5 The solid—gas interface
Adsorption of gases and vapours on solids
When a gas or vapour is brought into contact with a clean solid
surface, some of it will become attached to the surface in the form of
an adsorbed layer. The solid is generally referred to as the adsorbent,
adsorbed gas or vapour as the adsorbate and non-adsorbed gas as the
adsorptive. It is possible that uniform absorption into the bulk of the
solid might also take place, and, since adsorption and absorption
cannot always be distinguished experimentally, the generic term
sorption is sometimes used to describe the general phenomenon of
gas uptake by solids.
Any solid is capable of adsorbing a certain amount of gas, the
extent of adsorption at equilibrium depending on temperature, the
pressure of the gas and the effective surface area of the solid. The
most notable adsorbents are, therefore, highly porous solids, such as
charcoal and silica gel (which have large internal surface areas - up to
c. 1000 m^2 g"^1 ) and finely divided powders. The relationship at a
given temperature between the equilibrium amount of gas adsorbed
and the pressure of the gas is known as the adsorption isotherm
(Figures 5.1, 5.5-5.6, 5.8, 5.11, 5.13).
Adsorption reduces the imbalance of attractive forces which exists
at a surface, and, hence, the surface free energy of a heterogeneous
system. In this respect, the energy considerations relating to solid
surfaces are, in principle, the same as those already discussed for
liquid surfaces. The main differences between solid and liquid
surfaces arise from the fact that solid surfaces are heterogeneous in
respect of activity, with properties dependent, to some extent, on
previous environment.