Lynne Wong's PhD thesis

material on which adsorption occurs) and the adsorbate (adsorbable substance in
the fluid phase).
(b)
(c)
(d)
(e)
Physisorption generally occurs as a multilayer at high relative pressures whereas
chemisorbed molecules are linked to reactive parts of the surface and the adsorption
is confined to a monolayer.
A physisorbed molecule keeps its identity and on desorption returns to the fluid
phase in its original form, whereas a chemisorbed molecule undergoes reaction, it
loses its identity and cannot be recovered by desorption.
Physisorption is always exothermic, and the energy involved is not much larger
than the energy of condensation of the adsorbate, whereas the energy of
chemisorption is the same order of magnitude as the energy change in a comparable
chemical reaction.
Physisorption generally attains equilibrium fairly rapidly, in chemisorption, an
activation energy is often involved and at low temperature, the system may not
have sufficient energy to reach thermodynamic equilibrium.
The terms adsorption and desorption are used to indicate the direction from which the
equilibrium states have been approached. The former occurs when the material reaches
equilibrium by wetting and the latter by drying. Adsorption hysteresis arises when the
amount adsorbed is not brought to the same level by the adsorption and desorption
approach to a given 'equilibrium' pressure or bulk concentration.
The term ‘adsorption isotherm’ was first introduced by Freundlich (1907) to describe the
relationship, at constant temperature, between the amount adsorbed by a substrate
(adsorbent) and the equilibrium pressure, or concentrations of a fluid (adsorbate).
He proposed a general mathematical relationship for the isotherm, which is now
universally referred to as the Freundlich adsorption equation (1926).
a = kp s
where a is the fraction of the total surface area covered by an adsorbate. p is the pressure,
and k and s are constants. A plot of ln (a) against ln (p) is linear.
The above Freundlich equation only fits adsorption data taken over a small pressure range,
and breaks down at high pressures and low temperatures (Brunauer, 1945).
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