Page 29 - Adsorption by Powders and Porous Solids

14 ADSORPTION BY POWDERS AND POROUS SOLIDS

adsorbed molecules across the surface; whereas in the latter case, the location of the
adsorbed molecules is governed by the surface structure of the adsorbent.
Localization does not prevent the adsorbed molecules from 'hopping' from one
site to another (unlike the situation in immobile chemisorption), but it is not compat-
ible with the state of a close-packed completed monolayer.

1.7. Energetics of Physisorption

Provided that the experimental measurements are made under carefully controlled
conditions and that the adsorption systems are well characterized, energy of
adsorption data can provide valuable information concerning the mechanisms of
physisorption.
When a polar molecule is adsorbed on an ionic or polar surface various types of
specific interactions may contribute to the adsorption energy. A useful general
expression for the adsorption energy, E,, at very low surface coverage was first
proposed by Barrer (1966) in the form of the sum

in which ED and E, represent the non-specific dispersion and repulsion contributions
and the terms E,, E,, and EfiQ represent, respectively, the three types of specific
contributions: the polarization, field-dipole and field gradient-quadrupole energies.
For convenience, we may write Equation (1.6) in the form
Eo = En, + ESP
with Ens now in place of (ED + ER) representing the nun-specific contributions and ESP
representing the various specific contributions.
If we wish to study the adsorbent-adsorbate interactions we must undertake
adsorption calorimetry or analysis of the isotherm data at very low surface coverage.
It is only under these conditions that we can eliminate, or at least minimize, the
adsorbate-adsorbate interactions. At higher coverage, an additional (self-potential)
term, E, , must be added to E, to allow for the latter interactions.
It is evident that the adsorption energy is controlled by the nature of the adsorption
system (i.e. by both adsorbent and adsorptive). There are a few adsorbents which give
rise to essentially non-specific interactions with a wide range of different adsorp-
tive~. The most important non-porous adsorbent of this type is graphitized carbon
black, which in its most uniform state has a surface structure composed almost
entirely of the graphitic basal planes. When a polar molecule is adsorbed on this
surface, En, is much larger than ESP, which is unlikely to contribute more than 10% to
the total interaction energy. In the case of graphite, the E, contribution is largely due
to the polarization component arising from the interaction between a polar adsorptive
molecule and the induced dipoles in the graphite lattice.
It follows from Equations (1.2) and (1.5) that the magnitude of En, is dependent on
the polarizabilities of the adsorptive and the adsorbent and the density of the principal
force centres in the outer part of the adsorbent (i.e. in the surface layer). It is only