UNIT-V SURFACE CHEMISTRY Solid surfaces

VITS Surface Chemistry

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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. Adsorption Absorption

1.

2.

3.

4.

It is the phenomenon of concentration of

a gas or a liquid at a surface of a solid or

a liquid.

It is a surface phenomenon.

This process takes place rapidly.

Equilibrium is attained easily.

It is the phenomenon in which the substance

gets uniformly distributed throughout the

body of a solid or liquid.

It is a bulk phenomenon.

This is a slow process.

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. Physical adsorption Chemical adsorption

1.

2.

3.

4.

5.

6.

7.

Physical adsorption is caused by

Vanderwall forces of attraction.

Very weak forces are responsible for such

adsorption.

Heat of adsorption is very less

(5k.cal/mol)

Adsorption is completely reversible since

the molecules are not tightly retained by

the adsorbent.

Multilayer adsorption occurs.

Equilibrium is established rapidly.

The rate of adsorption increases with the

The chemical adsorption is formed by

chemical bond.

Strong forces are responsible for such

adsorption.

Heat of adsorption is large

(10 – 20 kcal/mol)

Adsorption is irreversible since the

molecules are tightly retained by the

adsorbent.

Monolayer adsorption occurs.

Equilibrium is attained slowly.

The rate of adsorption decreases with


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8.

9.

increase in the pressure.

Physical adsorption decreases with

increase in the temperature.

This adsorption involves small or little

activation energy.

increases the pressure.

Chemical adsorption increases with

increase in the temperature.

This generally involves appreciable

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

1/nx = K.P

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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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)

a

a da d

ad a

d

KP

K P K bPθ = = (where b = K /K )

KK +K P 1 + bP1 + P

K

=

The amount of gas adsorbed per gram of the adsorbent (x) is proportional to θ.

bP bPHence, x α or x = K

1 + bP 1 + bP

Where K is a constant.


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KbP K'PNow we have x = or x = where K' = Kb

1+ bP 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.

o m m o

P 1 C-1 P = +

V(P -P) V C V C P

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 o

P

V(P -P)against 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

o A

o

P V= N.S

RT

∑

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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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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Sl.No. Property True solution Colloidal solution Suspension

1. Nature Homogeneous Heterogeneous Heterogeneous

2. Particle size <1nm 1nm – 1000nm >1000nm

3. Separation by

1. Ordinary filtration

2. Ultra filtration

Not possible

Not possible

Not possible

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 Smoke

4. Liquid Solid Gel Jellies, curd, cheese

5. Liquid Liquid Emulsion Milk, cream, oil in water

6. Liquid Gas Aerosol Mist, fog, cloud

7. Gas Solid Solid foam Pumice stone, foam rubber

8. Gas Liquid Foam 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 Property Lyophillic colloids Lyophobic colloids

1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

11.

Affinity

Nature

Ease of

preparation

Nature of

Particles

Stability

Visibility

Action of

electrolytes

Charge on

particles

Hydration

Viscosity

Tyndall

effect

More affinity between dispersed

phase and dispersion medium due

to hydrogen bonding with water.

They are reversible in nature.

They are easily prepared by direct

mixing.

Colloidal particles are big in size

They are more stable.

Particles are not visible even under

ultra microscope.

Addition of small amount of

electrolyte causes coagulation.

Particles do not carry any charge

and migrate in any direction under

influence of electric field.

The particles are highly hydrated

due to attraction with solvent.

The viscosity and surface tension is

higher than that of dispersion

medium.

They do not show any Tyndall

effect.

Less or no affinity between

dispersed phase they and dispersion

medium.

They are irreversible in nature.

They are formed by special methods.

Particles are aggregates of many

molecules.

They are unstable and requires traces

if stabilizers.

Particles are detected under ultra

microscope.

Large quantity of electrolytes are

required for coagulation.

They possess either positive or

negative charge and migrate towards

oppositely charged electrodes under

influence of electric field.

These particles are less hydrated.

The viscosity and surface tension are

nearly same as that of dispersion

medium.

They show Tyndall effect.


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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

coming out from the chimney is free from dust.

Figure 11. Cottrell smoke precipitator

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 soccor-

ball 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? [June 2012]

2. Write a detailed account on the following: [June 2011]

(a) Origin of charge on colloids (b) Stability of colloids.

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. [June 2011]


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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? [June 2010]

(b) Differentiate the dispersed phase from dispersion medium.

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? [June 2010]

10. Give an account of the analytical applications of colloids. [Dec 2010]

11. Explain the Tyndall effect by Zsigmondy's ultra microscope and electron microscope.

[Dec 2010]

12. Write a detailed account on the following [Dec 2010]

(a) Quantum dots (b) Applications of nanotechnology to medicine.

13. (a) What are colloids? How are they classified? [Dec 2010]

(b) Differentiate the dispersed phase from dispersion medium.

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UNIT-V SURFACE CHEMISTRY Solid surfaces