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S Surface Chemistry 573
“The branch of physical chemistry, which deals the
nature of surfaces and also with the chemical and
physical processes which takes place on the surfaces, is
called surface chemistry”.
In surface chemistry, we study the phenomenon of
adsorption, catalysis and colloidal properties.
Adsorption
(1) Definition : The phenomenon of attracting
and retaining the molecules of a substance on the
surface of a liquid or solid resulting in to higher
concentration of the molecules on the surface is called
adsorption.
(2) Causes of adsorption : Unbalanced forces of
attraction or free valencies which is present at the
solid or liquid surface, have the property to attract and
retain the molecules of a gas or a dissolved substance
on to their surfaces with which they come in contact.
Example : Ammonia gas placed in contact with
charcoal gets adsorbed on the charcoal whereas
ammonia gas placed in contact with water gets
absorbed into water,
Table : 14.1 Some basic terms used in adsorption
Interface : Any surface is a
plane which separates any
two phases in contact with
each other. The plane which
separates any two phase is
generally called an
interface between the two
phases.
Adsorbate and Adsorbent :
The substance which gets
adsorbed on any surface is
called adsorbate for
example, if a gas gets
adsorbed on to the surface
of a solid, then the gas is
termed as the adsorbate.
The substance on the
surface of which adsorption
takes place is called
adsorbent.
Desorption : The removal
of the adsorbed substance
from a surface is called
desorption.
Absorption : When the
molecules of a substance are
uniformly distributed
throughout the body of a
solid or liquid. This
phenomenon is called
absorption.
Sorption : The phenomenon
in which adsorption and
absorption occur
simultaneously is called
sorption.
Mc. Bain introduced a
general term sorption
describeing both the
processes, however
adsorption is instantaneous
i.e. a fast process while
absorption is a slow process.
Occlusion : When
adsorption of gases occur on
the surface of metals this is
called occlusion.
(3) Difference between adsorption and absorption
Adsorption Absorption
It is a surface phenomenon. It concerns with the whole
mass of the absorbent.
Surface Chemistry
Chapter
574 Surface Chemistry
In it, the substance is only
retained on the surface and
does not go into the bulk or
interior of the solid or
liquid.
It implies that a substance is
uniformly distributed,
through the body of the
solid or liquid.
In it the concentration of the
adsorbed molecules is
always greater at the free
phase.
In it the concentration is
low.
It is rapid in the beginning
and slows down near the
equilibrium.
It occurs at the uniform
rate.
Examples : (i) Water
vapours adsorbed by silica
gel.
(ii) NH 3
is adsorbed by
charcoal.
Examples : (i) Water
vapours absorbed by
anhydrous CaCl 2
(ii) NH 3
is absorbed in water
forming NH 4
OH
(4) Surface forces : Only the surface atoms of an
adsorbent play an active role in adsorption. These
atoms posses unbalanced forces of various types such
as, Vander Waal’s forces and chemical bond forces.
Thus , the residual force-field on a free surface
which is responsible for adsorption is produced. For
example, when a solid substance is broken into two
pieces, two new surfaces are formed and therefore, the
number of unbalanced forces becomes more. As a result
the tendency for adsorption become large.
(5) Reversible and Irreversible adsorption : The
adsorption is reversible, if the adsorbate can be easily
removed from the surface of the adsorbent by physical
methods.If the adsorbate can not be easily removed
from the surface of the adsorbent is called irreversible
adsorption.
Example for reversible adsorption : A gas adsorbed
on a solid surface can be completely removed in vacuum.
Example for irreversible adsorption : Adsorption of
O 2 on tungusten adsorbent.
(6) Characteristics of adsorption
(i) Adsorption refers to the existence of a higher
concentration of any particular component at the
surface of a liquid or a solid phase.
(ii) Adsorption is accompanied by decrease in the
G (free energy change) of the system when G 0 ,
adsorption equilibrium is said to be established****.
(iii) Adsorption is invariably accompanied by
evolution of heat, i.e. it is an exothermic process. In
other words, H of adsorption is always negative.
(iv) When a gas is adsorbed, the freedom of
movement of its molecules becomes restricted. On
account of it decrease in the entropy of the gas after
adsorption, i.e. S is negative.
(v) For a process to be spontaneous, the
thermodynamic requirement is that G
must be
negative , i.e. there is decrease in free energy. On the
basis of Gibb’s Helmholtz equation , G H T S , G
can be negative if H has sufficiently high negative
value and T S has positive value****.
Classification of adsorption
Adsorption can be classified into two categories
as described below,
(1) Depending upon the concentration : In
adsorption the concentration of one substance is
different at the surface of the other substance as
compared to adjoining bulk or interior phase.
(i) Positive adsorption : If the concentration of
adsorbate is more on the surface as compared to its
concentration in the bulk phase then it is called
positive adsorption.
Example : When a concentrated solution of KCl is
shaken with blood charcoal, it shows positive
adsorption.
(ii) Negative adsorption : If the concentration of
the adsorbate is less than its concentration in the bulk
then it is called negative adsorption.
Example : When a dilute solution of KCl is shaken
with blood charcoal, it shows negative adsorption.
(2) Depending upon the nature of force existing
between adsorbate molecule and adsorbent
(i) Physical adsorption : If the forces of
attraction existing between adsorbate and adsorbent
are Vander Waal’s forces, the adsorption is called
physical adsorption. This type of adsorption is also
known as physisorption or Vander Waal’s adsorption****.
It can be easily reversed by heating or decreasing the
pressure.
(ii) Chemical adsorption : If the forces of
attraction existing between adsorbate particles and
adsorbent are almost of the same strength as chemical
bonds, the adsorption is called chemical adsorption.
This type of adsorption is also called as chemisorption
or Langmuir adsorption****. This type of adsorption
cannot be easily reversed.
Comparison between physisorption and
chemisorption
Physisorption
(Vander Waal's adsorption)
Chemisorption
(Langmuir adsorption)
Low heat of adsorption
usually in range of 20 - 40
kJ/mol
High heat of adsorption in
the range of 50- 400 kJ/mol
Force of attraction are
Vander Waal's forces.
Forces of attraction are
chemical bond forces.
It is reversible It is irreversible
576 Surface Chemistry
(iii) At high pressure , it becomes independent of
pressure.
0
p
m
x
(iv) At moderate pressure
m
x
depends upon
pressure raised to powers
n
p
m
x
1
(2) The Langmuir - adsorption isotherms
(i) One of the drawbacks of Freundlich adsorption
isotherm is that it fails at high pressure of the gas.
Irving Langmuir in 1916 derived a simple adsorption
isotherm, on theoretical considerations based on
kinetic theory of gases****. This is named as Langmuir
adsorption isotherm****.
(a) Adsorption takes place on the surface of the
solid only till the whole of the surface is completely
covered with a unimolecular layer of the adsorbed gas.
(b) Adsorption consists of two opposing
processes, namely Condensation of the gas molecules
on the solid surface and Evaporation (desorption) of
the gas molecules from the surface back into the
gaseous phase.
(c) The rate of condensation depends upon the
uncovered (bare) surface of the adsorbent available for
condensation. Naturally, at start when whole of the
surface is uncovered the rate of condensation is very
high and as the surface is covered more and more, the
rate of condensation progressively decreases. On the
contrary, the rate of evaporation depends upon the
covered surface and hence increases as more and more
of the surface is covered ultimately an equilibrium will
be set up at a stage when the rate of condensation
becomes equal to the rate of evaporation (adsorption
equilibrium).
(d) The rate of condensation also depends upon
the pressure of the gas since according the kinetic
theory of gases, the number of molecules striking per
unit area is proportional to the pressure****.
Mathematically,
bp
ap
m
x
, where a and b are
constants and their value depends upon the nature of
gas (adsorbate), nature of the solid adsorbent and the
temperature. Their values can be determined from the
experimental data.
Limitation of Langmuir theory
(a) Langmuir’s theory of unimolecular adsorption
is valid only at low pressures and high temperatures****.
(b) When the pressure is increased or
temperature is lowered, additional layers are formed.
This has led to the modern concept of multilayer
adsorption****.
Adsorption from solutions
(1) The process of adsorption can take place from
solutions also.
(2) In any solution, there are two (or more)
components ; solute and solvent. The solute may be
present in the molecular or ionic form.
(3) The extent of adsorption from solution
depends upon the concentration of the solute in the
solution, and can be expressed by the Freundlich
isotherm.
(4) The Freundlich adsorption isotherm for the
adsorption from solution is,
n
kc
m
x
1
where, x is the
mass of the solute adsorbed, m is the mass of the solid
adsorbent, c is the equilibrium concentration of the
solute in the solution, n is a constant having value
greater than one,
k is the proportionality constant, (The value of k
depends upon the nature of solid, its particle size,
temperature, and the nature of solute and solvent etc.)
(5) The plot of x/m against c is similar to that
Freundlich adsorption isotherm. The above equations
may be written in the following form,
c
n
k
m
x
log
1
log log where c, is the equilibrium
concentration of the solute in the solution.
Application of adsorption
The phenomenon of adsorption finds a number of
applications. Important applications are given as
follows.
(1) Production of high vacuum
(2) In Gas masks : This apparatus is used to
adsorb poisonous gases (e.g. , ,
2
Cl CO oxide of sulphur
etc.) and thus purify the air for breathing.
(3) For desiccation or dehumidification : These
substances can be used to reduce/remove water
vapours or moisture present in the air. Silica gel and
alumina are used for dehumidification in electronic
equipment.
(4 ) Removel of colouring matter from solution :
(i) Animal charcoal removes colours of solutions by
x/m
p
n
p
m
x
1 /
x/m
0
p
m
x
1
p
m
x
Freundlich adsorption
isotherm: plot of x/m
against p
Plot of log x/m against log p for
the adsorption of a gas on a solid
log
(x/m)
log p
intercept = log
k
n
1
slope
S Surface Chemistry 577
adsorbing coloured impurities. (ii) Animal charcoal is
used as decolouriser in the manufacture of cane sugar.
(5) Heterogeneous catalysis : Mostly
heterogeneous catalytic reactions proceed through the
adsorption of gaseous reactants on solid catalyst. For
example,
(i) Finely powdered nickel is used for the
hydrogenation of oils.
(ii) Finely divided vanadium pentaoxide ( )
2 5
VO
is
used in the contact process for the manufacture of
sulphuric acid.
(6) Separation of inert gases : Due to the
difference in degree of adsorption of gases by charcoal,
a mixture of inert gases can be separated by adsorption
on coconut charcoal at different low temperatures.
(7) Softening of hard water
(i) The hard water is made to pass through a
column packed with zeolite (sodium aluminium
silicate)
(ii) Ca
++
, Mg
++
ions which are responsible for
hardness, get adsorbed on zeolite, exchanging sodium
ions.
Na AlSiO CaCl CaAlSiO 2 NaCl
2 2 2 8 2 2 2 8
(iii) The exhausted zeolite is regenerated with
10% of sodium chloride solution.
2 2 8 2 2 2 8 2
CaAl SiO 2 NaCl NaAlSiO CaCl
(8) De-ionisation of water
(i) Water can be de-ionised by removing all
dissolved salts with the help of cation and anion-
exchanger resin.
(ii) Cation-exchanger is an organic synthetic resin
such as polystyrene-containing a macroanion
( etc.)
3
R SO which has adsorbed H
+
ions.
(iii) A resin containing a basic group ( etc.)
3
R N
which has adsorbed
OH ions acts as anion exchanger.
(9) In curing diseases : A number of drugs are
adsorbed on the germs and kill them or these are
adsorbed on the tissues and heat them.
(10) Cleaning agents : Soap and detergents get
adsorbed on the interface and thus reduce the surface
tension between dirt and cloth, subsequently the dirt is
removed from the cloth.
(11) Froth floatation process
A low grade sulphide ore is concentrated by
separating it from silica and other earthy matter by
this method.
(12) In adsorption indicators
Surface of certain precipitates such as silver
halide, have the property of adsorbing some dyes like
eosin, fluorescein etc.
(13) Chromatographic analysis
The phenomenon of adsorption has given an
excellent technique of analysis known as
chromatographic analysis.
(14) In dyeing : Many dyes get adsorbed on the
cloth either directly or by the use of mordants.
Catalysis
“Catalyst is a substance which speeds up and
speeds down a chemical reaction without itself being
used up.”
Berzelius (1836) introduced the term catalysis and
catalyst.
Ostwald (1895) redefined a catalyst as, “ A
substance which changes the reaction rate without
affecting the overall energetics of the reaction is termed
as a catalyst and the phenomenon is known as
catalysis. ”
Types of catalysis
Catalytic reactions can be broadly divided into the
following types,
(1) Homogeneous catalysis : When the reactants
and the catalyst are in the same phase ( i.e. solid, liquid
or gas). The catalysis is said to be homogeneous. The
following are some of the examples of homogeneous
catalysis.
(i) In the lead chamber process
2 () () 2 ()
3
()
2 2
SO g O g SO g
NO g
(ii) In the hydrolysis of ester
()
3 3 2
( ) ()
HCll
CHCOOCH l HO l
3 3
CH COOHl CHOHl
(iii) In the hydrolysis of sugar
()
2
(Sucrose solution)
12 22 11
2 4
HSO l
C H O l HO l
(Fructose solution)
6 12 6
(Glucose solution)
6 12 6
C H O ( l ) CH O ( l )
(2) Heterogeneous catalysis : The catalytic
process in which the reactants and the catalyst are in
different phases is known as heterogeneous catalysis.
Some of the examples of heterogeneous catalysis are
given below.
(i) In contact process for
2 4
HSO
3
()
2 2
25
SO g O g SO g
BrVO
Pts
(ii) In Haber’s process for
3
NH
( ) 3 () 2 ()
3
()
2 2
N g H g NH g
Fes
(iii) In Ostwald’s process for
3
HNO
4 () 5 () 4 () 6 ()
2
()
3 2
NH g O g NOg HOg
Pt s
(3) Positive catalysis : When the rate of the
reaction is accelerated by the foreign substance, it is
said to be a positive catalyst and phenomenon as
S Surface Chemistry 579
(5) The catalyst can not change the position of
equilibrium : The catalyst catalyse both forward and
backward reactions to the same extent in a reversible
reaction and thus have no effect on the equilibrium
constant.
(6) Catalytic promoters : Substances which
themselves are not catalysts, but when mixed in small
quantities with the catalysts increase their efficiency
are called as promoters or activators****.
(i) For example, in Haber’s process for the
synthesis of ammonia, traces of molybdenum increases
the activity of finely divided iron which acts as a
catalyst.
(ii) In the manufacture of methyl alcohol from
water gas ( )
2
CO H
, chromic oxide ( )
2 3
CrO
is used as a
promoter with the catalyst zinc oxide ( ZnO ).
(7) Catalytic poisons : Substances which destroy
the activity of the catalyst by their presence are known
as catalytic poisons****.
(i) For example, the presence of traces of
arsenious oxide ( ) 2 3
AsO in the reacting gases reduces
the activity of platinized asbestos which is used as
catalyst in contact process for the manufacture of
sulphuric acid.
(ii) The activity of iron catalyst is destroyed by
the presence of HS 2
or CO in the synthesis of
ammonia by Haber’s process.
(iii) The platinum catalyst used in the oxidation of
hydrogen is poisoned by CO.
(8) Change of temperature alters the rate of
catalytic reaction as it does for the same reaction in
absence of catalyst : By increasing the temperature,
there is an increase in the catalytic power of a catalyst
but after a certain temperature its power begins to
decrease. A catalyst has thus, a particular temperature
at which its catalytic activity is maximum. This
temperature is termed as optimum temperature****.
(9) A positive catalyst lowers the activation
energy
(i) According to the collision theory, a reaction
occurs on account of effective collisions between the
reacting molecules.
(ii) For effective collision, it is necessary that the
molecules must possess a minimum amount of energy
known as activation energy ( E a
(iii) After the collision molecules form an
activated complex which dissociate to yield the product
molecules.
(iv) The catalyst provides a new pathway
involving lower amount of activation energy. Thus,
larger number of effective collisions occur in
the presence of a catalyst in comparison to
effective collisions at the same temperature in
absence of a catalyst. Hence the presence of a
catalyst makes the reaction to go faster.
(v) Figure shows that activation energy
a
E ,
in absence of a catalyst is higher than the
activation energy Ea , in presence of a catalyst.
(vi)
R
E
and
p
E represent the average
energies of reactants and products. The difference
gives the value of G , i.e.,
R P
G E E
Theories of catalysis
There are two theories of catalysis which is
described as follows.
(1) Intermediate compound theory
Uncatalysed
complex
Catalysed
complex
Energy
barrier
E a
E a
Initial
state
Reactants
(A+B)
G° of
reaction
Final state Products (C +
D)
Reaction
sequence
E P
E R
Chemical potential
energy
Increases
E
a
Decreases
k
Increases
E
a
Decreases
RT
E
a
Increases
RT
e
- Ea/RT
Reaction speeds
up
580 Surface Chemistry
(i) This theory was proposed by Clement and
Desormes in 1806. According to this theory, the desired
reaction is brought about by a path involving the
formation of an unstable intermediate compound,
followed by its decomposition into the desired end
products with the regeneration of the catalyst.
(ii) The intermediate compund may be formed in
either of two ways
(a) When the intermediate compound is reactive
and reacts with the other reactants.
AB X BX A
intermediate
BX C CB X .....(i)
(b) When the intermediate is unstable and
decomposes to give the final product.
A B X ABX AB X
intermediate
.....(ii)
Where, A, B and C are the reactant molecules and
X is the molecule of the catalyst. The first type of
reaction sums up to, AB C CB A
While the second to, A B AB in many cases,
the intermediate compounds postulated to be formed
are known compounds and often their presence is
detected.
(2) Adsorption theory
(i) This theory is applicable to reactions between
gases in the presence of a solid catalyst. Some typical
examples are as follows.
(ii) The contact process for the oxidation of
2
SO
to
3
SO with atmospheric oxygen in the presence of
platinum as the catalyst.
(iii) The Haber’s process for the synthesis of
ammonia with iron as the catalyst.
(iv) Adsorption results in the loosening of the
chemical bonds in the reactant molecules, so that their
rupture becomes easier. This is confirmed by the
observed lower activation energies for heterogeneous
catalytic reactions in the presence of the catalysts as
compared to that for the same reaction in the absence
of the catalyst.
Enzyme catalysis
(1) Enzymes are complex nitrogenous substances
these are actually protein molecules of higher
molecular mass.
(2) Enzymes catalyse numerous reactions,
especially those connected with natural processes.
(3) Numerous reactions occur in the bodies of
animals and plants to maintain the life process. These
reactions are catalysed by enzymes. The enzymes are
thus, termed as bio-chemical catalysts and the
phenomenon is known as bio-chemical catalysis.
(4) Nitrogenase an enzyme present in bacteria on
the root nodules of leguminous plants such as peas and
beans, catalyses the conversion of atmospheric
2
N to
3
NH.
(5) In the human body, the enzyme carbonic
anhydrase catalyses the reaction of
2
CO with H O
2
2 2
CO aq HOl ( .) ( .)
3
H aq HCO aq
The forward reaction occurs when the blood takes
up
2
CO in the tissues, and the reverse reaction occurs
when the blood releases
2
CO in lungs.
(6) In manufacturing of ethyl alcohol
(i)
Invertase
12 22 11 2
C H O ( l ) HO ( l )
Fructose
6 12 6
Glucose
6 12 6
C H O ( l ) CHO ( l )
( ) 2 () 2 ()
2 5 2
Zy mase
6 12 6
C H O l CHOHl CO l
(ii) Starch ( l ) Maltose()
Diastase
l
Maltose Glucose Alcohol
Maltase Zyamase
Activity and Selectivity
(1) Activity : Activity is the ability of catalysts to
accelerate chemical reaction, the degree of acceleration
can be as high as
10
10 times in certain reactions. For
example reaction between
2
H and
2
O to form H O
2
in
presence of platinum as catalyst takes place with
explosive violence.
In absence of catalyst,
2
H
and
2
O
can be stored
indefinitely without any reaction.
(2) Selectivity : Is the ability of catalysts to direct
reaction to yield particular products (excluding other).
Example :
(i)
Pt
n heptane
(ii)
O
CH CH CH CH CHCH
BiMoO
||
Acrolein
3 2 2
4
Zeolite (Shape selective catalysis)
(1) Zeolite are alumino–silicates of the general
formula, M AlO SiO mHO
x / n 2 x 2 y 2
[ ].( ). , where, M may be
simple cation like
Na ,
K or
2
Ca , n is the charge on the
simple cation, m is the number of molecules of water of
crystallization.
(2) Some well known zeolites are as follows,
Erionite Na KCaMgAlO SiO HO
2 2 22 22 2
Gemelinite Na CaAlO SiO HO
2 22 24 2
3
CH
Toluen
e
582 Surface Chemistry
(2) Classification of colloids : The colloids are
classified on the basis of the following criteria
(i) Classification based on the physical state of
the dispersed phase and dispersion medium :
Depending upon the physical state of dispersed phase
and dispersion medium whether these are solids,
liquids or gases, eight types of colloidal systems are
possible.
Table : 14.3 Different types of colloidal systems
Disperse
d phase
Dispersio
n
Medium
Colloidal
System
Examples
Liquid Gas Aerosol of
liquids
Fogs, clouds, mists,
fine insecticide
sprays
Solid Gas Aerosol of
solids
Smoke, volcanic
dust, haze
Gas Liquid Foam or
froth
Soap lather.
Lemonade froth,
foam, whipped
cream, soda water
Liquid Liquid Emulsions Milk, emulsified
oils, medicines
Solid Liquid Sols Most paints, starch
in water, proteins,
gold sol, arsenic
sulphide sol, ink
Gas Solid Solid foam Pumice stone,
styrene rubber,
foam rubber
Liquid Solid Gels Cheese, butter, boot
polish, jelly, curd
Solid Solid Solid sols
(coloured
glass)
Ruby glass, some
gem stones and
alloys
(ii) Classification based on Nature of interaction
between dispersed phase and dispersion medium :
Depending upon the nature of interactions between
dispersed phase and the dispersion medium, the
colloidal solutions can be classified into two types as
(a) Lyophilic and (b) Lyophobic sols.
(a) Lyophilic colloids (water loving) : “The
colloidal solutions in which the particles of the
dispersed phase have a great affinity for the dispersion
medium, are called lyophilic collodis.”
(b) Lyophobic colloids (water hateing) : “The
colloidal solutions in which there is no affinity between
particles of the dispersed phase and the dispersion
medium are called lyophobic colloids. ”
Distinction between lyophilic and lyophobic sols
Property Lyophilic sols
(suspensoid)
Lyophobic sols
(Emulsoid)
Surface
tension
Lower than that of
the medium
Same as that of the
medium
Viscosity Much higher than
that of the medium
Same as that of the
medium
Reversibil
ity
Reversible Irreversible
Stability More stable Less stable
Visibility Particles can’t be
detected even under
ultramicroscope
Particles can be
detected under
ultramicroscope.
Migration Particles may
migrate in either
direction or do not
migrate in an
electric field
because do not carry
any charge.
Particles migrate either
towards cathode or
anode in an electric
field because they
carry charge.
Action of
electrolyt
e
Addition of smaller
quantity of
electrolyte has little
effect
Coagulation takes place
Hydration Extensive hydration
takes place
No hydration
Examples
Gum, gelatin,
starch, proteins,
rubber etc.
Metals like Ag and Au,
hydroxides like
3
Al ( OH ) ,
3
Fe ( OH ) metal
sulphides like
2 3
AS S
etc.
(iii) Classification based on types of particle of
dispersed phase : Depending upon the type of the
particles of the dispersed phase, the colloids are
classified as follows.
(a) Multimolecular colloids
When on dissolution, atoms or smaller
molecules of substances (having diameter less than
1 nm ) aggregate together to form particles of colloidal
dimensions, the particles thus formed are called
multimolecular colloids.
In these sols the dispersed phase consists of
aggregates of atoms or molecules with molecular size
less than 1 nm.
For example, sols of gold atoms and sulphur
8
S molecules. In these colloids, the particles are held
together by Vander Waal's forces. They have usually
lyophilic character.
(b) Macromolecular colloids
These are the substances having big size
molecules (called macromolecules) which on
dissolution form size in the colloidal range. Such
substances are called macromolecular colloids.
These macromolecules forming the dispersed
phase are generally polymers having very high
molecular masses.
Naturally occurring macromolecules are starch,
cellulose, proteins, enzymes, gelatin etc. Artificial
macromolecules are synthetic polymers such as nylon,
polythene, plastics, polystyrene etc.
S Surface Chemistry 583
They have usually lyophobic character.
(c) Associated colloids
These are the substances which on dissolved in
a medium behave as normal electrolytes at low
concentration but behave, as colloidal particles at
higher concentration due to the formation of
aggregated particles. The aggregates particles thus
formed are called micelles.
Their molecules contain both lyophilic and
lyophobic groups.
Micelles
Micelles are the cluster or aggregated particles
formed by association of colloid in solution.
The common examples of micelles are soaps
and detergents.
The formation of micelles takes place above a
particular temperature called Kraft temperature
k
T and above a particular concentration called critical
micellization concentration (CMC).
They are capable of forming ions.
Micelles may contain as many as 100 molecules
or more.
For example sodium stearate( )
17 35
C H COONa is a
typical example of such type of molecules.
When sodium stearate is dissolved in water, it
gives
Na and
C H COO
17 35
ions.
Sodiumstearate
17 35
C H COONa
C H COO Na
Stearateion
17 35
The stearate ions associate to form ionic
micelles of colloidal size.
It has long hydrocarbon part of
17 35
C H radical.
Which is lyophobic and
COO part which is lyophilic****.
In the figure, the chain corresponds to stearate
ion, ( ) 17 35
C H COO. When the concentration of the
solution is below from its CMC( 10 )
3 1
mol L , it behaves
as normal electrolyte. But above this concentration it is
aggregated to behave as micelles.
The main function of a soap is to reduce oily
and greasy dirt to colloidal particles (an emulsion).
Soap therefore, are known as emulsifying agents.
Some other examples of micelles are sodium
palmitate ( )
15 31
C H COONa , Sodium lauryl sulphate
[ ( ) ]
3 211 3
CH CH SOONa , Cetyl trimethyl ammonium
bromide
CH CH CH NBr
3 215 23
( ) ( ) etc.
General methods of preparation of colloids
Lyophilic and lyophobic colloidal solutions (or
sols) are generally prepared by different types of
methods. Some of the common methods are as follows.
(1) Preparation of Lyophilic colloids
(i) The lyophilic colloids have strong affinity
between particles of dispersed phase and dispersion
medium.
(ii) Simply mixing the dispersed phase and
dispersion medium under ordinary conditions readily
forms these colloidal solutions.
(iii) For example, the substance like gelatin, gum,
starch, egg, albumin etc. pass readily into water to give
colloidal solution.
(iv) They are reversible in nature become these
can be precipitated and directly converted into colloidal
state.
(2) Preparation of Lyophobic colloids :
Lyophobic colloids can be prepared by mainly two types
of methods.
(i) Condensation method : In these method,
smaller particles of dispersed phase are condensed
suitably to be of colloidal size. This is done by the
following methods.
(a) By oxidation : A colloidal solution of sulphur
can be obtained by bubbling oxygen (or any other
oxidising agent like
3 2
HNO , Br
etc.) through a solution
of hydrogen sulphide in water.
2 H S O (oranyotheroxidisingagent) 2 HO 2 S
2 2 2
(b) By reduction : A number of metals such as
silver, gold and platinum, have been obtained in
colloidal state by treating the aqueous solution of their
salts, with a suitable reducing agent such as
formaldehyde, phenyl hydrazine, hydrogen peroxide,
stannous chloride etc.
Goldsol
3 2 4
2 AuCl 3 SnCl 3 SnCl 2 Au
2 AuCl 3 HCHO 3 HO 2 Au 3 HCOOH 6 HCl
Goldsol
3 2
The gold sol, thus prepared, has a purple colour
and is called purple of cassius.
(c) By hydrolysis : Many salt solutions are rapidly
hydrolysed by boiling dilute solutions of their salts. For
example, ferric hydroxide and aluminium hydroxide
Na
Na
Na
Na
Na
Na
Na
Na
Fig. 14.2 Aggregation of several ions to form ionic
micelle
S Surface Chemistry 585
(1) Dialysis
(i) The process of separating the particles of
colloid from those of crystalloid, by means of diffusion
through a suitable membrane is called dialysis.
(ii) It’s principle is based upon the fact that
colloidal particles can not pass through a parchment or
cellophane membrane while the ions of the electrolyte
can pass through it.
(iii) The impurities slowly diffused out of the bag
leaving behind pure colloidal solution
(iv) The distilled water is changed frequently to
avoid accumulation of the crystalloids otherwise they
may start diffusing back into the bag.
(v) Dialysis can be used for removing HCl from
the ferric hydroxide sol.
(2) Electrodialysis
(i) The ordinary process of dialysis is slow.
(ii) To increase the process of purification, the
dialysis is carried out by applying electric field. This
process is called electrodialysis.
(iii) The important application of electrodialysis
process in the artificial kidney machine used for the
purification of blood of the patients whose kidneys
have failed to work. The artificial kidney machine
works on the principle of dialysis.
(3) Ultra – filtration
(i) Sol particles directly pass through ordinary
filter paper because their pores are larger (more than
1 or 1000 m ) than the size of sol particles (less than
200 m ).
(ii) If the pores of the ordinary filter paper are
made smaller by soaking the filter paper in a solution
of gelatin of colloidion and subsequently hardened by
soaking in formaldehyde, the treated filter paper may
retain colloidal particles and allow the true solution
particles to escape. Such filter paper is known as ultra
- filter and the process of separating colloids by using
ultra – filters is known as ultra – filtration****.
(4) Ultra – centrifugation
(i) The sol particles are prevented from setting
out under the action of gravity by kinetic impacts of the
molecules of the medium.
(ii) The setting force can be enhanced by using
high speed centrifugal machines having 15,000 or more
revolutions per minute. Such machines are known as
ultra–centrifuges.
Properties of colloidal solutions
The main characteristic properties of colloidal
solutions are as follows.
(1) Physical properties
(i) Heterogeneous nature : Colloidal sols are
heterogeneous in nature. They consists of two phases;
the dispersed phase and the dispersion medium.
(ii) Stable nature : The colloidal solutions are
quite stable. Their particles are in a state of motion and
do not settle down at the bottom of the container.
(iii) Filterability : Colloidal particles are
readily passed through the ordinary filter papers.
However they can be retained by special filters
known as ultrafilters (parchment paper).
(2) Colligative properties
(i) Due to formation of associated molecules,
observed values of colligative properties like relative
decrease in vapour pressure, elevation in boiling point,
depression in freezing point, osmotic pressure are
smaller than expected.
(ii) For a given colloidal sol the number of
particles will be very small as compared to the true
solution.
(3) Mechanical properties
(i) Brownian movement
(a) Robert Brown , a botanist discovered in 1827
that the pollen grains suspended in water do not
remain at rest but move about continuously and
randomly in all directions.
(b) Later on, it was observed that the colloidal
particles are moving at random in a zig – zag motion.
This type of motion is called Brownian movement.
(c) The molecules of the dispersion medium are
constantly colloiding with the particles of the dispersed
phase. It was stated by Wiener in 1863 that the impacts
of the dispersion medium particles are unequal, thus causing
a zig-zag motion of the dispersed phase particles.
(d) The Brownian movement explains the force of
gravity acting on colloidal particles. This helps in
providing stability to colloidal sols by not allowing
them to settle down.
(ii) Diffusion : The sol particles diffuse from
higher concentration to lower concentration region.
However, due to bigger size, they diffuse at a lesser
speed.
(iii) Sedimentation : The colloidal particles settle
down under the influence of gravity at a very slow rate.
This phenomenon is used for determining the molecular
mass of the macromolecules.
(4) Optical properties : Tyandall effect
(i) When light passes through a sol, its path
becomes visible because of scattering of light by particles.
586 Surface Chemistry
It is called Tyndall effect****. This phenomenon was studied
for the first time by Tyndall****. The illuminated path of the
beam is called Tyndall cone.
(ii) The intensity of the scattered light depends on
the difference between the refractive indices of the
dispersed phase and the dispersion medium.
(iii) In lyophobic colloids, the difference is
appreciable and, therefore, the Tyndall effect is well -
defined. But in lyophilic sols, the difference is very
small and the Tyndall effect is very weak.
(iv) The Tyndall effect confirms the
heterogeneous nature of the colloidal solution****.
(v) The Tyndall effect has also been observed by
an instrument called ultra – microscope.
Some example of Tyndall effect are as follows
(a) Tail of comets is seen as a Tyndall cone due to
the scattering of light by the tiny solid particles left by the
comet in its path.
(b) Due to scattering the sky looks blue.
(c) The blue colour of water in the sea is due to
scattering of blue light by water molecules.
(d) Visibility of projector path and circus light.
(e) Visibility of sharp ray of sunlight passing
through a slit in dark room.
(5) Electrical properties
(i) Electrophoresis
(a) The phenomenon of movement of colloidal
particles under an applied electric field is called
electrophoresis.
(b) If the particles accumulate near the negative
electrode, the charge on the particles is positive.
(c) On the other hand, if the sol particles
accumulate near the positive electrode, the charge on
the particles is negative.
(d) The apparatus consists of a U-tube with two
Pt - electrodes in each limb.
(e) When electrophoresis of a sol is carried out
with out stirring, the bottom layer gradually becomes
more concentrated while the top layer which contain
pure and concentrated colloidal solution may be
decanted. This is called electro decanation and is used
for the purification as well as for concentrating the sol.
(f) The reverse of electrophoresis is called
Sedimentation potential or Dorn effect****. The
sedimentation potential is setup when a particle is
forced to move in a resting liquid. This phenomenon
was discovered by Dorn and is also called Dorn effect****.
(ii) Electrical double layer theory
(a) The electrical properties of colloids can also
be explained by electrical double layer theory.
According to this theory a double layer of ions appear
at the surface of solid****.
(b) The ion preferentially adsorbed is held in
fixed part and imparts charge to colloidal particles.
(c) The second part consists of a diffuse mobile
layer of ions. This second layer consists of both the type
of charges. The net charge on the second layer is exactly
equal to that on the fixed part.
(d) The existence of opposite sign on fixed and
diffuse parts of double layer leads to appearance of a
difference of potential, known as zeta potential or
electrokinetic potential****. Now when electric field is
employed the particles move (electrophoresis)
(iii) Electro-osmosis
(a) In it the movement of the dispersed particles
are prevented from moving by semipermeable
membrane.
(b) Electro-osmosis is a phenomenon in which
dispersion medium is allowed to move under the
influence of an electrical field, whereas colloidal
particles are not allowed to move.
(c) The existence of electro-osmosis has suggested
that when liquid forced through a porous material or a
capillary tube, a potential difference is setup between
the two sides called as streaming potential. So the
reverse of electro-osmosis is called streaming
potential.
Origin of the charge on colloidal particles
The origin of the charge on the sol particles in
most cases is due to the preferential adsorption of
either positive or negative ions on their surface. The
sol particles acquire electrical charge in any one or
more of the following ways.
(1) Due to the dissociation of the surface
molecules : Some colloidal particles develope electrical
charge due to the dissociation / ionisation of the
surface molecules. The charge on the colloidal particles
is balanced by the oppositely charged ions in the sol.
For example, an aqueous solution of soap (sodium
palmitate) which dissociates into ions as,
Sodiumpalmitate
15 31
C H COONa
C H COO Na
15 31
The cations ( Na
+
) pass into the solution while the
anions( )
15 31
C H COO have a tendency to form aggregates
due to weak attractive forces present in the
hydrocarbon chains.
(2) Due to frictional electrification
(i) It is believed that the frictional electrification
due to the rubbing of the dispersed phase particles with
588 Surface Chemistry
the valency of the active ion are called flocculating ion,
which is the ion carrying charge opposite to the charge
on the colloidal particles. “ According to Hardy Schulze
rule, greater the valency of the active ion or flocculating
ion, greater will be its coagulating power ” thus, Hardy
Schulze law state:
(i) The ions carrying the charge opposite to that of
sol particles are effective in causing coagulation of the
sol.
(ii) Coagulating power of an electrolyte is directly
proportional to the valency of the active ions (ions
causing coagulation).
For example to coagulate negative sol of
2 3
AsS ,
the coagulation power of different cations has been
found to decrease in the order as,
Al Mg Na
3 2
Similarly, to coagulate a positive sol such as
3
Fe ( OH ) , the coagulating power of different anions has
been found to decrease in the order :
Fe CN PO SO Cl
2
4
3
4
4
6
[ ( )]
(7) Coagulation or flocculation value
“The minimum concentration of an electrolyte
which is required to cause the coagulation or
flocculation of a sol is known as flocculation value .”
or
“The number of millimoles of an electrolyte
required to bring about the coagulation of one litre of a
colloidal solution is called its flocculation value .”
Coagulation value or flocculating value
Coagulating power
(8) Coagulation of lyophilic sols
(i) There are two factors which are responsible
for the stability of lyophilic sols.
(ii) These factors are the charge and solvation of
the colloidal particles.
(iii) When these two factors are removed, a
lyophilic sol can be coagulated.
(iv) This is done (i) by adding electrolyte (ii) and
by adding suitable solvent.
(v) When solvent such as alcohol and acetone are
added to hydrophilic sols the dehydration of dispersed
phase occurs. Under this condition a small quantity of
electrolyte can bring about coagulation.
Protection of colloids and Gold number
Lyophilic sols are more stable than
lyophobic sols.
Lyophobic sols can be easily coagulated by
the addition of small quantity of an electrolyte.
When a lyophilic sol is added to any lyophobic
sol, it becomes less sensitive towards electrolytes.
Thus, lyophilic colloids can prevent the coagulation of
any lyophobic sol.
“The phenomenon of preventing the coagulation of
a lyophobic sol due to the addition of some lyophilic
colloid is called sol protection or protection of colloids.”
The protecting power of different protective
(lyophilic) colloids is different. The efficiency of any
protective colloid is expressed in terms of gold
number.
Gold number : Zsigmondy introduced a term
called gold number to describe the protective power of
different colloids. This is defined as, “weight of the
dried protective agent in milligrams, which when added
to 10 ml of a standard gold sol (0.0053 to 0.0058%) is
just sufficient to prevent a colour change from red to
blue on the addition of 1 ml of 10 % sodium chloride
solution, is equal to the gold number of that protective
colloid.”
Thus, smaller is the gold number, higher is the
protective action of the protective agent.
Goldnumber
Protective power
Table : 14.4 Gold numbers of some hydrophilic
substances
Hydrophili
c
substance
Gold
number
Hydrophilic
substance
Gold
number
Gelatin 0.005 -
Sodium oleate 0.4 – 1.
Sodium
caseinate
0.01 Gum tragacanth 2
Hamoglobi
n
0.03 – 0.07 Potato starch 25
Gum arabic 0.15 – 0.
Congo rubin number : Ostwald introduced congo
rubin number to account for protective nature of
colloids. It is defined as “ the amount of protective
colloid in milligrams which prevents colour change in
100 ml of 0.01 % congo rubin dye to which 0.16 g
equivalent of KCl is added.”
Mechanism of sol protection
(i) The actual
mechanism of sol
protection is very
complex. However it may
be due to the adsorption
of the protective colloid
on the lyophobic sol
particles, followed by its solvation. Thus it stabilises
the sol via solvation effects.
(ii) Solvation effects contribute much towards the
stability of lyophilic systems. For example, gelatin has
a sufficiently strong affinity for water. It is only
Lyophobi
c
particles
Lyophilic
protectin
g
particles
Fig. 14.7 Protection of
colloids
S Surface Chemistry 589
because of the solvation effects that even the addition
of electrolytes in small amounts does not cause any
flocculation of hydrophilic sols. However at higher
concentration, precipitation occurs. This phenomenon
is called salting out.
(iii) The salting out efficiency of an electrolyte
depends upon the tendency of its constituents ions to get
hydrated i.e, the tendency to squeeze out water initially
fied up with the colloidal particle.
(iv) The cations and the anions can be arranged in
the decreasing order of the salting out power, such an
arrangement is called lyotropic series.
Cations
Mg Ca Sr Ba Li Na K
2 2 2 2
NH Rb Cs
4
Anions :
SO Cl NH I CNS
3
2
4
3
Citrate
Ammonium sulphate, due to its very high
solubility in water, is oftenly used for precipitating
proteins from aqueous solutions.
(v) The precipitation of lyophilic colloids can also
be affected by the addition of organic solvents of non-
electrolytes. For example, the addition of acetone or
alcohol to aqueous gelatin solution causes precipitation
of gelatin. Addition of petroleum ether to a solution of
rubber in benzene causes the precipitation of rubber.
Emulsion
“The colloidal systems in which fine droplets of one
liquid are dispersed in another liquid are called
emulsions the two liquids otherwise being mutually
immiscible.” or
“Emulsion are the colloidal solutions in which both
the dispersed phase and the dispersion medium are
liquids.”
A good example of an emulsion is milk in which
fat globules are dispersed in water. The size of the
emulsified globules is generally of the order of
6
10
m.
Emulsion resemble lyophobic sols in some properties.
(1) Types of Emulsion : Depending upon the
nature of the dispersed phase, the emulsions are
classified as;
(i) Oil-in-water emulsions (O/W) : The emulsion
in which oil is present as the dispersed phase and
water as the dispersion medium (continuous phase) is
called an oil-in-water emulsion. Milk is an example of
the oil-in-water type of emulsion. In milk liquid fat
globules are dispersed in water. Other examples are,
vanishing cream etc.
(ii) Water-in-oil emulsion (W/O) : The emulsion
in which water forms the dispersed phase, and the oil
acts as the dispersion medium is called a water-in-oil
emulsion. These emulsion are also termed oil
emulsions. Butter and cold cream are typical examples
of this types of emulsions. Other examples are cod liver
oil etc.
(2) Properties of emulsion
(i) Emulsions show all the characteristic
properties of colloidal solution such as Brownian
movement, Tyndall effect, electrophoresis etc.
(ii) These are coagulated by the addition of
electrolytes containing polyvalent metal ions indicating
the negative charge on the globules.
(iii) The size of the dispersed particles in
emulsions in larger than those in the sols. It ranges
from 1000 Å to 10,000 Å. However, the size is smaller
than the particles in suspensioins.
(iv) Emulsions can be converted into two separate
liquids by heating, centrifuging, freezing etc. This
process is also known as demulsification.
(3) Applications of emulsions
(i) Concentration of ores in metallurgy
(ii) In medicine (Emulsion water-in-oil type)
(iii) Cleansing action of soaps.
(iv) Milk, which is an important constituent of
our diet an emulsion of fat in water.
(v) Digestion of fats in intestine is through
emulsification.
Gels
(1) “A gel is a colloidal system in which a liquid is
dispersed in a solid.”
(2) The lyophilic sols may be coagulated to give a
semisolid jelly like mass, which encloses all the liquid
present in the sol. The process of gel formation is
called gelation and the colloidal system formed called
gel.
(3) Some gels are known to liquify on shaking and
reset on being allowed to stand. This reversible sol-gel
transformation is called thixotropy.
(4) The common examples of gel are gum arabic,
gelatin, processed cheese, silicic acid, ferric hydroxide
etc.
(5) Gels may shrink by loosing some liquid help
them. This is known as synereises or weeping.
(6) Gels may be classified into two types
(i) Elastic gels : These are the gels which possess
the property of elasticity. They readily change their
shape on applying force and return to original shape
when the applied force is removed. Common examples
are gelatin, agar-agar, starch etc.
(ii) Non-elastic gels : These are the gels which
are rigid and do not have the property of elasticity. For
example, silica gel.
