Classification and Phases of Colloids Class 12 Notes | EduRev

Chemistry Class 12

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Class 12 : Classification and Phases of Colloids Class 12 Notes | EduRev

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Surface Chemistry

Surface chemistry deals with phenomena that occur at the surfaces or interfaces. Many important phenomena, noticeable among these being corrosion, electrode processes, heterogeneous catalysis, dissolution and crystallization occur at interfaces. The subject of surface chemistry finds many applications in industry, analytical work and daily life situations.

Distinction Between True Solution, Colloidal Solution  And Suspension :

 

Name of property

True solution

Colloidal solution

suspension

1. Particle size

< 10-7 cm

10-7 to 10-5 cm

> 10-5 cm

2. Visibility

Not visible with any of the optical means

Images are visible under Ultra microscope

visible with naked eye

3. Separation with filter

Not possible

Not possible

Possible

4. Diffusion

Diffuses readily

Diffuse very slowly

Does not diffuse

5. Nature

Homogeneous

Heterogeneous

Heterogeneous

6. Setting

Does not settle

Settle under centrifuge

Settles under influence of gravity

7. Tyndall effect

Does not show

Shows

May or may not show

8. Brownian movement

Does not show

Shows

may show

 

Particle size :

Classification and Phases of Colloids Class 12 Notes | EduRev

Phase of colloids :

A colloidal system is heterogeneous in character. It consists of two phases, namely a dispersed phase and a dispersion medium.

(a) Dispersed Phase (DP) : It is the component present in small proportion and is just like a solute in a true solution. For example, in the colloidal state of sulphur in water, the former acts as a dispersed phase.

(b) Dispersion Medium (DM) : It is normally the component present in excess and is just like a solvent in a solution.

The particles of the dispersed phase are scattered in the dispersion medium in a colloidal system.

Classification of colloids :

Colloids can be can be classified in a number of ways based upon some of their important characteristics.

(1) Physical state of Dispersed Phase & Dispersion Medium:

Depending upon whether the dispersed phase and the dispersion medium are solids, liquids or gaseous, eight types of colloidal system are possible. A gas mixed with another gas forms a homogeneous mixture and not a colloidal system. Typical examples of various type along with their characteristic names are given in table.

Common Colloidal System

DP

DM

Colloidal system

Examples

Gas

Liquid

Foam or froth

Soap sols, lemonade froth, whipped cream.

Gas

Solid

Solid foam

Pumice stone, styrene, foam, foam rubber.

Liquid

Gas

Aerosols of Liquids

Fog, clouds, fine insecticide spray, mist.

Liquid

Liquid

Emulsions

Milk, hair cream

Liquid

Solid

Gels

Cheese, butter, boot polish, table jellies, curd.

Solid

Gas

Aerosols of Solid

Smoke, dust

Solid

Liquid

Sols

Most paint, starch dispersed in water, gold sol, muddy water, cell fluids, inks.

Solid

Solid

Solid sols

Ruby glass, some gem stones.

 

(2) Based on dispersion medium

1. Water : Hydrosols

2. Alcohol :Alcohols

3. Gases : Aerosols

4. benzene :  benzosol

5. solid :   gel

Some colloids, such as gelatin, can behave both as a sol and a gel. At high temperature and low concentration of gelatin, the colloid is a hydrosol. But at low temperature and high gelatin concentration, the hydrosol can change into a gel.

(3) Based on interaction or affinity of phases : On the basis of the affinity or interaction between the dispersed phase and the dispersion medium, the colloids may be classified into two types :

(i) Lyophilic Colloids : The colloidal system in which the particle of dispersed phase have great affinity for the dispersion medium, are called lyophilic (solvent-loving) colloids. In such colloids, the dispersed phase does not get easily precipitated and the sols are more stable. Such colloidal systems, even if precipitated, may be reconverted to the colloidal state by simply agitating them with the dispersion medium. Hence lyophilic colloids are reversible. When the dispersion medium is water, these are called hydrophilic colloids. Some common examples of lyophilic colloids are gum, gelatin, starch, rubber, proteins, etc.

(ii) Lyophobic colloids : The colloidal system in which the dispersed phase have no affinity for the dispersion medium are called lyophobic (solvent hating) colloids. They are easily precipitated (or coagulated) on the addition of small amounts of the electrolyte, by heating or by shaking. They are less stable and irreversible. When the dispersion medium is water, these are known as hydrophobic colloids. Examples of lyophobic colloids include sols of metals and their insoluble compounds like sulphides and oxides.

The essential differences between the lyophilic and lyophobic colloids are summarised in table.

Difference between Lyophilic and Lyophobic sols

 PropertyLyophilic sols/hydrophilicLyophobic/hdrophobic sols

1.

Nature

Reversible

Irreversible

2.

Preparation

They are prepared very easily by shaking or warming the substance with dispersion medium. They do not required any electrolyte or stabilization.

They are difficult to prepare, Special methods are used. Addition of stabiliser is essential for their stability.

3.

Stability

They are very stable and are not easily coagulated by electrolytes.

They are generally unstable and get easily coagulated on addition of electrolytes.

4.

Charge

Particles carry no or very little charge depending upon the pH of the medium.

Colloidal particles have characteristic charge (positive or negative)

5.

Viscosity

Viscosity is much higher than that of the medium.

Viscosity is nearly the same as that of the medium

6.

Surface Tension

Surface tension is usually less than that of the medium.

Surface tension is nearly the same as that of the medium.

7.

Migration in

electric field

The particles may or may not migrate in an electric field.

The colloidal particles migrate either

towards cathode or anode in an electric field

8.

Solvation

Particles are heavily solvated.

Particles are not solvated.

9.

Visibility

The particles cannot be seen under ultra microscope.

The particles though invisible, can be seen under ultra microscope.

10.

Tyndall effect

Less distinct.

More distinct.

11.

Action of

electrolyte

Large amount of electrolyte is required to cause coagulation.

Small amount of electrolyte is sufficient to cause coagulation.

12.

Examples

Mostly organic substances e.g. starch, gums, proteins, gelatin, rubber etc.

Generally inorganic substance e.g., metal sols, sulphides and oxides sols.

 

(4) Based on type of particles of the dispersed phase : Depending upon the molecular size, the colloidal system has been classified into three classes :

(i) Multimolecular colloids : The multimolecular colloidal particles consists of aggregate of atoms of small molecules with diameters less than 10-9 m or 1 nm.

For example, a sol. of gold contains particles of various sizes having several atoms. A sol. of sulphur consists of particles containing a thousand or so S2 molecules. These particles are held together by vander Waal's forces. These are usually lyophobic sols for example gold sol.

(ii) Macromolecular colloids : The macromolecular colloidal particles themselves are large molecules. They have very high molecular weights varying from thousands to millions. These substances are generally polymers. Naturally occurring macro molecules are such as starch, cellulose and proteins. Artificial macro molecules are such as polyethylene, nylon, polysyrene, dacron, synthetic rubber, plastics, etc. The size of these molecules are comparable to those of colloidal particles and therefore, their dispersion known as macro molecular colloids. Their dispersion also resemble true solutions in some respects. For example - Starch, cellulose, proteins and enzymes.

(iii) The associated colloids or miscelles : These colloids behave as normal electrolytes at low concentrations but colloids at higher concentrations. This is because at higher concentrations, they form aggregated (associated) particles called miscelles. Soap and synthetic detergents are examples of associated colloids. They furnish ions which may have colloidal dimensions.

RCOONa  -  RCOO—  Na 

Sod. Stearate soap   = (R = C17H35)

The long-chain RCOO ions associates or aggregate at higher concentrations and form miscelles and behave as colloids. They may contain 100 or more molecules.

Sodium stearate C17H35COONa is an example of an associated colloid. In water it gives Na  and sterate, C17H35COO ions. These ions associate to form miscelles of colloidal size.

Colloids which behave as normal electrolytes at low concentration, but exhibit colloidal properties at higher concentration due to the formation of aggregated particles called micelles are referred to as associated colloids. The micelles are formed by the association of dispersed particles above a certain concentration and certain minimum concentration is required for the process of aggregation to take place. The minimum concentration required for micelle formation is called micellisation concentration (CMC) and its value depends upon the nature of the dispersed phase. For soaps CMC is 10-3 - 10-4 M.

Mechanism of Micelle Formation :

Micelles are formed by surface active molecules called surfactants such as soaps and detergents. These molecules have lyophilic group at one end and a lyphobic group at the other end. Let us take the example of a soap (say sodium oleate, C17H33COONa  ). The long hydrocarbon part of oleate radical (C17H33 -) is lyophobic end while COO part is lyophilic end. When the concentration of the solution is below its CMC, sodium oleate behaves as a normal electrolyte and ionises to give C17H33COO and Na  ions. When the concentration exceeds CMC, the lyophobic part starts receding away from the solvent and tends to approach each other. However, the polar COO ends tends to interact with the solvent (water). This finally leads to the formation of bigger molecules having the dimensions of colloidal particles. Thus 100 or more oleate ions are grouped together in a spherical way keeping their hydrocarbon parts inside and the -COO part remains projected in water.

  Classification and Phases of Colloids Class 12 Notes | EduRev

 

List of surfactants and their critical micelle concentration (CMC)

Surfactant

CMC (g/1)

Temp.(°C)

Anionic

 

 

CH3(CH2)6COONa

6.5 x 101

20

CH3(CH2)10COONa

5.6

20-70

CH3(CH2)7OSO3Na

3.0 x101

25-50

CH3(CH2)11OSO3Na

2.6

25-60

CH3(CH2)5C6H4SO3Na

9.8

75

CH3(CH2)11C6H4SO3Na

4.0 x10-1

50-75

Cationic

 

 

CH3(CH2)9NH2.HCl

8.5

25

CH3(CH2)11NH2HCl

2.7

30-50

CH3(CH2)7N(CH3)3Br

7.8 x 101

25

CH3(CH2)11N(CH3)3Br

5.4

25

Non-ionic

 

 

CH3(CH2)7C6H11O6

7.3

25

C12H20O9(C16H31O2)2

1.1. x 10-2

20

CH3(CH2)10COOC12H21O10

7.1 x 10-3

50

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