COLLOIDS

BY
DEVI PRIYA
SUGATHAN
MSc
 COLLOIDS

A colloid is a substance microscopically

dispersed throughout another substance.

The word colloid comes from a Greek

word 'kolla', which means glue thus
colloidal particles are glue like
substances.

These particles pass through a filter paper

but not through a semipermeable
membrane.

Colloids can be made settle by the process

of centrifugation.
The colloidal system consist of two phases:
A dispersed phase (A
discontinuous phase )
A dispersion medium ( A
continuous phase )

The dispersed-phase particles have a diameter of

between approximately 1nm 100nm .
Such particles are normally invisible in an optical,
though their presence can be confirmed with the
use of an ultramicroscope or an electron
microscope.
SOLUTIONS
Made up of particles or solutes and a solvent
The solvent part of the solution is usually a
liquid, but can be a gas.
The particles are atoms, ions, or molecules that
are very small in diameter.

COLLOIDAL MIXTURE
Has particles that are not as small as a solution
and not as large as a suspension.
The particles are intermediate in size.

SUSPENSIONS
Made up of particles and a solvent
its particles are larger than those found in a
solution.
The particles in a suspension can be distributed
throughout the suspension evenly by shaking the
mixture.
 Comparison of the Properties of
Solutions, Colloids, And Suspensions
Property True Solution Colloid Suspension
Particle Size Less than 1 nm 1 to 100 nm More than 100 nm

Appearance Clear Cloudy Cloudy

Homogeneity Homogeneous Homogeneous or Heterogeneous

Heterogeneous
Transparency Transparent but Often translucent Often opaque but
often coloured and opaque but can can be translucent
be transparent
Separation Does not separate Can be seperated Separates or settles

Filterability Passes through filter Passes through filter Particles do not

paper paper pass through filter
paper
 Examples of colloids are milk, synthetic
polymers, fog,
blood, jam, shoe polish, smoke, etc.
 The following forces play an important role in
the interaction of
colloid particles:

EXCLUDED VOLUME REPULSION IN

LIQUID THEORY :
In liquid state theory, the 'excluded
volume' of a molecule
is the volume that is inaccessible to
other molecules in
the system as a result of the presence
of the first
molecule.

The excluded volume of a hard sphere

is eight times its
VAN DER WAALS FORCE :
It is the sum of the attractive or
repulsive
forces between molecules other than
those due
to covalent bonds, the hydrogen bonds,
or the
electrostatic interaction of ions with one
another
ELECTROSTATIC INTERACTION
or with neutral molecules or charged :
molecules.
Colloidal particles often carry an electrical
charge and
therefore attract or repel each other.

The charges of both the continuous and

the dispersed phase,
as well as the mobility of the phases are
 STERIC
Steric effects arise
FORCES : from the fact
that each atom within a molecule
occupies a certain amount of
space.

If atoms are brought too close

together, there is an associated
cost in energy due to overlapping
electron clouds (Pauli or Born
repulsion), and this may affect
the molecule's preferred shape
(conformation) and reactivity.
 CLASSIFICATION OF COLLOIDS
Based of physical state of dispersed phase an dispersion medium.

Based of nature of interaction between dispersed phase and dispersion

medium.

Based on molecular size in the dispersed phase.

Based on appearance of colloids.

Based on electric charge on dispersion phase.

Based on physical state of dispersed phase and
dispersion medium
 BASED ON NATURE OF INTERACTION BETWEEN
DISPERSED PHASE AND DISPERSION MEDIUM

LYOPHILIC COLLOIDS

Colloidal solution in which the dispersed phase has a

great affinity for the dispersion medium.
They are also termed as intrinsic colloids.
Such substances have tendency to pass into colloidal
solution when brought in contact with dispersion
medium.
If the dispersion medium is water, they are called
hydrophilic or emulsoids.
The lyophilic colloids are generally self- stabilized.
Reversible in nature and are heavily hydrated.
Example of lyophilic colloids are starch, gelatin,
rubber, protein etc.
 LYOPHOBIC COLLOIDS
Colloidal solutions in which the dispersed phase has no
affinity to the dispersion medium.
These are also referred as extrinsic colloids.
Such substances have no tendency to pass into colloidal
solution when brought in contact with dispersion medium.
The lyophobic colloids are relatively unstable.
They are irreversible by nature and are stabilized by
adding small amount of electrolyte.
They are poorly hydrated.
If the dispersion medium is water, the lyophobic colloids
are called hyrophobic or suspenoids.
Examples: sols of metals like Au, Ag, sols of metal
hyroxides and sols of metal sulphides.
 Based on molecular size in the dispersed phase.

MULTIMOLECULAR
COLLOIDS
Individual particles of the dispersed phase
consists of aggregates of atoms or small
molecules having diameter less than 10-7cm .
The particles are held by weak vander waals
forces.
Example; gold sol, sulphur sol

MACROMOLECULAR
COLLOIDS
The particles of dispersed phase are sufficiently
large in size enough to be of colloidal solution.
These are called Natural Polymers.
SSOCIATED COLLOIDS
These colloids behave as normal electrolytes
at low concentrations but behave as colloids
at higher concentrations.
These associated colloids are also referred to
as micelles.
Sodium stearate (C18H35NaO2)behave as
electrolyte in dilute solution but colloid in
higher concentrations.
Examples: Soaps , higher alkyl sulphonates ,
polythene oxide.
 HOW DOES SOAP WORK?
When greasy dirt is mixed with
soapy water, the soap molecules
arrange themselves into tiny
clusters called micelles.

The water-loving (hydrophilic) part

of the soap molecules sticks to the
water and points outwards,
forming the outer surface of the
micelle.

The oil-loving (hydrophobic) parts

stick to the oil and trap oil in the
center where it can't come into
contact with the water. With the oil
tucked safely in the center, the
 Based on appearance of colloids
SOLS

When a colloidal solution appears as fluid.

The sols are generally named as dispersion
medium.
When the dispersion medium is water, the
sol is known as hydrosol or aquosol.
When the dispersion medium is alcohol or
benzene it is called alcosol and benzosol
respectively.

GELS

When a colloidal solution appear as solid.

The rigidity of gel varies from substance to
substance.
Examples : jelly, butter, cheese, curd.
 BASED ON ELECTRICAL CHARGE ON
DISPERSION PHASE

POSITIVE COLLOIDS
When dispersed phase in a colloidal
solution carries a positive charge.

Examples : Metal hyroxides like Fe(OH)3,

Al(OH)2, methylene blue sol etc.

NEGATIVE COLLOIDS

When dispersed phase in a colloidal

solution carries a negative charge.

Examples : Ag sol, Cu sol

 SEPERATION OF COLLOIDS

Mechanical Dispersion

Electrical dispersion or Bredigs Arc Method

Peptisation

Condensation Method
A) Mechanical dispersion:
In this method,

The substance is first ground to coarse particles.

It is then mixed with the dispersion medium to get

a suspension.

The suspension is then grinded in colloidal mill.

It consists of two metallic discs nearly touching

each other and rotating in opposite directions at a
very high speed about 7000 revolution per minute.

The space between the discs of the mill is so

adjusted that coarse suspension is subjected to great
shearing force giving rise to particles of colloidal size.

Colloidal solutions of black ink, paints, varnishes,

dyes etc. are obtained by this method.
(B) By electrical dispersion or Bredigs arc method:
This method is used to prepare sols of
platinum, silver, copper or gold.

The metal whose sol is to be prepared is made

as two electrodes which immerge in dispersion
medium such as water etc.

The dispersion medium is kept cooled by ice.

An electric arc is struck between the

electrodes.

The tremendous heat generated by this method

give colloidal solution.

The colloidal solution prepared is stabilized by

adding a small amount of KOH to it.
(C) By peptisation:
The process of converting a freshly prepared precipitate into colloidal form
by the addition of suitable electrolyte is called peptisation.

Cause of peptisation is the adsorption of the ions of the electrolyte by the

particles of the precipitate.

The electrolyte used for this purpose is called peptizing agent or stabilizing
agent.

Important peptizing agents are sugar, gum, gelatin and electrolytes.

(D) Condensation method

1) By change of physical state:

In condensation method, the smaller
particles of the dispersed phase are
aggregated to form larger particles of
colloidal dimensions.

Some important condensation

methods are described below:

a) Solutions of substances like mercury

and sulphur are prepared by passing
their vapours through a cold water
containing a suitable stabilizer such as
ammonium salt or citrate.
b)By excessive cooling:

A colloidal solution of ice in an organic solvent like ether or

chloroform can be prepared by freezing a solution of water in
solvent.

The molecules of water which can no longer be held in solution,

separately combine to form particles of colloidal size.

c) By exchange of solvent:
Colloidal solution of certain substances such as sulphur, phosphorus which are soluble
in alcohol but insoluble in water can be prepared by pouring their alcoholic solution in
excess of water.

For example alcoholic solution of sulphur on pouring into water gives milky colloidal
solution of sulphur.
d) Chemical methods:
Colloids can be prepared by following chemicals methods..

1) Oxidation:
Addition of oxygen and removal of hydrogen is called oxidation.

For example: Colloidal solution of sulphur can be prepared by oxidizing an aqueous

solution of H2S with a suitable oxidizing agent such as bromine water.

H2S + Br2 2HBr + S

2H2S + SO2 2H2O + 3S

2)Reduction:

Addition of hydrogen and removal of oxygen is called

reduction.

For example: Gold sol can be obtained by reducing a

dilute aqueous solution of gold with stannous chloride.

2AuCl3 + 3SnCl2 3SnCl4 + 2Au

3) Hydrolysis:

It is the break down of water.

Sols of ferric hydroxide and aluminium hydroxide can be
prepared by boiling the aqueous solution of the corresponding
chlorides.
For example.
FeCl3 + 3H2S Fe(OH)3 + 3HCl
4) Double Decompostion

The sols of inorganic insoluble salts such as arsenous sulphide, silver halide
etc may be prepared by using double decomposition reaction.

For example: Arsenous sulphide sol can be prepared by passing H2S gas
through a dilute aqueous solution of arsenous oxide.

As2O3 + 3H2S As2S3(OH)3 + 3H2O

 DONNAN MEMBRANE EQULIBRIUM
When two solutions containing
electrolytes are seperated by
membrane which is
impermeable to one of the ions
of the electrolytes.
It was found that at equilibrium
an unequal distribution occurs
for the permeable ion on both
sides of the membrane.
At equilibrium, the osmotic
pressure of two solutions will be
different and if two reference
electrodes such as calomel
electrodes are put into the two
solutions then a difference of
potential will be set up.
This type of equilibrium is
known as DONNAN
 COMPONENTS OF A SOLUTION
INTERNAL SOLUTION

It is a solution of an electrolyte
containing non penetrating ions of
about colloidal dimension and ions
small enough to be penetrable.

EXTERNAL SOLUTION
It is a solution of
electrolyte containing both penetrable
ions.

SEMIPERMEABLE MEMBRANE
It seperates the
two solutions and is permeable to all
 Purification of colloids

There are three common methods used

for purification of colloids:

Dialysis
Electrodialysis
Ultra filteration
 Dialysis
The process of separating the particles of
colloid from those of crystalloid, by means of
diffusion through a suitable membrane.

Its principle is that colloidal particles can

not pass through a cellophane membrane
while the ions of the electrolyte can pass
through it.

The impurities slowly diffused out of the

bag leaving behind pure colloidal solution.

The distilled water is changed frequently

to avoid accumulation of the crystalloids
otherwise they may start diffusing back into
the bag.

Dialysis can be used for

removing HCl from the ferric hydroxide sol.
Electro dialysis
The process of dialysis is very
slow.

The process is speeded up by

application of electrical
potential.

This is called electro dialysis.

Application of electro dialysis:

Artificial kidney machine make use of

electro dialysis.
Ultra filtration
Ultra filtration is a process of high
pressure
filtration through a semi
permeable membrane
in which colloidal particles are
retained while
the small sized solutes and the
solvent are
forced to move across the
membrane by
hydrostatic pressure forces.
 Application of ultra filtration:

Ultra filtration is a vital process that takes place in the

kidneys.
PROPERTIES OF COLLOIDS
 PHYSICAL PROPERTIES OF
COLLOIDS
Heterogeneity: Colloidal solutions consist of two phases-dispersed phase
and dispersion medium.

Visibility of dispersed particles: The dispersed particles present in them

are not visible to the naked eye and they appear homogenous.

Filterability: The colloidal particles pass through an ordinary filter paper.

However, they can be retained by animal membranes, cellophane
membrane and ultrafilters.

Stability: Lyophilic sols in general and lyophobic sols in the absence of

substantial concentrations of electrolytes are quite stable.

Colour: The colour of a colloidal solution depends upon the size of colloidal
particles present in it. Larger particles absorb the light of longer wavelength
and therefore transmit light of shorter wavelength.
 OPTICAL PROPERTIES
OFCOLLOIDS
TYNDALL EFFECT

When an intense converging beam of light is passed

through a colloidal solution kept in dark, the path of
the beam gets illuminated with a bluish light.

This phenomenon is called Tyndall effect and the

illuminated path is known as Tyndall cone.

The Tyndall effect is due to the scattering of light by

colloidal particles.

Tyndall effect is not exhibited by true solutions. This is

because the particles present in a true solution are too
small to scatter light.

Tyndall effect can be used to distinguish a colloidal

solution from a true solution. The phenomenon has
also been used to devise an instrument known as ultra
microscope. The instrument is used for the detection
of the particles of colloidal dimensions.
 MECHANICAL PROPERTIES OF
COLLOIDS
BROWNIAN MOVEMENT
The continuous zigzag movement of the colloidal
particles in the dispersion medium in a colloidal
solution is called Brownian movement.

Brownian movement is due to the unequal

bombardments of the moving molecules of
dispersion medium on colloidal particles.

The Brownian movement decreases with an

increase in the size of colloidal particle. This is why
suspensions do not exhibit this type of movement.
 ELECTRICAL PROPERTIES OF
COLLOIDS

ELECTROPHORESIS

The movement of colloidal particles

towards a particular electrode under the
influence of an electric field.

If the colloidal particles carry positive

charge, they move towards cathode when
subjected to an electric field and vice
versa.
 ELECTROSMOSIS

The movement of dispersion medium under the influence of an electric

field in the situation when the movement of colloidal particles is
prevented with the help of a suitable membrane.

During electrosmosis, colloidal particles are checked and it is the

dispersion medium that moves towards the oppositely charged electrode.
 COAGULATION OR
FLOCCULATION

Coagulation may be
defined as the
phenomenon involving the
precipitation of a colloidal
solution on addition of an
electrolyte.

Hardy-Schulze rule
The greater is the
valence of the oppositely
charged ion of the
 Flocculation value: The coagulating power of an electrolyte is usually
expressed in terms of its flocculation value which may be defined as the
minimum concentration (in millimoles per litre) of an electrolyte required to
cause the coagulation of a sol.
A smaller flocculation value indicates the greater coagulating power of the
electrolyte. Thus,

Coagulating power 1
Flocculation value

The coagulation of colloidal solution can also be achieved by any of the

following methods.
By electrophoresis
By mixing two oppositely sols
By persistent dialysis
 Biological significance of colloidal systems

Protoplasm is a Colloidal
system:
Protoplasm is
colloidal in
nature.

Amoeboid movements:
Amoeboid
movements in
amoeba occur
 Fruits:
Turns store a large amount of
proteins and
starch. They also exhibit
colloidal properties.

Blood:
High plasma protein forms a
colloidal
system in blood. This
system maintains the
pH and osmotic
concentration of blood.
 ADVANTAGES OF COLLOIDS
Colloids allow the dispersion of normally insoluble
materials, such as metallic gold or fats. These can
then be used more easily, or absorbed more easily.

Colloidal gold, for example, can be used in

medicine to carry drugs and antibiotics, because it is
highly non-reactive and non-toxic.

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.
In milk, the colloidal suspension of the fats prevents
the milk from being thick, and allows for easy
absorption of the nutrients.

Sewage water contains particles of dirt, mud etc.

which are colloidal in nature and carry some electrical
charge. These particles may be removed by using the
phenomenon of electrophoresis.

The sky is the empty space around earth and as such

has no colour. It appears blue due to the scattering of
light by the colloidal dust particles present in air
(Tyndall effect).

Asphalt emulsified in water and is used for building

roads.
The sugar present in milk produces lactic acid on
fermentation. Ions produced by acid, destroy the
charge on the colloidal particles present in milk,
which then coagulate and separate as curd.

Soap solution is colloidal in nature. It removes the

dirt particles either by adsorption or by emulsifying
the greasy matter sticking to the cloth.

Large numbers of food particles which we use in

our daily life are colloidal in nature.
Example: Milk, butter, & ice cream etc