Adsorption is the adhesion of molecules of gas, liquid, or dissolved solids to a surface. This process creates a film of the adsorbate (the molecules or atoms being accumulated) on the surface of the adsorbent. It differs from absorption, in which a fluid permeates or is dissolved by a liquid or solid.

1. Adsorption

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Content
• Adsorption
• Mechanism of Adsorption
• Types of Adsorption
• Factors affecting adsorption
• Adsorption Isotherm
• Adsorption of solute at the surface of
liquid
• Application of Adsorption

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The word coined by Henrich Kayser in 1881
• Adsorption is a surface phenomenon
• It is the deposition of molecular species onto the
surface
• Concentration of adsorbate on the surface of
adsorbent is high than that of bulk
Adsorbent : Surface on which adsorption occurs
Adsorbate : The molecular species that gets adsorbed on the surface
Adsorption is exothermic process in most of the cases
Adsorption
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Mechanism of
adsorption(WHY ADSORPTION OCCUR ? )
• It is a consequence of surface energy
• Molecules on the surface are not
wholly surrounded by other molecule
• These molecules experience some net
inward force of attraction which
cause surface tension
• In certain solids like transition metal,
the unutilized free valances at the
surface causes unbalanced inward
force

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Types Of
Adsorption
On the basis of concentration On the basis of nature
PHYSISORPTION
CHEMISORPTION
POSITIVE ADSORPTION
NEGATIVE ADSORPTION

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a. POSITIVE ADSORPTION NEGATIVE ADSORPTION
o Van Der Waals force (weak)
o Enthalpy : 4-40 KJ/mol
o Multilayer formation
o Not specific
o Reversible in nature
o Occurs at low temperature
o Chemical Interaction (strong)
o Enthalpy : 40-400 KJ/mol
o Monolayer formation
o Specific in nature
o Irreversible in nature
o Occurs at high temperature
b. PHYSICAL ADSORPTION CHEMICAL ADSORPTION
Conc of adsorbate on surface < bulk
Ex : Dil. KCl is shaken with blood charcoal
Conc of adsorbate on surface > bulk
Ex : Conc. KCl is shaken with blood charcoal

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Factors affecting
adsorption
 Temperature : TEMPERATURE ADSORPTION
GAS( adsorbate ) SOLID( adsorbent ) GAS ADSORBED HEAT
 Pressure : With in pressure, adsorption to an extent till the saturation
level is achieved
 Nature of the adsorbate : Easily liquefiable gas like NH3, HCl etc. are
adsorbed greater extent than permanent
gases like H2, O2, N2 etc.
 Surface area of the adsorbent : SURFACE AREA ADSORPTION
EVAPORATION
CONDENSATION
(GAS BY SOLID)

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ADSORPTION ISOTHERM
At constant temperature, a plot between x/m (amount adsorbed by an adsorbent) is a
function of pressure or concentration
After Saturation -- No Adsorption
LIMITED SITES
ADSORBATE + ADSORBENT ADSORPTION
x/m
P Ps (SATURATION PRESSURE)

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FREUNDLICH ADSORPTION ISOTHERM
If
x/m = amount of gas adsorbed per gm of the
adsorbent
P = equilibrium pressure of adsorbate,
then over small range of P,
= K.P
n & k = Const.
o At low pressure :  P
o At high pressure :  P 
X
m
1/n
m
x
x
m
PsPRESSURE (P)
x/m
m
x
= K.P
x
m = K.P
1/
n
=K.P
x
m
T= constant
EXTENTOFADSORPTION()
(FORM UNIFORM MOLECULAR LAYER)
0
A
B

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Plotting of FREUNDLICH ADSORPTION ISOTHERM
Limitation : Fails at high pressure
Plot Of log vs log P
x
m
log = log K + log P
x
m
1
n
INTERCEPT = log K
logx/m
log P
SLOPE = 1/n

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LANGMUIR ISOTHERM
ASSUMPTIONS
a. For monolayer only
b. Uniformity of sites(i.e. surface is
perfectly flat)
c. No interactions between
adsorbed moleculeA (gas) + M (surface) AM
Ka
Kd
Where,
Ka & Kd are rate const. for adsorption and desorption

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FORMULA
 =
LIMITATION : VALID AT LOW PRESSURE AND MODERATELY HIGH TEMPERATURE
K =
Ka
Kd
Where,
 =
NO OF ADSORPTION SITES OCCUPIED
NO OF ADSORPTION SITES AVAILABLE
At low pressure, K.P <<1,
At high pressure, K.P >>1,
 = K.P
 = 1
A
A
K.PA
1+K.PA
P =Partial pressure of AA
(atm
)
,
A

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BET ADSORPTION ISOTHERM
a. Multilayer adsorption is available
b. No interaction between adsorbed molecule
c. Surface is Homogeneous

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FORMULA
V (1 - Z) { 1 - (1 - C ) Z }
V C.Z
=
mono
Z =
V = Volume of monolayer
coverage
C = a const
P
P *
Where,
mono

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TYPE OF ADSORPTION ISOTHERM
Type I Adsorption
• Monolayer
adsorption
• Langmuir
Adsorption Isotherm.
•Examples:
Adsorption of
Hydrogen (H) on
charcoal at
temperature near to
-180C
Type II Adsorption
•Deviation from
Langmuir model of
adsorption.
•Monolayer
formation.
•Examples :
Nitrogen (N2 (g))
adsorbed at -
195C on silica gel.
Type III Adsorption
• Deviation from
Langmuir model.
• Multilayer
formation
• Examples :
Adsorption
Isotherm are
Bromine (Br2) at
79C on silica gel
or Iodine (I2) at
79C on silica gel.
Type IV Adsorption
•At lower pressure,
graph is quite
similar to Type II.
•Monolayer
followed by
multilayer.
•Examples :
adsorption of
Benzene on silica
gel at 50C.
Type V Adsorption
•Similar to Type
IV.
•Type IV and V
shows
phenomenon of
capillary
condensation of
gas.
•Example :
adsorption of
Water (vapors) at
100C on

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Adsorption of solute from solution
o For multicomponent system
o Used to relate the change in concentration of component in contact with a
surface
with change in surface tension
o Due to presence of solute, Gives an exact relationship between adsorption and
change in surface tension of a solvent
o Gives an exact relationship between adsorption and change in surface tension
of
solvent due to presence of solute
(Gibbs adsorption equation)

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If
T = excessive concentration of solute for unit area of surface of
liquid
 = surface tension
a = activity of solute
Then,
T =
For dilute solution,
T =
 For surface active substances like soaps, detergents T is +ve
 For surface inactive substances like most inorganic salts, sugar, T is -ve
a d 
RT d a
c d 
RT d c
2
2
2
2

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APPLICATION

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SEPERATION OF INNERT GAS ADSORPTION INDICATORGAS MASK
SILICA GEL : controlling humidity ACTIVATED CHARCOAL CHROMATOGRAPHY

20. CONCLUSION
o Adsorption is deposition of molecular species on the substance
o This process is seen in natural, biological, physical, chemical
system
o In industrial process like heterogeneous catalysis, in
pharmaceutical,
industry
o Thus adsorption plays a vital role in our day to day life requirements

21. Ref : KL Kapoor, Puri
Sharma Pathania,
Online platform
Thank You