Types of Colloids - TopperLearning

CHEMISTRY SURFACE CHEMISTRY

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Types of Colloids

Colloids

Differences between True Solutions, Colloidal Solutions and Suspensions

PROPERTY

TRUE SOLUTIONS

COLLOIDAL SOLUTIONS

SUSPENSIONS

Nature

Homogeneous

Heterogeneous

Heterogeneous

Particle size

<10−9

m

Between 10−9

and 10−6

m

>10−6

m

Filterability

Pass through ordinary

filter paper and animal

membrane

Pass through ordinary filter

paper but not through

animal membrane

Do not pass through

ordinary filter paper and

animal membrane

Settling

Do not settle

Do not settle

Settle on standing

Visibility

Particles are invisible

Scattering of light is

observed under ultra

microscope

Particles are visible to the

naked eye or under a

microscope

Diffusion

Diffuse quickly

Diffuse slowly

Do not diffuse

Appearance

Clear and transparent

Translucent

Opaque

Colloids: Non-crystalline substances which do not diffuse or diffuse at a very

slow rate through the parchment membrane in the dissolved state

Colloidal state of matter: The state in which the size of the particle is such that

it can pass through filter paper but not through the animal or vegetable

membrane



Dispersed Phase and Dispersion Medium

The dispersed phase is similar to that of the solute and the dispersion medium is similar to that of

the solvent in true solutions.

Types of Colloids

Colloids can be classified into three types:

A] Based on the physical state of the dispersed phase and dispersion medium:

1. There are eight types of colloids depending on the physical state of the dispersed phase

and dispersion medium—either they are solids, liquids or gases.

2. A gas mixed with a gas is not a colloidal system.

3. Of these eight types, sols, gels and emulsions are some common types.

DISPERSED PHASE

DISPERSION MEDIUM

NAME

EXAMPLES

Solid Solid Solid sol Coloured glass, gem stones

Solid Liquid Sol Paint, cell fluids, muddy water

Solid Gas Aerosol Smoke, dust

Liquid Solid Gel Cheese, butter, jelly

Liquid Liquid Emulsion Milk, hair cream

Liquid Gas Aerosol Fog, mist, cloud, insecticide

spray

Gas Solid Solid foam Pumice stone, foam rubber

Gas Liquid Foam Froth, whipped cream, soap

lather

Dispersed phase: Substance distributed in the dispersion medium in the form of

colloidal particles

Dispersion medium: Medium in which the substance is dispersed in the form of

colloidal particles



B] Based on the nature of interaction between the dispersed phase and the dispersion medium:

Colloids are classified into two categories—lyophilic and lyophobic.

(a) Lyophilic colloids:

1. They are also called intrinsic colloids. Examples: Gum, gelatine, starch, rubber

2. If we remove the dispersed phase from the dispersion medium, the sol can be made

again by remixing the dispersed phase with the dispersion medium; hence, these are

called reversible sols.

3. These sols are stable and cannot be precipitated.

(b) Lyophobic colloids:

1. They are also called extrinsic colloids. Example: Metals and their sulphides

2. These sols are easily precipitated and therefore are unstable.

3. If the dispersed phase is separated from the dispersion medium, then the sol cannot

be regained by the addition of the dispersion medium. Hence, they are called

irreversible sols.

C] Based on the type of particles of the dispersed phase:

They are classified into three types:

(a) Multimolecular colloids:

1. When a substance is dispersed in the dispersion medium, a large number of atoms

or smaller molecules of the substance (with diameters less than 1 nm) aggregate to

form species having size in the colloidal range. The species ; thus formed are

known as multimolecular colloids.

2. Examples: Gold sols, sulphur sols

3. Particles in multimolecular colloids are held together by van der Waals’ force of

attraction.

Lyophilic colloids: Substances which when mixed with a suitable liquid as the

dispersion medium directly form a colloidal sol

Lyophobic colloids: Substances which when mixed with the dispersion

medium do not form the colloidal sol but their sols can be prepared by special

methods



(b) Macromolecular colloids:

1. When substances with large molecules and molecular masses are dissolved in a

suitable liquid, they form a solution in which the molecules of the substance have a

size in the colloidal range. Such substances are known as macromolecular colloids.

2. These are usually polymers with very high molecular masses. Examples: Starch,

cellulose, proteins, enzymes, gelatin, nylon, polythene, synthetic rubber

3. These are stable and resemble true solutions.

(c) Associated colloids (Micelles):

1. Associated colloids are substances which behave as normal, strong electrolytes

when dissolved in a medium at low concentration but exhibit the colloidal state due

to the formation of coagulated particles at higher concentrations.

2. These coagulated particles are called micelles. Formation of micelles takes place

only above a particular temperature called Kraft temperature (Tk) and above a

particular concentration called critical micelle concentration (CMC).

3. Examples: Soap and synthetic detergents

Mechanism of Micelle Formation

Consider the example of soap. Soap is the sodium salt of higher fatty acids and represented as

RCOONa. Examples: Sodium stearate, sodium palmitate

When soap is dissolved in water, it dissociates into RCOO− and Na

+ ions. The RCOO

− consists of

two parts—a non-polar, long hydrocarbon chain R (which is hydrophobic and called the tail) and

the polar group COO−

(which is hydrophilic and called the head).

The RCOO− ions are present on the surface with their polar group in water and their non-polar

group away from water and remain at the surface.

At higher concentration, these ions do not remain on the surface but are pulled into the bulk of

solution. Because of this, RCOO− ions coagulate to form a spherical shape with their hydrocarbon

chains pointing towards the centre and the COO− part outwards on the surface of the sphere. This

coagulate formed is called ionic micelle.

Each of these micelles contains up to 100 ions.



Stearate ions present on the surface of water at

low concentration of soap solution

Stearate ions pulled together to form a

spherical ionic micelle inside the bulk of water

at high concentration (CMC)

Cleansing Action of Soap

Some oil particles stick to the surface of clothes. When it comes in contact with soap solution, the

stearate ions arrange themselves around it in such a way that the hydrophobic part of the stearate

ions are in the oil and the hydrophilic part heading outside the oily droplets.

Because the hydrophilic part is polar, these polar groups interact with water molecules present

around the oil droplet.

So, the oil droplet is pulled away from the surface of clothes into water to form ionic micelle. This is

then washed out with excess of water.

The sheath of negative charge around the oil droplet prevents them from coming together and

forming coagulates.

Oil on a surface of cloth

Stearate ions arranged around

the oil droplet

Ionic micelle formed surrounded

by a sheath of negative charge



Preparation of Colloids

A] Dispersion or disintegration methods:

These methods involve the breaking of larger particles into colloidal size.

(a) Mechanical disintegration:

1. This is carried out in a colloid mill or ball mill or ultrasonic disintegrator.

2. A colloid mill consists of two steel discs with a small gap in between and is capable

of rotating in the opposite direction at a very high speed.

3. A suspension of substance in water is injected into the mill. The size of the

suspension particles is reduced to the colloidal size.

Colloid mill

(b) Electro-disintegration (Bredig’s method):

1. This method is mainly useful for obtaining colloidal solutions of metals such as gold,

silver and platinum.

2. An electric arc is struck between the two metallic electrodes suspended in a trough

of water.

3. The high heat of the arc converts the metal into vapours which are condensed

immediately in the ice cold water bath giving particles of colloidal size.



Bredig’s method

(c) Peptisation:

1. When an electrolyte is added to a freshly precipitated substance, the particles of the

precipitate preferentially absorb one particular type of ions of the electrolyte.

2. As a result, they get dispersed because of electrostatic repulsions. This gives

particles of a colloidal size.

3. For example, peptisation of freshly precipitated ferric hydroxide with ferric chloride

solution.

Preparation of colloids by peptisation

Peptisation: Process of converting a fresh precipitate into colloidal particles by

shaking it with the dispersion medium in the presence of a small amount of a

suitable electrolyte. The electrolyte added is called peptising agent.



B] Condensation or aggregation methods:

(a) By chemical reaction:

1. By double decomposition:

When H2S is passed through a dilute solution of arsenious oxide in water, a colloidal

solution of arsenious sulphide is obtained.

2 3 2 2 3 2Colloidal solution

As O 3H S As S 3H O

2. By reduction:

Gold, silver and platinum are obtained in the colloidal form by the reduction of dilute

solutions of their salts by a suitable reducing agent.

3 2 4Gold Sol

2AuCl 3SnCl 2Au 3SnCl

3. By oxidation:

Sulphur is obtained in the colloidal form when H2S is passed through a solution of an

oxidising agent like bromine water.

2 2H S Br S 2HBr

4. By hydrolysis:

Hydroxide sols are obtained by boiling the solution of their corresponding chlorides.

3 2 3Colloidal sol

FeCl 3H O Fe OH 3HCl

(b) By exchange of solvent:

Substances such as sulphur and phosphorus are more soluble in alcohol but less soluble in

water. If their alcoholic solutions are poured in water, then colloidal solutions of sulphur and

phosphorus are formed.

(c) By condensing vapours of substance into the solvent:

Colloidal solutions of sulphur and mercury in water are prepared by passing their vapours in

cold water containing a small amount of stabilising agent such as ammonium sulphate.

(d) By excessive cooling:

A colloidal solution of ice in an organic solvent such as ether is obtained by freezing a

mixture of the solvent and water.



Purification of Colloidal Solutions

A] Dialysis:

1. Separation of crystalloids from colloids is based on the principle that particles of

crystalloids pass through the animal membrane or parchment paper but particles of

colloids do not.

2. The apparatus used for separation is called a dialyser.

3. A parchment paper is turned into a bag with a funnel fitted in the mouth of the bag

for the addition of impure sol.

4. The impure sol is introduced into a bag which is then suspended into a vessel

containing distilled water.

5. After sometime, all the crystalloids in the solution pass out and the colloids are left

behind.

6. The distilled water is renewed from time to time to avoid the accumulation of

crystalloids; as otherwise, they will again start diffusing back into the bag.

7. If an electric field is applied around the membrane, then the process will be faster.

Dialysis

Purification of colloidal solutions: Process of reducing impurities of

electrolytes to the minimum required level

Dialysis:

Process of removing a dissolved substance from a colloidal solution by means

of diffusion through a suitable membrane.



Electro-dialysis

B] Ultrafiltration:

1. In this method, a special type of filter paper is used which is made by treating ordinary filter

paper with collodion or gelatin solution followed by hardening by dipping in formaldehyde

solution.

2. The collodion solution is 4% solution of nitrocellulose in a mixture of alcohol and ether.

3. As a result of this treatment, the pore size of the filter paper is reduced and the filter paper

obtained is called ultrafilter.

4. By using this ultrafiltration, solute impurities of different sizes can be effectively removed.

5. The sol is poured over the ultrafilter which allows the solution of the electrolyte to pass

through but retains the colloidal particles in the form of slime.

6. Slime in contact with water disperses spontaneously to form a colloidal system.

C] Ultra-centrifugation:

1. Impure sol is taken in a tube which is placed in an ultracentrifuge.

2. In this device, the tube is rotated at a very high speed.

3. As a result, the colloidal particles settle at the bottom of the tube and the crystalloids and

other soluble impurities remain in the solution.

4. This solution is decanted and the colloid particles are remixed with the dispersion medium

to give the pure colloidal sol.

Types of Colloids - TopperLearning

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