Page 24 - Adsorbents fundamentals and applications

POTENTIAL ENERGIES FOR ADSORPTION 9

The three basic types of contributions to the adsorbate–adsorbent interactions
are dispersion, electrostatic, and chemical bond. The latter, chemical bond, has
been explored for adsorption only recently. Weak chemical bonds, particularly the
broad type of bonds involving π electrons or π-complexation, offer promising
possibilities for designing new and highly selective sorbents. The subject of π-
complexation sorbents will be discussed separately, in Chapter 8. For physical
adsorption, the adsorbate–adsorbent potential is

(2.3)
φ = φ D + φ R + φ Ind + φ Fµ + φ ˙ FQ

where φ D = dispersion energy, φ R = close-range repulsion energy, φ Ind =
induction energy (interaction between electric field and an induced dipole),
φ Fµ = interaction between electric field (F) and a permanent dipole (µ),
= interaction between field gradient (F) and a quadrupole (with quadrupole
˙
φ ˙ FQ
moment Q).
The first two contributions (φ D + φ R ) are “nonspecific” (Barrer, 1978), which
are operative in all sorbate–sorbent systems. The last three contributions arise
from charges (which create electric fields) on the solid surface. (This is a sim-
plified view, because an adsorbate molecule with a permanent dipole can also
induce a dipole in the sorbent if the sorbent is a conductor [Masel, 1996]). For
activated carbon, the nonspecific interactions dominate. For metal oxides, zeo-
lites, and ionic solids, the electrostatic interactions often dominate, depending
on the adsorbate. For adsorbate with a quadrupole, the net interaction between
a uniform field and the quadrupole is zero. However, the quadrupole interacts
.
strongly with the field gradient, thus the term φ ˙ FQ
The individual contributions to the total potential have been reviewed and
discussed in detail in the literature (Barrer, 1978; Masel, 1996; Razmus and Hall,
1991; Gregg and Sing, 1982; Steele, 1974; Adamson and Gast, 1997; Rigby et al.,
1986; Israelachvili, 1992; Young and Crowell, 1962; Ross and Olivier, 1964).
Their functional forms are summarized below. All interactions are given between
an atom (or a charge) on the surface and the adsorbate molecule.

Dispersion:
A
φ D =− (2.4)
r 6
Repulsion:
B
φ R =+ (2.5)
r 12
Field (of an ion) and induced point dipole:

1 2 αq 2
φ Ind =− αF =− (2.6)
4
2 2r (4π ∈ 0 ) 2