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SURFACE CHEMISTRY  Adsorption is effective when the surface of

SYNOPSIS adsorbent is pure.
 The process of removal of impurities from the
 Surface chemistry is a branch which deals with the surface of adsorbent is called activation.
study of the phenomena occuring at the surface or Ex : Charcoal is activated by heating at 3000C-
interface, i.e., at the boundary separating the two 10000C in vacuum or presence of inert gas.
bulk phases.
Absorption: Absorption is the bulk
 Surface area means the top most layer upto the
phenomenon i.e., gas, liquid or solid molecules
100nm depth.
distributed uniformly in solid or liquid.
Adsorption: Adsorption is the phenomenon  Ex: i) Piece of chalk dipped in ink (chalk absorbs
of attracting and retaining the molecules of a ink) Sponge in water (sponge absorbs water)
substance on the surface of a liquid or a solid Sorption: If both adsorption and absorption
resulting into a higher concentration of the takes place simultaneously then it is known as
molecules on the surface. sorption.
 The molecular species or substances, which  The term sorption was introduced by MC Bain.
concentrates or accumulates at the surface is
termed as adsorbate. Desorption: The process of removing an
 Adsorbate can be gas, liquid or solid. adsorbed substance from a surface on which it
 The material or substance on the surface of is adsorbed is called desorption.
which the adsorption takes place is called Positive and Negative Adsorption:
adsorbent.
When the concentration of the adsorbate is more
 Adsorbent can be solid or liquid but not gas
on the surface of the adsorbent than in bulk, it
Ex : Activated Charcoal adsorbs gases (Cl2, SO2, is called positive adsorption. On the other hand,
CO2, Noble gases etc.) if the concentration of the adsorbate is less
 Ni or Pt adsorbs H2 gas (hydrogenation of oils). relative to its concentration in bulk,it is called
 Animal charcoal adsorbs acetic acid molecules. negative adsorption.
 Coloured particles of molasses gets adsorbed Eg : when a concentrated solution of KCl is
on act ivat ed charcoal hence molasses shaken with wood charcoal, it shows positive
decolourises. adsorption but with a dilute solution of KCl, it
 If a gas like O2 , H 2, CO, Cl2 , NH 3 or SO2 is shows negative adsorption.
taken in a closed vessel containing powdered Mechanism of Adsorption: Adsorption
charcoal, then the pressure of the gas in the arises due to the fact that the surface particles
vessel decreases due to adsorption of gases on of the adsorbent are not in the same environment
charcoal. as the particles inside the bulk.adsorption is due
 In a solution of an organic dye say methylene to unbalanced forces acting on the surface.
blue, when animal charcoal is added and the  During adsorption, there is always a decrease
solution is well shaken, then the filtrate turns in residual forces of surface i.e. there is decrease
colourless. Because the molecules of the dye are in surface energy which appears as heat. There
adsorbed on the surface of charcoal. fore Adsorption is an exothermic process.
 The air becomes dry in the presence of silica
gel because water molecules get adsorbed on
 H  ve 
the surface of the gel.  During the process of adsorption, free
 Adsorption is due to unbalanced molecular movement becomes restricted i.e. entropy of
forces or vanderwaals forces or valence forces. system decreases.  S  ve  .
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 Adsorption is thus accompanied by decrease in  On the basis of Thermodynamics
enthalpy  H  ve  and entropy  S  ve  G  H  T S , G can be negative if H
has sufficiently high negative value as  T S
of the system.
 But adsorption process is spontaneous, the is positive.
 As the adsorption proceeds, H becomes less
thermodynamic requirement for this is G must
and less negative ultimately H becomes equal
be negative  i.e. H TS  ve . to T S and G becomes zero. At this state
equilibrium is reached.

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The extent of Adsorption of Gases
depends on the following factors
50 83K
i) Surface area of the Adsorbent: With the 195K
40

Volume of N2
3
244K

Adsorbed cm
increase of surface area of the solid, rate of
30 273K
adsorption of gases increases.
 Finely divided transition metals like Co,Ni, Pt x 20
m
act as good adsorbents. 10

 Greater the surface area, greater is the extent of
adsorption. P
20 40 60
 The process of increasing the surface area of an
Pressure in cm of Hg
adsorbent and making it a better adsorbent is
called activation of the Adsorbent. x 1
 log = log p + log k
 Porous charcoal, silica gel contain large surface m n
area.
 Purified adsorbent is called activated adsorbent x slope = 1/n
log m
Intercept = log k
ii) Nature of Gas (Adsorbate): Easily log k
liquifiable gases which contain low boiling P
point adsorb more than non liquifiable gases.  Freundlich isotherm explains. the behaviour of
adsorption in approximate manner. Here the
 SO2 , NH3 ,HCl and CO2 adsorb more than H2, 1
O2 & N2 factor can have value between 0 and 1
n
 Higher the critical temperature, greater the ease (probable range 0.1 to 0.5)
of liquification of the gas and more is adsorption.
1 x
 1 g. of activated charcoal adsorbs about 400 ml  If  0 , then  constant, then adsorption is
n m
of SO2 (Tc=430 K), 20 ml of CH4 (Tc = 356K) independent of pressure.
iii) Pressure of the Gas: At constant 1 x x
 If  1, then  K .P i.e.  . p, adsorption
temperature, increase of pressure of a gas leads n m m
to increase of the extent of physical adsorption. varies directly with pressure.
 The effect of pressure on chemisorption is zero.  Both t he conditions are supported by
experimental result s. The experimental
At low pressure the physically adsorbed gas isotherms always seem to approach saturation
forms a monolayer. at high pressure. This cannot be explained by
Freundlich isotherm. Thus, it fails at high
Freundlich isotherm: At constant pressure.
temperature the amount of the gas adsorbed (x)  Freundlich theory is applicable to physical
on given mass of adsorbent (m) is directly adsorption at medium pressures only.
proportional to its pressure.
Langmuir Adsorption Isotherm (For
1
x n
Advance) :A solid surface is considered
 KP homogeneous. But it contains a fixed number
m
of adsorption sites on the surface of it.
k, n are constants of Freundlich adsorption  Each such site adsorbs a single molecule. This
isotherm means that adsorption is confined to a mono
molecular layer.
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 Adsorption is considered as an equilibrium process
comprising of vapourization and condensation
occuring simultaneously at a given temperature.
 At equilibrium the rate of evaporation is equal to
rate of condensation

(x/m)
 Rate of evaporation is proportional to the area of
the surface covered by the adsorbed gas
Rate  area covered. temperature (t°C)

Rate of evaporation = K d   a. Physical Adsorption

 Rate of condensation is proportional to the product
of the pressure of the gas and area uncovered on 50 83K
adsorbent. 195K
40

Volume of N2
3
Rate  P  area uncovered. 244K

Adsorbed cm
30 273K
Rate of condensation = K a p 1   
20
At equilibrium bot h rates are equal
10
K d  K a p 1   
If  is covered fraction of the surface.
20 40 60
bp ka Pressure in cm of Hg
 where b  k
1  bp d

X kbp X ap
 ; 
m 1  bp m 1  bp
 where x = mass of a gas adsorbed
(x/m)

m = mass of adsorbent
a = kb, b = ka/kd, p = pressure.
temperature (t°C)
The above equat ion is called Langmuir
adsorption Isotherm. b. Chemisorption

Temperature: Low temperatures favour Adsorption from solutions: Freshly

precipitated inorganic precipitates (e.g. metal
physical adsorption and high temperatures
hydroxides) act as good adsorbents for the dye
favour chemical adsorption or chemisorption.
stuff.
 N2 is physically adsorbed on iron at 463k but it  During the adsorption if the concentration of
is chemisorbed at 723k the solution decreases, it is called positive
 When temperature increases, rate of physical adsorption.
adsorption decreases.  The extent of Adsorption (x/m) is related to the
 In Chemical adsorption with increase of concentration of the solution through the
1
temperature the magnitude of adsorption first x n
increases and then decreases. mathematical formula  k .c
m
 The graph plotted between x / m vs temperature c = concentration of the solution.
at constant pressure is called Freundlich
adsorption isobars. x 1
log  log k  log c
m n
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 Extent of adsorption decreases with an increase in  Separation of inert gases: Due to the
temperature. difference in degree of adsorption of gases by
 Extent of adsorption depends on concentration charcoal, a mixture of noble gases can be
of solute in solution, nature of adsorbent, separated in Dewar’s method by adsorption on
adsorbate and surface area of the adsorbent. coconut charcoal at different temperatures.
Ex:- The precipitate of Mg  OH  2 attains Froth floatation process: A low grade
blue colour when precipitated in presence of sulphide ore is concentrated by separating it
Magneson reagent. The colour is due to from silica and other earthy matter by this
adsorption of Magneson. method using pine oil and froathing agent.

W.E-1: Per two gram of charcoal, a gas is adsorbed In curing diseases: A number of drugs are
by 0.1g and 0.2g at 10 torr and 80 torr pressure used to kill germs by getting adsorbed on them.
respectively. Calculate the n value in Adsorption indicators: Surfaces of certain
Freundlich adsorption isotherm. precipitates such as silver halides have the
1/ n
x x P property of adsorbing some dyes like eosin,
Sol.  K .P1/ n (or) 1   1  fluorescein, etc. and thereby producing a
m x2  P2 
characteristic colour at the end point.
Substituting the values,
Chromatographicanalysis:
1/ n 1 3/ n
0.1  10  1 1 Chromat ographic analysis based on t he
   (or)     
0.2  80  2 2 phenomenon of adsorption finds a number of
applications in analytical and industrial fields.
Therefore, the value of n is 3
Applications of Adsorption Catalysis: The name catalysis (Kata = wholly,
Lysis = to loosen) was first given by Berzelius
 Production of high vacuum: The remaining
in 1836.
traces of air can be adsorbed by charcoal from a
 A catalyst is that which increases the rate of
vessel evacuated by a vacuum pump to give a
very high vacuum. reaction without itself undergoing any change.
 Gas masks: Gas mask (a device which consists Characteristics of Catalyst:
of activated charcoal or mixture of adsorbents)  A catalyst does not initiate a reaction.
is usually used for breathing in coal mines to  A catalyst remains chemically uneffected at the
adsorb poisonous gases. end of the chemical reaction.
 Control of humidity: Silica and aluminium  Small amount of the catalyst are generally
gels are used as adsorbents for removing sufficient to speed up a chemical reaction
moisture and controlling humidity.  A catalyst does not effect the position of
 Removal of colouring matter from equilibrium. It helps to attain the equilibrium
solutions: Animal charcoal removes colours of quickly. It catalyses both the forward and the
solutions by adsorbing coloured impurities. backward reactions to the same extent.
 Heterogeneous catalysis: Adsorption of  A catalyst generally functions under the
reactants on the solid surface of the catalysts optimum conditions only (temperature, pressure,
increases the rate of reactants.There are many pH etc.,)
gaseous reactions of industrial importance
 A catalyst may get poisoned (loss of its activity)
involving solid catalysts. Manufacture of
by the presence of even traces of impurities.
ammonia using iron as a catalyst, manufacture
This is called catalytic poison. H2S or CO are
of H 2 SO4 by contact process Pt/V2 O5 as poison for Fe catalyst (Haber’s process). As2O3
catalyst and use of finely divided nickel in the is poison for Pt catalyst. (contact process).
hydrogenation of oils are excellent examples of  Catalytic poison is specific for a catalyst.
heterogeneous catalysis.
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 The action of catalyst in many instances is selective. 4) Hydrolysis of sucrose in presence of mineral acids
Change of catalyst may give changed products. A 
H
catalyst gives specific products only. For example. C12 H 22O11 l   H 2 O l   

Ni
CO  3H 2 
catalyst
 CH 4  H 2 O C6 H12O6  C6 H12O6
Cu
glu cos e Fructose
CO  H 2 
catalyst
 HCHO
 The substance which increases the activity of Heterogeneous Catalysis: If the reactants
catalyst is called promoter. Promoter is selective and catalysts are present in different phases, it
for a catalyst. For example, molybdenum is called heterogeneous catalysis
(Mo),Iron oxide,Potassium oxide and Alumina Eg:Preparation of SO3 by contact process
is promoter to the catalyst Fe in the Haber's Pt / V O
2SO2( g )  O2( g ) 
2 5( s )
 2 SO3( g )
process.
 Finely divided substances function as more 2) The preparation of ammonia by Haber's process
effective catalysts than the coarsely divided  
N2(g) + 3H2(g) 
Fe + Mo s
 2NH3(g)
substances. For example, finely divided Ni
 Oxidation of ammonia into nitric oxide in the
functions as a good catalyst in the hydrogenation
presence of platinum gauze in Ostwald’s
of oils.
process.
 If the rate of reaction is decreased in the presence
4 N H 3  g   5 O 2  g    
Pt s
of catalyst then the catalyst is called negative  4 N O g   6 H 2O g 
catalyst.  Hydrogenation of vegetable oils in the presence
Ex: :Decomposition of H 2O 2 is retarded by the of finely divided nickel as catalyst.
Ni  s 
Vegetableoils  s   H 2  g    Vegetableghee  s 
presence of glycerol or acetanilide.
Glycerol One of the reactants is in liquid state and the
2 H 2O2    2 H 2O  O2
other in gaseous state while the catalyst is in
 Change in temperature may alter the rate of a the solid state.
catalytic action. Bio-catalysts (enzymes) may
lose their activity at higher temperatures. Auto catalysis: When one of t he
Catalysts thus function at optimum temperatures intermediates formed in a reaction itself acts as
a catalyst, it is called auto catalysis.
Types of Catalysis Homogeneous
Catalysis: If the reactants and catalyst are  Oxidation of oxalic acid by acidified KMnO4
present in a same phase, it is called 2 KMnO4  5 H 2C2O4  3H 2 SO4
homogeneous catalysis.
 2 MnSO4  K 2 SO4  8 H 2O  10CO2
Eg:1) The preparation of SO3 by lead-chamber
process In this reaction Mn  2 ions act as autocatalyst
 In Hydrolysis of esters carboxylic acid act as
2 S O 2 ( g )  O 2 ( g )  N
O(g)
 2 S O 3 ( g )
 N O2 ( g ) auto catalyst .
2) The conversion of Carbon monoxide to Carbon CH 3COOC2 H 5  H 2 O  CH 3COOH  C2 H 5OH
dioxide in the presence of NO
NO
(g)
2CO( g )  O2( g )   2CO2 g  W.E-2:Why ester hydrolysis is slow in the beginning
3) Hydrolysis of ester in presence of acid. and becomes faster after some time?

Sol: In the hydrolysis of ester, carboxylic acid formed
H
CH 3COOC 2 H 5l   H 2 Ol   in the reaction acts as autocatalyst.. Thus the
reaction is slow in the begining and becomes
CH 3COOH  l   C 2 H 5 OH  l 
faster after some time due to the formation of
acid which acts as catalyst..

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 Decomposition of arsenic hydride to As and  Metals with maximum activity being shown by
groups 7-9 elements of the periodic table.
H 2  2 AsH 3  2 As  3H 2  Arsenic formed
initially is acting as autocatalyst 2 H 2  g   O 2  g   P
t
 2 H 2 O l 

Adsorption theory of Heterogeneous  B) Selectivity: The ability of catalysts to direct

Catalysis (Mechanism) reaction to give particular product is called
selectivity of that catalyst.
(Mechanism of heterogeneous catalysis)
 The modern adsorpt ion theory is t he Ex:- Different catalysts yield different products
combination of Int ermediat e compound for the same reacting substances.
formation theory and the old adsorption theory. i) C O  g   3 H 2  g   Ni  C H 4  g   H 2 O  g 
 The catalytic activity is localised on the surface
of the catalyst. ii) C O  g   2 H 2  g   C
u / Zno  C r O
   C H 3 O H  g 
2 3

 Diffusion of reactants to the surface of the

iii) C O  g   H 2  g   C
u
 H C H Og 
catalyst.
 Adsorption of reactant molecules on the surface
of the catalyst.
Shape-Selective Catalysis by Zeolites
 Formation of Intermediate due to the chemical  The catalytic reaction that depends upon the
reaction on the surface of catalyst. porestructure of the catalyst and the size of the
 Desorption of reaction products from the catalyst reactant and product molecules is called shape-
surface and there by catalyst whose surface selective catalysis.
available to further reaction to occur.  Zeolites are alumino silicates i.e., t hree
 Diffusion of reaction products away from the dimensional network silicates in which some
surface of catalyst. silicon atoms are replaced by aluminium atoms
 This theory explains why the catalyst remains giving Al  O  Si frame work.
unchanged in mass and chemical composition  Zeolites are good shape-selective catalysts
at the end of the reaction and is effective even because of their honeycomb-like structures.
in small quantities.  Zeolites, before using as catalysts, are heated in
 This theory does not explain the action of vaccum so that the water of hydration is lost.
catalytic promoters and catalytic poisons. As a result, zeolite becomes porous i.e., the
Adsorption of
cavities in the honey-comb like structure which
reacting molecules were occupied by the water molecules become
A vacant.
+A+B
B The size of the pores generally varies between
Reacting
molecules 260 pm and 740 pm. Thus only those molecules
Adsorption of can be adsorbed in these pores whose size is
Catalyst surface reacting molecules
having free valencies small enough to enter these cavities and also
Desorption of leave easily.
product molecules A
+A-B  The reactions taking place in zeolites depend
B upon the size and shape of reactant and product
Catalyst Intermediate molecules as well as upon the pores and cavities
of the zeolites, that is why these types of
Solid catalysts possess reactions are called ‘shape-selective catalysis’
reactions.
Two important features
 A) Activity B) Selectivity  Zeolites are being very widely used as catalysts
in petrochemical industries for cracking of
 A) Activity: The ability of a catalyst to hydrocarbons and isomerisation. An important
accelerate chemical reactions is called activity zeolite catalyst used in the petroleum industry
of a catalyst. is ZSM-5. It converts alcohols directly into
Catalytic activity increases from group 5 to gasoline (petrol) by dehydrating them so that a
group 11. mixture of hydrocarbons is formed.
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Catalysis: called the optimum temperature. On either side of
 Enzyme catalysis: Enzymes are complex the optimum temperature, the enzyme activity
nitrogenous organic compounds which are decreases. The optimum temperature range for
produced by living plants and animals. They are enzymatic activity is 298-310K. Human body
actually protein molecules of high molecular temperature being 310 K is suited for enzyme
mass and form colloidal solutions in water. The catalysed reactions.
enzymes are also sometimes called biocatalysts.  Highly active under optimum pH: The rate
 Though enzymes are produced by living beings, of an enzyme-catalysed reaction is maximum
they themselves are non-living and can act as at a particular pH called optimum pH, which is
catalysts even outside the living bodies. between pH values 5-7.
 Numerous reactions that occur in the bodies of  Increasing activity in presence of
animals and plants to maintain the life process activators and co-enzymes: The enzymatic
catalysed by enzymes. The enzymes are, thus, activity is increased in the presence of a certain
termed as biochemical catalysts and the small non-protein (vitamin) present along with
phenomenon is known as biochemical catalysis. an enzyme, the catalytic activity is enhanced
Enzymes are capable of bringing about complex considerably.
reactions at body temperature. Some examples Activators are generally metal ions such as
of enzyme catalysed reactions are: Na  , Co 2  , Cu 2  . etc. These metal ions, when
Some enzymatic reactions weakly bonded to enzyme molecules, increase
their catalytic activity. Amylase in presence of
sodium chloride i.e., Na  ions are catalytically
very active.
 Influence of inhibitors and poisons :Like
ordinary catalysts, enzymes are also inhibitors
or poisons interact with the active functional
groups on the enzyme surface and often reduce
or completely destroy the catalytic activity of
the enzymes. The use of many drugs is related
to their action as enzyme inhibitors in the body.
Mechanism of enzyme Catalysed
Reaction
 The molecules of the reactant (substrate) which
have complementary shape, comparing with the
Characteristics of Enzyme Catalysis shape of cavities on the surface of colloidal
particles of enzymes.
 High Efficiency: One molecule of an enzyme
 There fore molecules of reactant fit into cavities
may transform one million molecules of the
of colloidal particles of enzymes just like key
reactant per minute.
fits into lock.
 Highly specific nature: Each enzyme is The enzyme-catalysed reactions are considered
specific for a given reaction, i.e., one catalyst to proceed in two steps.
cannot catalyse more than one reaction. For
Step-I: Binding of enzyme to substrate
example, the enzyme urease catalyses the
(reactant) to form an activated complex.
hydrolysis of urea only. It does not catalyse
hydrolysis of any other amide. E  S  ES *
Step-II: Decomposition of the activated
 Highly active under optimum
complex to form product.
temperature: The rate of an enzyme reaction
becomes maximum at a definite temperature, ES   E  enzyme   P  product 

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Active site E+S [E-S] E+P

Colloidal State:
 The substances are classified into crystalloids
P and colloids by Thomas Graham.
E + S E S E +  The substance which passes through the
P membrane is called crystalloid.
Enzyme Enzyme-Substrate Products Eg: Sugar in water, Salt in water, Urea in water,
(catalyst) complex
Substrate Enzyme acids and bases etc.,
(reactants) . The substance which disperses in a solvent and
The Differences Between Enzymes can not pass through membrane is called colloid.
Eg: Glue : gum arabic, gelatin, agar etc
and Catalyst
 If the particle size of the solute in the binary
system is in the range 1m  - 1  a colloidal
solution is formed.
 Colloidal solutions are - Starch paste, gelatin
(or) glue added to hot water.
 The system cont ains two phases one is
“dispersed phase”, the other is “dispersion
medium”.
 Solutions of chemical substances are broadly
classified into true solutions and colloidal
solutions depending on the particle size.
 If the particle size of the solute in the binary
system is less than 1m  10 9 m  , a true solution
is formed.
 Examples for true solutions are - solutions of
Catalyst in Industry: Some of t he Common salt, sugar, acids,bases etc.
important technical catalytic processes are listed  Colloidal particles have an enormous surface
in below table. area per unit mass as a result of their size.
Consider a cube with 1cm side. It has a total
surface area of 6cm 2 . If it were divided equally
into 1012 cubes, the cubes would be the size of
colloidal particles and will have a total surface
area of 60, 000cm 2 or 6m 2 . This large surface
area leads to some special properties of colloids
to be discussed later in this chapter.
(Volume of big cube is 1cm3 . Volume of small
1
cube is 12
cm3 i.e. 1012 cm3 . Side is
10
3
1012  104 cm . Total surface area is
6  10 4  104  1012  6  104 cm 2 )

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Types of colloidal system based on
Physical state of Dispersed phase and
Dispersion medium.

Dispersed Phase: The substance which is in

smaller proportion and is distributed as the
colloidal particles in the colloidal solution is
known as dispersed phase.
The dispersed phase can be a gas, liquid (or)
solid.

Dispersion Medium: The continuous phase

of the heterogeneous colloidal solution, in which Smoke :
the colloidal particles are dispersed is called  Is a lyophobic sol.
dispersion medium.  This is a solid in gas sol.
 dispersed phase : Carbon particles
 Dispersion medium can be a gas or a liquid or a
dispersion medium : air (Aerosol)
solid
Cloud: It is a lyophobic sol.
 Gas in gas system doesnot form a colloidal
 It is liquid in gas sol.
solution.
dispersed phase: water drops dispersion
 The dispersion medium in colloidal solution is medium : air (Aerosol)
named as lyo (solvent), hence the colloidal Blood : It is lyophilic sol
solutions are named as lyo sols.  It is a solid in liquid sol.
 If the dispersion medium is air, colloidal  dispersed phase is albuminoid substance (RBC:
solutions are named as aerosols. erythrocytes)
. If the dispersion medium is water, these are dispersion medium:Water(Aquasol)
named as hydrosols (or) aqua sols. Milk: It is lyophilic sol .
 If the dispersion medium is alcohol the sol is It is liquid in liquid type of colloid
called “alcosol” Disperse phase : Liquid fat
Dispersion medium : water(Aquasol)
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Starch Solution: It is lyophilic sol. Classification of Colloids based on the
 It is a solid in liquid sol.
Disperse phase : starch
type of particles of dispersed Phase
Dispersion medium :water(Aquasol)  a) Multimolecular colloids : On dissolution,
a large number of atoms or smaller molecules
Gold sol: It is a lyophobic sol of a substance aggregate together to form species
 It is a solid in liquid sol. having size in the colloidal range (diameter
Disperse phase: Gold particles >1nm). The species thus formed are called
Dispersion medium : water (aqua sol) multimolecular colloids.
Classification based on Nature of Ex: A gold sol many contain particles of various
sizes having many atoms, sulphur sol.(S8)
Interaction between dispersed phase
 b) Macromolecular colloids :
and dispersion medium Lyophillic Sol Macromolecules in suitable solvents form
 If great affinity exists between the dispersed solutions in which the size of the
phase and the dispersion medium it is called macromolecules may be in the colloidal range
lyophillic sol. such systems are called macromolecular
 These are called solvent -loving collodial colloids.
solutions. Ex : Starch, cellulose, proteins and enzymes,
 High molecular weight carbon compounds form polythene, nylon, polystyrene, synthetic rubber
lyophillic sol etc.
 These are very stable and cannot be coagulated  c) Associated colloids ( Micelles): There are
easily. some substances which at low concentrations
 An important characteristic of these sols is that behave as normal strong electrolytes, but at
if the dispersion medium is separated from the higher concentration exhibit colloidal behaviour
dispersed phase (say by evaporation), the sol can due to the formation of aggregates The
be reconst ituted by simply remixing the aggregated particles thus formed are called
dispersed phase with the dispersion medium. micelles. These are also known as associated
That is why these sols are also called reversible colloids.The formation of micelles takes place
sols. only above a particular temperature called kraft
Eg: starch solution, aqueous protein solutions, temperature (T k) and above a particular
polymer solutions. concent ration called critical micelle
concentration (CMC). On dilution, these
Lyophobic Sol: If there is low affinity
colloids revert back to individual ions.
existing between the dispersed phase and
Ex : Surface active agents like soaps and
dispersion medium it is called lyophobic sol.
synthetic detergents.
 These are called solvent hating colloidal
 For soaps the CMC is 10-4 to 10-3 mol L-1.
solutions.
 Low molecular weight inorganic substances Micelles : A colloidal sized particle
form lyophobic sol. (aggregate) formed in water by the association
. These are not very stable of normal simple molecules, each having a
 Lyophobic colloids needs stabilising agents for hydrophobic end and a hydrophillic end is
their preservation. micelle.
 These sols are readily precipitat ed (or  A colloid in which the dispersed phase consists
coagulated) on the addition of small amounts of micelles or aggregated colloidal particles is
of electrolytes. an associated colloid.
 The precipitate does not give back the colloidal  Substance whose molecules aggregate
sol by simple addition of the dispersion medium spontaneously in a given solvent to form large
to it. Hence, these sols are also called irreversible particles of colloid dimensions are called
sols. associated colloids.
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 Ordinary soap, synthetic detergents dissolved in Preparation of colloidal solutions
water belong to this class. Lyophobic sols and lyophilic sols are prepared by
 Micelles contain generally as many as 100 or more
different methods.
molecules of the substance forming the micelle.
Preparation of lyophobic sols
Mechanism Of Micelle Formation
Ex:Soap The methods employed are :
 Sodium stearate (soap) with the formula I) Condensation methods
C17 H35COONa is one example which forms II) Dispersion methods.
micelles. III) Chemical method
 Stearate ion CH 3 (CH 2 )15 CH 2 .COO   contains IV) Peptization
at its ends a hydrophobic group (alkyl group end) I) Condensation methods: In these methods
and a hydrophilic group (carboxylate ion end) small ions or molecules are induced to combine
together to form aggregates of colloidal size
 Working of soap (micelles): The COO
either by using chemical or physical methods.
group has an affinity for water. It is therefore
called hydrophilic group and is called head of II) Dispersion methods: Here lumps of the
the stearate anion substance is broken down to colloidal size in
 The hydrocarbon chains of the anion has an presence of dispersion medium and suitable
affinity for grease, oil or dirt. This is the stabilizer.
hydrophobic part of the ion and is called tail of  Mechanical Dispersion: In this method
the anion. The tail part dissolves the grease or
colloid mill, ball mill or ultrasonic disintegrater
dirt.
are used.
Cleaning Action of Soap : The cleaning
action of soap is due to the fact that soap Bredig’s arc method (Electrical
molecules form micelle around the oil droplet disintegration)
in such a way that hydrophobic part of the
stearate ions is in the oil droplet and hydrophilic + -
part projects out of the grease droplet like the
bristles.
 Since the polar groups can interact with water,
the oil droplet surrounded by stearate ions is now
pulled in water and removed from the dirty Ice-bath
surface.
 Thus soap helps in emulsification and washing Despiratio
away of oils and fats. The negatively charged Medium
sheath around the globules prevents them from
coming together and forming aggregates.
 An arc is struck between two metal electrodes
of silver, gold or platinum held at the surface of
cold water containing traces of alkali when sol
of metal is obtained.
Purple of cassius a colloidal sol of gold is
obtained.
 Gold sol is prepared by Bredig arc’s method
Hydrophobic  Alkali stabilises the gold sol, but electrolytes
head destabilises it.
 The gold sol is coloured blue if the particle size
is big and is coloured red when the particle size
Hydrophobic tail is small.

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III) Chemical methods: (iv) Cellulose nitrate is peptized by a mixture of

ethanol and water. The product obtained is called
 Double Decomposition: An arsenic sulphide
“COLLOIDION”.
(As2S3) sol is prepared by passing H2S through
cold solution of As2O3 till yellow colour deepens to Purificaion of Colloidal Solutions
its maximum  Generally colloidal solutions associate with
As2 O3  3H 2 S  As2 S3 ( sol )  3H 2O excessive amount of electrolyte and some other
 Oxidation: A sol of sulphur is prepared by passing soluble substances, which act as impurities.
H2S into solution of SO2  The process of removal of impurities from
colloidal solution is known as purification of
2H 2 S  SO2  2H 2O  3S  colloids.
 Reduction: Gold, platinum and silver sols are Colloidal solutions can be purified by the
prepared by reduction of their compounds in following methods.
water by using formaldehyde (or) hydrogen or a) Dialysis b) Electrodialysis c) ultrafiltration
tannic acid. a)Dialysis: The process of removal of dissolved
AgNO3 + Tannic acid  Ag Sol substance (Impurities) from a colloidal solution
AuCl3 + Tannic acid  Au Sol by means of diffusion through a suitable
2 AuCl3  3HCHO  3H2O  2 Au  3HCOOH  6HCl membrane iscalled dialysis.
 Hydrolysis: Ferric hydroxide sol is prepared
Dialysing
by pouring dilute solution of ferric chloride into Crystalloid membrane
boiling water
FeCl3  3H 2O  Fe(OH )3 3HCl
Re d sol ( Positivein nature ) - Water +
+ Electrolyte
Sols of chromium and aluminium can also be
prepared by this method. Water Sol particles
 Change of solvent: When ethanolic solution
of sulphur is added to an excess of water, the
sol of sulphur is obtained. This is physical W.E-3:How to save a patient suffering from kidney
method. failure?
Sol. Blood is a colloidal solution. In case of kidney
IV) Peptization:The process of conversion
failure, blood cannot be purified. Under such
of a precipitated substance into colloidal
conditions, the blood is separated from dissolved
solution by the addition of a small amount of
electrolyte is called peptization. toxic impurities by dialysis and reintroduced in
 The electrolyte used for the peptization process the blood stream.
is called peptizing agent. b) Electro dialysis: Ordinarily the process of
 During peptization the precepitate adsorbs one dialysis is slow. It can be made faster by
of the ions of electrolyte on its surface. This electrodialysis.
causes the development of positive or negative  Definition: If dialysis is carried out in presence
charge on precipitate which ultimately breakup of electric field is called electro dialysis.
to colloidal particle. This method is used only when colloidal
(i) Ferric hydroxide Fe(OH)3 is peptized by solutions possess electrolytic impurities
ferric chloride giving positive sol. of
c) Ultra filteration:The process of seperating
[ Fe(OH ) 3 ]Fe3
the collidal particles from the solvent and
(ii) Silver chloride AgCl is peptized by HCl soluble solutes present in the colloidal solution
giving negative sol of (AgCl)Cl- by specially prepared filters (ultra filter) known
(iii) Cadmium sulphide CdS is peptized by H2S as ultra filteration.
giving negative sol of [CdS]S-2
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 The pore size of ordinary filter paper can be reduced d) Smaller the size and lesser the viscosity, faster is
by impregnating with colloidion solution to stop the motion of colloidal particles
the flow of colloidal particles e) Brownian movement increases with rise in
 Colloidion is a 4% solution of nitro cellulose in a temperature
mixture of alcohol and ether.
 Ultrafilter paper may be prepared by soaking the Electrical properties
filter paper in a colloidion solution, which is then
 Presence of charge: All the particles of
hardened by formaldehyde.
dispersed phase carry a positive or negative
Properties of colloids: The charge on the charge and dispersion medium carry the opposite
colloidal particle (disperse phase) is usually charge. sol as a whole is neutral. The origin of
taken as the charge of the colloid the charge may be due to electron capture by
Properties are classified into Five types: sol particles during electrodispersion of metals,
A) Optical properties (size of the colloidal due to preferential adsorption of ions from
particles) solution and / or due to formulation of electrical
B) Kinetic properties (random motion of the double layer.
particles)  The charge on the sol particles is due to one or
C) Electrical properties (charge on the colloidal
more of the following reasons.
particle)
D) Colour. i) Due to presence of acidic or basic
E) Colligative properties. groups: A protein molecule has a carboxylic
Tyndall Effect: It is an optical property group and a basic NH 2 group. The particles of
 The phenomenon of the scattering of light by proteins in sols can either have positive charge
the colloidal particles is called Tyndall effect or negative charge depending upon the pH of
 The illuminated beam or cone formed by the the medium.
scattering of light by the sol particles is reffered
as Tyndall beam or tyndall cone. NH 2 NH 3
(acidic
 Tyndall effect is observed only when CH 2  HCl CH 2  Cl medium)
a) The diameter of the dispersed particles is not COOH COOH
Glycine Positively charged
much smaller than the wave length of light used
NH 2 NH 2
b) The refractive indices of the dispersed phase + NaOH CH 2  Na   H 2O
CH 2
and dispersion medium differ greatly in COO 
COOH
magnitude Glycine Negatively charged (alkaline
medium)
Examples of Tyndall Effect
a) Blue appearance of sky and sea water  Isoelectric point of a colloid: In case of
b) Visibility of tails of comets colloidal solution of proteins, the nature of
Brownian Movement (Kinetic charge depends on the pH of the solution. Above
Property) this pH, the particles are negatively charged and
a) The continuous rapid zig - zag movement below this pH, they have positive charge. At
executed by colloidal particle in a liquid isoelectric point, colloidal particles exist in the
dispersion medium is called Brownian motion. form of Zwitter ion hence they do not migrate
b) All collodial particles in colloidal solution under the influence of external electric field.
exhibits Brownian motion.This is due to Examples:
bombardment of the particles of the dispersion Colloidal sol Isoelectric pH
medium on the particles of dispersed phase.
c) Brownian motion is independent of the nature Haemoglobin 4.3-5.3
of colloid but depends on the size of the particles Casein from human milk 4.1-4.7
and viscosity of solution Gelatin 4.7

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ii) Due to self-dissociaiton: When colloidal SnO2  4H   Sn4  2H 2O

particles such as soaps or detergents are dissolved
SnO2  Sn 4    SnO2  Sn 4 
in water, ionised molecules associate to form a Positive colloid
micelle. The outer surface will be thus charged
depending on the charge of the ions from which it is On the other hand in alkaline medium, SnO2
formed. Thus, sodium palmitate solution will have forms negatively charged colloidal sol due to
negative charge on its sol particles. adsorption of SnO32  ions formed.
 
C15 H 31COONa  C15 H 31COO  Na SnO2  2OH   SnO32   H 2O
Sodium palmitate

iii) Due to electron capture by sol SnO2  SnO32   SnO2  SnO32 

particles: e.g., during electro-dispersion of Negative colloid
metals.
iv) Due to preferential adsorption of ions:  Frictional electrification is due to rubbing
of particles of dispersed phase with particles of
This is the most accepted view. The sol particles
dispersion medium.
acquire positive or negative charge by
preferential adsorption of +ve or -ve ions. When  Positively Charged Sols Negatively
two or more ions are present in the dispersion Charged Sols
medium, preferential adsorption of the ion
common to the colloidal particle usually takes
place. This can be explained by taking the
following examples.
a) If silver nitrate solution is added to potassium
iodide solution taken in excess, the precipitated
silver iodide will adsorb iodide ions from the
dispersion medium and negatively charged
colloidal solution will result. However, when
KI solution is added to AgNO3 solution taken
in excess, positively charged sol will result due
to adsorption of Ag  ions from dispersion
medium. Helmholtz electrical double layer
AgI / I  AgI / Ag   The each sol particle is surrounded by either
Negatively charged Positively charged positive or negative ions in the form of fixed
layer or compact layer. The second layer is
b) If FeCl3 is added to excess of hot water, a diffuse or mobile layer consisting of ions of both
positively charged sol of ferric hydroxide is the signs but net charge being equal and opposite
formed due to adsorption of Fe3 ions. to the fixed layer. This is known as Helmholtz
electrical double layer.
Fe2O3 xH2O/ Fe3  Zeta potential: The potential difference
Positively charged
developed between the two layers is known as
How ever, when ferric chloride is added to zeta potential or electric kinetic potential.
NaOH a negatively charged sol is obtained with
-
adsorption of OH  ions. - - + -
+ + +
- + + -
Fe2O3 xH 2O / OH  -ve Stationary
Negatively charged - sol phase
+
+ + -
SnO2 is positively charged colloidal sol due to - + -+
-
+ - Mobile
adsorption of Sn 4  ions.
phase

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 Electrophoresis: The movement of sol particles  E) Colligative Properties: Colloidal particles
under an applied electric potential is called being bigger aggregates, the number of particles
electrophoresis or cataphoresis. The phenomenon in a colloidal solution is compartively small as
helps in compared to a true solution. Hence, the values of
(a) Removing suspended impurities colligative properties (osmotic pressure, lowering
(b) Removing smoke from chimney gases in vapour pressure, depression in freezing point and
(c) Electroplating of rubber elevation in boiling point) are of small order as
compared to values shown by true solutions at same
(d) Painting metals with colloidal pigments
concentrations.
(e) Coagulation of sols
(f) determination of charge Coagulation of colloidal solution
+ 
a. The precipitation of colloidal particles by adding
Ag +
Ag Ag
+
I

 I
Ag
+ Ag
+
I  I a suitable electrolyte is called coagulation (or)
 +
I  Ag flocculation.
Ag I
+

I

I Ag+ Ag+ 
I b. The particles of the colloidal solutions possess
  + + +
I AgI I Ag Ag AgI Ag
I

electrical charge i.e., positive or negative
  + +
+
Ag I I +
Ag Ag Ag  c. Because of the presence of charge on the
I

Ag
+
I

Ag
+
Ag
+ I I
 colloidal particles, which can be converted into

precipitation by the addition of electrolyte

AgI / I negatively AgI / I positively (oppositely charged ion)
charged sol charged sol
d. The ion which responsible for the coagulation
Reservior of colloid solution is known as effective ion or
active ion.
Cathode + - Anode e. The effectiveness of an ion or electrolyte in
Water
causing coagulation is dependent on the charge
(dispensing
medium) Colloidal sign and charge magnitude. This fact is
solution enunciated by Hardy and Schulze.
Initial level f. At lower concentrat ion of elect rolyte
flocculation takes place and can be reversed by
Stop cock
shaking. At higher concentration, coagulation
 Electro-Osmosis: The movement of the takes place and the process cannot be reversed
dispersion medium under the influence of simply by shaking.
applied electric potential is known as electro Hardy - Schulze law
osmosis. The phenomenon helps in 1. The ion with charge opposite to the charge of
(a) removing water from peat (Coal) the colloidal particles is effective in coagulating
(b) dewatering of moist clay the colloid
(c) drying dye pastes. 2. Greater the charge of the ion, greater is the
 D) Colour: The colour of colloidal solution coagulating ability of the ion
depends on the wavelength of light scattered by  Positive colloids are coagulated by negative ions
the dispersed particles. The wavelength of light or anions of the salt added.
scattered further depends on the size and nature a) Cl   SO42  PO43
of the particles. The colour of colloidal solution 
b)  Fe(CN )6 4  PO43  SO42  C l
also changes with the manner in which the
observer receives the light. For example, a Negative colloids are coagulated by positive ions
mixture of milk and water appears blue when or cations of the salt added. a) K   Ba2  Al 3
viewed by the reflected light and red when  Blood is positively charged sol (pH=7.4) and is
viewed by the transmitted light. Finest gold sol coagulated by alum, Al2 (SO4 )3 and FeCl3 .
is red in colour; as the size of particles increases, These salts lower the pH and denaturate globular
it appears purple, then blue and finally golden. proteins.

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Flocculation value (or) Precipitation . Gold numbers of some protective colloids
value (or) Flocculation value Protective colloid Gold number
 It is the minimum amount of the electrolyte in Gelatin 0.005-0.01
millimoles that must be added to one litre of the Haemoglobin 0.03 - 0.07
colloidal sol to bring about complete coagulation Caesin 0.01 - 0.02
or precipitation Albumin 0.1 - 0.2
Smaller is the flocculation value of an Gum Arabic 0.15 - 0.25
electrolyte, greater is its coagulating or Potato Starch 25
precipitating power.
Coagulating power is inversely proportional to W.E-4:One gram of charcoal adsorbs 100ml of
coagulation or flocculation value.
The relative coagulating powers of two 0.5M CH 3COOH to form a monolayer and
electrolytes for the same colloidal sol, we have thereby the molarity of acidic acid is reduced
the relation to 0.49M. Calculate the surface area of the
Coagulating power of electrolyte 1 charcoal adsorbed by each molecule of acetic

Coagulating power of electrolyte 2 acid (surface area of charcoal is
Coagulation value of electrolyte 2
3.01 102 m 2 / g )
Coagulation value of electrolyte 1
Sol. Number of moles of acetic acid before the
Coagulation can also be caused by
electrophoresis, mutual precipitation (mixing 100
colloidal sols of opposite charge), prolonged adsorption = 0.5   0.05
1000
dialysis or by heating or cooling the sol. Number of moles of acetic acid after the
Protective colloids & Gold Number 100
 Lyophobic sols are less stable than lyophilic adsorption = 0.49   0.049
colloids 1000
 On addition of electrolytes lyophobic colloids Number of moles of acetic acid adsorbed =
are precipitated. This phenomenon is called 0.05  0.049  0.001
coagulation or flocculation Number of molecules of acetic acid adsorbed =
 Positively charged colloid is coagulated by
negative ion and negatively charged colloid is 0.001 6.023  1023  6.023  1020
coagulated by the positive ion of the added salt Surface area of the charcoal occupied by each
 The coagulating effect is more when the charge acetic acid molecule
of the ion is more
3.01 102
. A lyophobic sol can be prot ected from = 20
 5 1019 m 2
coagulation by adding a lyophilic colloid to the 6.023  10
lyophobic sol
. The lyophilic sol added is called protective W.E-5:For the coagulation of 100ml of arsenious
colloid or protective agent sulphide solution, 5ml of 1M NaCl is required.
. Zigmondy introduced the term gold number to Calculate the flocculation value.
measure the protective power of different Sol: Number of millimoles of electrolyte
NaCl
colloids
. Weight in milligrams of a protective colloid required to coagulate 100ml of sol = 1 5  5
which prevents the coagulation of 10 ml of a Number of millimoles of electrolyte required to
given gold solution on adding 1 ml of a 10 % 1000
solution of sodium chloride is called gold coagulate 1000ml of sol = 5   50
number. 100
. Smaller the gold number of a lyophilic colloid, The minimum number of moles of electrolyte
greater is its protective power per litre required to cause precipitation is called
. In the given examples flocculation value. Flocculating value of
Gelatin is the most effective protective colloid
and starch is the least effective protective colloid NaCl  50milli mole L1  0.05 mol L1

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Emulsions: The dispersion of finely divided  The droplets in emulsions are often charged and
liquid droplets in a immiscible liquid dispersion can be precipitated by electrolytes. They also
medium is defined as emulsion. show Brownian movement and Tyndall effect.
 Generally, in emulsion one of the liquids is water  Emulsions can be broken into constituent liquids
and the other is a liquid immiscible with water by heating, freezing, centrifuging etc.
generally oils. Applications of Emulsions
 When a mixture of water and oil is shaken  In washing process of clothes and crockery.
thoroughly, an emulsion is formed  In the digestion of fats in intestines. A little
 Milk is a naturally occuring emulsion in which amount of fat in the intestines forms a soap with
the liquid fat is dispersed in water. alkaline solution of intestines. This soap
 Milk: Liquid fat (oil) in water emulsifies the rest of the fat.
Vanishing cream: fat in water  As lot ions, creams and ointments in
 Oil in water (O/W) type Emulsions: If the pharmaceuticals and cosmetics.
dispersed phase is oil and the dispersion medium  As drugs of oily type in the form of emulsions
is water, it is called oil in water emulsion. milk, to facilitate their easy absorption.
vanishing cream are examples.  In the metallurgy, concentration of ores is carried
 Water Oil (W/O) Emultion: In these the out through emulsification process.
dispersed phase is water and the dispersion  In the conversion of cream into butter by
medium is oil. Greases, codliver oil, cold cream, churning. This is breaking of emulsion of fats
butter and creams are examples. in water.
 Stiff greases: Water in lubrication oils  In natural oil wells, oils and water form
Cod liver oil: Water in cod liver oil. emulsions. Hence de-emulsificat ion is
Cold cream: Water in fat necessary.
 The third substance added in small amounts to  Stability of sols: It is mainly due to two factors
an emulsion to keep the emulsion stable is (i) Presence of like charge: On sol particles.
known as emulsifier or emulsifying agent. It prevents them from aggregating and settling
Eg: Soaps, egg albumin, solid Hg I 2 , graphite down under the influence of gravity.
powder, caesin, gelatin etc. (ii) Solvation of sol particles: In case of
 For kerosene in water emulsion, soap is lyophilic sols a protective layer of solvent is
emulsifier formed around sol particles in addition to charge.
 For an olive oil in water emulsion, Egg albumin Hence they are more stable than lyophobic sols.
is emulsifier Gels: The liquid solid system is called gel.
 Solid mercuric iodide is emulsifier for water in They are of two types
benzene emulsion.  (a) Elastic gels: They can be temporarily
 Soap emulsifies o/w type emulsion
deformed by applying force.
 Casein and silica emulsifies oil in water Ex: Gelatin, starch and soaps.
emulsion.
 (b) Non elastic gels: They are rigid
 Soaps & Graphite powder act as an
Ex : silica gel.
emulsifying agent for both types collides.
 The emulsifier reduces the surface tension on Applications of colliods
the side of one liquid and this roles into droplets. (1) Industrial applications
 The emulsifying agent forms an interfacial film  (a) Purification of drinking water: By
between suspended particles and the medium. adding alum, the suspended impurities in water
 The principal emulsifying agents for o/w are coagulated and removed.
emulsions are proteins, gums, natural and
 (b) Electrical precipitation of smoke:
synthetic soaps etc. & for w/o emulsions, heavy
Cottrell’s precipitator, Smoke carry negative
metal salts of fattyacids, long chain alcohols,
charge and is removed by the principle of
lamp black etc.
electrophoresis in cottrell’s precipitator.
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 (c) Sewage disposal: It is passed through big  (m) Fog, mist and rain : When a large mass of
tanks fitted with electrodes. The colloidal particles air containing dust particles, is cooled below its
lose their charge and settle down and removed. dewpoint, the moisture in the air condenses on the
 (d) Electroplating of rubber: Latex is colloidal surfaces of these particles forming find droplets.
suspension of negatively charged rubber particles these droplets being colloidal in nature continue to
in water and can be deposited on metals by float in air in the form of mist or fog. Clouds are
electrophoresis. aerosols having small droplets of water suspended
in air. On account of condensation in the upper
 (e) Artificial rains: Clouds are aerosols atmosphere, the colloidal droplets of water grow
(water dispersed in air). Aggregates of particles of bigger and bigger in size, till they come down in the
water cause the rain fall which can be artificially form of rain. Sometimes, the rainfall occurs when
achieved by throwing electrified sand or AgI on two oppositely charged clouds meet.
clouds and cause the artificial rain. AgI has similar
It is possible to cause aritificial rain by throwing
crystal structure as that of ice.
electrified sand or spraying a sol carrying charge
 (f) Leather tanning: Skin of animals is positively opposite to the one on clouds from an aeroplane.
charged colloidalsystem. Extract of barks, wood
 (n) Blood :It is a colloidal solution of albuminoid
leaves is negatively charged colloidal solution of
substances. The styptic action of alum and ferric
tannin. When latter is applied on the surface of skin
chloride solution is due to coagulation of blood
(leather). This results in the hardening of leather.
forming a clot which stops further bleeding.
This process is called tanning. Chromium salts are
used in place of tannin.  (o) Soils :Fertile soils are colloidal in nature in
which humus acts as a protective colloid. On
 (g) In warfare: Animal charcoal is used in gas account of colloidal nature, soils adsorb moisture
masks to adsorb poisonous gases. Smoke screens and nourishing materials.
are titanium oxide particles dispersed in air.
 (p) Formation of Deltas :River water is a
 (h) In everday life: Blood, milk, butter, cheese, colloidal solution of clay. Sea water contains a
clothes, shoes all are colloidal system. number of electrolytes. When river water meets the
 (i) In medicines: Colloidal medicines are easily sea water, the electrolytes present in sea water
adsorbed and assimilated hence are widely used. coagulate the colloidal solution of clay resulting in
Colloidal antimony is effective medicine for kalazar. its deposition of clay with the formation of delta.
Blood is coagulated by FeCl3. Colloidal sols of Ag  (q) Industrial products :Paints, inks, synthetic
(Argyrol and protargol) are used as eye lotions. plastics, rubber, graphite, lubricants, cement, etc.,
 (j) In nature: Blue colour of sky, tails of comets are all colloidal in nature.
are due to scattering of light.
Properties of gels (Additional information)
Formation of deltas in rivers is due to coagulation
of negatively charged sand particles by Na+, Mg2+  (a) Syneresis: Shrinkage of gels on standing by
etc present in sea water. exudation of solvent is known as syneresis
 (k) Photographic plates and films:  (b) Thixotropy: Certain gels when shaken form
Photographic plates or films are prepared by a sol and on standing are converted into the form
coating an emulsion of the light sensitive silver of gel They are known as thixotropic gels and sol-
bromide in gelatin over glass plates or celluloid films. gel transformation is known as thixotropy.

 (l) Blue colour of the sky : Dust particles along  (c) Swelling or Imbibition of gels: The
with water vapour suspended in air, scatter blue property of adsorbing definite amount of water and
light which reaches our eyes and hence the sky looks causing the volume of gel to increase is known as
blue to us. swelling or imbibition.

NARAYANAGROUP 105
SURFACE CHEMISTRY JEE-ADV CHEM-VOL-II
SR-MAIN-CHEM-VOL-II

ADVANCED Applications of adsorption:

MAIN POINTS 1. In dyeing of cloth: Mordants such as alums are
used in dying of cloth. They adsorb the dye particles
ADSORPTION which, otherwise, do not stick to the cloth.
 Adsorption from Solution: Apart from 2. In ion-exchange resins: The organic polymer
adsorption of the gases, solids have also the capacity containing groups like –COOH, –SO3H, –NH2
to adsorb substances present in solutions. For etc., possess the property of selective adsorption
example, if we place a piece of charcoal in a litmus of ions from solutions. These are quite useful in
solution taken in a test tube and shake, the solution the softening of water and also in the separation
becomes colourless. It is because of adsorption of of the elements of the Lanthanide series (also
the litmus which is in fact a dye, by the charcoal. called rare earths).
Similarly, animal charcoal decolorizes impure sugar 3. Softening of hard water: The hard water is made
solution by adsorbing colouring dye. The actual to pass through a column packed with
mechanism of adsorption from the solution is not zeolite. Ca 2 , Mg 2 ions which are responsible for
definite. However, it is believed that it continues till
a unimolecular layer is built up on the surface of the hardness get adsorbed on zeolite
adsorbent. The Freundlich dsorption isotherm as 4. Surfactants: Surfactants will act as emulsifier
well as Langmuir adsorption isotherm are applicable (which work on the principle of adsorption) in the
to the adsorption from the solutions in the same manufacture of emulsions
way as from the gases. The effect of temperature is  Osmotic pressure is the collegative property used
also similar. However, equilibrium concentration (C) to determine the molecular weight of a colloid
is used in place of equilibrium pressure and the Colloids:
mathematical expressions for the two types of Coagulation of lyophobic sol:Following are
isotherms are: some general methods used for coagulation of sols.
x  Coagulation occurs by
 k  C1/ n (Freundlich Adsorption Isotherm) (i) Additional of electrolite
m
(ii) Electrophoresis
x aC (iii) Mixing oppositely charged sol
or  (Langmuir Adsorption Isotherm)
m 1  bC (iv) Prolonged dialysis
Competing Adsorption:When more than one (v) Heating or coolling
type of the adsorbate species are in contact with a b) Mutual coagulation: A deep red +ve sol of
particular adsorbent, there is a competition between Fe(OH)3, on mixing with bright yellow –ve sol of
their molecules or particles to be adsorbed on the As2S3 shows mutual coagulation of both leading to
solid surface. This depends upon their nature. a colourless solution with precipitate settled down
However, the one which can form stronger bonds due to neutralization of charges by each other.
is adsorbed more in preference to the other. The coagulation of sols by electrolytes has been
This is known as competing adsorption and it also dealt in terms of Hardy-Schulze rule. According to
happens in the reverse process also i.e. desorption. this rule:
In this case, the species which is weakly adsorbed  One of the ions furnished by an electrolyte, carrying
are dislodged first from the solid surface. It may be
charge opposite to that of sol particles is responsible
noted that the adsorption chromatography is based
for coagulation of sols and is known as effective
upon this principle of adsorption.
ion or counter ion.
 Determination of surface area of
 The greater is the valency of the counter-ions, the
adsorbent:If V litre of gas is adsorbed at greater will be its coagulating power. For example,
temperature T and Pressure P then the number of
in coagulating, a negatively charged sol (e.g., As2S3
PVN A sol) the order of efficiency of coagulating ions will
gas molecules adsorbed =
RT be Ti4+ > Fe3+ > Ca2+ > Na+. Similarly, for a
 surface area of adsorbent = number of molecules positively charged sol say Fe(OH)3 sol, the order
x Area of cross section of molecule(  r 2 ) will be [Fe(CN)6]4– > [Fe(CN)6]3– > SO42– > Cl–.
106 NARAYANAGROUP
SURFACE CHEMISTRY JEE-ADV CHEM-VOL-II
SR-MAIN-CHEM-VOL-II
 More is the coagulating power, lesser is coagulation  After CMC the colligative properties such as
or flocculation value (i.e., the minimum amount of osmotic pressure shows a slow rate of increase than
electrolyte required to coagulate a definite amount the rate at which they increase below CMC
of (sol). Usually, the flocculating value of these ions  The surface tension of solution shows a sharp
1 decrease after CMC
are quantitatively described as where x is the  Process of micellisation is exothermic, but CMC
(x)6 decreases or increases with Temperature.
valency of the coagulating ion. For example, for
For soap the CMC is 104 to 103 mol / lt
mono-, di-, tri- and tetravalent ions, the ratio in the
flocculation value will be  The conductance of an emulsion increases by adding
an electrolyte
1 1 1 1  Swelling of a gel when placed in water is called
6
: 6
: 6
: 6
or
1  2   3  4  Inbibition
1.00 : .156 : 0.137 : 0.025  Some gels on shaking change into liquid which on
long standing again changes into gel. This property
 Surfactants:These are the substances which are called Thixotropy.
preferentially adsorbed at the interfaces like oil -  Some more example of micelle system are
water , air-water etc. and Surfactants are
responsible for micellisation. Sodium lauryl sulphate CH 3  CH 2 11 SO3 Na 
Types of surfactants Sodium oleate C 17 H 33 COO  Na 
 Cationic surfactants: substances on ionisation P-dode cyl benzene sulphonate
giving a cation with hydrophilic & hydrophobic
 +
group. C12H25 SO 3 Na
Ex: 1. Cetyl trimethyl ammonium chloride
Cetyl trimethyl ammonium bromide
[ C16 H 33 (CH 3 )3 N  Cl  ]
CH 3  CH 2 15 N   CH 3 3 Br 
2. Octa decyl ammonium chloride
  Bredig’s arc method involves both dispersion as
[C18 H 37 NH 3 Cl  ] well as condensation.
 Anionic surfactants: Substances giving anion  During peptization, The ppt adsorbs one of the ion
which acts as surfactant of the electrolyte on its surface. The adsorbed ion
Ex: 1.Sodium palmitate [C15 H 31COONa ] is generally common with those of ppt. This causes
the dovelopment of positive or negative charge on
2.Sodium oleate [C17 H 35COONa] the ppt, which ultimately breaks into particles of
 Non ionogenic Surfactants: Substances not the collidel dimensions.
Ionisable in aqueous medium but have hydrophobic  The colour of the colloidal solution depends on the
and hydrophillic end. wave length of the light scattred by dispersed
 Micelle or Associated colloid: The minimum particles, size and nature of the particles.
concentration of surfactant at which micelle  The protective action of a lyophilic colloid
formation starts is Critical Micelle Concentration sometimes expressed interms of congo rubin
number given by Ostwald.
(CMC)
It is defined as the minimum amount of the protective
 The formation of micelle takes place above a
colloid in milligrams that prevent the colour change
particular temparature called Kraft temperature of 100ml of 0.01% congo rubin dye to which 0.16gr
( Tk ) equivalent of KCl is added.
 Lesser the CMC of surfactant, more is its surface  The order of the temperature to which the gel must
activity be heated in the presence of various anions, before
 After CMC, the rate of increase of conductivity is if changes into sol is called Hofmeister series or
slower than the rate at which lyotropic series.
it increases bellow CMC. The order is Citrate 3  Tatarate2  SO42  PO43
 After CMC the rate of increase of turbidity with
concentration becomes more than at CMC  Acetate  Cl   NO3  Br   I   CNS 

NARAYANAGROUP 107