Page 57 - Gas Adsorption Equilibria

1. Basic Concepts 43

2. If preliminary saturation is reached, increase of the helium gas pressure
does not lead to gravimetrically measurable increase of adsorption for
several hours.

3. However, long time adsorption experiments lasting days and months show
a slow but steady increase of helium being sorbed in the material. This also
was observed for nonporous sorbent materials like dense polymers.

4. Helium adsorption experiments at gas adsorption systems in equilibria
states show that the volume of the combined sorbent/sorbate phase is
not constant but depends, i. e. increases with the amount of gas adsorbed,
cp. next Section 5 of this chapter and also Chaps. 2-4.

Consequently, the helium volume of a sorbent material can be considered
as one of its characteristic quantities describing that volume of the material
impenetrable for helium molecules on a short time scale of minutes, may be
hours. It seems to be kind of lower boundary to the sorbent volumes
impenetrable to other, bigger molecules.

Helium volume measurements are also very sensitive to the state of
activation of the sorbent material and to preadsorption of – for example –
other gases or water vapor. Hence measurements of this type may be used on
an industrial level for quality control.

4.3 Gas Adsorption Ar,

Similarly to the helium adsorption experiments described in Sect. 4.2,
adsorption experiments with other gases, preferably nitrogen, argon or carbon
dioxide at high purity (> 4.0), can be used to characterize the pore system of a
porous sorbent material. Early experiments have been performed by I.
Langmuir, W. Ostwald and others prior to World War I. Meanwhile gas
adsorption either by volumetric/manometric or by the gravimetric
measurement method, has become a standard technology, though discussion
of the physio-chemical interpretation of experimental data gained is still going
on [5.51]. The reasons for this are threefold.

First, measurements are fairly sensitive to experimental circumstances,
namely the preparation of the sorbent sample, i. e. activation, outgassing or
still remnant preadsorbed substances such as water in zeolitic materials. Also
gas pressure measurements, i. e. calibration of pressure gauges, leaks in the