3. o ptical p roperty

3.3. O Opticalptical p property 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 .

The same thing happens if they are separated by distances comparable to or greater than the wavelength () of light .

3.3. C Commonommon O Opticalptical PPhenomena henomena

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

G3 : Rayleigh Scattering G3 : Rayleigh Scattering G4 : RamanG4 : Raman ScatteringScatteringG5 : Mie ScatteringG5 : Mie ScatteringG6 : Bragg ScatteringG6 : Bragg Scattering

(10 students per group)

TTurbidity (urbidity ())

• 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 = 84 2 (1+cos2 );Io 4 r2

: polarizability of particle

Is =turbidity, Io

IIo,

For a small particle (Rayleigh scattering)

Scattering>>>Absorption

Scattering :-ln I/Io = -ln (Io – Is) = -ln (1- Is) = (Is/Io) Io Io

Turbidity = = Is/Io = -ln I/Io

isAbsorption :

(b=1 cm, C=1M)Beer’s law A = bC

A = = - ln I/Io

ln(1-x)=-x; x<<<1

Experimentally

Kc1 cos2

r2(1/M 2Bc)o2 r2sin d

( )

= Is

= Io 16 Kc

3(1/M 2Bc)

=

Where K = 22[ndn/dc)]2; n = refractive index

NA4 related to

Experimentally

16 Kc3(1/M 2Bc)

=

Given :

H = 16K = 323(n2(dn/dc)2

3 3NA4

Hc = 1 + 2Bc M

Hc

c (g cm-3)

Intercept = 1/M

3.C3.Colloidal olloidal SStability and tability and Its Its Applications Applications

Stabilization serves to prevent

colloids from aggregating.

IInteraction Between Colloid nteraction Between Colloid

ParticlesParticles 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 covere -d surfaces or in solutions containing non adsor

bing 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 .

CColloidal olloidal SStabilizationtabilization

• Self stabilization

• Stabilization by other substances

Adding emulsifying agent

Fe(OH)Fe(OH)33H+

H+

H+

H+

H+

H+

H+

H+

Adsorption on charged surface(electrical double layer)

The The eelectrical double lectrical double layerlayer

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

Variation of potential with particle separation

Zeta potential,

the electrostatic potentia l generated by the accumu

lation of ions at the surfac e of a (colloidal) particle th at is organized into an elec

- trical double layer, consisti ng of the Stern layer and t he diffuse layer.

provide information about the charge on the surface

Diffuse layer

Stern layer

Assignmemt-5

GGroup roup PPresentationresentationG7G7 : : zeta potentialzeta potentialG8 :G8 : application of the zeta application of the zeta potentialpotential

(10 students per group)

(5 min per group)