ABSTRACT
From physicochemical principles it is known that any pure surface, especially a solid, means that it is surrounded by no other foreign molecule (which means it is under a vacuum). However, as soon as there is a foreign molecule in the gas phase, the latter will adsorb to some extent dependent on the physical conditions (temperature and pressure). On any solid surface, gas will adsorb or desorb under speci†c conditions (for example, in gas recovery from shale deposits). Gas molecules will adsorb and desorb at the solid surface as determined by different parameters. At equilibrium the rates of adsorption (Rads) and desorption (Rdes) will be equal. The surface can be described as consisting of different kinds of surfaces:
Total surface area = At = Ao + Am Area of clean surface = Ao Area covered with gas = Am Enthalpy of adsorption = Eads (energy required to adsorb a molecule from gas
phase to the solid surface)
One can write the following relations:
Rads = kapAo (5C.1)
Rdes = kb Am exp(–Eads/RT) (5C.2)
where ka and kb are constants. At equilibrium:
Rate of adsorption (Rads) = Rate of desorption (Rdes) (5C.3)
and the magnitude of Ao is a constant. Further we have:
Amount of gas adsorbed = Ns Monolayer capacity of the solid surface = Nsm
By combining these relations and
Ns/Nsm = Am = At (5C.4)
we get the well-known Langmuir adsorption equation (Birdi, 2008):
Ns = Nsm/(a p)/(1 + (a p)) (5C.5)
Additionally, the heat of adsorption has been investigated. For example, the amount of Kr adsorbed on AgI increases when the temperature is decreased from 79 K (0.13 cc/g) to 77 K (0.16 cc/g). This data allows one to estimate the isosteric heat of adsorption (Jaycock and Par†tt, 1981; Birdi, 2008):
d (Ln P/dT) = qads/RT2 (5C.6)
The magnitude of qads was in the range of 10 to 20 kJ/mol.
