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3. O ptical p roperty. 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 .

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3 o ptical p roperty
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 distancescomparable to or greater than the wavelength () of light.


3 c ommon o ptical p henomena
3. Common Optical Phenomena

the interaction of light from the sun or moon with the atmosphere, clouds, water, or dust and other particulates


Sizecomparable to the wavelength of light


G roup p resentation
Group Presentation

(10 students per group)

G3 : Rayleigh Scattering G4 : RamanScatteringG5 : Mie ScatteringG6 : Bragg Scattering


T urbidity
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


V arious parameters influencing the cloudiness of water
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 


iS()

Io, 

Is =turbidity, 

Io

is = 84 2 (1+cos2 );

Io 4 r2

 : polarizability of particle


For a small particle (Rayleigh scattering)

Scattering>>>Absorption

Absorption :

(b=1 cm, C=1M)

Beer’s law A=bC

A =  = - ln I/Io

is

Io, 

I

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

 =Is =

Io

=

Where

K = 22[ndn/dc)]2; n = refractive index

NA4 related to 


Experimentally

=

Given :

H = 16K = 323(n2(dn/dc)2

33NA4

Hc

Hc = 1 + 2Bc

 M

Intercept = 1/M

c (g cm-3)


3 c olloidal s tability and its applications
3.Colloidal Stability and Its Applications

Stabilization serves to prevent colloids from aggregating.


I nteraction between colloid particles
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 m ain m echanisms for colloid stabilization
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.


C olloidal s tabilization
Colloidal Stabilization

  • Self stabilization

  • Stabilization by other substances

H+

H+

H+

Fe(OH)3

H+

H+

H+

H+

H+

Adsorption on charged surface

(electrical double layer)

Adding emulsifying agent


The e lectrical 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

  • the electrostatic potential generated by the accumulation of ions at the surface of a (colloidal) particle that is organized into an electrical double-layer, consisting of the Stern layer and the diffuse layer.

  • provide information about the charge on the surface

Diffuse layer

Variation of potential with particle separation


A ssignmemt 5
Assignmemt-5

(10 students per group)

Group PresentationG7 : zeta potentialG8 : application of the zeta potential

(5 min per group)