Page 32 - Gas Adsorption Equilibria

18 Chapter 1

experimental methods used to measure adsorption equilibria and enthalpies of
pure and mixed gases in rigid and swelling sorbent materials is given. In view
of space limitations we have to restrict the discussion to classical methods like
volumetry/manometry and gravimetry. However, introduction to on some new
measurement methods like oscillometry and impedance spectroscopy
emerging in today’s literature also will be given and their pros and cons will
be discussed in brief.

2. ADSORPTION PHENOMENA

Molecules of fluid phases (f), i. e. gases, vapors, and liquids, can stick to
the surface of solids (s) or other liquid phases (1). This phenomenon is called
adsorption. It occurs in principle at any temperature and pressure and for all
chemical species known so far [1.1-1.3]. The adsorbed molecules may have
their place on the surface of the solid and return to the gaseous phase. This
phenomenon is called desorption. Often one can observe dynamic equilibrium
between the number of molecules adsorbed and those desorbed in a certain
time interval. Such a situation is called adsorption equilibrium. If these
molecular flows to and from the surface do not match, we have either an
adsorption process or a desorption process [1.4-1.6].

Additionally, in highly porous solids like zeolites and activated carbons
there may be internal diffusion processes of the adsorbed molecules
(admolecules). These can occur without external exchange of mass, i. e. at
constant mass adsorbed, cp. Sects. 4, 5. An example for such a phenomenon is
presented in Chap. 6, Fig. 6.29, [1.4, 1.7-1.9].

In Figure 1.1 a schematics of the molecular situation of an adsorption
system is presented [1.3, 1.10].

Nomenclature:
Adsorptive: Gas or liquid whose molecules are interacting with the surface
atoms of a solid phase.

Adsorbent: Solid phase with external and internal surfaces exposed to the
molecules of a gas or liquid phase.

Adsorbate: Set of molecules being adsorbed on the surface of an (often
porous) solid material and forming a separate phase in the sense
of thermodynamics, cp. Sect. 5.