Adsorption of gases/vapors on
solid surface
The adsorption
If a gas or a vapor contact with a solid surface it
might attach to the surface . The enrichment of a
gas (vapor) on the surface is called adsorption. The
solid phase is called adsorbent, while the gas is
referred as adsorbate.
If the binding also takes place in the bulk of the
solid (or liquid) the process is called absorption.
When the adsorbate is being released from the
surface, we call it desorption
Classification
Classification of the adsorption can be made on
the basis of binding.
Binding types
Physical adsorption
chemisorption
van der Waals,
Chemical bond
second order
Strength
Small sorption
Large sorption enthalpy
enthalpy
Distance
Long range
Short range
Number of layers
Multilayer
Monolayer
Specific
-
+
Molecular structure of
Doesn’t change
changes
adsorptive
Dependence of adsortion
on temperature
Adsorption isotherms
Quantitative description
The surface
excess(Γ) or
the coverage
(θ) is plotted
as a function
of gas (vapor)
pressure (or
solute
concentration)
at constant
temperature.
Langmuir isotherm (Type I.)
Physical adsorption of gases
Conditions:
- The adsorbate covers the surface in monolayer
- The binding sites are independent, and have a same
binding capacity (the surface is uniform)
- The adsorption and desorption are in dynamic equilirium
a
bp


a m 1  bp
θ: coverage, a: adsorbed amount (mol/g), am:monolayer capacity (mol/g),
b: sorption constant, p: pressure (Pa)
B.E.T isotherm (Type II. and III.)
Brunauer, Emmett, Teller
Physical adsorption of gases
Conditions:
- Multilayer adsorption
- Each layer is described by the Langmuir isotherm
- The adsorption and desorption are in dynamic equilirium
a
Zp


a m ( p0  p )1  ( Z  1) p / p0 
θ: coverage, a: adsorbed amount (mol/g), am:monolayer capacity (mol/g),
Z: constant depending on sorption enthalpy, p: pressure (Pa)
Capillary condensation (Type IV.and V.)
Adsorption of vapors on porous
materials
Conditions:
- The adsorbent is porous
- The adsobate is wetting the surface in liquid phase(rm<0)
- The vapor condenses in the pores, and a meniscus appear
- Usually there is a hysteresis on the isotherm