Adsorption Adsorption isotherms

1. Adsorption
Dr. Hasan Tamuos
Al-Azhar University of Gaza
Faculty of Science
Chemistry Department
Preparation by:
Mohammed H. Rida
Supervised by:
23-3- 2015

2. Contents
Adsorption and orientation at interfaces.
(Liquid-gas and liquid-liquid interfaces)
The solid—gas interface.
- Adsorption of gases and vapours on solids
- Differentiation between adsorption and absorption
- Physical adsorption and chemisorption
- Measurement of gas adsorption
- Adsorption Isotherms basics
- Adsorption Isotherms types
The solid-liquid interface
- Adsorption from solution
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3. Adsorption and orientation
at interfaces.
(Liquid-gas and liquid-liquid interfaces)
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4. Liquid-gas and liquid-liquid interfaces
Adsorption and orientation at interfaces:
Adsorption of surface-active molecules as an orientated monolayer
at air-water and oil-water interfaces. The circular part of the
molecules represents the hydrophilic polar head group and the
rectangular part represents the non-polar hydrocarbon tail. At the
air-water interface, the hydrocarbon chains will tend to lie
horizontally on top of the water surface at low coverage.
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5. The strong adsorption of such materials at surfaces or interfaces in
the form of an orientated monolayer is termed surface activity.
Surface-active materials (or surfactants):
consist of molecules containing both polar and non-polar parts.
The tendency for surface-active molecules to pack into an interface
favours an expansion of the interface; this must, therefore, be
balanced against the tendency for the interface to contract under
normal surface tension forces.
If  is the expanding pressure (or surface pressure) of an adsorbed
layer of surfacant, then the surface (or interfacial) tension will be
lowered to a value  =  o - 
Liquid-gas and liquid-liquid interfaces
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6. the effect of lower members of the homologous
series of normal fatty alcohols on the surface tension
of water.
The longer the hydrocarbon chain, the greater is the
tendency for the alcohol molecules to adsorb at the
air-water surface and, hence,lower the surface
tension.
Liquid-gas and liquid-liquid interfaces
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7. 7

8. The solid—gas interface.
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9. Adsorption Absorption
adsorption is a process in which a
substance takes another substance
on its surface , i.e. they remain on
the surface
Absorption is a process in which one
substance penetrates into another
substance, i.e. the molecules
penetrate into the body of the solid.
The process is fast in the beginning
and gradually becomes slower
It happens at an uniform rate.
At the end, molecules of the
adsorbate are found only on the
surface
At the end, absorbed molecules
penetrate into the body of the solid.
The solid—gas interface.
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10. Adsorbent and adsorbate
• The solid on the surface of which gas or a liquid
molecules accumulate is called ‘adsorbent’.
• The substance (gas or liquid) whose molecules
accumulate on the solid surface is called
‘adsorbate’.
• non-adsorbed gas is called ‘adsorptive’.
The solid—gas interface.
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11. - Any solid is capable of adsorbing a certain amount of gas, the
extent of adsorption at equilibrium depending on:
temperature, the pressure of the gas and the effective surface area
of the solid.
- The most notable adsorbents are, therefore, highly porous solids,
such as charcoal and silica gel (which have large internal surface
areas) and finely divided powders.
- The relationship at a given temperature between the equilibrium
amount of gas adsorbed and the pressure of the gas is known as the
adsorption isotherm
The solid—gas interface.
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12. The solid—gas interface.
When adsorption takes place, the gas molecules are
restricted to two-dimensional motion. Gas adsorption
processes are, therefore, accompanied by a decrease in
entropy.
Since adsorption also involves a decrease in free energy,
then, from the thermodynamic relationship,
G = H-TS
it is evident that Hads. must be negative - i.e. the
adsorption of gases and vapours on solids is always an
exothermic process.
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13. Types of adsorption
1> Physical or Van der Waal’s adsorption
2> Chemical adsorption or chemisorption.
The solid—gas interface.
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14. The solid—gas interface.
Physical adsorption Chemical adsorption
Surface area of
the adsorbent
The extent of the adsorption of gases
on the surface is directly proportional to
the surface area of the solid
The extent of the adsorption of gases on
the surface is directly proportional to the
surface area of the solid.
Nature of the gas Since the forces that bind the molecule
are Van der Waal’s forces, is easily
liquefiable gases like SO2 and NH3 are
readily adsorbed .
Chemical adsorption is highly specific and is
formed if only the adsorbed molecules are
capable of forming a chemical bond with
the surface.
Effect of Pressure Pressure is directly proportional to
extent of adsorption.
Pressure is directly proportional to extent
of adsorption only until the surface gets
saturated ,after that pressure has no effect
Heat of
Adsorption
Since the Van der Waal’s forces is weak,
the heat of adsorption is low
Since the chemical bond is much stronger,
the heat of adsorption is much higher
Reversible Nature At high pressure adsorption takes place
whereas decrease in pressure causes
desorption
Decrease in pressure does not cause
desorption as molecules are strongly bound
to the surface.
Effect of
temperature
Van der Waal’s force of attraction is
weak at high temperature. Hence
physical adsorption is favored at low
temperatures
As any chemical reaction is fast at high
temperature, chemisorption is favored at
high temperature.
Thickness of the
adsorbed layer
At low pressure unimolecular thick layer
is formed on the surface. However at
high pressure the multi-molecular thick
layer is formed.
In chemisorption , there is only a
unimolecular thick layer on the surface
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15. adsorption
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16. The solid—gas interface.
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17. The solid—gas interface.
The adsorption of a gas or vapour can be measured by admitting a
known amount of the adsorbate into an evacuated, leak-free space
containing the outgassed adsorbent. The extent of adsorption can
then be determined either volumetrically or gravimetrically
The volumetric method is mainly used for the purpose of
determining specific surface areas of solids from gas (particularly
nitrogen) adsorption measurements. The gas is contained in a gas
burette, and its pressure is measured with a manometer All of the
volumes in the apparatus are calibrated so that when the gas is
admitted to the adsorbent sample the amount adsorbed can be
calculated from the equilibrium pressure reading. The adsorption
isotherm is obtained from a series of measurements at different
pressures.
Measurement of gas adsorption
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18. The solid—gas interface.
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19. The solid—gas interface.
The gravimetric method
can be used for studying both gas and vapour sorption.
Purified gas or vapour is introduced into an evacuated apparatus in
which the sorbent sample is contained in a weighing pan, the
pressure is noted and the extent of sorption is measured directly as
the increase in the weight of the sorbent sample.
An alternative gas-adsorption method, which does not require
vacuum equipment, is that developed by Nelsen and Eggersten .
a gas mixture containing the adsorbate (usually nitrogen) and a
carrier gas (usually helium) is passed over the solid under test at
room temperature. The gas flow into and out of the sample
container is monitored by means of a pair of thermal conductivity
detectors. When equilibrium has been established, the sample is
cooled Owing to gas adsorption, the outlet stream is depleted for
a time in adsorbate, the thermal conductivity detectors are thrown
off balance and the amount of gas adsorbed can be measured in
terms of the area under a peak on a recording potentiometer. 19

20. The solid—gas interface.
ADSORPTION ISOTHERMS basics
Adsorption process is usually studied through graphs known as: Adsorption Isotherm.
The amount of adsorbate on the adsorbent as a function if its pressure (gas) or
concentration (liquid) at constant T
In the given Adsorption Isotherm , after saturation pressure Ps adsorption does not increases
Vacancies on the surface of the adsorbent are
limited, at Ps a stage is reached when all the
sites are occupied and further increase in
pressure does not increases adsorption extent
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21. Types of Adsorption Isotherms :
1- Freundlich Isotherm
2- Langmuir Isotherm
3- Temkin Isotherm
4- Brunauer, Emmett And Teller (BET) Isotherm
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22. Empirical equation for representing the isothermal variation of
adsorption of a quantity of gas adsorbed by unit mass of solid adsorbent
with pressure is Freundlich Adsorption Isotherm
Freundlich Adsorption Isotherm
x/m = adsorption per gram of adsorbent which is obtained by dividing the amount of
adsorbate (x) by the weight of the adsorbent (m).
P is Pressure, k and n are constants whose values depend upon adsorbent and gas at
particular temperature .
Taking the logarithms of Freundlich Isotherm
• Freundlich Isotherm correctly
established the relationship of
adsorption with pressure at lower
values,
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23. Langmuir Adsorption isotherm
Assumes independent sites, monolayer coverage
The dynamic equilibrium is A(g) + M(surface) ⇌ AM(surface)
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24. Langmuir Adsorption isotherm
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25. Temkin Isotherm takes into account all the indirect adsorbate
adsorbate interactions on adsorption isotherms
where c1 and c2 are constants, corresponds to supposing that the
adsorption enthalpy changes linearly with pressure
Temkin Isotherm
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26. BET ISOTHERM
Most widely used isotherm dealing with multilayer adsorption is BET Isotherm
-In physical adsorption , if initial adsorbed layer can act as substrate instead of
the isotherm levelling off to some saturated value at high pressures, it can be
expected to rise indefinitely
- Under the condition of high P & low T , thermal energy of gaseous molecules
decreases & more & more gaseous molecules would be available per unit
surface area of adsorbent and this leads to multilayer adsorption
p* is the vapour pressure above a
layer of adsorbate that is more
than one molecule thick & which
resembles a pure bulk liquid ,
Vmon is the monolayer coverage vol.,
c is a constant
-When c >> 1, the BET isotherm takes
the simpler form:
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27. The solid-liquid interface
- Adsorption from solution
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28. The theoretical treatment of adsorption from solution, however, is,
in general, more complicated than that of gas adsorption, since
adsorption from solution always involves competition between
solute(s) and solvent or between the components of a liquid
mixture
for the adsorption sites. Consider, for example, a binary liquid
mixture in contact with a solid.
Zero adsorption refers to uniform mixture composition right up to
the solid surface.
If the proportion of one of the components at the surface is
greater than its proportion in bulk, then that component is
positively adsorbed and, consequently, the other component is
negatively adsorbed.
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29. apparent adsorption isotherms are, therefore, calculated from changes in
solution concentration.
Examples of apparent adsorption isotherms for binary liquid mixtures are
given in the next Figure.
Composite (surface excess)
isotherms for the adsorption of
(a) benzene
from solution in methanol on to
charcoal and,
(b) chloroform from solution in
carbon tetrachloride on to
charcoa
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30. Isotherm equations, surface areas
*In adsorption from solution, physical adsorption is far more common
than chemisorption. However, chemisorption is sometimes possible;
for example, fatty acids are chemisorbed from benzene solutions on
nickel and platinum catalysts.
* Solute adsorption is usually restricted to a monomolecular layer,
since the solid-solute interactions, although strong enough to
compete successfully with the solid-solvent interactions in the first
adsorbed monolayer, do not do so in subsequent monolayers
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31. The Langmuir and Freundlich equations are frequently applied to
adsorption from solution data, for which they take the form:
respectively, where x is the amount of solute adsorbed by a mass m
of solid, c is the equilibrium solution concentration, and a, k and n
are constants , (x/m)max monolayer capacity .
Adsorption from solution has the merit of being experimentally
less demanding than gas adsorption; however, the problems in
interpretation are far greater.
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32. References:
1- Introduction to Colloid and Surface Chemistry,4th edition by Duncan J. Shaw
2- Physical Chemistry , 9th edition by Peter Atkins
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