Adsorption at solid interface

1. ADSORPTION AT SOLID
INTERFACE

2. ABSORPTION

3. ADSORPTION
It is the phenomenon in which one substance accumulates on the the surface of another substance in higher concentration.
❏ The substance which is being adsorbed is called adsorbate.
❏ The substance which adsorbs called adsorbent.
For example:
➢ Writing on the board,
On one substance(board) , the another substance ( chalk) accumulates on the surface of another substance in higher concentration.
➢ Silican gel - adsorbed moisture.

4. Adsorption
❖ NEGATIVE ADSORPTION
If the concentration of
adsorbate is less in the surface
layer then in the bulk phase such
adsorption is called NEGATIVE
ADSORPTION.

5. DIFFERENCE BETWEEN ADSORPTION AND ABSORPTION
ABSORPTION
● Absorption is bulk phenomenon.
● Uniform distribution
● Neither exothermic nor endothermic.
● Rate of absorption is uniform
● Independent of surface area.
ADSORPTION
● Adsorption is surface phenomenon.
● Non- uniform distribution ( higher
concentration on surface).
● Exothermic
● Rate of adsorption is:
● RAPID -due to availability of surface area.
Falls down later since surface area is
occupied.
● Depends on the surface area.

6. ➔ DESORPTION: Removal of adsorbed molecule called
desorption.
➔ SORPTION: It is the process in which absorption and
adsorption both takes place.
For example: Dyeing of fabric.

7. Types of adsorption
On the basis of the nature of force between adsorbate and adsorbent,
adsorption is of two types
➢ Physisorption
Weak force of attraction - vander waals force
Physical adsorption.
➢ Chemisorption
Strong force of attraction- Covalent bond.
Chemical adsorption

8. TYPES OF ADSORPTION
PHYSISORPTION
● The molecules are attracted to the surface simply by VANDER
WAAL’S FORCE.
● It is relatively weak.
● The heat of adsorption is low and lie between 20 to 40 kJ/mole.
● It is reversible.
● It involves the formation of Multilayer of adsorbed particles.
● It is not specific in nature. ( hence may be found in all instance.)
● It occur Appreciably only at low temperature ( the of adsorption
decreases as the temperature of the surface is raised.)
● FOR EXAMPLE:
adsorption of various gases on charcoal.
CHEMISORPTION
● The molecules are held to the surface by chemical bonds forming surface
compunds by COVALENT BONDING.
● It is relatively strong.
● The heat of adsorption is high lying in the range of 40 to 200 kJ/ mole.
● It is irreversible
● It is specific in nature. ( occurs only when there isva tendency to form a
chemical bond between adsorbate particles.
● It occurs at All temperature but extent of adsorption decreases with rise in
temperature.
● FOR EXAMPLE:
Oxygen react with Au, Ag and Pt to form there oxide.
When atomic oxygen is adsorbed on graphite surface., on heating the surface
after adsorption carbon monoxide is desorbed but not the atomic oxygen.

9. ADSORPTION AT SOLID GAS INTERFACE
Adsorption of gas on to a solid surface is very important in pharmacy
in many areas .
● Removal of objectionable odours from room .
● Prevention of obnoxious gases entering in to the body .
● Estimation of surface and particle size of powders.

10. FACTORS AFFECTING THE EXTENT ADSORPTION
NATURE OF ADSORBENT
The adsorption of the gas depends on the nature of the adsorbent. A gas can be adsorbed on different
absorbent surfaces in different amounts. For example, Hydrogen is weakly adsorbed on the alumina
surface whereas it is strongly adsorbed on the nickel surface under certain conditions.
SURFACE AREA
When we increase the surface area of the adsorbent there is an increase in the adsorption of gases.
This is because when we increase the surface area there is more number of adsorbing sites. So finely
divided solids and some porous substances are good adsorbents.

11. NATURE OF GAS
In general, if a gas is more liquefiable it will be more easily absorbed. For example, gases like
NH3, HCl, Cl2, CO2, which can be liquefied easily are more readily adsorbed on the solids
surface rather than permanent gases like O2, H2, etc.
EXOTHERMIC NATURE
The heat of adsorption can be defined as the energy liberated when 1 g mol of a gas is
adsorbed on a solid surface. When the temperature is increased the kinetic energy of the gas
molecules also increases which results in more number of collisions between the molecules and the
surface.
PRESSURE
On the solid surface, there is a fixed number of adsorption sites where gas molecules can be
adsorbed. Initially when the pressure has increased the rate of adsorption increases due to an
increase in the gas molecules striking on the surface. Thus, an increase in the pressure
increases the rate of adsorption linearly. But after sometime, it will reach a point when the
pressure has no effect on the rate of adsorption as the number of adsorption sites is fixed and
no more adsorption can happen in those sites. Hence, at that point, the extent of adsorption will
be independent of the pressure.

12. ADSORPTION ISOTHERM
When graph is plot between pressure and concentration of adsorption at constant
temperature called adsorption Isotherm.
TYPES OF ADSORPTION ISOTHERM
● Freundlich Isotherm
● Langmuir Isotherm
● Temkin Isotherm
● Brunauer, Emmett and Teller ( BET) Isotherm.

14. LANGMUIR ISOTHERM
According to Langmuir Isotherm rate of adsorption is
directly proportional to rate of desorption.