Colloidal dispersions

1. Colloidal dispersions
Unit 5

2. Dispersed systems consist of particulate matter known as dispersed
phase, dispersed throughout a continuous or dispersion medium.
Dispersed systems are classified according to particle size
Coarse dispersion > 1 m suspension & emulsion
Colloidal dispersion 1 nm – 1000 nm (1µm) colloids
Colloidal System is defined as the heterogeneous biphasic system in which
dispersed phase is subdivided into nano size range (1-1000 nanometer).
Nanoparticles are small colloidal particles, but not all small colloidal
particles are nanoparticles.”
If all particles in a colloidal system are of (nearly) the same size the system
is called monodisperse; in the opposite cases the systems are heterodisperse
/polydisperse.
19 March 2020

4. Types of colloids:
On the basis of nature of interaction between dispersed
phase and dispersion medium.
A-Lyophilic colloids (solvent attracting) (solvent loving) –
• The particles in a lyophilic system have a great affinity for
the solvent. i.e dispersed particle and dispersion medium
interact to a great extent.
• If water is the dispersing medium, it is often known as
hydrophilic colloids .E.g. gelatin , acacia and albumin in
water
• Rubber and polystyrene form lyophilic colloids in non
aqueous organic solvents such as benzene and thus
known as lipophilic colloids.

5. Types of colloids
B- Lyophobic (solvent repelling) (solvent hating) - The particles
resist solvation and dispersion in the solvent.
• Very little attraction between dispersed particle and
dispersion medium.
• When dispersion medium in these colloids is water they are
known as hydrophobic colloids. E.g. gold and silver in water.
• These are thermodynamically unstable

6. Types of colloids:
C- Association / amphiphilic colloids
- Certain molecules termed amphiphiles or surface active
agents, characterized by two regions of opposing solution
affinities within the same molecule.

7. Types of colloids:
- At low concentration: amphiphiles exist separately
(subcolloidal size)
- At high concentration: form aggregates or micelles (50 or
more monomers) (colloidal size)

8. Association colloids

9. Comparison of colloidal sols
PROPERTY LYOPHILIC
COLLOIDS
LYOPHOBIC
COLLOIDS
ASSOCIATION
COLLOIDS
Dispersed Phase Mostly organic
molecules
Largely inorganic
particles
Aggregation of
surface active agents
Nature of Interaction Stronger solvent
sheath around
particle
Little interaction Aggregates are
solvated
Presence of charge Less charged Highly charged Charged micelles
Method of
Preparation
Readily form sol Special methods are
required
Readily forms when
concn is equal to cmc
Viscosity of
Dispersion
Higher than that of
medium alone
Nearly same as
dispersion medium
Viscosity increases
but not appreciably.
Reversibility reversible irreversible reversible

10. Optical Properties of Colloids
1-Faraday-Tyndall effect
– when a strong beam of light is passed through a colloidal sol,
the path of light is illuminated (a visible cone formed).
- This phenomenon resulting from the scattering of light by the
colloidal particles.
- Tyndall effect is observed due to interaction of particles with
light.
- When intense narrow beam is passed through colloidal
dispersions, its path is visible due to scattered light. Scattered
beam is called tyndall beam.

12. Optical Properties of Colloids
2- Electron microscope
- Ultra-microscope has declined in recent years as it does not
able to resolve lyophillic colloids.
- so electron microscope is capable of yielding pictures of
actual particles size, shape and structure of colloidal particles.
- Electron microscope has high resolving power, as its radiation
source is a beam of high energy electrons with wavelength of
0.1 Angstrom (smaller the wavelength of radiation greater is
the resolving power), while that of optical microscope is
visible light.

13. Electron Microscope

14. Optical Properties of Colloids
3- Light Scattering / Turbidity
When a beam of light is passed through a colloidal dispersions
some of it is absorbed, some scattered and remainder transmitted
undisturbed through sample.
- Absorbed light is responsible for highly coloured nature of some
colloids.
- Scattered light causes the solution to appear turbid.
- Turbidity is proportional to the molecular weight of lyophilic
colloid.
- If amount of dispersed solids is high light is scattered more and
transmitted light will have less intensity.
- Light scattering measurements are important in estimating particle
size, shape and interaction of dissolved molecules. Mol. Wt. of
colloids can be determined using following formula

15. Optical Properties of Colloids
Hc / T = 1/M + 2Bc
T: turbidity
C: conc of solute in gm / cc of solution
M: molecular weight
B: interaction constant
H: optical constant, i.e. constant for a particular system
Plot of HC/T against concentration gives as straight line whose slope
is equal to 2B and intercept is equal to 1/M.

16. Kinetic Properties of Colloids
1-Brownian motion
- The zig-zag movement of colloidal particles
continuously and randomly.
• This brownian motion arises due to
bombardment between the colloidal particles in
dispersion and molecules of dispersion medium.
- Brownian movement was more rapid for smaller
particles.
- It decrease with increase the viscosity of the
medium.
- It is observed with particles as large as about 5
μm.

17. Kinetic Properties of Colloids
2- Diffusion
- Particles diffuse spontaneously from a region of higher conc.
To one of lower conc. Until the conc. of the system is
uniform throughout.
- Diffusion is a direct result of Brownian motion.
- Fick's first law used to describe the diffusion:
(The amount of Dq of substance diffusing in time dt across a
plane of area A is directly proportional to the change of
concentration dc with distance traveled
dq /dt= -DA (dc / dx)

18. Kinetic Properties of Colloids
D  diffusion coefficient
- The measured diffusion coefficient can be used to determine the
radius of particles or molecular weight using the equation
D = RT . 4 N
6  rηN 3Mv
- Where M= molecular weight
- V= partial specific volume
- η= viscosity of solvent
- R= molar gas constant
- T= absolute temperature
- R= radius of spherical particles
- N= Avagadro’s No.

19. Kinetic Properties of Colloids
3- Osmotic pressure
• It depends on no. of particles in dispersion
- Can be used to determine the molecular weight of
colloid in dilute solution using following equation.
P/C = RT/M
Where
P = osmotic Pressure
C= concn. in grams solute per litres solvent
M= Molecular weight
R= Gas constant
T= temperature in kelvin

20. Kinetic Properties of Colloids
4- Sedimentation
Stronger force must be applied in order to bring about sedimentation of
colloidal particles using ultracentrifuge where particles settle according to
their molecular weight
- The velocity of sedimentation is given by Stokes‘ Law:
v = d (i-e)g/18η
V = rate of sedimentation
D = diameter of particles
 = density of internal phase and external phase
g = gravitational constant
η = viscosity of medium

21. Molecular weight can be determined using the
following equation
M= RTS/ D(1-vρo)
Where R= Gas constant
T= Absolute temperature
V= Partial specific volume of polymer
ρo= Density of solvent
S= svedberg sedimentation coefficient
determined at 200 C
D= diffusion coefficient

22. Kinetic Properties of Colloids
5- Viscosity:
- It is the resistance to flow of system under an applied stress. The more
viscous a liquid, the greater the applied force required to make it flow at
a particular rate.
- The viscosity of colloidal dispersion is affected by the shape of particles
of the disperse phase:
Spherocolloids dispersions of low viscosity
Linear particles more viscous dispersions

23. Schulze Hardy Rule
• Coagulation is a process which involves coming together of
colloidal particles so as to change into large sized particles
which ultimately settle as a precipitate or float on the surface.
• Coagulation is generally brought about by the addition of
electrolytes. When an electrolyte is added to a colloidal
solution, the particles of the sol take up the ions which are
oppositely charged and thus get neutralized. The neutral
particles then start accumulating to form particles of a larger
size which settle down.
• The quantity of the electrolyte which is required to coagulate a
definite amount of a colloidal solution depends upon the
valency of the ion having a charge opposite to that of the
colloidal particles. This observation of Hardy and Schulze are
known as Hardy Schulze law.

24. • It can be defined as: Greater is the valency of the
oppositely charged ion of the electrolyte being
added, the faster is the coagulation
• Al3+ is more effective than Mg++ and Na+ .
Negatively charged arsenious sulfide will be
coagulated rapidly with a smaller concentration of
Alcl3 than that of Bacl2 or Nacl. Similarly for
positively charged sol such as Fe(OH)3 , PO4
3- is
more effective than SO4
2- and cl-

25. Protective Action
• When a large amount of lyophilic colloid
carrying opposite charge is added to the
lyophobic particles they get adsorbed on
hydrophobic particles and forms a protective
layer around it.
• This phenomenon is known as protective
action of lyophilic colloid and the lyophilic
colloid is called as protective colloid.

26. Gold Number
• It is the protective ability of hydrophilic colloid
and may be defined as ‘least quantity of
protective colloid in milligrams, which is just
sufficient to prevent the coagulation of 10 ml
of standard gold sol (containing 0.0053% gold)
by the rapid addition of 1 ml of 10% NaCl
solution. The Coagulation of gold sol is
indicated by change in color from red to
violet. Thus small the gold number, the
greater is protective power of lyophilic colloid.

27. Measurement of Gold Number
• A series of test tubes containing 10ml of gold
solution are taken. To each of test tubes is added a
protective colloid in increasing concentrations. To
each of the test tubes is then added 1ml of 10%
sodium chloride solution. The test tubes are left
undisturbed. At higher concn. of protective
colloids the gold soln does not change its colour
while at lower concn. the gold sol changes colour
from red to violet. The test tube containing
minimum qty of colloid which prevents the
change in colour of the gold sol is the gold no. of
the protective colloid.

28. Effect of Electrolytes
• Precipitation or coagulation is the condition in
which settling of dispersed phase and
flocculation (aggregation) is noticed.
Electrolytes have significant effect on this
instability of colloidal dispersions.
- Addition of excess electrolytes: particles
precipitate after a specific concentration. It
takes place owing to accumulation of
oppositely charged particles.

29. • Hofmeister or Lyotropic Series: it states that
the precipitating power of an ion is directly
related to the ability of that ion to separate
water molecules from the colloidal particles.
• Coagulating power of anions or cations on
hydrophilic colloids are arranged by
hofmeister series.
• Cations: Mg2+ > Ca2+ > Ba2+ >Na+ > K+
• Anions: Citrates > tartrates > sulfates >
chlorides > nitrates > bromides

30. Applications of colloidal solutions:
1- Therapy--- Colloidal system are used as therapeutic agents
in different areas.
e.g- Silver colloid-germicidal
Copper colloid-anticancer
Mercury colloid-Antisyphilis
2- Stability---e.g. lyophobic colloids prevent flocculation in
suspensions.
e.g- Colloidal dispersion of gelatin is used in coating over
tablets and granules which upon drying leaves a uniform
dry film over them and protect them from adverse
conditions of the atmosphere.

31. Applications of colloidal solutions:
4- Absorption--- As colloidal dimensions are small enough,
they have a huge surface area. Hence, the drug constituted
colloidal form is released in large amount.
e.g- sulphur colloid gives a large quantity of sulphur and this
often leads to sulphur toxicity
5-Targeted Drug Delivery--- Liposomes are of colloidal
dimensions and are preferentially taken up by the liver and
spleen.

32. Applications of colloidal solutions:
6- Photography:
A colloidal solution of silver bromide in gelatin is applied on
glass plates or celluloid films to form sensitive plates in
photography.
7- Clotting of blood:
- Blood is a colloidal solution and is negatively charged.
- On applying a solution of Fecl3 bleeding stops and blood
clotting occurs as Fe+3 ions neutralize the ion charges on the
colloidal particles.

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