Page 35 - Gas Adsorption Equilibria

1. Basic Concepts 21

2. Multilayer adsorbates
Sorbent offers many adsorption sites which energetically can be
homogenous or inhomogeneous.
Sorptive gas pressure (p) may approach the saturation pressure at system
temperature. * )

3. Pore fluids
Sorbent has cylindrical or slit like pores which gradually can be filled with
admolecules depending on sorptive gas pressure.
In sorptive gas mixtures molecular sieve or size exclusion effects may
occur as only the smaller molecules can enter the pores whereas the bigger
ones are prevented from doing this due to their size.

4. Steric sorbates
Sorbent offers specially formed adsorption sites provided for example by
organic molecules impregnating an activated carbon. These anchor
molecules only accept (biochemical) admolecules having an appropriate
complementary atomic group (key-lock-mechanism).

5. Ionic sorbates
Sorbent has ions on its surface which can be replaced by other ions
diffusing freely in a sorptive liquid or are part of molecules being dissolved
in the liquid (ion exchange).

6. Quantum sorbates
Sorbent has very narrow submicropores – so-called nanotubes – the
diameter of which is about [1.17], in which light molecules
like hydrogen deuterium tritium or helium can
penetrate. As the pore diameter at low temperatures becomes comparable
with the de Broglie wave length of the admolecules, quantum sieve effects
may occur allowing separation of the different types of admolecules due to
different diffusion velocities (quantum resonance effect), [1.13]. **)

Technical adsorbents often are heterogeneous, i. e. include pores of very
different size, shape, and connectivity. Hence, the above mentioned types of
adsorbates may occur simultaneously or in a mixed way, one of the other

*) For supercritical temperatures the so-called Riedel pressure
should be considered, the index “c” indicating the critical state of
the sorptive gas.
**)
The de Broglie wavelength of is at (1 atm) = 20 K about 0.5 nm and at 300 K about
0.1 nm.