3. Optical property
A colloidal system contains particles
that affect a light beam by scattering and
absorption.
 If the particles are of a size comparable to
the wavelength () of light or larger, they
scatter or absorb light independently.
 The same thing happens if they are separated
by distances comparable to or greater than
the wavelength () of light.
3. Common Optical Phenomena
the interaction of light from the sun or
moon with the atmosphere, clouds, water, or
dust and other particulates
Size comparable to the wavelength of light
Group Presentation
(10 students per group)
G3
G4
G5
G6
:
:
:
:
Rayleigh Scattering
Raman Scattering
Mie Scattering
Bragg Scattering
Turbidity ()
• a measure of the degree to which the
water looses it’s transparency due to the
presence of suspended particulates
• Measurement of turbidity is a key test of
water quality
Various parameters influencing
the cloudiness of water
•Sediments from erosion
•Resuspended sediments from the bottom
(frequently stir up by bottom feeders like carp)
•Waste discharge
•Algal growth
•Urban runoff
Io, 
is = 84 2 (1+cos2 );
Io
4 r2
iS()
Is =turbidity, 
Io
 : polarizability of particle
For a small particle (Rayleigh scattering)
Scattering>>>Absorption
Io, 
is
I
Absorption :
(b=1 cm, C=1M)
Beer’s law A = bC
A = = - ln I/Io
ln(1-x)=-x; x<<<1
Scattering :
-ln I/Io = -ln (Io – Is) = -ln (1- Is) = (Is/Io)
Io
Io
Turbidity =  = Is/Io = -ln I/Io
Experimentally
 Kc ( 1  cos 2  )
2
 = Is =  2
2  r sin  d 
Io o r (1/M  2Bc)
=
16  Kc
3(1 / M

2Bc)
Where
K = 22[ndn/dc)]2; n = refractive index
N 4
related to 
A
Experimentally

=
16  Kc
3(1 / M

2Bc)
Given :
H = 16K = 323(n2(dn/dc)2
3
3NA4
Hc = 1 + 2Bc

M
Hc

Intercept = 1/M
c (g cm-3)
3.Colloidal Stability and Its
Applications
Stabilization
serves to
prevent colloids
from
aggregating.
Interaction Between Colloid Particles
Excluded Volume Repulsion:
any overlap between hard particles.
Electrostatic interaction:
an electrical charge attract or repel
van der Waals forces :
two dipoles interaction which are either permanent, induced
or temporary dipole induces a dipole
Entropic forces :
according to the second law of thermodynamics,
resuling in effective forces even between hard spheres.
Steric forces :
interparticle forces between polymer-covered surfaces
or in solutions containing non-adsorbing polymer,
producing an additional repulsive steric stabilization
force or attractive depletion force between them.
Two main mechanisms for colloid
stabilization
 Steric stabilization interparticle forces
between polymer-covered surfaces or in
solutions containing non-adsorbing polymer
 Electrostatic stabilization the mutual
repulsion of like electrical charges. Different
phases generally have different charge
affinities, so that a charge double-layer forms
at any interface. Small particle sizes lead to
enormous surface areas, and this effect is
greatly amplified in colloids.
Colloidal Stabilization
• Self stabilization
• Stabilization by other substances
H
H+
H+
H+
Adding emulsifying agent
+
Fe(OH)3
H+
H+
H+
H+
Adsorption on charged surface
(electrical double layer)
The electrical double layer
the variation of
electric potential
near a surface, and
has a large bearing on
the behaviour of
colloids and other
surfaces in contact
with solutions
Stern Model
Helmholtz Capacitor Model
Zeta potential, 
Stern layer

Diffuse layer
 the electrostatic potential
generated by the
accumulation of ions at the
surface of a (colloidal)
particle that is organized
into an electrical doublelayer, consisting of the
Stern layer and the diffuse
layer.
 provide information about
the charge on the surface
Variation of potential with particle separation
Assignmemt-5
(10 students per group)
Group Presentation
G7 : zeta potential
G8 : application of the zeta potential
(5 min per group)