Page 21 - Adsorption by Powders and Porous Solids

6 ADSORPTION BY POWDERS AND POROUS SOLIDS

often difficult. Although many isotherms have a similar shape to the classical
Langmuir isotherm, they rarely obey the Langmuir equation over an appreciable
range of concentration. It is evident that consideration must be given to the competi-
tion between solute and solvent, the solvation of solute and, in many cases, lack of
thermodynamic equilibration.

1.3. General Definitions and Terminology

Some of the principal terms and properties associated with adsorption, powders and
porous solids are defined in Tables 1.1, 1.2 and 1.3. These definitions are consistent
with those proposed by the International Union of Pure and Applied Chemistry
(IUPAC) (see Sing et al. 1985; Haber, 1991; Rouquerol et al., 1994) and by the
British Standards Institution (1958, 1992) and other official organizations (see
Robens and Krebs, 1991).
As noted earlier, the term adsorption is universally understood to mean the enrich-
ment of one or more of the components in the region between between two bulk
phases (i.e. the interfacial layer). In the present context, one of these phases is neces-
sarily a solid and the other a fluid (i.e. gas or liquid). With certain systems (e.g. some
metals exposed to hydrogen, oxygen or water), the adsorption process is accom-
panied by absorption, i.e. the penetration of the fluid into the solid phase. As already
indicated, one may then use the term sorption (and the related tenns sorbent, sorptive
and sorbate). This is the convention that we shall adopt in the present book. The term
sorption is used by some authors to denote the uptake of gas or liquid by a molecular
sieve, but we do not favour this practice.
The terms adsorption and desorption are often used to indicate the direction from
which the equilibrium states have been approached. Adsorption hysteresis arises
when the amount adsorbed is not brought to the same level by the adsorption and
desorption approach to a given 'equilibrium' pressure or bulk concentration. The
relation, at constant temperature, between the amount adsorbed and the equilibrium
pressure, or concentration, is known as the adsorption isotherm.
A powder is easily recognized as a mass of small dry particles, but the precise
definition is inevitably somewhat arbitrary. The tern fine powder is also used in an

Table 1.1. Definitions: adsorption
Term Definition
Adsorption Enrichment of one or more components in an interfacial layer
Adsorbate Substance in the adsorbed state
Adsorptive' Adsorbable substance in the fluid phase
Adsorbent Solid material on which adsorption occurs
Chemisorption Adsorption involving chemical bonding
Physisorption Adsorption without chemical bonding
Monolayer capacity eitherchernisorbed amount required;o occupy a11 surface sites
or Physisorbed amount required to cover surface
Surface coverage Ratio of amount of adsorbed substance to monolayer capacity
' Translated into French as 'adsorbable'.