3 o ptical p roperty
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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

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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. 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


Group Presentation

(10 students per group)

G3 : Rayleigh Scattering G4 : RamanScatteringG5 : Mie ScatteringG6 : 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 


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.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+

Fe(OH)3

H+

H+

H+

H+

H+

Adsorption on charged surface

(electrical double layer)

Adding emulsifying agent


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


Assignmemt-5

(10 students per group)

Group PresentationG7 : zeta potentialG8 : application of the zeta potential

(5 min per group)


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