Adsorption, desorption
and hysteresis
Since zeolite molecular sieves rely on
strong physical forces rather than
chemisorption to retain adsorbates,
their adsorption is characterized by a
Langmuir-type isotherm (the amount
of a given compound adsorbed
increases rapidly to a saturation
value as its pressure or concentration
increases in the external bulk phase).
Any further increase in pressure at
constant temperature causes no
further increase in the amount
adsorbed. With zeolite molecular
sieves, this equilibrium saturation
value typically corresponds to a
complete filling of the internal void
volume with the adsorbate. When
adsorbed molecules are desorbed
via heat or by displacement with
another material, the crystal’s
chemical state remains unchanged.