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Surface Chemistry
UNIT-V
SURFACE CHEMISTRY
Solid surfaces
Adsorption
Adsorption is a surface phenomenon. When a solid surface is exposed to a gas or liquid then
the molecules of gas or liquid accumulates or concentrates over the surface. This phenomenon
of concentration of a gas or liquid molecules at a solid surface is called as adsorption.
The substance which concentrates at surface is called adsorbate and the solid surface is called
adsorbent.
Figure 1. Adsorption of gas
Figure 2. Adsorbate and adsorbent
When the concentration of adsorbate is more on the surface of adsorbent than in the bulk it is
called as positive adsorption.
When the concentration of the adsorbate is less on the surface of the adsorbent than in the
bulk it is called as negative adsorption.
Adsorption of a gas on a solid is also called as occlusion.
The amount of heat evolved when one mole of any gas or vapour is adsorbed on a solid
adsorbent surface is called enthalpy of adsorption or heat of adsorption.
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Examples for adsorption
i. Adsorption of a dye by charcoal: If finely divided charcoal is stirred into a dilute solution of
methylene blue (an organic dye), the depth of color of the solution decreases appreciably. The
dye molecules adsorbed be the charcoal particles.
ii. Adsorption of a gas by charcoal: If a gas (CO2, Cl2, NH3) treated with powdered charcoal in a
closed vessel the gas pressure is found to decrease. The gas molecules concentrate on charcoal
surface.
Absorption
The phenomenon of uniform distribution of a substance throughout the body of a solid or a
liquid is known as absorption.
Eg. A piece of chalk dipped in ink and crystals of CaCl2 immersed in water.
Sl.No.
1.
Adsorption
Absorption
It is the phenomenon of concentration of It is the phenomenon in which the substance
a gas or a liquid at a surface of a solid or gets uniformly distributed throughout the
a liquid.
body of a solid or liquid.
2.
It is a surface phenomenon.
It is a bulk phenomenon.
3.
This process takes place rapidly.
This is a slow process.
4.
Equilibrium is attained easily.
Equilibrium is attained slowly.
Types of adsorption
Adsorption is of two types: physical adsorption and chemical adsorption.
Physical adsorption (Physisorption)
Physical adsorption is due to the adsorption of gas molecules on the solid surface by
Vanderwall forces of attraction.
Forces responsible for such adsorption are very weak. Heat of adsorption is very small (5
k.cal/mol).
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Adsorption is completely reversible since the molecules are not tightly retained by the
adsorbent.
Due to physical adsorption multilayer adsorption occurs.
This adsorption involves very small or activation energy.
Physical adsorption occurs rapidly at low temperature.
As the physical adsorption is increased desorption takes place.
Chemical adsorption (Chemisorption)
In this adsorption the gas molecules are held on the solid surface by a chemical bond.
These bonds may be covalent or ionic in nature.
Heat of adsorption is very high (10-20 k.cal/mol).
This adsorption is irreversible because the molecules are tightly adhered on the surface of the
adsorbent.
The rate of adsorption decreases with the increase of pressure or concentration of the
adsorbate.
Chemisorption like most of the chemical changes generally increases with temperature.
Comparison of physical adsorption and chemical adsorption
Sl.No.
1.
Physical adsorption
Chemical adsorption
Physical adsorption is caused by
The chemical adsorption is formed by
Vanderwall forces of attraction.
chemical bond.
Very weak forces are responsible for such
Strong forces are responsible for such
adsorption.
adsorption.
Heat of adsorption is very less
Heat of adsorption is large
(5k.cal/mol)
(10 – 20 kcal/mol)
Adsorption is completely reversible since
Adsorption is irreversible since the
the molecules are not tightly retained by
molecules are tightly retained by the
the adsorbent.
adsorbent.
5.
Multilayer adsorption occurs.
Monolayer adsorption occurs.
6.
Equilibrium is established rapidly.
Equilibrium is attained slowly.
7.
The rate of adsorption increases with the
The rate of adsorption decreases with
2.
3.
4.
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increase in the pressure.
increases the pressure.
Physical adsorption decreases with
Chemical adsorption increases with
increase in the temperature.
increase in the temperature.
This adsorption involves small or little
This generally involves appreciable
activation energy.
activation energy.
Adsorption isotherms
The relation between the amounts of gas adsorbed by a solid and the equilibrium pressure or
concentration at a given temperature is called an adsorption isotherm. It is obtained by plotting
the adsorbate concentration in the solid phase as a function of pressure of the gas.
1. Freundlich adsorption isotherm
x
= K.P1/n
m
Where x is the mass of the gas adsorbed; m is the mass of the adsorbent; P is the pressure
2. Langmuir adsorption isotherm
The postulates of Langmuir’s theory are as follows:
1) The surface of the solid consists a fixed number of adsorption sites per unit surface area.
2) Each site can adsorb only one gas molecule. Hence the solid surface is covered by a single
layer or monolayer of gas molecules.
3) Initially the rate of adsorption is high as the number of vacant sites is quite large compared to
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Surface Chemistry
the filled sites.
4) After the adsorption of all the surface area then the evaporation of gas molecules takes place.
This process is called as desorption.
5) At equilibrium point the rate of adsorption is equal to rate of desorption.
Based on the above postulates the rate of adsorption depends on the pressure and the unadsorbed
surface area.
Rate of adsorption = Ka (1- θ) P
Where θ is fraction of adsorbed surface area.
(1- θ) is the fraction of vacant sites available for adsorption.
Ka is adsorption rate constant.
Rate of desorption = Kd θ
where Kd is the desorption rate constant
At equilibrium state, the rate of adsorption is equal to rate of desorption.
Ka (1- θ) P = Kd θ
KaP - Ka. θ.P = Kd θ
KaP = Kd θ + Ka. θ.P
KaP = θ (Kd + KaP)
Ka
P
Ka P
Kd
bP
θ=
=
=
(where b = K a /K d )
K d +K a P 1 + K a P 1 + bP
Kd
The amount of gas adsorbed per gram of the adsorbent (x) is proportional to θ.
Hence, x α
bP
or
1 + bP
Where K is a constant.
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x=K
bP
1 + bP
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Surface Chemistry
Now we have x =
KbP
1+ bP
or x =
K'P
where K' = Kb
1+ bP
The above equation is Langmuir adsorption isotherm which gives the relation between the
amount of gas adsorbed to the pressure of the gas at constant temperature. Dividing the above
equation by P and then taking the reciprocal of that, we get
P
1
b
=
+  P
x
K'  K' 
Since K’ and b are constants for a given system, a plot of P/x against P should give a straight line
with a slope equal to b/K’ and an intercept equal to 1/K’.
Figure 3. Langmuir adsorption isotherm
However, Langmuir adsorption isotherm holds good at low pressure but fails at high pressure.
If pressure is very low, the factor K’ may be ignored and the isotherm becomes
x = K’P
Hence at low pressure the amount of gas adsorbed (x) is directly proportional to the pressure
(P).
If pressure is very high, the factor 1/K’ may be ignored and the isotherm becomes x = K’.
At high pressure the adsorbent surface is completely covered with a unimolecular layer of gas.
At this stage the adsorption is independent of pressure.
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3. BET adsorption isotherm
This theory is proposed by Brunauer, Emmett and Telier.
It is applicable for multilayer adsorption.
Adsorption at one site does not effect adsorption at another site.
The molecules can be adsorbed in the second, third……and nth layers.
P
1
C-1  P 
=
+
 
V(Po -P)
Vm C
Vm C  Po 
Where, V is the volume of the gas adsorbed; P is pressure; Vm is the volume adsorbed when the
solid surface is completely covered with a monolayer of the adsorbed molecules of the gas; C is
constant depending upon the nature of the gas.
Since C is a constant for a given gas and Vm is a constant for a given gas-solid system, the plot
of
P
against
V(Po -P)
P/Po should be a straight line.
The slope of the linear plot evidently gives the value of (C-1)/ VmC while the intercept yields
the value of 1/VmC. Thus from the slope and intercept both Vm and C can be evaluated.
Figure 4. BET adsorption isotherm
Having the value of Vm, the surface area adsorbed may be calculated by multiplying it with the
number of molecules required to form a monomolecular layer.
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Determination of surface area of adsorbents by BET equation
As given in the BET equation, the surface area of the solid per definite weight of the adsorbent is
given by
PV
∑=  o A
 RTo

 N.S

Where, Σ is the surface area in Ao; Po is the pressure in atmospheres; R is the gas constant; To is
absolute temperature; VA is the volume; N is the Avagadro number; s is the area occupied on
the surface by a single gas molecule.
Applications of Adsorption
The activated charcoal is used in gas masks in which all undesirable gases are adsorbed by
charcoal.
The phenomenon of adsorption of is useful in heterogeneous catalysis. Eg. Haber’s process
(preparation of NH3) and hydrogenation of oils.
Colloidal silica and alumina are used as adsorbents for removing moisture and controlling
humidity in the room.
Adsorption is also employed in the softening and deionization of water by permutit process.
When hard water is passed through permutit (zeolite) the replacement of Ca2+ and Mg2+ ions
by Na+ ions takes place there by removing the hardness of water.
Adsorption is also applied in chromatographic analysis. This analysis is used in separation of
a mixture of substances into its various components by passing through the column of
suitable adsorbents like alumina, silica, magnesia etc.
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Surface Chemistry
Colloids
Substances are divided in to two types depending upon their ability to diffuse in liquid
medium. (i) Crystalloids (ii) Colloids.
Solutes which can diffuse rapidly in solutions and can pass through parchment membrane are
known as crystalloids.
Eg. NaCl, gelatin, starch and proteins.
Solutes which cannot diffuse in solutions and pass through parchment membrane are known
as colloids.
However, the above classification of solutes into crystalloids and colloids is unsatisfactory as
some solutes behave as crystalloids in some solutions and colloids in other solutions.
Colloid is not a substance but it is a state of substance which depends upon the molecular size.
Colloidal solutions are intermediate between true solutions and suspensions.
A true solution is a clear and homogeneous having particle size below 1 nanometer.
Suspensions are heterogeneous and their suspended particle size is above 1000 nanometer.
Colloids are heterogeneous having particle size which are in the range of 1nm to 1,000nm.
Figure 5. True solution-colloid-suspension
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Surface Chemistry
Property
Sl.No.
True solution
Colloidal solution
Suspension
1.
Nature
Homogeneous
Heterogeneous
Heterogeneous
2.
Particle size
<1nm
1nm – 1000nm
>1000nm
3.
Separation by
1. Ordinary filtration
Not possible
Not possible
Possible
2. Ultra filtration
Not possible
Possible
Possible
4.
Settling of particles
Do not settle
By centrifugation
Under gravity
5.
Diffusion of particles
Diffuses rapidly
Diffuses slowly
Do not Diffuse
The colloids are two phased systems which possess dispersed phase and dispersion medium.
Dispersed phase: It is also known as discontinuous phase or inner phase. It consists of small
particles dispersed in solvent.
Dispersion medium: It is also known as a continuous phase or outer phase and constitutes of
medium (solvent) in which the colloidal particles are dispersed. This phase usually forms the
larger fraction of colloids.
Brownian movement: The random motion of colloidal particles in a zig-zag motion is called
Brownian movement. This is due the collisions among the colloidal particles in the dispersion
medium.
Figure 6. Brownian movement
Coagulation: The phenomenon of the precipitation of colloidal solution by the addition of the
excess of an electrolyte is called coagulation or flocculation.
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Classification of colloids
1. Based on the stage of aggregation of the two phases
Based on the stage of aggregation of the dispersed phase and dispersion medium, the colloidal
systems are classified as solid sol, colloidal sol, aerosol, gels, emulsions, solid foam and foam.
Sl.No. Dispersed phase Dispersion medium
Colloid
Examples
1.
Solid
Solid
Solid sol
Ruby glass, precious stones
2.
Solid
Liquid
Sol
Gold in water, clay in water
3.
Solid
Gas
Aerosol
4.
Liquid
Solid
Gel
5.
Liquid
Liquid
Emulsion
6.
Liquid
Gas
Aerosol
7.
Gas
Solid
Solid foam
8.
Gas
Liquid
Foam
Smoke
Jellies, curd, cheese
Milk, cream, oil in water
Mist, fog, cloud
Pumice stone, foam rubber
Lather in soap solution,
shaving cream, soda water
A colloidal system of two gases is not possible because it is a homogeneous system.
2. Based on the dispersion medium
Sl.No.
Dispersion medium
Colloid
1.
Fluid
Sol
2.
Gas
Aerosol
3.
Water
Hydrosol
4.
Alcohol
Alcosol
5.
Benzene
Benzosol
3. Based on affinity between dispersion medium and dispersed phase
Based on the affinity of the dispersed phase to dispersion medium, colloids are classified into
two types (i) Lyophilic colloids( solvent loving) (ii) Lyophobic colloids (solvent hating).
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Lyophilic colloids
In these colloids, the dispersed phase has a great affinity for the dispersion medium due to
the hydrogen bonding with water.
Eg. Gelatin , glue, starch, proteins and other organic substances
Lyophilic colloids are also called as reversible colloids because once these colloids
precipitated from colloidal form then can be again reconverted into the colloidal form.
Lyophilic colloids are prepared by simple method i.e mixing the material with a suitable
solvent.
Lyophilic sol particles have little or no charge at all.
Each lyophilic sol particle is surrounded by solvent molecules giving them high solubility.
Lyophilic sols are viscous as the particle size increases due to solvation.
Large amount of electrolyte is required for coagulation.
Lyophilic sols show high conductance.
The particles of lyophilic sols cannot be easily seen under the ultra microscope
On account of relatively small particles, lyophilic sols donot show Tyndall effect.
Lyophilic particles can migrate only one direction when placed in electric field.
In lyophilic sols, there is no Brownian movement.
Lyophobic colloids
These are insoluble substances which do not readily yield colloidal substances, when comes in
contact with solvent.
In these colloids the dispersed phase has very little affinity for dispersion medium.
These are called irreversible colloids because once precipitated they cannot be directly
reconverted back into colloidal form.
Lyohophic sols are not obtained by simple mixing of the solid material with the solvent.
These colloidal particles carry either positive or negative charge which gives them stability
The viscosity of lyophobic salts almost same as the viscosity of dispersion medium.
The particles of lyophobic sols can easily be detected under ultra microscope.
Lyophobic sol particles show Tyndall effect.
When lyophobic sol is placed in electric field, the particles migrate towards electrodes.
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Differences between lyophillic and lyophobic colloids
Sl.No
1.
Property
Affinity
Lyophillic colloids
Lyophobic colloids
More affinity between dispersed
Less or no affinity between
phase and dispersion medium due
dispersed phase they and dispersion
to hydrogen bonding with water.
medium.
2.
Nature
They are reversible in nature.
They are irreversible in nature.
3.
Ease of
They are easily prepared by direct
They are formed by special methods.
preparation mixing.
4.
Nature of
Colloidal particles are big in size
molecules.
Particles
5.
Stability
Particles are aggregates of many
They are unstable and requires traces
They are more stable.
if stabilizers.
6.
7.
Visibility
Action of
Particles are not visible even under
Particles are detected under ultra
ultra microscope.
microscope.
Addition of small amount of
Large quantity of electrolytes are
electrolytes electrolyte causes coagulation.
8.
required for coagulation.
Charge on
Particles do not carry any charge
They possess either positive or
particles
and migrate in any direction under
negative charge and migrate towards
influence of electric field.
oppositely charged electrodes under
influence of electric field.
9.
Hydration
The particles are highly hydrated
These particles are less hydrated.
due to attraction with solvent.
10.
11.
The viscosity and surface tension is
The viscosity and surface tension are
higher than that of dispersion
nearly same as that of dispersion
medium.
medium.
Tyndall
They do not show any Tyndall
They show Tyndall effect.
effect
effect.
Viscosity
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Optical properties of colloids
Tyndall effect
When a beam of light is passed through a true solution no scattering of light is observed.
But when a beam of light is passed through a colloidal solution, then the path of the light is
scattered which can be seen in the form of cone.
This is due to the colloidal particles which are present in the solution absorb light energy and
then emit it in all the directions. This is called as scattering of light.
This phenomenon of scattering of light by the colloidal particles is known as Tyndall effect.
The beam or cone formed by the scattering of light by the sol particles is called as Tyndall
cone or Tyndall beam.
The Tyndall effect can be observed by an electron microscope.
In true solutions the particle size is very less and they cannot scatter light. Hence the beam of
light is invisible.
But in colloidal solutions the particle size is very large so they exhibit Tyndall effect.
Hydrophobic sols show Tyndall effect but hydrophilic sols do not show Tyndall effect.
Figure 7. Tyndall effect
Electrical properties of colloids
The most important property of colloidal solutions is that the sol particles carry an electric
charge.
All particles in a given colloidal systems carry the same charge and the dispersion medium
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has an opposite and equal charge thus making the system electrically neutral.
Positively charged colloids: Fe(OH)3, Al(OH)3, TiO2, Methylene blue.
Negatively charged colloids: Sols of Au, Ag; As2S3, Prussian blue.
The colloidal sols exhibit four electrical properties which are as follows:
1. Electrical double layer or zeta potential
2. Electrophoresis
3. Electro-osmosis
4. Isoelectric point
1. Electrical double layer or zeta potential
The surface of sol particles acquire positive charge by
adsorption of layer of positive ions from dispersion
medium and form second layer of negative charge.
The combination of positive and negative charge
around sol particles was called as Helmoltz double
layer. According to this positive layer is fixed and
negative layer is movable. There exists a potential
difference between the layers and bulk of the solution
which is called as zeta potential.
Figure 8. Electrical double layer of AgI
2. Electrophoresis
When the colloidal sol particles are taken up in an electrical field, they migrate towards one of
the electrodes. Therefore electrophoresis involves migration of sol particles towards opposite
electrode. If a sol particle migrates towards the positive electrode, then the sol particles must be
carrying negative charge. If sol particles carry positive charge, then they migrate towards the
negative electrode. Thus by knowing the direction of movement of sol particles gives the
information of charge of sol.
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Eg. Ferric hydroxide sol will migrate towards negative
electrode. Therefore it can be concluded that sol
particles carry positive charge.
Eg. As2S3 sol will migrate towards positive electrode.
Therefore it can be concluded that sol particles carry
negative charge.
Figure 9. Electrophoresis of As2S3
3. Electro-osmosis
When two portions of water are separated by moist clay or sand and a difference of potential is
applied in between the two electrodes, the water flows from one side to another. This
phenomenon is called as electro-osmosis. The colloidal system is placed in the central
compartment ‘ A’ which is separated from the compartments ‘ B’ and ‘C’ compartments which
are extended to the side tubes T and T’. The compartments are separated by dialyzing membrane
M and M’. These membranes prevent to passage of the colloidal particles through them.
When a potential difference is applied
across the electrodes, the water begins to
move from the ‘B’ and ‘C’ compartments. If
the particles carry positive charge, then the
water will carry negative charge and
therefore it would start moving towards
anode and hence the level of water in side
tube T would rise. If the particles carry
negative charge, the water carries positive
Figure 10. Electro osmosis
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charge which will start moving towards the cathode and the water level in the side tube T’ would
start rising.
4. Isoelectric point
Naturally occurring macro molecules acquire a charge when dispersed in water. In lyophilic
colloids the overall charge of particles depends on the pH of the medium. Lyophilic colloids
carry positive charge in strongly acidic solutions and carry a negative charge in the basic
medium. At isoelectric point, the pH is such that there is no net charge on the macromolecules.
Hence they do not migrate under the influence of the electric field but coagulation of colloidal
particles occurs.
Protection of colloids
Lyophobic colloids can be easily precipitated out by addition of a small amount of electrolytes.
They can be prevented from undergoing coagulation by the previous addition of some stable
lyophillic colloids like gelatin, glue etc. this process of protecting the lyophobic colloids from
coagulation by the electrolytes due to previous addition of some lyophobic colloids is called
protection of colloids. The colloid which is added to prevent coagulation of the colloidal sol is
called protecting colloid.
Eg. Addition of gelatin (protecting colloid/lyophillic colloid) to gold sol (lyophobic colloid).
The protecting power of different protecting (lyophillic) colloids is expressed in terms of gold
number. The minimum amount of the protective colloid required in milligrams to prevent
coagulation is called coagulation. Smaller the value of gold number, greater will be the power of
protecting colloid.
Associated colloids (Micelles)
Some substances which undergo formation of colloids by aggregation or clustering at
reasonably higher concentrations are known as associated colloids or micelles.
Thus micelles are the cluster or aggregated particles formed by association of colloids in
solution.
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These substances behave as normal electrolytes at low concentrations and behave as colloids
at higher concentrations.
Eg. Soap (sodium strearate – C17H35COONa) and detergents (sodium dodecyl sulphate –
C12H25SO2ONa)
The formation of micelle takes place above certain concentration called critical micellization
concentration (CMC). Below CMC, they dissociate into individual ions.
C17H35COONa
C17H35COO- + Na+ (at low conc.)
Micelles are generally formed by association of individual ions or molecules which possess
both lyophillic and lyophobic parts.
Stearate ion has long hydrocarbon chain (C17H35-) which is lyophobic in nature and COO- end
is lyophillic in nature.
The stearate ions associate themselves above CMC to form ionic micelle of colloidal size.
Figure. Individual sodium stearate ion below CMC
and associated colloid above CMC.
Applications of colloids
1. Cleaning action of soap: Soap has tendency to form micelle and emulsion. Soap (sodium
stearate) is composed of long hydrocarbon chain (tail) and polar group (head). The polar head is
soluble in water and non-polar tail is soluble in organic solvents/oil.
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The dirt of the clothes is due to the presence of dust particles in fat or grease which stick to the
cloth. When the cloth is soaked into a aqueous soap solution the soap particles surround the oil
particles. The non-polar end (tail) dissolves in the grease deposit while the polar end (head) is
directed towards water. In this manner, each oil droplet is surrounded by a number of negatively
charged carboxylate ions. The hand rubbing or agitation due to washing causes dispersion of oil
throughout the soapy solution. Since the charges reel each other, the oil droplets breakup and
form small droplets which get dispersed in water to form emulsion. In this way the grease/oil/dirt
are removed from the surface of the cloth.
2. Cottrell smoke precipitator: the smoke coming out of
the chimneys contain a lot of unburnt carbon particles.
These are quite injurious to health and have to be
precipitated out from the smoke. In fact, smoke is a
colloidal system of carbon particles suspended in air. The
carbon particles are charged in nature and they do not get
coagulated. The charge on the carbon particles is
neutralized by bringing them in contact with the
oppositely charged metal plates. The smoke is allowed to
pass through a chamber having series of plates charged to
very high potential (20,000-70,000V) in a Cottrell smoke
precipitator. Thus they get precipitated and smoke
Figure 11. Cottrell smoke precipitator
coming out from the chimney is free from dust.
3. Purification of drinking water: the drinking water can be purified by precipitation of
suspended colloidal particles. For this purpose, a small amount of alum [K2SO4. Al2(SO4)3.
24H2O] is added. The Al+3 ions neutralize the charge on the particles and they get coagulated.
4. Quanitative analysis: in gravimetric analysis of BaSO4, little amount of HCl is added before
adding the precipitating reagent. This is to avoid the colloidal formation during analysis.
5. Identification of noble metal traces: The traces of noble metal are identified by colloidal
nature if some metals. They exhibit bright and intense colours in colloidal forms.
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Nano-materials
“Nano” means one billionth of a meter i.e 1nm = 10-9 meter.
When a bulk material is transformed into a nano sized particle, its physical (Hardness, M.P),
chemical (Reactivity, reaction rates), electrical (conductivity) and optical properties (Color,
transparency) also change.
Eg. When a bulk gold is transformed into 12nm sized gold particles, its optical properties
change from yellow to red.
Nano particles are of great specific interest as they effectively bridge between bulk materials
and atomic/molecular structures.
They possess very high surface area to volume ratio compared with larger particles which
provides a tremendous driving force for diffusion.
Two principle factors which cause the properties of nano materials from other materials are
increased relative surface area and quantum effects.
Nano particles made of metals, oxides, semiconductors are of particular interest for their
mechanical, electrical, magnetic, optical chemical and other properties.
The characterization of nano materials is done by X-Ray Diffraction (XRD), Scanning
Electron Microscope (SEM), Transmission Electron Microscope (TEM), Atomic Force
Microscope (AFM).
Nano materials are produced by two methods:
i. Top-down approach: Producing very small structures from large pieces of materials.
ii. Bottom-up approach: Self assembly of atoms/molecules which arrange themselves into a
nano structure.
Nano materials are used in sunscreens, cosmetics, food additives, packaging, scratch-proof,
self-cleaning paints, glass, clothing, sports equipment, disinfectants, fuel additives, batteries.
Carbon nano tubes (CNT’s)
They are allotropes of carbon with a cylindrical nano structure.
CNT’s are sheets of graphite rolled up to make a few nano meters in diameter and upto few
hundreds of micrometers long. They possess length to diameter ratio of 28,000,000:1.
If a nano tube consists of one type of graphite it is called as single walled nano tube (SWNT)
and if they consists number of concentric tubes, they are called as multi walled nano tubes
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(MWNT).
They have novel properties that make them potentially useful in applications of nano
technology, electronics, optics, architecture and other fields of material science.
They are good conductors of heat and electricity.
Because of varying electrical properties they can be doped with other elements giving them
electrical properties like insulating, conducting and semiconducting.
They are extremely strong, resilient and flexible. Due to these properties, they are used as
ultra light structural materials for wings of advanced aircrafts.
They are hydrophobic and bind easily to proteins.
When incorporated into a polymer matrix, they can produce composites with high strength
elastic module.
Applications: (i.) Electrical energy storage (ii.) Conductive/reinforced plastics (iii.) Catalyst
support etc.
Fullerenes
They are class of allotropes of carbon which are conceptually grapheme sheets rolled up into
spheres.
They are classic three dimensional nano materials.
C60-fullerene (Buck minister fullerene) is made of sixty carbon atoms arranged into a soccorball like structure.
It has hallow interior which can fit a molecule of a particular drug inside, while outside bulky
ball is resistant to interaction with other molecules in the body.
Applications: Catalysis, drug-delivery systems, optical devices, sensors separation devices etc.
Quantum Dots (QD)
They are often referred as artificial atoms.
QD is a semiconductor that exhibit Quantum confinement property in all three dimensions.
Their size is in the range of 1-10nm.
They can be either metallic or chalcogenide.
The display chosen color in U.V region which is used in the development of multi-color
lasers.
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QD have high potential for photo-voltaic applications.
Applications: New generation computers, light emitting diodes, cellular imaging.
Figure 12. (a) Carbon nano tube (b) C60-fullerene (c) Quantum dots
Applications of nano materials
Silver nano particles are used as anti-bacterial agents.
ZnO nano particles are incorporated into plastic packaging to block U.V rays and provide
anti bacterial protection.
Gold nano particles used in drug delivery systems.
Aliminosilicate nano particles in medical gauze which help blood clot faster in open wounds.
QD’s can identify the location of cancer cells in the body.
Electrodes composed of nano sized filters used in purification of water.
Previous exam questions
1. (a) Outline the difference between physisorption and chemisorption with examples?
(b) What are protective colloids? Give their significance?
(c) What are fullerenes? Give an account of their applications?
2. Write a detailed account on the following:
(a) Origin of charge on colloids (b) Stability of colloids.
[June 2012]
[June 2011]
3. (a) State and explain BET equation for multilayer adsorption?
[June 2011]
(b) How is the surface area of an adsorbent is determined with the help of BET equation?
4. Write an account of the following: (a) Micelles (b) Dialysis.
22
[June 2011]
VITS
Surface Chemistry
5. Write an account on the following: (a) Electrophoresis (b) BET Adsorption isotherm. [June2011]
6. What is meant by coagulation of colloids? How is it brought out?
[June 2010]
7. (a) What are colloids? How are they classified?
(b) Differentiate the dispersed phase from dispersion medium.
[June 2010]
8. Explain the following statements with proper illustrations.
[June 2010]
(a) Tyndall cone is observed when a beam of light is concentrated on colloidal systems.
(b) Alums are used for the treatment of water supplied by municipalities
9. What are fullerenes? Present an account of applications of fullerenes?
10. Give an account of the analytical applications of colloids.
[June 2010]
[Dec 2010]
11. Explain the Tyndall effect by Zsigmondy's ultra microscope and electron microscope.
[Dec 2010]
[Dec 2010]
12. Write a detailed account on the following
(a) Quantum dots (b) Applications of nanotechnology to medicine.
13. (a) What are colloids? How are they classified?
(b) Differentiate the dispersed phase from dispersion medium.
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23
[Dec 2010]