
of molecules in the proximity of a surface arises because the surface
forces of an adsorbent solid are unsaturated. Both short range (repulsive)
and longer range (attractive) forces between adsorbate and adsorbent
become balanced when adsorption occurs. For reasons which will be re-
vealed later, adsorption is nearly always an exothermic process. Physical
adsorption (as distinct from chemisorption involving the sharing or ex-
change of electrons between adsorbate and adsorbent) of a gas or vapour
is normally characterized by the liberation of between 10 and 40 kJ mo1-1
of heat which is close to values associated with heats of liquefaction of
gases. The heat evolved on adsorption of a solute onto a solid from a
liquid, however, is strongly dependent on the source and history of the
solid adsorbent. Nevertheless the heat evolved when a porous solid is
immersed in a liquid solvent containing an adsorbable solute is of the
same order of magnitude as the heat of adsorption of a saturated vapour
onto a porous solid.