Page 66 - Gas Adsorption Equilibria

52 Chapter 1

Consequently, the temperature of the gas remained nearly constant and a
monotonous increase of the mass of adsorbed could be observed. More
examples of gas adsorption processes are given in Chaps. 3, 4 and 6.

In concluding this section we want to emphasize that gas adsorption
experiments also can be done at elevated temperatures. Preferently they
should be carried out gravimetrically using a highly sensitive microbalance.
These so-called thermogravimetric measurements can be used to monitor
surface and structural changes of the sorbent material caused by physical or
chemical modifications [1.3].

5. MASS AND VOLUME OF ADSORBED PHASES

The set of molecules of a fluid phase, i. e. a gas or a liquid, which is within
the field of forces of the atoms or molecules of the external or internal surface
of a solid sorbent is called the adsorbate of the fluid on this sorbent material.
This set may be considered as a thermodynamic system or phase in the sense
of W. Schottky [1.63]. Such a system is defined as set of bodies or molecules
divided from its surroundings by clearly defined boundaries and exchanging
* )
with its surroundings only mechanical work, heat and mass. Obviously, any
adsorbate is an inhomogeneous phase as – by definition – its molecules are
subject to the surface forces of the sorbent atoms. Hence the conditions for
local thermodynamic equilibrium conditions, derived from
the Second Law of thermodynamics, are [1.64]

Here p, T, indicate the pressure, mass density, temperature and chemical
potential of the adsorbate at location is the temperature of
the sorptive fluid. The quantity is the potential energy per unit mass
at location (x) of the surface forces of the sorbent atoms. As this quantity
normally is an unknown function of space coordinates (x), so are the local
thermodynamic quantities (p, T, For given model function of the surface

*) This definition may be extended to include also electric loads, electromagnetic work,
radiation energy and information [1.64, 1.65].