Surface chemistry notes

1
SURFACE CHEMISTRY
Adsorption
Adsorption is due to the presence of unbalanced forces, believed to have developed either during
crystallisationofsolidsordue to presence of unpaired, electrons or free valencies in solids having d–orbital.
In liquids it is due to surface tension.
Difference between adsorption and absorption
Sr. No. Adsorption Absorption
1 Adsorption is a surface phenomenon. The adsorbing
substance is called adsorbate and is only concentrated
on the surface of adsorbent.
It is a bulk phenomenon. In
absorption the substance
penetrates into the bulk of the
other substance.
2 The rate of adsorption is rapid to start with and its rate
slowly decreases.
Absorption occurs at a uniform
rate
Theamountof heatevolvedwhen one mole of an adsorbate gets adsorbed on the surface of an adsorbate is
called molar heat (enthalpy) of adsorption.
Adsorption in Terms of Gibb’s Helmholtz Equation
Adsorption is an exothermic reaction. Therefore, adsorption is accompanied by release of energy or H is
always negative and favours the process. Also absorbate molecules get lesser opportunity to move about
on the surface of adsorbent. Thus, entropy factor opposes the process. According to Gibb’s Helmholtz
equation,
G = H – T S
Since adsorption does actually take place, H is greater than T  S  G is negative. As the adsorption
continues, the difference between the two opposing tendencies becomes lesser and lesser till they are
equal i.e., H = TS or G = 0. At this stage, equilibrium called adsorption equilibrium gets established
and there is no net adsorption taking place at this stage.
Types of Adsorption
There are two types of adsorption.
a) Physical adsorption (or Vander waal’s adsorption) or physiosorption.
b) Chemisorption
Characteristics of Physical and Chemisorption
Physio–sorption Chemisorption
1. Adsorption by weak vander waal’s forces. By chemical force (covalent or ionic bond)
2. Multimolecular layer may be formed Unimolecular layer will be formed
3. Low heat of adsorption viz. about 20–40
kJ/mol
High heat of adsorption viz. about (200–400
kJ/mol)
4. Easily reversed Not reversed
5. Molecular state of adsorbate on
adsorbent is same. No surface
compounds are found.
Molecular state may be different. Surface
compounds are found.
6. Usually occurs rapidly at low
temperature and decreases with increase
in temperature.
It occurs at high temperature initially but then
decreases.
7. It increases with pressure Change in pressure will have less effect on
chemisorptions
8. Not specific Highly specific
9. Extent of adsorption depends on ease of
adsorption depends on ease of
liquefaction i.e. surface area, critical
temperature, inversion temperature
Vanderwaal’s constant ‘a’, e.g. Adsorption
of gases on charcoal
There is no relative between extent of
adsorption and ease of liquefaction of gas.
e.g. Adsorption of H2 on Pt; decomposition of
NH3 in presence of tungsten, decomposition of
Hl on gold (zero order)
Application of Adsorption
1. In gas mask:

2
Finally divided coconut charcoal is used as gas masks for absorbing toxic gases like CH4, CO, COCl2.
It is usually used for breathing in coal mines to adsorb poisonous gases.
2. In preserving Vacuum:
In Dewar flasks, activated charcoal is placed between the walls of the flask so that any gas which
enters into annular space either due to glass imperfection of diffusion through glass is adsorbed.
3. As dehumidizer e.g silica gel
4. Fe(OH)3 antidote adsorbent in Arsenic poisoning.
5. In clarification of sugar: Sugar decolorized by treating sugar solution with charcoal powder. The
latter adsorbs the undesirable colours present.
6. Heterogeneous catalysis: Adsorption of reactants on the solid surface of the catalysts increases
the rate of reaction. There are many gaseous reactions of industrial importance involving solid
catalysts. Manufacture of ammonia using iron as a catalyst, manufacture of H2SO4 by contact
process and use of finely divided nickel in the hydrogenation of oils are excellent examples of
heterogeneous catalysis.
7. In Chromatography: The different chromatographic techniques such as adsorption, paper or
column chromatography which are used for the purification and the separation of the substances
available in small amounts, are based upon the theory of selective adsorption.
Catalysis
Catalyst is a substance which changes the speed of a reaction, and usually, can be recovered completely
unchanged at the end of a reaction. However it may take part in a reaction consumed in one step and
regenerated in another. This phenomenon is known as Catalysis.
A substance is termed a positive catalyst or simply as catalyst if it accelerates the rate of chemical
reaction. On the other hand, the added substance is termed as negative catalyst if it retards the rate of a
chemical reaction.
Example of Negative Catalysts:
i) H3PO4 or acetanilide in the decomposition of H2O2.
ii) Alcohol in the oxidation of chloroform leading to the formation of phosgene.
iii) Tertraethyl lead or nickel carbonyl acting as antiknock material in internal combustion engines.
iv) Antifreezes like glycerol which retard the rusting of the machines.
Catalytic Promoters
There are certain substances which when added only in small quantity to a catalyst enhance its activity.
This substance itself may not be catalyst. Such substances which enhance the activity of a catalyst are
called catalytic promoters. E.g. Molybdenum is used as Promoter for Fe catalyst in Haber’s process.
Catalytic Inhibitors or poisons
The rates of some reactions are reduced considerably by the presence of small amounts of other
substances called inhibitors. The substance whose presence decreases or destroy the activity of a
catalyst are called poison. CO or H2S in hydrogen gas acts as a poison for Fe catalyst in Haber process.
Types of Catalysis
Broadly, two types of catalysis are known:
(a) Homogeneous catalysis (b) Heterogeneous catalysis
(a) Homogeneous catalysis
If the catalyst is present in the same phase as the reactants, it is called a homogeneous catalyst
and this type of catalysis is called homogeneous catalysis.
Ex:
FructoseecosGluSucrose
61266126
SOH
2112212 )aq(OHC)aq(OHC)l(OH)aq(OHC 42
 
)g(SO2)g(O)g(SO2 3
)g(NO
22  
)aq(OHCH)aq(COOHCH)(OH)(COOCHCH 33
)(HCl
233   

b) Heterogeneous catalysis:
In this type of catalysis the catalyst is present in a difference phase than that of the reactants. In
heterogeneous catalysis, catalyst is generally a solid and the reactants are generally gases but
sometimes liquid reactants are also used. It is also known as surface catalysis.
Ex:
i) Synthesis of methyl alcohol (CH3OH) from CO and H2 using a mixture of copper, ZnO and Cr2O3 as
catalyst.
CO(g) + 2H2(g)    32OCrZnO,Cu
CH3OH(l)
ii) Manufacture of ammonia from N2 and H2 by Haber’s process using iron as catalyst.
N2(g) + 3H2(g)
Fe
2NH3(g)
Theories of Catalysis
Intermediate Compound Formation Theory
This theory explains homogeneous catalysis mainly.

3
According to this theory, the catalyst combines with one of the reactants to give an intermediate
compound. This compound intermediately reacts with the other reactants and given the product and
regenerates the catalyst in its original form.
Thus the reactants do not directly combine with each other, instead they react through the catalyst which
provides an alternative pathway which involves lesser energy of activation.
For example, the function of nitric oxide [NO] as a catalyst in the formation of SO3 is explained as follows:
2NO + O2 2NO2
Catalyst Reactant Intermediate
NO2 + SO2 SO3 + NO
Intermediate Reactant Product Catalyst
regenerated
Adsorption Theory:
This theory explains the heterogeneous catalysis. The role of a solid catalyst in enhancing the reaction
rate is explained on the basis of this theory in the following steps:
(i) The reactant molecules are adsorbed on the surface of the catalyst at adjacent point. Adsorption
leads to higher concentration of the adsorbed reactant on the surface of a catalyst.
(ii) As adsorption is an exothermic process, the heat of adsorption provides the necessary activation
energy for the chemical reaction to proceed and enhance rate of greater.
(iii) The product molecules rapidly leave the catalyst surface to make room for the other reactant
molecules to get adsorbed. Thus the chemical combination between reactant molecules occurs at
the surface of the catalyst at a must faster rate.
e.g. Hydrogenation of ethane in presence of Ni
H – H + 2M  2M – H
C2H4 + 2M – H  C2H6 + 2M
Modern Adsorption Theory of Heterogeneous Catalysis
The modern adsorption theory is the combination of intermediate compound formation theory and the
old adsorption theory. The catalytic activity is localized on the surface of the catalyst. The mechanism
involves five steps:
O
O
O
O
O
O
Catalyst surface
having free valencies
+ A + B
Reacting molecuels
Adsorption of
reacting molecules O
O
O
O
O A
O B
O
O
O
O
O
O
Catalyst
+ A - B
Product
Desorption of product
molecules
O
O
O
O
O A
O B
|
Intermedaite
Adsorption of reacting molecules, formation of intermediate and desorption of products
i) Diffusion of reactants to the surface of the catalyst.
ii) Adsorption of reactant molecules on the surface of the catalyst.
iii) Occurrence of chemical reaction on the catalyst’s surface through formation of an intermediate.
iv) Desorption of reaction products from the catalyst surface, and thereby, making the surface
available again for more reaction to occur.
v) Diffusion of reaction products away from the catalyst’s surface. The surface of the catalyst unlike
the inner part of the bulk, has free valencies which provide the basis for chemical forces of
attraction. When a gas comes in contact with such a surface, its molecules are held up there due to
loose chemical combination. If different molecules are adsorbed side by side, they may react with
each other resulting in the formation of new molecules. Thus, formed molecules may evaporate
leaving the surface for the fresh reactant molecules.
This theory explain why the catalyst remains unchanged in mass and chemical composition at the
end of the reaction and is effective even in small quantities. It however, does not explain the action
of catalytic promoters and catalytic poisons.
General Characteristics of Catalytic Reactions
1. The catalyst is unchanged chemically at the end of the reaction:
2. Only a small quantity of the catalysts in generally needed:
3. The catalyst does not alter the position of equilibrium in a reversible reaction
4. The catalyst does not initiate the reaction
Important feature of solid catalysts
(a) Activity
It is ability of a catalyst to catalyse a process. The activity of a catalyst depends upon the strength of
chemisorption to a large extent.
(b) Selectivity

4
The selectivity of a catalyst is its ability to direct a reaction to yield a particular product. For
example, starting with H2 and CO using different catalysts we get different products.
i) CO(g) + 2H2(g) Ni
CH4(g) + H2O(g)
ii) CO(g) + 2H2(g)   32OCrZnO/Cu
CH3OH(g)
iii) CO(g) + H2(g) Cu
HCHO (g)
Shape – Selective Catalysis by Zeolites
The catalytic reaction that depends upon the pore structure of the catalyst and the size of the reactant
and product molecules is called shape selective catalysis.
The catalysis that depends upon the pore–structure of the catalyst and molecular sizes of
reactants and product molecules is called shape selective catalysis. e.g. zeolites are shape selective
catalysts due to their honey–comb structure. ZSM–5 is used for converting methanol to gasoline.
Zeolites are micro–porous aluminosilicates of the general formula Mx/n [(AlO2)x (SiO2)y]. zH2O.
where n is the charge on the metal cation Mn+ which is usually Na+, K+ or Ca2+ and z is the number of
water molecules of hydration which are highly variable. They are three dimensional network silicates in
which some silicon atoms are replaced by Al giving Al – O – Si frame work.
Enzymes
Thus, enzymes are defined as biological catalysts. Chemically, all enzymes are globular proteins with high
molar mass ranging from 15,000 to 1,000,000g mol–1 and form colloidal solution in water.
Properties of Enzymes: Some important properties of enzymes are:
(i) Specificity
Each enzyme catalyses only one chemical reaction.
(ii) Efficiency
Enzymes are very efficient catalysts. They speed up rate of a reaction by factors of upto 1020.
(iii) Small quantity
Only small amounts of enzymes can be highly efficient.
(iv) Optimum temperature and pH
Enzyme catalysed reactions is maximum at particular pH called optimum pH, which is between 7.4
and temperature of 298–310K under one atmospheric pressure.
(v) Enzyme activators (Co–enzymes)
The activity of certain enzymes is increased in the presence of certain substances called co–
enzymes. e.g. if a protein contains a small amount of vitamin as the non–protein part, its activity is
enhanced. The activators are generally metal ions like Na+, Cu+2, Mn+2. Co+2
(vi) Influence of Inhibitors and poisons:
The action of enzymes is controlled by a number of mechanisms and are inhibited by certain
organic molecules called inhibitors. Ex drugs
Mechanism of Enzyme Catalysis
Thus, the enzyme–catalysed reactions may be considered to proceed in two steps.
Step 1: Binding of enzyme to substrate to form an activated complex.
E + S  ES*
Step 2: Decomposition of the activated complex to form product.
ES*  E + P
Mechanism of enzyme catalysed reaction
Colloidal State
Comparison of Suspensions Colloids and True Solutions
S.No. Property True Solution Colloids Suspension
(i) Particle size < 10Å 10Å to 103 Å > 103Å
(ii) Visibility Not visible with any of
the optical means
Visible with
ultramicroscope
Visible with naked
eye
(iii) Separation
(a) with filter paper
(b) with membranes
Not possible
Not possible
Not possible
Possible
Possible
Possible
(iv) Diffusion Diffuses rapidly Diffuses very slowly
does not settle
Does not diffuse
(v) Settling Does not settle but it may settle
under Centrifuge
Settles under
gravity
(vi) Nature Hemogeneous heterogeneous Heterogeneous

5
(vii) Appearance Clear Generally clear Opaque
Colloids and their Classification
(i) Classification Based on Physical State of Dispersed Phase and Dispersion Medium
Depending on the physical states of dispersed phase or dispersion medium, colloidal solutions are
of eight types:
Dispersed
phase
Dispersion
Medium
(appearance) Name (e.g.) alloys
Solid Solid (solid) Solid Sol Coloured glasses, Pearl, Ruby,
alloys, gems
Solid Liquid (liquid) Sol Ag, Sol, Au, Sol., S. Sol., Muddy
water, gelatin in water, paint
Solid Gas (gas) Aerosol Smoke, dust, strom
Liquid Liquid (liquid) Emulsion Milk, medicines, oil water, blood
Liquid Solid (solid) Gel shampoo, jelly, cheese, butter, all
fruits and veg, polish, curd
Liquid Gas (gas) Aerosol of liquid Cloud, fog, mist, spray
Gas Liquid (liquid) Foam Soap leather, whipped cream,
shaving cream, soda water, froath
Gas Solid (solid) Solid foam Styrene foam, Foam rubber
A colloidal dispersion of one gas in another is not possible since the two gases would give a
homogenous molecular structure.
(ii) Classification Based on Nature of Interaction between Dispersed Phase and Dispersion
Medium
(a) Lyophilic Sols
Colloidal solutions in which the dispersed phase has considerable affinity for the dispersion
medium, are called lyophilic sols (solvent–linking). For example – dispersion of gelatin starch, gum
and proteins in water. Such colloidal solutions can be easily prepared in water directly so called
intrinsic colloids. These solutions are stable known as reversible colloids since the residue left on
evaporating can be readily transferred back into colloidal state by adding water. These sols are
quite stable and cannot be easily coagulated.
(b) Lyophobic Sols
Colloidal solution in which the dispersed phase has no affinity or attraction for the dispersion
medium are called Lyophobic colloid (solvent hating) solutions. Colloidal solutions of metals which
have negligible affinity for solvents are examples of this type. Lyophobic colloidal solutions are less
stable. On evaporation of solvent the residue cannot be easily transferred back into colloidal state
by ordinary mean hence also called extrinsic colloid. Therefore, lyophobic colloids are also called
irreversible colloids. Lyophobic sols need stabilizing agents for their preservation. Such sols are
readily precipitated on addition of small amount of electrolytes by heating or by shaking.
(iii) Classification Based on Type of Particles of the Dispersed Phase
Depending upon the type of the particles of the dispersed phase, colloids are classified as:
(a) Multimolecular Colloids:
On dissolution, a large number of atoms or smaller molecules of a substance aggregate together to
form species having size in the colloidal range (diameter < 1 nm). The species thus formed are
called multimolecular colloids. For example, a gold sol may contain particles of various sizes
having many atoms. Sulphur sol consists of particles containing a thousand or more of S8 sulphur
molecules.
(b) Macromolecular Colloids
These are formed by macromolecules which have bigger size than the colloidal particle but as soon
as they are put in suitable solvents, they get dissociated to form smaller particle of the colloidal
range. Such systems are called macromolecular colloids. These colloids are quite stable and
resemble true solutions in many respects. Examples of naturally occurring macromolecules are
starch, cellulose, proteins and enzymes; and those of man–made macromolecules are polythene,
nylon, polystyrene, synthetic rubber, etc.
(c) Associated Colloids (Micelles)
There are some substances which at low concentrations behave as normal strong electrolytes, but
at higher concentrations exhibit colloidal behaviour due to the formation of aggregates. The
aggregated particles thus formed are called micelles. These are also known as associated
colloids. The formation of micelles takes place only above a particular temperature called kraft
temperature (Tk) and above a particular concentration called critical micelle concentration
(CMC). Surface active agents such as soaps and synthetic detergents belong to this class.
Mechanism of Micelle Formation
Note: The soaps and detergents form micelles in water only because of the presence of charge on their
molecules. Micelles formation does not occur in solvent like ethyl alcohol since it is not as polar as soaps.
That is why only water is used for the washing of dirty clothes.

6
Method of Preparation of Colloidal Solution
(a) Dispersion Method
1. Mechanical dispersion (e.g. black ink, paint, varnish) dye
Solid material is first finely grounded by usual methods. It is
then mixed with dispersion medium which gives a coarse
suspension. The suspension is now introduced into the colloid
mill. The simplest form of colloid mill consist of two metal
discs held at a small distance apart from one another and
capable of revolving at a very high speed (about 7000
revolutions per minute) in opposite directions. The particles
are grounded down to colloidal size and are then dispersed in
liquid.
2. Electrical Dispersion (Bredig’s arc Method) (e.g. metal sol.)
This method is suitable for the preparation of colloidal
solutions of metals like gold, silver, platinum, etc. An arc is
struck between the metal electrodes under the surface of
water containing some stabilizing agent such as a trace of
KOH. The water is cooled by immersing the container in a ice
bath. The intense heat of the arc vaporizes some of the metal
which condenses under cold water.
3. Peptization
The dispersion of a freshly precipitated material into colloidal
solution by the action of an electrolyte in solution is termed
peptization, the electrolyte used is called a peptizing agent.
During Peptization, the precipitate adsorbs one of the ions of
the electrolyte on its surface. This causes the development of
positive or negative charge on precipitates, which cause
electrostatic repulsion and ultimately break up into smaller
particles of the size of a colloid.
Ex:
Freshly prepared ferric hydroxide on treatment with a small amount of ferric chloride
solution at once forms, a dark raddish brown solution. Ferric chloride acts as a peptizing
agent.
Purification of Colloidal Solutions
 Dialysis:
ialysis is a process of removing a dissolved substance
(impurities) from a colloidal solution by means of
diffusions through semipermiable which is a bag of
suitable membrane containing colloidal solution to be
purified, placed in a vessel (or continuous flow of
water).
When potential difference is applied across the membrane,
ions in the solution move faster towards opposite electrode.
This process is called electrodialysis. The colloidal solution
is placed in a bag of suitable membrane while pure water is
taken outside. Electrodes are fitted in the compartment as
shown in figure. The ions present in the colloidal solution
migrate out towards the oppositely charged electrodes.

Ultra–filtration: Such filter are called ultrafilters. However, the pores of filter paper can be
reduced in size by impregnating with colloidion solution to stop the flow of colloidal particles.
The usual colloidion is a 4 % solution of
nitro–cellulose in a mixture of alcohol and ether. An ultra–filter paper may be prepared by
soaking the filter paper in a colloidal solution, hardening by formaldehyde and then finally drying
it.
 Ultracentrifugation: In this method, the impure sol is taken in a tube which is placed in an
ultracentrifuge. In this machine, the tube is rotated at a very high speed. As a result, the colloidal
particles settle down at the bottom of the tube whereas the crystalloids and other soluble
impurities remain in the solution. This solution is decanted off and the colloid particles are
remixed with the dispersion medium to give the pure colloidal sol.
Properties of Colloidal Solution
Colloidalmill

7
 Colour:
The colour of hydrophobic sol depends on the wavelength of the light scattered by the dispersed
particles. The wavelength of the scattered light again depends on the size and the nature of
particles. The colour of colloidal solution also changes with the manner in which the observer
receives the light.
For example, a mixture of milk and water appears blue when viewed by reflected light and red
when viewed by the transmitted light. Finest gold sol is red in colour, as the size of particles
increases, it appears purple, then blue and finally golden. When light emitted by the setting sun
passes through the blanket of dust, the blue part of the light is scattered away from out eyes and
at the same time the red colour is seen. This sun appears red while setting.
 Optical Properties: Tyndalll effect
The path of the light shows up a hazy beam of cone. This was first observed by Farraday and later
by Tyndall and is known as Tyndall effect. It may be defined as the scattering of light by the
colloidal particles in a colloidal sol. The bright cone of the light is called “Tyndall cone”. The
Tyndall effect is due to the fact that the colloidal particles absorb light and scatter it in all colloidal
dispersion. Tyndall effect is observed only when the following two conditions are satisfied
(i) The diameter of the dispersed particles is not much smaller than the wavelength of light used.
(ii) The refractive index of dispersed phase & the dispersion medium differ greatly in magnitude.
Some example of Tyndall effect are:
(i) Blue colour of sky and sea water.
(ii) Visibility of tails of comets.
(iii) Twinkling of stars.
 Kinetic Properties: (Brownian Movement)
The continuous rapid zig–zag motion of a colloidal particle in the dispersion medium is called
“Brownian movement of motion” (first observed by British botanish Robert Brown).
The Brownian movement has been explained to be due to the unbalanced bombardments of the particles
by the molecules of dispersion medium. The Brownian movement has a stirring effect which does not
permit the particles to settle and thus, is responsible for the stability of sols.
 Charge on Colloidal Particles:
Preferential adsorption of ions is the most accepted reason. The sol particles acquire positive or negative
charge by preferential adsorption of +ve or –ve ions. When two or more ions are present in the dispersion
medium, preferential adsorption of the ion common to the colloidal particle usually takes place. This can
be explained by taking the following examples:
(a) When silver nitrate solution is added to potassium iodide solution, the precipitated silver iodide
adsorbs iodide ions from the dispersion medium and negatively charged colloidal solution results.
However, when Kl solution is added to AgNO3 solution, positively charged sol results due to
adsorption of Ag+ ions from dispersion medium.
Agl/l– Agl/Ag+
Negatively charged Positively charged
(b) If FeCl3 is added to excess of hot water, a positively charged sol of hydrated ferric oxide is formed
due to adsorption of Fe3+ ions. However, when ferric chloride is added to NaOH a negatively
charged sol is obtained with adsorption of OH– ions.
Fe2O3.xH2O/Fe3+ Fe2O3.xH2O/OH–
Positively charged Negatively charged
Having acquired a positive or a negative charge by selective adsorption on the surface of a colloidal
particle as stated above, this layer attracts counter ions from the medium forming a second layer,
as shown below.
Agl/l– K+ Agl/Ag+ l–
The combination of the two layers of opposite
charges around the colloidal particle is called
Halmholtz electrical double layer.
This potential difference between the fixed layer
and the diffused layer of opposite charges is called
the electrokinetic potential or zeta potential.
The presence of equal and similar charges on colloidal particle is largely responsible in providing
stability to the colloidal solution, because the repulsive forces between charged particles having
same charge prevent them from coagulating or aggregating when they come closer to one another.
Electrical Properties:
(a) Stability of colloidal sols – Electrical charge on colloidal particles: The dispersed phase
particles carry either +ve and –ve charge and dispersion medium has an equal and opposite charge.
The particles repel one another and hence do not coagulate, thus making the colloid stable.

8
(b) Cataphoresis or Electrophoresis is the movement of
colloidal particles either towards cathode or anode,
depending on their charge, under the influence of an electric
field. Electrophoresis is used to determine nature of charge
Positive sol. Sol Fe(OH)3, Ca(OH)2, Al(OH)3, basic dye stuffs
(methylene blue, methyl violet, Haemoglobin).
Negative Sol. Metal & sulphides: CdS, As2S3, Cu, Ag, Pt and
dyes (congored, Prussian blue, silicic acid, gum, charcoal).
(c) Electro Osmosis:
The movement of dispersion medium under the influence of applied potential is known as
“Electro–osmosis.”
(d) Coagulation or flocculation: [Process of setting of colloidal particle]
The flocculation and setting down of the discharged sol particles is called coagulation or the
precipitation and can be brought about in following ways:
(a) By addition of electrolyte. (b) By electrophoresis
(c) By mixing two oppositely charged sols. (mutual precipitation)
(d) By boiling (e) Persistent dialysis.
(a) By Addition of Electrolytes:
When an electrolyte is added in excess to a sol, the electrolyte furnishes both types of ions in
solution. The oppositely charged ions get adsorbed on the surface of colloidal particles. This causes
neutralization and thereby the size and mass of colloidal particle increases and it becomes a
suspension particle. Due to greater volume and greater mass these suspension particles settle
down i.e., they coagulate. The ion irresponsible for neutralization of charge on the particle is called
flocculating ion.
 Coagulation or flocculation is the process of bringing colloidal particles closer so that they
aggregate to form larger particle that precipitate and settle down or float on the surface. It is
usually done by addition of an electrolyte. Effective ions or electrolyte are those which carry
charge opposite to colloids.
 Hardy– Schulzerule states that “greater is the valency of the oppositely charged ion of electrolyte
being added, faster is the coagulation: e.g. for a negatively charged sol, the order is: Al3+ > Ba2+ >
K+, for a positively charged sol the order is: 
 ClSOPO)CN(Fe 2
4
3
4
4
6
 Coagulating value is the minimumamountof electrolyteinmilli moles required to coagulate a 1
lit of solution of sol in two hours. The smaller the quantity needed, the higher will be the
coagulating power of anion.
(b) By Electrophoresis:
During electrophoresis the charged sol particles migrate towards the electrode of opposite sign.
There they deposit their charge and then get coagulated (As the neutral particles can aggregate and
change to suspension particles).
(c) By Mixing Two Oppositely Charged Sols:
The mutual coagulation of two sols of opposite charge can be effected by mixing them. For e.g.
Fe(OH)3 (positive sol) and Arsenious sulphide (negative sol) when mixed join and coagulate.
(d) By Boiling
Sols such as sulphur and silver halides disperse in water, get coagulated when boiled, due to
increased collisions between sol particles and water molecules, which remove the adsorbed layer
from the sol and therefore the sol particles settle down.
(e) Persistent dialysis:
The stability of a colloidal sol is due to presence of a small amount of the electrolyte. On prolonged
dialysis, the electrolyte is completely removed. As a result, the colloidal sol becomes unstable and
gets coagulated.
Coagulation of Lyophillic Sols:
Lyophilic sols are stable due to charge and solvation of the colloidal particles. When these two factors
are removed, a lyophilic sol can be coagulated. This is done (i) by adding electrolyte; (ii) by adding
suitable solvent.
When solvents such as alcohol and acetone are added to hydrophilic sol, the dehydration of dispersed
phase occurs. Under this condition a small quantity of electrolyte can bring about coagulation.
Protection of Colloids:
 Lyophilic sols are more stable than lyophobic sols hence they
are used as protective colloids to increase the stability of
lyophobic sols, e.g. addition of gums, gelatin etc. to certain metal
sols. These sols are called Protective colloids.
 Protective action of lyophilic sols is explained due to formation
of a thin layer around the lyopobic sol particles, thus preventing
them from coming closer.

9
Emulsions
Emulsionisa colloidalsysteminvolvingoneliquiddispersedinanother, provided both are immiscible. Some
commonly known emulsions are Milk, butter, milk cream, cold cream, vanishing cream, etc. There are
many drugs and medicines which are also in the form of emulsions e.g. various ointments, codliver oil etc.
 Types of emulsions:
Depending upon the type of proportions in which these constituents are present, emulsion have
been classified into two types.
1. Oil in water emulsions (O/W) are those in which oil is the dispersed phase and water is the
dispersion medium. Milk is a common example in which liquid fats are dispersed in water.
Similarly, if a few drops of nitrobenzene (oil) is added to water, an emulsion results. Varnishing
cream is another example of this type.
2. Water in oil emulsions (W/O) are those in which water is the dispersed phase and oil is the
dispersion medium, Butter is an example of water in oil emulsion in which water is dispersed in oil.
Cod liver oil and cream are the other examples of these emulsions.
 Emulsifier and Emulsifying Agents
An emulsifier or emulsifying agent is a substance which helps in stabilizing an emulsion of oil and
water and, thus, prevents them from getting apart. The commonly used emulsifying agents for
O/W emulsion are soaps, detergents, lyphilic colloids, proteins, gums and agar etc. & for W/O
emulsion are heavy metal salt of long chain fatty acids etc. The preparation of emulsion in the
presence of an emulsifier is known emulsification. The role of an emulsifier in stabilizing an
emulsion can be explained as :
Gels
 These are colloidal systems where liquids are the dispersed phase and solids act as dispersion
medium, e.g. curd, cheese etc. When a warm solution of gelatin is cooled it sets to a semisolid mass
which is gel. This process is called gelation.
Applications of Colloidal System
Colloids are widely used in the industry. Following are some examples.
1. Medicines in colloidal form can be easily adsorbed by body tissues & hence are more effective e.g.
colloidal antimony is used in curing kalaazar, argyrols is a silver sol used as an eye lotion.
Colloidal gold is used for intramuscular injection Neutralization of excess acidity by Al (OH)3, Milk
of magnesia, an emulsion, is used for stomach disorders, colloidal sulphur – germ killer, kalolin
used to remove toxins from intestine. Cod liver oil is emulsion of oil in water. Some ointment,
antibiotics, Penicillin, streptomycin are produced in colloidal form. Colloidal medicines are more
effective because they have large surface area and are therefore easily assimilated.
2. Food: Gelatin in ice–cream. It preserve smoothness and prevent ice crystals formation, protein,
milk, cheese are colloids, fruits, jelly, cream, bread is dispersion of air in baked dough.
4. Purification of water by alums: Water from rivers or lakes are sometimes used for domestic and
industrial purpose after purification. The water from lake or river is turbid due to the presence of
fine clay particles which are negatively charged. These can be removed by adding potash alum or
aluminium sulphate. Al3+ ions from potash alum o aluminium sulphate neutralize the negative
charge on clay particles. This causes the coagulation of clay particles which settle down leaving
water which is clear of further treatment.
5. Stoppage of bleeding from a fresh cut: The bleeding from a fresh cut can be immediately
stopped by applying a concentrated solution of ferric chloride or potash alum. Blood consists of
colloidal particles of haemoglobins which carry positive charge on them. When ferric chloride or
alum is applied on the cut these colloidal particles get their positive charge neutralized by the
anions available from these substances in solution. In the absence of the charge, they get
coagulated and the bleeding stops.
6. Delta formation: Formation of delta due to coagulation of colloidal particles of river water by sea
salt. The river water contains colloidal particles of sand and clay which carry negative charge. The
sea water, on the other hand, contains positive ions such as Na+, Mg2+ and Ca++. As the river water
meets sea water, these ions discharge the sand or clay particles which are precipitated as delta.
7. Photographic plates and films: Photographic plates or films are prepared by coating an
emulsion of the light sensitive silver bromide in gelatin over glass plates or celluloid films.
8. Fog, mist and rain. In extremely cold weather, the temperature is very low. The water droplets
(moisture) present in air condense on the surface of the dust particles that are suspended. Since
these are colloidal in nature, they float in air in the form of mist or fog or extremely low, these
colloidal droplets grow in size. They become bigger and bigger and when atmosphere is not in a
position to hold them, they come down as rain.
In the winter season, the atmosphere generally becomes very foggy and visibility is quite poor.
This leads to lot of inconvenience for vehicular movement and result in major accidents. Farmers
are in the habit of burning husk in the open fields. This results in the release of unburnt carbon
particles in the atmosphere. The water droplets condense on them and they appear as fog. But
actually this is smog and not fog and is extremely injurious to health. This can lead to asthama,
lung as well as throat cancer. It is very essential to educate our farmer community about these
harzards.

10
9. Purification of Blood: Blood is a colloidal solution. It contains toxic waste products such as urea
and uric acid which pass through the membrane while colloidal sized particles of blood proteins
hemoglobin are retained. Kidney failure, therefore, leads to death due to accumulation of
poisonous waste products in the blood. Blood is purified by dialysis.
10. Chemical warfare: (Smoke screen): Smoke is a colloidal solution of solid particles such as
carbon, arsenic compounds, dust, etc., in air. The smoke, before it comes out from the chimney, is
lead through a chamber containing plates having a charge opposite to that carried by smoke
particles. The particles on coming in contract with these plates lose their charge and get
precipitated. The particles thus settle down on the floor of the chamber. The precipitator is called
Cottrell precipitartor. Colloidal dispersions of certain substance, like TiO2 (as dispersed phase) in
air (as the dispersion medium) are used a smoke screens in warfare for the purpose of
concealment. The dispersed phase particles (i.e. TiO2) being very heavy rapidly drops down in the
form of a curtain of drizzling whiteness.
11. Artificial rain: Tiny water droplets in clouds are electrically charged. In any cloud all such water
particles carry the same charge. Artificial rain can be caused by spraying oppositely charged dust
or fine sand or precipitates like Agl (which has a crystal structure similar to ice and as such
particles of Agl can act as nuclei for precipitation) on to a cloud. Even certain salts in t he form of
fine particles can be sprayed. The neutralization of charge results in coagulation of water droplets
or rain. Cloud–brust–a natural disaster resulting in a very heavy down pour over a very short time
is believed to occur due to the mutual discharge of oppositely charged clouds.
12. Cleaning Action of Soaps and Detergents: Dust and dirt particles on clothes are colloidal in
nature. Soaps being sodium salts of higher fatty acids coagulate them which become suspension
particles called micelle and roll out due to greater volume and greater mass.
13. Rubber Industry: Latex is a colloidal solution of rubber particles which are negatively charged.
Rubber is obtained by coagulation of Latex. Rubber is electroplated on metals when they act as
anode.
14. Tanning: Process of hardening of leather is called tanning. Tannin obtained from plants is
negatively charged sol and animal hides are also sol containing positively charged particle. Mutual
coagulation results in hardening of leather.

Surface chemistry notes

More Related Content

What's hot (20)

Similar to Surface chemistry notes (20)

More from KarunaGupta1982 (20)

Recently uploaded (20)

Surface chemistry notes