3. o ptical p roperty
DESCRIPTION
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 . - PowerPoint PPT PresentationTRANSCRIPT
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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 .
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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
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Size comparable to the wavelength of light
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Group Presentation
G3 : Rayleigh Scattering G3 : Rayleigh Scattering G4 : RamanG4 : Raman ScatteringScatteringG5 : Mie ScatteringG5 : Mie ScatteringG6 : Bragg ScatteringG6 : Bragg Scattering
(10 students per group)
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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
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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
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iS()Io,
is = 84 2 (1+cos2 );Io 4 r2
: polarizability of particle
Is =turbidity, Io
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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
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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
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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
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3.C3.Colloidal olloidal SStability and tability and Its Its Applications Applications
Stabilization serves to prevent
colloids from aggregating.
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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.
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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 .
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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)
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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
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Stern Model
Helmholtz Capacitor Model
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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
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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)