INDEX

Name of the content..................................................Page No.

1) Introduction...................................................................... 01
2) Application of Colloids.................................................... 01
3) Disperesed Phase and Dispersion medium.................... 02-03
4) Types of Colloid................................................................ 03
5) Preparation of Colloids.................................................... 04-05
6) Lyophobic and Lyophilic Sols......................................... 05-06
7) Physical Properties of Colloids....................................... 06
8) Electrical Properties of Colloids..................................... 06-07
9) Optical Properties of Colloids......................................... 07
10) Kinetic Properties of Colloids.................................... 07
11) Purification of colloidal solution................................ 08-09
12) Coagulation.................................................................. 09
13) Peptization.................................................................... 09
14) Emulsion...................................................................... 10
15) Gel................................................................................. 10
16) Importance of colloid................................................... 11

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 COLLOIDS
The term colloid, first used by Graham (1861), has been used for more than a century, but the
art of the preparing and processing colloids had been known for thousands of years. In fact,
colloids, as we understand today, were known, prepared and used by man as early as our
civilization itself. The various paints, and varnishes used in early days, making of earthen
wares from clay and its painting and glazing, preparation of butter and cheese from milk, the
use of wood and timber for various purposes are only a few examples of man’s use of
colloids.

Modern views of colloids and the tremendous development that has taken place in this branch
of physical science owe a great deal to Graham although his classification of colloids is no
longer fully acceptable. But the interest he created amongst the scientists is primarily
responsible for subsequent developments.

A colloid maybe defined as a substance in peculiarly fine state of subdivision dispersed in

another continuous medium giving rise to a large increase in surface area of the dispersed
phase. In other words, we can say that, when the diameter of a particle dispersed in a
solvent range from about 10 Å to 2000Å, the system is then termed as Colloid.

 Application of Colloids:
i. Colloid is used as thickening agent in industrial products such as lubricants, lotions etc.
ii. It is used in the manufacturing of paints and inks, in ball-point pens.
iii. It is used in sewage disposal.
iv. In purification of water.
v. In the cleaning action of soap.
vi. In the artificial Kidney machine.

 Dispersed phase and Dispersion Medium

The phase that is scattered or present in the form of colloidal particles is called dispersed
phase and the medium in which the colloidal particles are dispersed is called the dispersion
medium. For example, in a starch solution, starch represents the dispersed phase while water
represents the dispersion medium. Yet another example is milk where small fat globules are

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present as discrete particles in water. Here the fat globules are the disperse phase and water is
the dispersion medium. In butter, water is the dispersed phase and fat is the dispersion
medium.

Fig: Example of colloids

 Types of Colloids
Disperse phase Disperse medium Type Examples

Gas Liquid Foam Shaving cream, whipped

cream
Gas Solid Solid foam Foam rubber, pumice stone

Liquid Gas Aerosol Fog, mist, cloud

Liquid Liquid Emulsion Hair cream, milk

Liquid Solid Solid emulsion Butter, cheese

(gel)
Solid Gas Smoke Dust, soot in air

Solid Liquid Sol Ink, paint, prepared sol

Solid Solid Solid sol Alloys, ruby glass

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 Preparation of Colloids

There are two principal ways of preparation of colloids:

Dispersion of large particles or droplets to the colloidal dimensions by milling, spraying,

or application of shear (e.g. shaking, mixing, or high shear mixing).

Condensation of small dissolved molecules into larger colloidal particles by precipitation,

condensation, or redox reactions. Such processes are used in the preparation of colloidal
silica or gold.

 Chemical Methods of preparation of Colloids: Hydrophilic or Lyophobic

colloidal solutions can be prepared by various chemical techniques such as:
a. Double Decomposition Technique: When hydrogen sulphide is passed through a
solution of arsenious oxide in distilled water, we get a colloidal solution of
arsenious chloride.
As2O3 + 3H2S → As2S3 + 3H2O

b. Oxidation Technique: A colloidal solution of Sulphur is made to pass through an

aqueous solution of sulphur dioxide. It can also be obtained by passing the gas
through a solution of an oxidization agent such as bromine water as well as nitric
acid.

SO2 + 2H2S → 2H2O + 3S

H2S + [O] → H2O + S

c. Reduction Technique: Another technique of preparing colloidal solutions of metals

such as silver, gold as well as platinum involves the use of reducing agent for
reduction of the salt solutions of these metals. Example of reducing agent include
stannous chloride.
d. Hydrolysis Technique: It involves the use of boiling water to obtain a reduced
solution of ferric chloride.

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 FeCl3 + 3H2O → Fe(OH)3 + 3 HCl
 Physical Methods of Preparation: Various physical methods can also be employed
to obtain colloidal dispersions. Some of them include,
a. Exchange of Solvent: It involves the formation of a colloidal solution of an element
by addition of its alcoholic solution to excess water. This colloidal formation can take
place only when the element is more soluble in alcohol as compared to water.
Example: When an alcoholic solution of sulphur is made to pass through excess water
it yields a colloidal solution of sulfur. This is because the solubility of sulphur is more
in water as compared to alcohol.
b. The Technique Involving Excessive Cooling: This technique involves freezing a
solution of water in organic solvents such as chloroform, ether, etc. to form colloidal
solutions of ice. The molecules of water find it impossible to exist separately combine
to form colloidal molecules.

 Lyophobic and Lyophilic Sols

A colloid can be defined as a phase separated mixture in which one substance one
substance of a microscopically soluble or insoluble dispersed particle is suspended
throughout another substance. Every colloid consists of two parts: the colloidal particles
and the dispersing medium. The colloids can be of two types. These are lyophobic
colloids and lyophilic colloids.
 Lyophilic colloids: Lyophilic colloids are the colloidal solution in which the dispersed
phase or the particles have a very strong affinity with the liquid. The solution of this
colloid is known as lyophilic sol. These can be easily prepared by direct mixing or
heating. The lyophilic sols are reversible in nature. They are quite stable and cannot be
easily coagulated. The affinity of the dispersed particles with the dispersion medium is
high due to the formation of a large number of hydrogen bonds.
 Lyophobic colloids: Lyophobic colloids are the colloidal solution in which the dispersed
phase has very little affinity for the dispersion medium. The solution of this colloid is
known as lyophobic sol and cannot be directly prepared. They are prepared by special
methods. The lyophobic sols are irreversible in nature. They are unstable and can be
easily coagulated by adding a small amount of suitable electrolyte.
The lyophilic sols do not easily coagulate with the addition of a small amount of
electrolyte. However, adding a large amount of electrolyte may be able to do so. Whereas,

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 the lyophobic sols can be easily coagulated with the addition of a small amount of
electrolyte.
 Physical Properties of Colloids

1. The nature of the colloidal solution is heterogeneous, i.e. unlike. These solutions
dwell in two different phases:

 Dispersed medium

 Dispersed phase

2. Despite the fact that colloidal dispersions are unlike in description (nature), the
dispersed fragments are not detectable by the human eye. This is due to the
microscopic size of the particles in the solution.

3. The colour of the colloidal dispersion is determined by particles in the solution based
on their size. The wavelengths of light that are absorbed will be longer if the size of
the particle is large.

4. As a result of its size, the colloidal fragments can easily be passed through a
traditional filter paper. However, these particles can be filtered by using membranes
such as animal, cellophane, and ultrafilters.

 Electrical Properties of Colloids

i. Electrical double layer theory: In this theory, the charge is imparted to the
particles by placing ions which are adsorbed preferentially at immovable points
which form the first layer. The second layer consists of diffused mobile ions. The
charge present on both layers is equal. This two-layer arrangement leads to a
development of potential called zeta or Electrokinetic potential. As a result of this
potential developed across the particles, under the influence of an electric field these
particles move.
ii. Electrophoresis: It is a process in which an electric field is applied to a colloidal
solution which is responsible for the movement of colloidal particles. Depending
upon the accumulation near the electrodes the charge of the particles can be
predicted. The charge of the particles is positive if the particles get collected near a
negative electrode and vice versa.

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iii. Electro-osmosis: It is a process in which the dispersing medium of the colloidal
solution is brought under the influence of an electric field and the particles are
arrested.

Fig: Electrophoresis & Electro-osmosis

 Optical Properties of Colloids

Tyndall’s effect is defined as the phenomenon in which light is scattered by the
colloidal particles. The light is absorbed by the particles present in the solution. Once
the light is absorbed a part of the light gets scattered in all directions. The result of
scattering exhibits this effect.

Fig: Optical Properties

 Kinetic Properties of Colloids
During the observation of the colloidal dispersion under an ultra-microscope, it is
clearly seen that the particles are in a continuous movement in the solution. This

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 random zigzag movement of the particles in the colloidal solution is called the
Brownian effect. This movement is mainly due to the unique bombardment of the
molecules present in the dispersed medium on the colloidal particle.
 Purification of Colloidal Solutions
Freshly prepared colloidal solutions usually contain the impurities of electrolytes.
The presence of electrolytes in smaller concentrations stabilizes a sol but their presence in
large concentration tends to destabilize the colloidal solution.
Therefore, it is necessary to purify colloidal solutions by removing the impurities of
electrolytes present in them.
Following methods are generally used for the purification of colloidal solutions (sols).
i. Dialysis: An animal membrane allows the passage of crystalloids but retains the larger
colloidal particles. This property of animal membranes is utilized for the purification of
sols. The process involved is called dialysis. It may be defined as follows. The process of
separating the impurity particles of true solution dimensions (crystalloids) from an impure
sol by means of diffusion through a suitable membrane such as parchment paper or
cellophane membrane is called dialysis. The apparatus used in this method is called
dialyzer. It consists of a bag made of parchment or cellophane. The bag is filled with the
impure sol to be purified and is suspended in a tank through which pure water is
circulated. The impurities of electrolytes present in the sol diffuse out of the bag leaving
behind pure sol in the bag.
ii. Electrodialysis: Dialysis is a slow process. However, it can be expedited by applying an
electric field. Under the influence of electric field, the impurity ions move faster to the
oppositely charged electrodes and the process gets quickened.This process is referred to
as electrodialysis.

Fig: Dialysis & Electrodialysis

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iii. Ultrafiltration: The pores of an ordinary filter paper are large enough to allow the passage
of both impurity particles as well as colloidal particles. Therefore, an ordinary filter paper
cannot be used for removing the impurities of electrolytes from an impure sol. However,
if the pore size of ordinary filter paper is reduced, it can be used for separating the
impurities from impure sols. This is achieved by treating an ordinary filter paper with
collodion or gelatin followed by its hardening by dipping it in formaldehyde solution.
This treatment reduces the pore size and enables it to check the passage of colloidal
particles through it. Filter papers thus obtained are called ultrafilters. Filtration through
ultrafilters is called ultrafiltration. In ultrafiltration, the ultrafilter is supported over a wire
mesh and the impure sol is poured over it. The impurity particles (electrolytes) pass
through the ultrafilter while the larger colloidal particles are retained. The process is very
slow. However, it can be expedited by applying pressure on sol side or by using a suction
pump on the filtrate side. By using a series of graded ultrafilters, impurities of different
size can easily be removed and it is even possible to separate colloidal particles of
different size from one another.

 Coagulation
Coagulation is the process of particles clumping together when their oppositely charged
particles aggregate together and then gravity causes them to settle on the bottom. Just like
opposite charges on a magnet will pull together, the opposite charges on the particles in
the colloid will pull together, until aggregates that are too large to be held in suspension
are formed. These then fall out of the colloid due to gravity. Example of coagulation that
you're probably already familiar with include blood clotting to stop a cut from bleeding
and gravy thickening as it cools.

 Peptization

Peptization is the process responsible for the formation of stable dispersion of colloidal
particles in the dispersion medium. In other words, it may be defined as a process of
converting a precipitate into a colloidal sol by shaking it with dispersion medium in the
presence of a small amount of electrolyte. In some ways we can think of peptization as the
opposite of coagulation. In order to perform peptization we need an electrolyte, which is

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called a peptizing agent. The precipitate particles adsorb onto the ions of the peptizing agent,
and then electrostatic repulsion (repulsion between two compound with like charges) causes
the precipitate particles combined with the electrolyte ions to mix with the colloidal solution,
thereby forming a stable colloid.

 Emulsion

An emulsion is a colloid of two or more immiscible liquids where one liquid contains a
dispersion of the other liquids. In other words, an emulsion is a special type of mixture made
by combining two liquids that normally don't mix. The word emulsion comes from the Latin
word meaning "to milk" (milk is one example of an emulsion of fat and water). The process
of turning a liquid mixture into an emulsion is called emulsification.

 An emulsion is a type of colloid formed by combining two liquids that normally don't
mix.
 In an emulsion, one liquid contains a dispersion of the other liquid.
 Common examples of emulsions include egg yolk, butter, and mayonnaise.
 The process of mixing liquids to form an emulsion is called emulsification.
 Even though the liquids that form them may be clear, emulsions appear cloudy or
colored because light is scattered by the suspended particles in the mixture.
 Example:
 Oil and water mixtures are emulsions when shaken together. The oil will form drops
and disperse throughout the water.
 Egg yolk is an emulsion containing the emulsifying agent lecithin.
 Cream on espresso is an emulsion consisting of water and coffee oil.
 Butter is an emulsion of water in fat.
 Mayonnaise is an oil in water emulsion that is stabilized by the lecithin in egg yolk.
 The photosensitive side of photographic film is coated with an emulsion of silver
halide in gelatin.

 Gel:

If the sol is concentrated and the discrete particles are present in a state of bridged or cross-
linked structure the system is called gel. Gel is mechanically stable and possesses elasticity.

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Ordinary jelly is an example of gel. All gels are basically sols but all sol are not necessarily
gel.

 Importance of Colloids

The study of colloids, lyophobic, lyophilic, molecular, or colloidal electrolytes is important

not only for the satisfaction of an inquisitive mind but also from the practical point of view.
There is hardly any field where these little particles, with their electric charge and incessant
zigzag motion, do not show their influence and importance. The human body with all its
intricacies is principally composed of colloids. Blood is a dispersion colloid where corpuscles
are dispersed in a liquid medium known as blood serum. The serum itself is a colloidal
solution of a number of molecular colloids of proteins.

Thus blood is a colloidal suspension in another molecular colloid. Digestion of most food we,
eat, are caused by enzymes, again a colloid; the food particles are broken down to the
colloidal dimension or absorption in the body. Even the food we take is mostly colloids,
notable amongst them are the proteins which are associated with the very process of our life.
No less important is milk, butter, and cheese. The garments we wear to protect ourselves are
made of colloids, whether it is made of cotton or Dacron, wool or nylon, rayon or silk. The
paints and varnishes that decorate a multitude of things and protect materials against
corrosion are colloids. The entire family of cosmetics is colloids of different types and finds a
variety of use. The paper we write on, the ink we write with, the ink we print with, are only a
few of many colloids we use every day. Around us, we have our atmosphere, which is an
aerosol, below us we have soil and clay which are alto colloids. Thus we ourselves are
colloids, we live on colloids.

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