INTRODUCTION  Surface chemistry is the study of processes that occur at the interface of two bulk phases.

The bulk phases can be of the type :

Liquid - liquid

 

 Adsorption is a surface phenomenon

THE PHENOMENON OF CONCENTRATION OR ASSIMILATION OF A GAS OR LIQUID AT THE SURFACE OF A SOLID OR LIQUID WITH WHICH IT IS IN CLOSE PROXIMITY OR CONTACT IS CALLED ADSORPTION .

The material providing the surface upon which adsorption occurs is known as the adsorbent

The substance adsorbed or attached is called adsorbate

Types of Adsorption

 Depending on the nature of attractive forces existing between the adsorbate and adsorbent,

adsorption can be classified as:         

Physical AdsorptionPhysical Adsorption

Chemical AdsorptionChemical Adsorption

In physical adsorption,the forces of attraction between the molecules of the adsorbate and the adsorbent are of the weak van der Waals' type.

Since the forces of attraction are weak, the process of physisorption can be easily reversed by heating or decreasing the pressure of the adsorbate (as in the case of gases).

Physical Adsorption - PHYSISORPTION

Chemical adsorption - CHEMISORPTION

In chemisorption, the forces of attraction between the adsorbate and the adsorbent are very strong; the molecules of adsorbate form chemical bonds with the molecules of the adsorbent present in the surface.

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 Forces of attraction are vander Waals’ forces

 Forces of attraction are chemical bond forces

 Low enthalpy of adsorption (20 - 40 k.J/mole)

 High enthapy of adsorption (200 - 400 k.J/mole)

 This process is observed under conditions of low temperature

 This process takes place at high temperatures

 It is not specific  It is highly specific Multi-molecular layers may be formed

 Generally, monomolecular layer is formed

 This process is reversible  This process is irreversible

Difference between

Physisorption  Chemisorption

POSTULATES: The surface of the solid consists of a fixed

number of adsorption sites as per unit area. Each site can adsorb only one gas molecule .

Hence the solid surface could be covered only a single layer or monolayer of gas molecules.

According to this theory all the adsorption sites are considered to be equivalent i.e. the surface of the solid is homogeneous so that each adsorption site has the same affinity for the gas molecules.

Further adsorption of a gas molecule at a site was assumed to be independent of whether the neighbouring sites are vacant or occupied.

The adsorbed gas molecules remain localized with out any interaction between them.

Initially the rate of adsorption is high as the number of vacant sites is quite large compared to the filled sites.

As adsorption progresses the initial rate of adsorption falls since the area available for adsorption decreases.

As the surface becomes saturated , the rate of the desorption increases

The two opposing rates eventually become equal and dynamic equilibrium exists between the adsorption and desorption processes.

Consider a surface on which the molecules of a gas are condensing and evaporating

let θ = fraction of the total covered at a particular instant 1-θ =fraction of surface still bare

ө Ө)(1-

Fraction adsorbed

Adsorbed molecules evaporate Gas molecules

to be adsorbedFraction bare

Similarly, Rate of backward reaction or Rate of Desorption depends upon number of sites occupied by the gaseous molecules on the surface of adsorbent.

At equilibrium, rate of adsorption is equal to rate of desorption.Ka P (1 – θ) = Kd θ

We can solve the above equation to write it in terms of θ.

KaP – KaP θ = Kd θ

KaP = KaP θ + Kd θ

KaP = (Kd + KaP) θ

Divide numerator and denominator on RHS by Kd, we get

Now put

in above equation we get d

a

k

ka

aP

aP

1

But quantity of gas adsorbed per unit mass of the adsorbent w is proportional to the surface covered w=k . θ

This equation is called Langmuir's adsorption equation. The above equation is written as:

)(11

kabwhereaP

bP

aP

kaPw

Pb

a

bw

P1

Thus ,if we plot P/w against P, we should get a straight line. This is found to be valid in all cases.

CASE I:At low pressure aP becomes negligible in comparison with 1, hence equation reduces to : w=bPi.e the amount of adsorption per unit weight of adsorption at a given temperature is directly proportional to the gas at low pressures.

CASE II: At high pressures , aP is very high as compared with 1 and, therefore , takes the form

tconsa

b

aP

bPw tan

At high pressures , the extent of adsorption at a given temperature is independent of the gas, because the surface becomes completely

covered.

LANGMUIR’S ADSORPTION ISOTHERM

Purification of water Silica and alumina gels are used as

adsorbents for removing moisture and for controlling humidity of rooms.

Charcoal is used as a decoloriser Treatment of Diahroea

Activated charcoal is used in gas masks as it adsorbs all the toxic gases and vapours and purifies the air for breathing. Etc….

   It is well known, that solutions are homogenous systems while suspensions are heterogeneous systems, i.e., they consist of more than one phase. In between the extremes of suspensions and solutions lies the colloidal system.

COLLOIDS

Colloids, which diffuse very slowly in solution and whose solution can’t pass through animal or veg membrane.

Eg: starch, glue, albumin, gelatin, proteins etc……Colloidal state: a substance is said to be in the colloidal state,

when it is dispersed in another medium in the form of very small particles having diameter 2×10-4 to 1×10-7 cm.

Solute and solvent are replaced by dispersed phase & dispersion medium

Sols( solid in liquid),gels(liquids in solids), emulsions (liquid in liquid)

Size of particles lies between that of true solution and suspension, i.e. 10 Ao to 1000 Ao

Classification is based on following criteria

Physical state of dispersed phase and dispersion medium.

Nature of interaction between dispersed phase and dispersion medium.

Based on the affinity of dispersed phase and dispersion medium.

Eight types of colloidal systems are possible.

Dispersed phase 

Dispersion medium 

Type of colloid 

Example

Solid Solid Solid sol Some coloured glasses, and gem stones 

Solid Liquid Sol Paints, cell fluids 

Solid Gas Aerosol Smoke, dust 

Liquid Solid Gel Cheese butter, jellies 

Liquid Liquid Emulsion

Milk, hair cream 

Liquid Gas Aerosol Fog, mist, cloud, insecticide sprays 

Gas Solid Solid sol Pumice stone, foam rubber 

Gas Liquid Foam Froth, whipped cream, soap-lather 

Multimolecular colloids : Consists of aggregates of a large number of atoms or smaller molecules whose diameter is less than 1 nm. In these aggregates the atoms or molecules are held together by weak vanderwalls forces of attraction they are lyophbic in nature. Ex: gold sol in water.

Macromolecular colloids: In these colloids, the molecules have sizes and dimensions very large comparable to colloidalidal particles. For example: proteins, starch, cellulose.

Lyophilic colloids ( solvent loving)In these colloids the dispersion phase has great affinity for the dispersion medium. The affinity or attraction of the sol particles for the medium , in a lyophilic sol, is due to hydrogen bonding with water .Certain substances like gum, gelatine ,rubber and other organic substances directly pass into colloidal solution when brought in contact with water. when once precipitated from colloidal form, they can directly be reconverted into colloidal form again and hence termed as reversible colloids

• self-stabilizing • reversible sols • For example, gums, gelatin, starch, albumin in water.

Lyophobic colloids (solvent hating colloids )Insoluble substances which do not readily yield colloidal solutions when brought in contact with solvent are termed as lyophobic colloids. here the dispersed phase has a very little affinity for dispersion medium.These lyophbic colloids are also known as suspensoids and when once precipitated they cannot be directly obtained into colloidal form and therefore termed as irreversible colloids.

When metals and their sulphides simply mixed with dispersion medium, they don’t form colloids.• need stabilizing to preserve them. • irreversible. • For example, colloidal solutions of gold,silver, Fe(OH)3,

As2S3, etc.

Optical properties: Tyndall effect

When a beam of light falls at right angles to the line of view through a solution, the solution appears to be luminescent and due to scattering of light the path becomes visible.

Quite strong in lyophobic colloids while in lyophilic colloids it is quite weak.

If a powerful beam of light is passed through a colloidal solution placed in a dark room, the path of beam becomes visible, when viewed through a microscope placed at right angles to the path of light.

The colloidal particles appears as pin-points of light moving against the dark background. This phenomenon is known as tyndal effect.

Cause of tyndal effect: It is believed to be due to scattering of

light by the coloidal particles. The particles absorb the incident light

energy, becomes self-luminous and scatter this absorb light from their surfaces.

As the intensity of scattering is maximum in the plane at right angles to the direction of incident beam, so path is visible, hen viewed from the sides.

Electrical properties

Colloids have three electrical properties, they are

1) Electrophoresis2) Electro-osmosis3) Iso electric point

Electrophoresis: Colloidal particles are electrically

charged either positive or negative.

When a high potential gradient is applied between a U-tube, filled partly with a colloidal solution and rest with distilled water.

The colloidal particles move towards oppositely charged electrode with a speed of the order of about 1 micron per second per unit potential gradient.

On reaching the electrode, they lose their charge and get precipitated.

The movement of the colloidal particles under the influence of an electric field, is known as electrophoresis.

Movement of colloidal particles underinfluence of electric field

Electro-osmosis: when an electric current is passed through a

colloidal solution in such a way that the dispersed particles are prevented from movement, it is observed that the dispersion medium moves.

This phenomenon of movement of the dispersion medium of a colloidal solution, under the influence of an electric field, when the dispersed particles are prevented from moving, it called ELECTRO-OSMOSIS.

Isoelectric point:Lyophilic colloids are +vely charged in strongly acidic solution and –vely charged in alkaline solution.

However at a certain H+ion concentration is called the iso-electric point.

At which the dispersed particles are neutral and hence they don't migrate even when subjected to an electric field.

At the iso electric point, coagulation of colloidal particles starts.

MICELLES:

Molecules of substances like soap and detergents are smaller than the colloidal particle size.

These substances behave as normal strong electrolytes at low concentrations.

In concentrated solutions, these substances exhibit colloidal properties

The colloidal size aggregates of soap or detergent molecules formed in a concentrated solution, are referred to as MICELLES.

The minimum concentration at which the micelle formation starts is designated as C.M.C

The protective power of lyophilic sol is expressed in terms of gold number. Gold number is defined as the number of milligrams of a lyophilic colloid that will just prevent the precipitation of 10 ml of gold sol on addition of 1 ml of 10% solution of NaCl. The start of precipitation of gold sol is indicated by a colour change from red to blue with increase in particle size. The smaller the gold number, the greater is the protective action of the lyophilic colloid.

GOLD NUMBER

An emulsion is a dispersion in which the An emulsion is a dispersion in which the dispersed phase is composed of small dispersed phase is composed of small globules of a liquid distributed throughout a globules of a liquid distributed throughout a vehicle in which it is immiscible.vehicle in which it is immiscible.

IntroductionIntroduction

EMULSIONSEMULSIONSEMULSIONSEMULSIONS

Based on dispersed phase Oil in Water (O/W): Oil droplets dispersed in water Water in Oil (W/O): Water droplets dispersed in oil

Classification of emulsions :Classification of emulsions :

Emulsions encountered in everyday life! Emulsions encountered in everyday life!

Metal cutting oils Margarine Ice cream

Pesticide Asphalt Skin cream

Stability of emulsions may be engineered to vary Stability of emulsions may be engineered to vary from seconds to years depending on application from seconds to years depending on application

General Types of Pharmaceutical General Types of Pharmaceutical Emulsions:Emulsions:

1) Lotions 1) Lotions 2) Liniments2) Liniments 3) Creams3) Creams 4) Ointments4) Ointments 5) Vitamin drops5) Vitamin drops

Methods of Preparation of Emulsions:Methods of Preparation of Emulsions:

1) Continental or Dry Gum Method:1) Continental or Dry Gum Method:

"4:2:1" Method"4:2:1" Method

4 parts (volumes) of oil4 parts (volumes) of oil

2 parts of water2 parts of water

1 part of gum1 part of gum

2) English or wet Gum Method:2) English or wet Gum Method:

4 parts (volumes) of oil4 parts (volumes) of oil

2 parts of water2 parts of water

1 part of gum1 part of gum

3) Bottle or Forbes Bottle Method:3) Bottle or Forbes Bottle Method: useful for extemporaneous preparation of useful for extemporaneous preparation of

emulsion from volatile oils or oleaginous emulsion from volatile oils or oleaginous substance of low viscosity.substance of low viscosity.

powdered acaciapowdered acacia

+ Dry bottle+ Dry bottle

2 parts of oil2 parts of oil

This method is not suitable for viscous oils This method is not suitable for viscous oils (i.e. high viscosity oil).(i.e. high viscosity oil).

Emulsifiers are molecules that have two different ends: A hydrophilic end (water-loving) that forms chemical

bonds with water but not with oils A hydrophobic end (water-hating) that forms chemical

bonds with oils but not with water

The hydrophilic 'head' dissolves in the water and the hydrophobic 'tail' dissolves in the oil

In this way, the water and oil droplets become unable to separate out – the mixture formed is called an emulsion

An emulsion is a mixture of oil and water An emulsifier is a specific molecule able to bind the two

ends so they ‘stick together’ (i.e. the oil and water bind)

E.g. Lecithin is an emulsifier which binds the emulsion of water and oil

Gels: the colloidal solution having a liquid dispersed in a soloid of high concentration , is called gel. Ex : curds ,jellies, cheese, bread dough , fruit jam etc.

Formation of gels:Gels are mostly hydrophilic colloids, but some hydrophobic sol also form gels. They are formed by the interlocking of the dispersed particles in the form of loose- frame work, inside which the liquid dispersion medium is entrapped. For Ex: when hot 50% solution of gelatin is allowed to cool, it sets into a semi-solid mass(or jelly) . Similarly, when ferric hydroxide and silicic acid precipitates are left on filter paper for some time, they set into gels.

Application of gels:1.For making cheap soaps with silicate gels2.Curds and edible jellies are forms gels3.Boot polishes and animal tissue have the gel structure.4.In conversion of alcohol into solid fuel by forming a gel with calcium oxalate.

  Colloids have uses in our daily life as well as in various industrial processes. Some of the applications where colloids are present are listed below.

      Pharmaceutical industry makes use of colloidal solution preparation in many medicines. A wide variety of medicines are emulsions. An example is Cod Liver Oil      Paint industry also uses colloids in the preparation of paints.

Applications of colloids

Applications of colloids 1. Electroplating of rubber: Rubber is a colloidal suspension of negatively charged

particles in water. By electrophoresis, it can be made to deposit on various tools. These negative particles migrate towards anode and get deposited on it during electrolysis.

2. Leather tanning: The coagulating ability of colloids is used in leather tanning. Leather is a positively charged colloid when mixed with wood, mutual coagulation takes place and leather surface gets hardened.

3. Chrome tanning: Leather can be subjected to chrome tanning when hydrated chromic oxide penetrates into leather under the influence of electric field.

4. Medicine: The ease of adsorption and assimilation of colloids makes their use in a number of medicinal and pharmaceutical preparations where colloidal gold, iron, calcium, etc., are administered (orally or injected) to raise the vitality of human system. Trivalent Al+3 and Fe+3 colloids are used in the coagulation of blood. Many skin ointments consist of physiologically active components dissolved in oil and made emulsion with water. Penicillin and streptomycin antibiotics are produced in colloidal form suitable for injection. and scatter the light of blue colour. This is an application of the Tyndall effect.

5. Food: Many food materials are colloidal in nature. For example, butter-milk (emulsion of fat in water), cheese, fruit jelly, eggs, whipped cream, protoplasm, blood, etc. Ice cream is a dispersion of ice in cream. Bread is a dispersion of air in baked dough.

6. Artificial rain is due to the aggregation of minute colloidal particles. Clouds are charged particles of water dispersed in air. Electrified sand is thrown into clouds to get rain.

7. Blue colour of the sky: The outer atmosphere contains colloidal dust particles dispersed in air. As the rays of the sun strike the colloidal particles, they absorb sunlight

SMOKE PRECIPITATIONREMOVAL OF DIRT FROM SEWAGEPURIFICATION OF WATERELECTROPLATING OF RUBBERLEATHER TANNINGARTIFICIAL RAINSIN WARFARE

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