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COLLOIDS
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• Dispersed systems consist of
a) Particulate matter (dispersed phase,
discontinuous phase, internal phase).
b) Dispersion medium (continuous medium, external
phase).
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Classification of dispersed systems (according to
particle size):
Molecular Colloidal Coarse
Dispersion Dispersion Dispersion
- Less than 1 nm - 1 nm to 500 nm - Greater than
- Particles - Very slow 500 nm
undergo rapid diffusion -Don’t diffuse
diffusion e.g. O2 & e.g. suspension
glucose & emulsion
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Molecular Colloidal Coarse
Dispersion Dispersion Dispersion
-Particles invisible -Particles resolved -Particles are
in electron by electron visible under
microscope. microscope. ordinary
Microscope.
-Pass through - Pass through - Do not pass
semipermeable filter paper but through filter
membranes and not paper or
filter paper. pass through semipermeable
semipermeable membrane.
membrane.
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Types of colloids
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Types of colloidal systems
According to the interaction between particles
of dispersed phase & those of dispersion
medium:
1. Lyophilic (solvent loving): the dispersed phase has
high affinity for the dispersion medium
2. Lyophobic (solvent hating): the dispersed phase has
low affinity for the dispersion medium
3. Association (amphiphilic)
# Hydrophilic and hydrophobic are used when the
dispersion medium is aqueous.
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1) Lyophilic colloids
• Colloidal particles interact to an appreciable extent with
the molecules of the dispersion medium (solvent loving).
• Obtained spontaneously by dissolving the material in the
solvent ( due to the high affinity)
•Hydrophilic colloids
•Solvent: water
Types of lyophilic •Example: acacia, insulin, gelatin
colloids;
(According to type of
solvent) •Lipophilic colloids
•Solvent: non- aqueous, organic
•Example: polystyrene
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1) Lyophilic colloids (cont.)
• 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.
• The solutions are quite stable as the solute particle
surrounded by two stability factors:
a-Negative or positive charge
b-Layer of solvent :
solvating layer represent a protective layer that prevent
adherence and coagulation of particles.
• The dispersed phase does not precipitate easily (need
neutralization of charge and removal of water layer)
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1) Lyophilic colloids (cont.)
• Addition of liquid less polar than water (acetone or alcohol)
with small amount of electrolytes facilitate coagulation due
to dehydration of the particles
• 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
• Similarly charged hydrophilic colloid easily mixed with no
precipitation
• While that of different charge interact together with
precipitation (e.g antigen antibody reaction)
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1) Lyophilic colloids (cont.)
• Lyophilic colloids are more viscous than the dispersion
medium due to swelling of the particles by the solvent
(concentrated sols form gels)
• Gels: e.g. lyophilic sol of gelatin or agar in water
which is elastic in behavior and the sol-gel
transformation is reversible (depend on temp.).
• Gel structure is broken on shaking but gel reform on
stand (thixotropy)
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2) Lyophobic colloids
✓Colloidal particles have very little or no attraction for the
dispersion medium (solvent hating).
✓Colloidal particles: inorganic particles (e.g. gold, silver, sulfur….)
dispersed in water.
✓These colloids are easily precipitated on the addition of small
amounts of electrolytes, by heating or by shaking
✓Addition of small amount of lyophilic or lyophobic colloid to a
lyophobic colloid of opposite charge make the latter more sensitive
to coagulation by electrolytes
✓Addition of excess lyophilic sol to a lyophobic sol (less
precipitation of lyophobic by electrolyte)
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2) Lyophobic colloids (cont.)
✓e.g. tincture of benzoin + ammonium chloride soln lead to
coagulation while in the presence of water mucilage of tragacanth
no coagulation occur (mucilage of tragacanth is a lyophilic sol that
protect the lyophobic resin sol).
✓Less stable as the particles surrounded only with a layer of positive
or negative charge which lead to repulsion between particles.
✓Once precipitated, it is not easy to reconstitute the sol by simple
mixing with the dispersion medium. Hence, these sols are called
irreversible sols.
✓Viscosity of the dispersion medium does not increase by the
presence of the dispersed particles.
✓Not obtained simply i.e need special method for preparation
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3) Association colloids:
• Certain molecules or ions termed
amphiphile (surface active agent SAA) are
characterized by two distinct regions of
opposing solution affinities within the
same molecules or ions.
• At low concentration: amphiphiles exist
separately (subcolloidal size)
• At high concentration: form aggregates or
micelles (50 or more monomers) (colloidal
size)
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➢ As with lyophilic sols, formation of association colloids is
spontaneous, provided that the concentration of the
amphiphile in solution exceeds the CMC.
➢ Amphiphiles may be
1. Anionic (e.g., Na. lauryl sulfate)
2. Cationic (e.g., cetyl triethylammonium bromide)
3. Nonionic (e.g., polyoxyethylene lauryl ether)
4. Ampholytic (zwitterionic) e.g., dimethyl dodecyl
ammoniom propane sulfonate.
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Lyophilic Lyophobic Association (amphophilic).
(solvent-loving). (solvent-hating).
Dispersed Large organic Inorganic particles Aggregates (micelles) of small
phase molecules lying such as gold or organic molecules or ions whose
within colloidal size silver size is below the colloidal size
Solvation Solvated little Hydrophilic or lipophilic portion
of the molecules is solvated
depending on the medium
Preparation Spontaneous by Needs special Spontaneous when conc. of
dissolving in solvent procedure amphiphiles exceeds CMC
Viscosity Viscosity increased Not greatly Increased as conc. of amphiphile
as the conc. increase. increased increase as micelles no. increase &
At certain conc. Gel become asymmetric.
sol gel formation
Effect of Stable in presence of Unstable due to CMC is reduced and salting out
electrolytes small amount of neutralization of occur at high salt conc
electrolytes charges on particles
Desolvation and
salting out in high
conc.
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Preparation of colloids
✓Lyophilic sols and association colloidal sols are usually
prepared by dissolving the dispersed phase in water.
✓While lyophobic colloids were prepared by :
Dispersion methods, in which coarse particles are reduced in
size, and condensation methods, in which materials of
subcolloidal dimensions are caused to aggregate into particles
within the colloidal size range.
A. Condensation process:
1) Change of solvent 2) Double decomposition
3) Reduction 4) Oxidation
B. Dispersion methods:
1) Ultrasonic generator 2) Electric arc.
3) Colloid mill. 4) peptization
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2) Lyophobic colloids (Dispersion methods)
1)Ultrasonic generator:
• Dispersion achieved by high intensity UG at frequency more than
20,000 cycles/second e.g. colloidal dispersions containing
graphite, mercury, metal oxides and sulphur
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2) Lyophobic colloids (Dispersion methods)
2) Electric arc:
• This process is also known as
Bredig’s arc method. It involves
dispersion as well as condensation.
• Colloidal solutions of metal can be
prepared by this method. Electric arc
is struck between electrodes of metal
immersed in the dispersion medium
• The intense heat produced vaporizes
some of the metal, which then
condenses to form particles of
colloidal size
e.g. colloidal solutions of metals as
copper, gold, mercury, silver.
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2) Lyophobic colloids (Dispersion methods)
3) Colloidal mill:
• Material sheared between 2 rapidly rotating close
plates (in opposite directions).
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2) Lyophobic colloids (Dispersion methods)
4) peptization:
• Achieved by addition of electrolytes to precipitates
that carry a small net charge.
e.g. negatively charged precipitates may be redispersed
by the addition of alkali
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2) Lyophobic colloids (Condensation methods)
• Materials of sub colloidal dimensions are caused to
aggregate into particles with colloidal size range by:
1) Change in solvent:
• Change in solvent leading to super saturation that will
result in formation & growth of nuclei. (colloidal system
formation) e.g. sulfur and alcohol in excess of water,
simple tincture of benzoin is poured in water.
2) Oxidation:
• Hydrogen sulfide by oxidation result in sulfur atoms
that give sulfur sol
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2) Lyophobic colloids (Condensation methods)
3) Double decomposition:
Hydrogen sulfide is passed through arsenous
oxide solution in distilled water Double decomp.
Arsenous sulfide sol
4) Reduction:
A number of metals (silver, gold, platinum) can
be prepared in the form of colloidal sol when the
solutions of their salts are treated with reducing
agents (formaldehyde, hydrogen peroxide)
e.g. Colloidal silver is prepared by reduction of
silver oxide with tannic acid
