This document discusses adsorption chromatography, which involves the adsorption of solutes onto the surface of a stationary phase. Key points include: - Adsorption is a surface phenomenon where interaction occurs only on the surface of one substance, unlike absorption. - Common stationary phases used in adsorption chromatography include silica gel, alumina, charcoal, and kieselguhr. Silica gel is the most widely used, with hydroxyl groups facilitating hydrogen bonding. - Forces between solutes and the adsorbent like dipole-dipole interactions, hydrogen bonding, and van der Waals forces determine how strongly compounds are retained on the stationary phase. Mobile phase composition impacts the balance of

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

2.  The stationary phase in adsorptionThe stationary phase in adsorption
Chromatography is called "Adsorbent"Chromatography is called "Adsorbent"
 Adsorption Chromatography is the oldest type ofAdsorption Chromatography is the oldest type of
chromatography. Actually Tswett's work was achromatography. Actually Tswett's work was a
kind of adsorption.kind of adsorption.
 When a liquid is used as mobile phase is liquid itWhen 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 HPLCe.g. TLC and HPLC
 If the mobile phase is gas it is called "Gas-SolidIf the mobile phase is gas it is called "Gas-Solid
Chromatography (GSC) e.g. GasChromatography (GSC) e.g. Gas
Chromatography (GC).Chromatography (GC).

3. In adsorption chromatography there areIn adsorption chromatography there are
two types of forces:two types of forces:
Forces attracting solutes to adsorbentForces attracting solutes to adsorbent
(Stationary Phase).(Stationary Phase).
 Forces tending to remove solutes fromForces 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 betweenDipole–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 thanHydrogen bonding: It is a type of bond weaker than
covalent bonds. Hydrogen bonds are formed between thecovalent bonds. Hydrogen bonds are formed between the
OH group hydrogen (as in silica) and electronegativeOH group hydrogen (as in silica) and electronegative
atoms such as Oxygen ,nitrogen in solutes.atoms such as Oxygen ,nitrogen in solutes.

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

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

7. Forces cause solutesForces cause solutes
movements:movements:
 Elution:Elution: It is the tendency of solutes to dissolve and moveIt is the tendency of solutes to dissolve and move
with the mobile phase. The solvent used as mobile phasewith the mobile phase. The solvent used as mobile phase
must be just good enough to dissolve the solutes to allowmust be just good enough to dissolve the solutes to allow
competition with the adsorption power of the stationarycompetition with the adsorption power of the stationary
phase. If very strong solvents are used they will wash out allphase. 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 competeIn this case solvent molecules compete
with the solutes for the adsorption sites of the stationarywith the solutes for the adsorption sites of the stationary
phase. This competition makes solutes move in differentphase. This competition makes solutes move in different
speeds.speeds.

8. Elutropic Series of solvents:Elutropic Series of solvents:
Solvent are arranged in this seriesSolvent are arranged in this series
according to their strength in ascendingaccording 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: ( StationaryTypes of adsorbents: ( Stationary
phase)phase)
The Ideal adsorbent must fulfill the followingThe 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 withColourless especially when work with
coloured mixtures.coloured mixtures.
 Suitable particle size enough to give goodSuitable 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 columnIt is the most widely used adsorbent in both column
and thin layer Chromatography. Silica gel isand thin layer Chromatography. Silica gel is
prepared by acidification of sodium silicate withprepared by acidification of sodium silicate with
sulphuric acid followed by washing with water andsulphuric acid followed by washing with water and
drying.drying.
 The active sites of silica gel are the hydroxyl groupsThe active sites of silica gel are the hydroxyl groups
attached to silicon atoms "Silanol groups" .Theseattached to silicon atoms "Silanol groups" .These
groups are 5 0A apart and form hydrogen bondinggroups are 5 0A apart and form hydrogen bonding
with solutes. Silica gel reaches its maximum powerwith solutes. Silica gel reaches its maximum power
when heated between 150 -250when 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 notIf silica gel contains water it then act by partition not
by adsorption. Decrease particle size increases theby adsorption. Decrease particle size increases the
surface area and consequently increases separationsurface area and consequently increases separation
power.power.

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

13. Derivatives of silica gel:Derivatives of silica gel:
All are based on reaction with the Si – OH groups (SilanolAll 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 (Al22OO33). Alumina). Alumina
activated by heating at 400activated by heating at 400 00
C overnight.C overnight.
Advantages of alumina: -Advantages of alumina: -
1- large capacity1- large capacity 2- Insoluble2- Insoluble
3- Relatively inert3- Relatively inert 4- Available4- Available
5- Adsorption is different from silica gel due to5- Adsorption is different from silica gel due to
the strong positive field of Althe strong positive field of Al++++++
and theand the
influence of basic sites which affect easilyinfluence of basic sites which affect easily
polarized compounds. It is good inpolarized 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 ofCause 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 by2- Acidic alumina pH 4. It is prepared by
washing aluminum oxide with 2N HCl thenwashing aluminum oxide with 2N HCl then
with distilled water.with distilled water.
3 – Basic alumina pH 10. This type is3 – Basic alumina pH 10. This type is
prepared by washing with NaOH then distilledprepared by washing with NaOH then distilled
water.water.

18. 3- Charcoal:3- Charcoal:
 There are two types of charcoal based onThere are two types of charcoal based on
temperature of activation:temperature of activation:
1-Non–polar of Charcoal prepared by1-Non–polar of Charcoal prepared by
activation at 1000activation at 1000 00
C and act by adsorptionC and act by adsorption
through hydrogen bonds and electrostaticthrough hydrogen bonds and electrostatic
forces.forces.
2- Polar charcoal prepared at lower2- 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.