ADSORPTION ISOTHERMS
The magnitude of adsorption depends on the pressure of the gas and the temperature
of the experiment for a given gas and adsorbent.
Mathematically, it can be expressed as

a = f (P,T) ------------- (1)
Where



a – amount of gas adsorbed
p – pressure and
T – temperature
When equation (1) is represented graphically, three different curves are obtained.
I)
II)
III)
If the temperature is kept constant and pressure is change, the curve between a and P is
known as adsorption isothern.
a = f (P) if T is constant.
If pressure is kept constant and temperature is varied, the curve between a and T is called
the adsorption isobar.
a = f (T) if P is constant
If the amount adsorbed is kept constant, the curve between P and T is known as adsorption
isostere.
P = f (T) if a is constant.
Graphs- refer
From the graph, it is clear that the extent of adsorption x/m increases with increasing
pressure (P) and becomes maximum at Ps (Saturation pressure). At Ps the rate of adsorption
becomes equal to rate of desorption, there by dynamic equilibrium is established.
Free gas ↔ Gas adsorbed on solid.
Further increase of pressure does not favour the adsorption.
Types of Adsorption Isotherms
Seven types of physical adsorption isotherms have been reported. These are given below:
Figure 4.4
Type:1
This types of curve is obtained in such cases where mono molecular layer is formed on the
surface of the adsorbent.
This curve shows that a saturation state is reached. It means that there is no change in the value
of ‘a’ (the amount adsorbed) with the increase in pressure onwards. This type of the curve is rare.
Example:
Adsorption of N2 on charcoal at -1950 C
Type: 2
This type of isotherm has transition point ‘B’ which represents the pressre at which the
formation of mono layer is complete and that of the multi layer is being started. It represents a case of
multi-molecular layer physical adsorption on non-porous materials. This curve shows that there is a
transition point.
Example:
Adsorption of N2 on silica get at -1950C.
Type :3
In this types of isotherm there is no transition point. In this the multi layer formation starts even
before the formation of mono molecular layer is complete.
Example:
Adsorption of bromine or iodine vapours on sillies get at 800C.
Type III is relatively rare and a recent example of this is that of the adsorption of nitrogen on ice.
Type: 4
In this case there is a tendency for a saturation state to be reached in the multi molecular region
as well. In fact this can be regarded as a duplication of the 2nd type.
Example:
Adsorption of benzene vapours on ferric oxide get at 500C.
Type: 5
This isotherm indicates multi molecular layer formation in the beginning. At high pressure,
there is a tendency for ‘a’ (amount adsorbed) to remain constant. It means that the saturation state has
been reached.
Example:
Adsorption of water vapour on charcoal at 1000C.
Type: 6 & 7
There is a need to recognize at least the two additional isotherms shown in fig.4.5. there are
expected for non-wetting adsorbate – adsorbent systems.
Fig 4.5 types of adsorption Isotherms