Lecture 20.
Adsorption Phenomena
Adsorption is the accumulation of atoms
or molecules on the surface of a material.
This process creates a film of the
adsorbate (the molecules or atoms being
accumulated) on the adsorbent's surface.
It is different from absorption, in which a
substance diffuses into a liquid or solid to
form a solution. The term sorption
encompasses both processes, while
desorption is the reverse process of
"adsorption".
Isotherms
Adsorption is usually described through
isotherms, that is, the amount of adsorbate
on the adsorbent as a function of its
pressure (if gas) or concentration (if liquid)
at constant temperature.
some of these isotherms are:
• Langmuir
• BET
Langmuir
Langmuir derived a relationship for q; weight
adsorbed per unit wt of adsorbent and C;
concentration in fluid based on some quite
reasonable assumptions. These are: a
uniform surface, a single layer of adsorbed
material, and constant temperature. The rate
of attachment to the surface should be
proportional to a driving force times an area.
The driving force is the concentration in the
fluid, and the area is the amount of bare
surface. If the fraction of covered surface is
F , the rate per unit of surface is:
Langmuir
rate going on = k1 C ( 1 - ϕ )
The evaporation from the surface is
proportional to the amount of surface
covered:
rate leaving = k2 ϕ
where k1 and k2 are rate coefficients
C = concentration in the fluid
ϕ = fraction of the surface covered
Langmuir
At equilibrium, the two rates are equal,
and we find that:
A plot of
versus should indicate a
straight line of slope
and an
intercept of
. The graph shows data
points and lines fitted to both Freundlich
and Langmuir equations.
Langmuir Isotherm
BET Theory
BET theory is a rule for the physical
adsorption of gas molecules on a solid
surface and serves as the basis for an
important analysis technique for the
measurement of the specific surface area
of a material. The concept of the theory is
an extension of the Langmuir theory,
which is a theory for monolayer molecular
adsorption, to multilayer adsorption with
the following hypotheses:
BET Theory
(a)gas molecules physically adsorb on a
solid in layers infinitely;
(b) there is no interaction between each
adsorption layer; and
(c) the Langmuir theory can be applied to
each layer.
BET Theory
The resulting BET equation is expressed
by:
P and P0 are the equilibrium and the
saturation pressure of adsorbates at the
temperature of adsorption,
v is the adsorbed gas quantity (for
example, in volume units), and
BET Theory
vm is the monolayer adsorbed gas
quantity.
c is the BET constant.
The BET method is widely used in surface
science for the calculation of surface areas
of solids by physical adsorption of gas
molecules.
BET Isotherm