Page 80 - Gas Adsorption Equilibria

66 Chapter 1

only part of the total mass adsorbed, cp. (1.17), and may become
zero or even assume negative values for sorbent materials with very
narrow pores or inkbottle-like pores [1.70].
d) In view of the ambiguity of curves (3), (4) in Figure 1.22 it is still
desirable to have an experimental method at hand allowing one to
measure the absolute amount of a fluid adsorbed on the surface of a
porous solid without introducing any hypotheses on the void
volume of the sorbent or the volume of the sorbate phase
Indeed such a method exists [1.54]. Its thermodynamic principle
will be outlined below. However, the method is very laborious from
an experimental point of view. Hence only few data exist today
[1.54]. Also no commercial instruments allowing automated and
reliable measurements are available – up to now.

5.2 Outline of Calorimetric-Dielectric Measurements of
Absolute Masses of Adsorbates

Absolute masses of adsorbates defined by Eq. (1.14) in principle can
experimentally be determined by combined dielectric and calorimetric
measurements. In this section we only will present the basic thermodynamic
idea of this method and give an example. Details can be found in the literature
[1.54].

If an electric field is applied to a sorbent material exerted to a sorptive gas,
the dielectric polarizability of the combined sorbate/sorbent system can be
measured, cp. Chap. 6 and literature cited herein. Combining these
measurements with dielectric measurements of the empty sorbent material in
vacuum, the dielectric polarizability of the sorbate phase itself can be
calculated. This is a thermodynamic quantity of state, i. e. it can be
represented by its dielectric equation of state (DEOS) as function of the gas
pressure (p), temperature (T), mass of sorbent and mass of sorbate

As is an extensive quantity it always can be written as