Dispersed systems consist of particulate matter dispersed in a continuous medium and are classified based on particle size as molecular dispersions, colloidal dispersions, or coarse dispersions. Colloidal dispersions have particle sizes between 1-1000 nm that are not visible under an ordinary microscope but can be seen under an electron microscope. Colloidal dispersions exhibit Brownian motion, diffusion, sedimentation, osmotic pressure, viscosity, and optical properties. The document then provides details on these various properties of colloidal dispersions.

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3. Dispersed Systems
Dispersed systems consist of :
 Particulate matter (dispersed phase).
 Dispersion medium (continuous medium).
Classification of dispersed systems (according to
particle size):
 MOLECULAR DISPERSION
 COLLOIDAL DISPERSION
 COARSE DISPERSION
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 Particle size: from 1nm to 0.5nm
 Particles not resolved by ordinary
microscope,can be detected by electron
microscope.
 Pass through filter paper but not pass through
semipermeable membrane.
 Particles made to settle by centrifugation
 Diffuse very slowly
 E.g. colloidal silver sols, natural and synthetic
polymers , cheese, butter, milk
COLLOIDAL DISPERSION

5. Properties of colloids
 Kinetic properties.
 Optical properties.
 Electrical properties.

6. Brownian motion.
Diffusion.
Sedimentation.
Osmotic pressure.
Viscosity.
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7. Colloidal particles are subjected to
random collision with molecules of the
dispersion medium so each particle move
in irregular and complicated zigzag
pathway.
First observed by Robert Brown (1827)
with pollen grains suspended in water.
The velocity of particles increases with
decreasing particle size and viscosity.
Increasing the viscosity of dispersion
medium (by glycerin) decrease then stop
Brownian motion.
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9. Due to Brownian Motion particles move from region
of higher concentration to region of lower
concentration.
According to the Fick’s first law:
dm/dt = -DA dc/dx
D = Diffusing constant
dm = mass of substance diffusing in time dt across an
area A under influence of concentration gradient dc/dx
The minus sign denotes that diffusion takes place in the
direction of decreasing concentration.
The measured diffusion coeffecient can be used to
determine the radius of particles or molecular weight.
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11. Sutherland-Einstein Equation
This equation is used to obtain radius of a spherical
colloidal particles much larger than the sovent
molecules.
D= diffusion coefficient
K= Boltzmann constant
T= Absolute temperature
η = viscosity of solvent
r= radius of spherical particle
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12. At small particle size (less than 0.5 um) Brownian
motion is significant & tend to prevent sedimentation
due to gravity & promote mixing instead.
So, we use an ultracentrifuge which provide stronger
force so promote sedimentation in a measurable
manner.
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13. The velocity of sedimentation is given by Stokes‘ Law:
V = 2r2( p-po) g / 9 η
V = rate of sedimentation
r= radius of particles
p & po = density of internal phase and external phase
g = gravitational constant
η = viscosity of medium
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14. Osmosis is a passage of particles across semi-
permeable membrane against concentration gradient.
The minimum pressure needed to nullify osmosis is
osmotic pressure.
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15. 104 _ 106 Pascal
Larger particles have smaller osmotic pressure.
The method is based on Van's Hoff's law
 = cRT
Can be used to determine the molecular weight of
colloid in dilute solution.
Replacing c by C / M (where C = the grams of solute /
liter of solution, M = molecular weight)
 /C = RT/M
 = osmotic pressure
R= molar gas constant
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16. 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
 If linear colloidal particles coil up into spheres then
the viscosity of the system falls due to changing the
shape.
 Viscosity is used to obtain molecular weight of
material comprising disperse phase
 Shape of particles in solution
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17. Above is low viscous
solution
Below is high viscous
solution
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18. Einstein develpoed equation of flow applicable to
colloidal dispersion of spherical particles:
η= ηo ( 1+2.5φ)
ηo= viscosity of dispersion medium
η= viscosity of dispersion when volume fraction is φ
η. Can be measured by using viscometer
Relative viscosity= ηrel = η/ ηo= 1+2.5 φ
Specific viscosity= ηsp = (η/ ηo)-1= 2.5 φ
ηsp / φ = 2.5
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19. When you do not succeed in taking giant
steps on the road to your goal,
be satisfied with little steps,
and wait patiently till the time that you are
able to run, or better still, to fly.
Be satisfied to be a little bee in the hive who
will soon become a big bee capable of making
honey…
Thank you …
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