2. INCLUDES:
1) Introduction to Colloidal Systems
2) Classification of Colloids
3) Stabilization Of Colloids
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3. Dispersed Phase = Solute
Dispersion Medium = Solvent
Colloid is a mixture in which one substance is divided
into minute particles (colloidal particles) and
dispersed throughout a second substance(dispersion
medium).
The mixture is also called colloidal system, colloidal
solution, or colloidal dispersion.
Examples are : Milk, fog, jellies and much more.
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4. COLLOIDAL PARTICLES are smaller than
SUSPENSION SOLUTES (<1000nm) and bigger than
Solutes of a TRUE SOLUTION (>1nm).
So, for determining a mixture is a Colloid we can see the
particle size of mixtures’ solute.
They cannot be seen by microscope directly but the size
of its particles can be seen by an electron microscope.
Hence the size of colloidal particles ranges in between :
1-1000nm
NOTE: Colloids are not only for liquid mixtures. The
medium could be solid, liquid or gas, and the
dispersed substance can also be solid, liquid or gas.
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5. The colloids are classified on the basis of:
1. State of Dispersion.
2. Nature of Interaction.
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6. a) Dispersion medium is SOLID.
b) Dispersion medium is LIQUID.
c) Dispersion medium is GAS.
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7. When dispersion medium is solid, the dispersed phase
can be solid, liquid or gas. Based on the state of
dispersed phase, colloids are called Solid Sols or Gels.
Different types of colloids and examples of each type are
given here:
Dispersion
Medium
Dispersed
Phase
Names of
Colloids
Examples
are:
SOLID SOLID SOLID SOL GEMSTONE, PEARLS,
COLOURED GLASS.
SOLID LIQUID GEL CHEESE, BUTTER,
JELLY, JAM, SHOE
POLISH.
SOLID GAS SOLID SOL PUMICE STONE,
FOAM, RUBBER.
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MILLING AND BAKING HANDOUTS
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9. When dispersion medium is liquid, the dispersed
phase can be solid, liquid or gas. Based on the state
of dispersed phase, colloids are called Sols,
Emulsions or Foam.
Different types of colloids and examples of each type
are given here:
Dispersion
Medium
Dispersed
Phase
Name of
Colloids
Examples
are
LIQUID SOLID SOLS PAINTS, GUM,
MUDDY WATER.
LIQUID LIQUID EMULSION MILK, HAIR CREAM,
COD-LIVER OIL.
LIQUID GAS FOAM FROTH, SOAP
LEATHER, WHIPPED
CREAM.
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MILLING AND BAKING HANDOUTS
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11. When dispersion medium is gas, the dispersed phase can
be solid or liquid but not the gas.
It is important to note that colloid is a heterogenous
mixture. When gas is mixed with another gas, it forms
a completely miscible homogeneous mixture and not
a colloidal solution
Colloids of gas with dispersed phase as solids or liquids
are called Aerosols. Different types of colloids and
examples of each type are given here:
Dispersion
Medium
Dispersed
Phase
Name of
Colloids
Examples are
GAS SOLID AEROSOLS SMOKE, DUST.
GAS LIQUID AEROSOLS INSECTICIDE, FOG,
CLOUDS.
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13. The stability of colloids is very important as it
helps our daily useful colloids to become
much effective, also it is necessary to remove
useless colloids from our environment.
This stability is achieved by:
Balance of forces
Electrical forces
Interaction with Solvent
Stabilization by Cloaking
Steric Stabilization
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14. It is achieved by weak forces called Vander
Waals Forces.
And not total attraction by the thermal motion
which keep the very small atomic molecules
apart.
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15. Electrical forces keep the colloids dispersed. When particles of colloidal dimension
suspended in a liquid collide with each other, they do so with much smaller kinetic
energies than is the case for gases, so in the absence of any compensating repulsion
forces, we might expect van der Waals or dispersion attractions to win out. This
would quickly result in the growth of aggregates sufficiently large to exceed
colloidal size and to fall to the bottom of the container. This process is
called coagulation.
Each particle with its double layer is more or less electrically neutral. However,
when two particles approach each other, each one "sees" mainly the outer part
[shown here in blue] of the double layer of the other. These will always have the
same charge sign (which depends on the type of colloid and the nature of the
medium), so there will be an electrostatic repulsive force that opposes the
dispersion force attractions.
Electrostatic (columbic) forces have a strong advantage in this respect because they
act over much greater distances do van der Waals forces.
But as we will see further on, electrostatic repulsion can lose its effectiveness if the
ionic concentration of the medium is too great, or if the medium freezes. Under
these conditions, there are other mechanisms that can stabilize colloidal
dispersions.
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16. According to the interaction between particles of
dispersed phase & those of dispersion medium:
1) Lyophilic (solvent loving).
2) Lyophobic (solvent hating).
3) Association (amphiphilic).
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17. Colloidal particles interact
to an appreciable extent
with the molecules of the
dispersion medium
(solvent loving).
Obtained simply by
dissolving the material
in the solvent ( due to
the high affinity).
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18. NOTE : The material that form Lyophilic colloid in
a certain solvent may not do so in another
solvent,
e.g.;
Acacia + water Lyophilic colloid
(hydrophilic type).
Acacia + benzene NO Lyophilic colloid
formed.
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19. The dispersed phase does not precipitate
easily
The sols are quite stable as the solute particle
surrounded by two stability factors: a- negative
or positive charge b- layer of solvent
If the dispersion medium is separated from the
dispersed phase, the sol can be reconstituted by
simply remixing with the dispersion medium.
Hence, these sols are called reversible sols
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20. Colloidal particles have very little or no attraction for
the dispersion medium (solvent hating).
Colloidal particles: inorganic particles (e.g. : Gold ,
silver, sulfur….) Dispersion medium: water.
These colloids are easily precipitated on the addition of
small amounts of electrolytes, by heating or by shaking
Less stable as the particles surrounded only with a
layer of positive or negative charge.
Once precipitated, it is not easy to reconstitute the sol
by simple mixing with the dispersion medium. Hence,
these sols are called irreversible sols.
Not obtained simply i.e.: Need special method for
preparation.
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21. As LYOPHOBIC COLLOIDS are difficult to form,
the special methods for their formation
includes:
A) Condensation method.
B) Dispersion method.
Condensation
Method
Dispersion
Method
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22. "Stabilization by stealth" has unwittingly been
employed since ancient times through the use
of natural gums to stabilize pigment particles
in inks, paints, and pottery glazes. These gums
are also widely used to stabilize foods and
personal care products.
A Lyophobic colloid can be made to
masquerade as Lyophilic by coating it with
something that itself possesses suitable
Lyophilic properties.
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23. Alternatively, attaching a Lyophobic material to a
colloid of any type can surround the particles with
a protective shield that physically prevents the
particles from approaching close enough to join
together. This method usually employs synthetic
polymers and is often referred to as Steric
Stabilization.
Synthetic polymers, which can be tailor-made for
specific applications, are now widely employed for
both purposes. The polymer can be attached to the
central particle either by simple adsorption or by
chemical bond formation.
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24. Surfactants are molecules consisting of a hydrophilic "head" connected to
a hydrophobic chain. Because such molecules can interact with both
oil and water phases, they are often said to be amphiphilic. Typical of
these is the well known cleaning detergent sodium dodecyl
sulfonate ("sodium laurel sulfate“).
Amphiphiles possess the very important property of being able to
span an oil-water interface. By doing so, they can stabilize emulsions
of both the water-in-oil and oil-in-water types. Such molecules are
essential components of the lipid bilayers that surround the cells and
cellular organelles of living organisms.
Emulsions are inherently unstable; left alone, they tend to separate
into "oil" and "water" phases.
When a detergent-like molecule is employed to stabilize an emulsion,
it is often referred to as an emulsifier. The resulting structure is
known as a micelle.
Emulsifiers are essential components of many foods. They are widely
employed in pharmaceuticals, consumer goods such as lotions and
other personal care products, paints and printing inks, and numerous
industrial processes.
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25. Brownian Movement
Tyndall Effect
Adsorption
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26. BROWNIAN
MOVEMENT:
If a colloid is viewed
under a special
microscope, the
dispersed particles will
be seen moving in a
rapid, random, zigzag
motion through the
dispersion medium.
This motion in a colloid
is one of the reasons
why particles remain
suspended indefinitely.
27. A beam of light passing through a
transparent pure colourless solid, liquid or
gas cannot be seen except at the point
where it enters and leaves the object.
scattering of light
28. The binding of
molecules to a
surface
It is different
from absorption.
(ex. Activated
carbon – binds
molecules to
purify)
29. ADSORPTION
Adsorption is the process of collecting a
thin layer of molecules or ions of a
substance with which it is in contact.
Surfaces of solids and liquids can adsorb
molecules from fluid phases (liquids and
gases) onto their surface.
Chemisorptions – fairly strong,
resembling chemical bonds, and
difficult to break
ex: activated charcoal
Physisorption – weak, involves only
Van der Waals forces and the
adsorbed material can usually
removed again.
ex: silica gel
activated carbon
silica gel
30. Electric Charge
A dispersed colloidal particle can adsorb electrically
charged particles on its surface.
opposite attracts; same repel
prevents coagulation and precipitation of dispersed
colloidal particles
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