This document discusses different types of adsorption - physisorption and chemisorption - and how they differ. Physisorption involves weak van der Waals forces between adsorbate and adsorbent molecules, while chemisorption involves chemical bond formation. It also discusses isotherm models like the Langmuir and Freundlich isotherms that describe the relationship between amount of gas adsorbed and pressure or concentration. Finally, it mentions that adsorption can change the work function of materials by altering charge distribution and dipole formation at the surface.

1. PHYSISORPTION,
CHEMISORPTION AND WORK
FUNCTION CHANGE INDUCED
BY ADSORBATES
ANEETTA DAVIS
MSc NS & T, 2nd SEM
MOUNT CARMEL COLLEGE

2. ADSORPTION
The accumulation of molecular
species at the surface of a solid or
liquid rather than in the bulk is
called adsorption.

3. Free materials
Adsorbed material

5. Physisorption Chemisorption

7. If accumulation of gas molecules on
the surface of solids occurs due to
weak van der Waals’ forces of
attraction, the adsorption is called
physisorption.

8. Characteristics of Physisorption
1. Non-specific nature: An adsorbent does not show any preference
for a gas as the van der Waals’ forces are universal.
2. Easily liquefiable gases like CO2, SO2, NH3 etc, are readily
adsorbed.
3. Reversible nature: Physisortion of a gas by a solid is reversible.
4. Increases by increase of pressure.
5. Surface area of adsorbant: When the surface area of the adsorbent
increases, more gas is adsorbed, ie extent of adsorption increases.
Since adsorption is exothermic, physisorption takes place
readily at low temperature and desorption takes place at higher
temperature.

10. When atoms or molecules of gases
are held by solids on its surface by
chemical bonds, the adsorption is
called chemisorption.

11. Characteristics of Chemisorption
1. High specificity: It is highly specific and will occur only
if chemical bond formation takes between adsorbate and
adsorbent.
2. Irreversibility: Chemisorption is irreversible because the
chemical bond formed is difficult to break.
3. Chemisorption increases with temperature.
4. Increases by increase of pressure.
5. Surface area of adsorbent: When the surface area of the
adsorbent increases, more gas is adsorbed, ie extent of
adsorption increases.

12. Physisorption Chemisorption
1 Occurs due to van der Waals’ force Chemical Bond
2 Reversible Irreversible
3 Not specific Specific
4 Enthalpy of adsorption is low Enthalpy of adsorption is high
5 More liquefiable gases are adsorbed
readily
Gases which form compounds with
adsorbent alone undergo chemisorption
6 Decreases with increase of temperature Increases with increase of temperature
7 Low temperature is favorable. High temperature is favorable
8 High pressure favors physisorption and
decrease of pressure causes desorption
High pressure is favorable but decreases of
pressure does not cause desorption
9 Results in multimolecular layers Only unimolecular layer are formed
10 No activation energy is needed High activation energy is needed
11 It is instantaneous It is a slow process

13. Isotherm models

14. Langmuir isotherm
In 1916, Irving Langmuir published an isotherm for gases adsorbed on
solids, which retained his name. It is an empirical isotherm derived from
a proposed kinetic mechanism. It is based on four hypotheses:
1. The surface of the adsorbent is uniform, that is, all the adsorption
sites are equal.
2. Adsorbed molecules do not interact.
3. All adsorption occurs through the same mechanism.
4. At the maximum adsorption, only a monolayer is formed: molecules
of adsorbate do not deposit on other, already adsorbed, molecules of
adsorbate, only on the free surface of the adsorbent.

15. Shape of Langmuir Isotherm

16. The Freundlich (or Classical) adsorption
isotherm
The relation between the concentration of a solute on the surface of an
adsorbent to the concentration of the solute in the liquid with which it is
in contact.
The variation of adsorption with pressure can often be represented
(especially at moderately low pressures) by the equation
where k and n are constants, n usually being greater than unity.
Taking logarithms,

17. Shape of Freundlich Isotherm
V

18. Work Function change induced by adsorption
Different material have different f. Moreover, chemisorptions
change the charge distribution and cause f change. For
semiconductor even band bending is changed.
The dipole of the adsorbate can
change f.
Under different condition (T for
example), different adsorption
bonding for the same adsorbate can
be formed on the same surface with
different f.