Adsorption chromatography

adsorption

1. Adsorption chromatographyAdsorption chromatography
Adsorption versus Absorption: Adsorption versus Absorption:
In absorption one substance penetrate in to the In absorption one substance penetrate in to the
bulk of another substance.bulk of another substance.
Adsorption is a surface phenomenon where Adsorption is a surface phenomenon where
interaction takes place only on the surface of interaction takes place only on the surface of
one substance. one substance.

2. The stationary phase in adsorption The stationary phase in adsorption
Chromatography is called "Adsorbent" Chromatography is called "Adsorbent"

Adsorption Chromatography is the oldest type of Adsorption Chromatography is the oldest type of
chromatography. Actually Tswett's work was a chromatography. Actually Tswett's work was a
kind of adsorption.kind of adsorption.
When a liquid is used as mobile phase is liquid it When a liquid is used as mobile phase is liquid it
is called "Liquid-Solid Chromatography (LSC) is called "Liquid-Solid Chromatography (LSC)
e.g. TLC and HPLC e.g. TLC and HPLC
If the mobile phase is gas it is called "Gas-Solid If the mobile phase is gas it is called "Gas-Solid
Chromatography (GSC) e.g. Gas Chromatography (GSC) e.g. Gas
Chromatography (GC). Chromatography (GC).

3. In adsorption chromatography there are In adsorption chromatography there are
two types of forces: two types of forces:
Forces attracting solutes to adsorbent Forces attracting solutes to adsorbent
(Stationary Phase).(Stationary Phase).
 Forces tending to remove solutes from Forces tending to remove solutes from
adsorbent to move with the mobile phase.adsorbent to move with the mobile phase.

4. Forces of attraction:Forces of attraction:
They may be classified into according to their strength:They may be classified into according to their strength:
Dipole–dipole attraction: It is a force takes place between Dipole–dipole attraction: It is a force takes place between
polar adsorbent and polar solutes.polar adsorbent and polar solutes.
Hydrogen bonding: It is a type of bond weaker than Hydrogen bonding: It is a type of bond weaker than
covalent bonds. Hydrogen bonds are formed between the covalent bonds. Hydrogen bonds are formed between the
OH group hydrogen (as in silica) and electronegative OH group hydrogen (as in silica) and electronegative
atoms such as Oxygen ,nitrogen in solutes.atoms such as Oxygen ,nitrogen in solutes.

5. OHOH
SiSi O O
R -C - OHR -C - OH

6. Polarizability forces: A force occurs between polar Polarizability forces: A force occurs between polar
adsorbents and solutes that can polarize such as adsorbents and solutes that can polarize such as
aromatic compounds. aromatic compounds.
Weak covalent bonds: As those take place during Weak covalent bonds: As those take place during
complex formation. complex formation.
Van der Waals forces: Non polar attraction forces Van der Waals forces: Non polar attraction forces
occur between the atoms of nuclei and electrons occur between the atoms of nuclei and electrons
of another atoms. of another atoms.

7. Forces cause solutes Forces cause solutes
movements:movements:
Elution:Elution: It is the tendency of solutes to dissolve and move It is the tendency of solutes to dissolve and move
with the mobile phase. The solvent used as mobile phase with the mobile phase. The solvent used as mobile phase
must be just good enough to dissolve the solutes to allow must be just good enough to dissolve the solutes to allow
competition with the adsorption power of the stationary competition with the adsorption power of the stationary
phase. If very strong solvents are used they will wash out all phase. If very strong solvents are used they will wash out all
solutes together without separation. Ether/ hydrocarbons / solutes together without separation. Ether/ hydrocarbons /
carbonyl solvents are of common use.carbonyl solvents are of common use.
Displacement:Displacement: In this case solvent molecules compete In this case solvent molecules compete
with the solutes for the adsorption sites of the stationary with the solutes for the adsorption sites of the stationary
phase. This competition makes solutes move in different phase. This competition makes solutes move in different
speeds. speeds.

8. Elutropic Series of solvents:Elutropic Series of solvents:

Solvent are arranged in this series Solvent are arranged in this series
according to their strength in ascending according to their strength in ascending
(increasing) order. (increasing) order.

9. Arrangement of polar groups
according to their binding to adsorbent
Elutropic series of solvents
(increasing strength)
-COOH carboxylic Light petroleum & Hexanes
-OH hydroxyl Cyclohexane
-NH2 amines Carbon tetrachloride
-CHO aldehydes Trichloro ethylene
-C=O ketones Toluene
-COOR esters Benzene
-OCH3 ethers Dichloromethane
-C=C- olifens Chloroform
Ethyl ether
Ethyl acetate
Acetone
n-Propanol
Ethanol
Methanol
Water

10. Types of adsorbents: ( Stationary Types of adsorbents: ( Stationary
phase)phase)
The Ideal adsorbent must fulfill the following The Ideal adsorbent must fulfill the following
requirements:requirements:
Insoluble in mobile phase. Insoluble in mobile phase.
Inert to solutes (adsorptive).Inert to solutes (adsorptive).
Colourless especially when work with Colourless especially when work with
coloured mixtures. coloured mixtures.
Suitable particle size enough to give good Suitable particle size enough to give good
separation and reasonable flow rate. separation and reasonable flow rate.

11. Some examples of adsorbents are:Some examples of adsorbents are:
1- Silica gel - Silica - Silica acid:1- Silica gel - Silica - Silica acid:
It is the most widely used adsorbent in both column It is the most widely used adsorbent in both column
and thin layer Chromatography. Silica gel is and thin layer Chromatography. Silica gel is
prepared by acidification of sodium silicate with prepared by acidification of sodium silicate with
sulphuric acid followed by washing with water and sulphuric acid followed by washing with water and
drying.drying.
The active sites of silica gel are the hydroxyl groups The active sites of silica gel are the hydroxyl groups
attached to silicon atoms "Silanol groups" .These attached to silicon atoms "Silanol groups" .These
groups are 5 0A apart and form hydrogen bonding groups are 5 0A apart and form hydrogen bonding
with solutes. Silica gel reaches its maximum power with solutes. Silica gel reaches its maximum power
when heated between 150 -250 when heated between 150 -250
00
C to get rid of water. C to get rid of water.
If silica gel contains water it then act by partition not If silica gel contains water it then act by partition not
by adsorption. Decrease particle size increases the by adsorption. Decrease particle size increases the
surface area and consequently increases separation surface area and consequently increases separation
power.power.

12. OHOH OH OH
Si---------O-------SiSi---------O-------Si

13. Derivatives of silica gel:Derivatives of silica gel:
All are based on reaction with the Si – OH groups (Silanol All are based on reaction with the Si – OH groups (Silanol
groups) to block them.groups) to block them.
1- Reversed phase silica gel ( RP):
In this type a straight chain aliphatic groups are attached to the
OH of silica gel by silylation. RP silica gel are named according to
the length of the carbon chains.

C4 (RP4) C8 (RP8) C18 (RP18)

Si-O-Si–(CH2)3 –CH3 Si-O-Si-(CH2)7-CH3 Si-O-Si-(CH2)17-CH3

14. 2- Cyano silica gel:2- Cyano silica gel:
O

Si-O–Si –(CH2)3-CN

O

Si

15. 2- Alumina: 2- Alumina:
It is aluminum oxide (AlIt is aluminum oxide (Al
22OO
33). Alumina ). Alumina
activated by heating at 400 activated by heating at 400
00
C overnight.C overnight.
Advantages of alumina: - Advantages of alumina: -
1- large capacity 1- large capacity 2- Insoluble 2- Insoluble
3- Relatively inert 3- Relatively inert 4- Available 4- Available
5- Adsorption is different from silica gel due to 5- Adsorption is different from silica gel due to
the strong positive field of Althe strong positive field of Al
++++++
and the and the
influence of basic sites which affect easily influence of basic sites which affect easily
polarized compounds. It is good in polarized compounds. It is good in
separation of aromatics from olefins.separation of aromatics from olefins.

16. Disadvantages : -Disadvantages : -
Not suitable for base labile compounds.Not suitable for base labile compounds.
Cause rearrangement and ring expansion of Cause rearrangement and ring expansion of
unsaturated compounds. unsaturated compounds.
React chemically with acidic compounds. React chemically with acidic compounds.

17. Types of commercial alumina : Types of commercial alumina :
1- Neutral alumna pH 7– 7.5.1- Neutral alumna pH 7– 7.5.
2- Acidic alumina pH 4. It is prepared by 2- Acidic alumina pH 4. It is prepared by
washing aluminum oxide with 2N HCl then washing aluminum oxide with 2N HCl then
with distilled water. with distilled water.
3 – Basic alumina pH 10. This type is 3 – Basic alumina pH 10. This type is
prepared by washing with NaOH then distilled prepared by washing with NaOH then distilled
water. water.

18. 3- Charcoal:3- Charcoal:
There are two types of charcoal based on There are two types of charcoal based on
temperature of activation: temperature of activation:
1-Non–polar of Charcoal prepared by 1-Non–polar of Charcoal prepared by
activation at 1000 activation at 1000
00
C and act by adsorption C and act by adsorption
through hydrogen bonds and electrostatic through hydrogen bonds and electrostatic
forces.forces.
2- Polar charcoal prepared at lower 2- Polar charcoal prepared at lower
temp and contains water so act by partition.temp and contains water so act by partition.
4- Kieselguhr (Diatomaceous earth):4- Kieselguhr (Diatomaceous earth):
It have relatively low adsorption power.It have relatively low adsorption power.