Adsorption refers to the binding of molecules or particles to a surface. It occurs when there is a higher concentration of substances at an interface compared to the bulk phase. Common adsorbents used industrially include activated carbon, silica gel, and alumina due to their large surface areas. Adsorption can be physical or chemical in nature. Physical adsorption involves weak van der Waals forces while chemical adsorption forms chemical bonds. Adsorption finds applications in gas masks, ammonia production, and removing colors from solutions.

1. Adsorption
DEFINITIONS: "It refers strictly to the
existence of higher concentration of substances
on the interface as compared to that in bulk
phase."
OR
"It is a surface phenomenon and refers to the
uniform distribution of the substance at the
surface of another substance."
e.g H2 in palladium (pd)
OR
"Adsorption is the concentration of the
substance at the interference or boundary of the
two heterogenous layers."
e.g solid/gas, gas/liquid, liquid/solid, gas/solid.
OR
"Adsorption is a phenomenon in which a layer
of ions, molecules or aggregates condenses upon
the surface of the substance with which they
come in contact with."
Two terms;

2. Adsorbate:
The phase which is going to be adsorb is called
adsorbate"
Adsorbent:
The phase which is going to adsorb.
Examples:
Solid/liquid: strychnine HCL in activated
charcoal.
Solid/gas: Activated charcoal used in respirator.
Liquid gas: surface active agents in solutions.
Liquid/liquid: emulsifying agents in emulsion.
(the mixture of two immiscible liquids is called
emulsion.)
Positive adsorption: If the concentration of
adsorbate is greater in the surface as compared
to bulk is called positive adsorption.
Negative adsorption: if the concentration of
adsorbate is greater in the bulk as compared to
surface is called neg. adsorption.
Adsorption, the binding of molecules or
particles to a surface, must be distinguished

3. from absorption, the filling of pores in a
solid. The binding to the surface is usually
weak and reversible. Just about anything
including the fluid that dissolves or suspends
the material of interest is bound, but
compounds with color and those that have
taste or odor tend to bind strongly.
Compounds that contain chromogenic
groups (atomic arrangements that vibrate at
frequencies in the visible spectrum) very
often are strongly adsorbed on activated
carbon. Decolorization can be wonderfully
efficient by adsorption and with negligible
loss of other materials.
The most common industrial adsorbents are
activated carbon, silica gel, and alumina,
because they present enormous surface areas
per unit weight. Activated carbon is
produced by roasting organic material to
decompose it to granules of carbon -
coconut shell, wood, and bone are common
sources. Silica gel is a matrix of hydrated
silicon dioxide. Alumina is mined or

4. precipitated aluminum oxide and hydroxide.
Although activated carbon is a magnificent
material for adsorption, its black color
persists and adds a grey tinge if even trace
amounts are left after treatment; however
filter materials with fine pores remove
carbon quite well.
A surface already heavily contaminated by
adsorbates is not likely to have much
capacity for additional binding. Freshly
prepared activated carbon has a clean
surface. Charcoal made from roasting wood
differs from activated carbon in that its
surface is contaminated by other products,
but further heating will drive off these
compounds to produce a surface with high
adsorptive capacity.
Temperature effects on adsorption are
profound, and measurements are usually at a
constant temperature. Graphs of the data are
called isotherms. Most steps using
adsorbents have little variation in

5. temperature.
Depending on the nature of attractive forces
existing between the adsorbate and
i) Physical adsorption
ii) Chemical adsorption
i) Physical adsorption (Physisorption)
In physical adsorption, the forces of
attraction between the molecules of the
adsorbate and the adsorbent are of the
weak van der Waals' type. Since the forces
of attraction are weak, the process of
physisorption can be easily reversed by
heating or decreasing the pressure of the
adsorbate (as in the case of gases).
ii) Chemical adsorption (Chemisorption)
In chemisorption, the forces of attraction
between the adsorbate and the adsorbent
are very strong; the molecules of adsorbate
form chemical bonds with the molecules of
the adsorbent present in the surface.
Adsorption is generally accompanied by
release of energy, that is, most adsorption
processes are exothermic in nature.
Adsorption is a spontaneous process;

6. therefore its free energy change is negative
(DG<0). However, the entropy change
associated with adsorption is generally
negative because the adsorbate molecules
lose their translation freedom when they get
attached to the surface of the adsorbent.
Therefore, in order for DG to be negative,
the enthalpy change (DH) must be
sufficiently negative, such that, (DG=DH-
TDS)<0. This explanation accounts for
exothermic adsorption processes. In cases,
where endothermic adsorption occurs as in
the case of hydrogen adsorption on glass,
the entropy change DS is sufficiently
positive such that DG remains negative.
Enthalpy of adsorption, which is the
enthalpy change for the adsorption of one
mole of an adsorbate on an adsorbent
surface, is usually in the range of 20
kJ/mole to 40kJ/mole while for
chemisorption, the values are an order of
magnitude high, that is, 200 kJ/mole to 400
kJ/mole.
The differences between physisorption and
chemisorption are summarized in the below

7. table.
Comparison between Physisorption and
Chemisorption
Physisorption Chemisorption
Forces of attraction Forces of attraction
are vander Waals’ are chemical bond
forces forces
Low enthalpy of High enthapy of
adsorption (20 - 40 adsorption (200 -
k.J/mole) 400 k.J/mole)
This process is
This process takes
observed under
place at high
conditions of low
temperatures
temperature
It is not specific It is highly specific
Generally,
Multi-molecular layers
monomolecular layer
may be formed
is formed
This process is This process is
reversible irreversible
Since chemisorption is specific in nature, a
gas is chemisorbed only when it forms
chemical bonds with the adsorbent. A gas

8. which is physisorbed at a certain
temperature can be chemisorbed when the
temperature is increased dramatically. For
e.g., at 83 K nitrogen (N2) is physisorbed on
iron surface as N2 molecules. At room
temperature there is no physisorption of N2
on iron surface. However, at 773K nitrogen
is chemisorbed on iron surface as nitrogen
atoms.
ii) Nature of the adsorbent
The nature of the adsorbent has profound
effect on the process of adsorption. Solids
with porous structure are potentially good
adsorbents. Among them, activated carbon
is the most common adsorbent for gases
which are easily liquefied. Activated carbon
is used in gas masks to adsorb poisonous
gases such as methane (CH4).
Activated carbon is prepared from a variety
of raw materials including wood, lignite,
coal, bone, nut shells and petroleum
residues. The new material is activated in
an atmosphere of CO2, CO, O2 water vapor
air or other selected gases at a temperature
between 573 K and 1273 K. This is often

9. followed by quenching in air or water.
Adsorption on activated carbon is
associated with the presence of small and
uniform pores. The surface area of activated
carbon is approximately 1000 m2/g.
Apart from activated carbon, silica gel,
aluminium oxide and clay are used as
adsorbents. It is common knowledge that
small satchets containing silica gel are used
in electronic equipments such as camera to
keep the lenses dry. As mentioned earlier,
silica gel is used to adsorb water vapor,
which is always present in the humid
weather conditions.
iii) Specific area of the adsorbent.
Specific area of an adsorbent is the surface
area available for adsorption per gram of
the adsorbent. As discussed before, specific
area of the adsorbent increases
tremendously when finely divided forms of
the solid adsorbent are used. Porous solids
are very good adsorbents too, however, the
pores should be large enough to allow the
gas molecules to enter them.
iv) Pressure of the gas - Adsorption

10. Isotherms
The extent of adsorption of a gas on a solid
generally increases with pressure. The
extent of adsorption is given by x/m, the
ratio of the mass (x) of the adsorbate and
the mass (m) of the adsorbent when
dynamic equilibrium has been attained. The
plot of extent of adsorption versus pressure
of the gas at a given temperature is called
the adsorption isotherm. Adsorption
isotherm of different shapes have been
observed for various experiments, that is
different adsorption isotherms are usually
described by different empirical equations.
Here, the two most common adsorption
isotherms namely, Freundlich isotherm and
Langmuir isotherm will be discussed.
IMPORTANT APPLICATION OF
ADSORPTION:
Examples and few Applications of Physical
Adsorption
* Charcoal is used in the adsorption of many
gases as it has more surface area.

11. * Small particles of Platinum adsorb
hydrogen gas.
* Bone black and charcoal adsorb colors.
Applications:
* Physical adsorption has many applications
in various industries. Few of them are
* In the gas masks, where charcoal is used
to adsorb poisonous gases
* In the preparation of ammonia and
sulphric acid, where platinum is used as a
adsorbent.
* Removal of colors from different solutions
where charcoal and bone black are used.
In air conditioning and other processes,
adsorption chillers are used.
WHAT IS THE DIFFERENCE BETWEEN
?ADSORPTION ON ABSORPTION
In the process of absorption molecules of
one phase interpenetrate uniformly among
those of another phase to form a solution

12. with the second phase. In adsorption,
molecules of one phase are present in
higher concentration at the surface of the
second phase. Water vapor is absorbed by
calcium chloride while it is adsorbed by
silica gel. In the same manner, ammonia is
absorbed by water and is adsorbed by
charcoal.
Sorption, which includes both absorption
and adsorption is a process in which a
substance moves from one phase to be
accumulated in another phase. The
difference between adsorption and
absorption is illustrated graphically as
shown in below figure, for the case where a
substance moves from a liquid phase to a
solid phase.

13. fig 7.2 - Types of sorption separations
In the figure, m is the amount of the
substance that has moved across the
interface. In adsorption, the process is rapid
in the beginning and slows down near
equilibrium while absorption takes place at
uniform rate.
:Sorption
The phenomenon when it is not clear that
wether it is adsorption or absorption then it
.is called sorption
There is sometimes difficult to define clearly
the interference of highly porous solid then
the term sorption has been used by
workers, notabley by J.W Mcbain to
.describe uptake of gases by porous solid