electrical double layer theory for colloidal chemistry

1. Electrical Double Layer
H.P PATHIRAGE
12/AS/073
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2. Content
1. Introduction
2. Application of EDL
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3. Introduction
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4. Electrical double layer
• Occurs between ions/molecules in solution
and electrode surface.
• There is strong attraction between both.
• It uses to visualize the ionic environment in
charged surface.
• Here ion distribute surrounding the charged
surface. Therefore increases concentration of
counter ions.
• Liquid droplet, solid particles, gas bubble use
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5. Followings are developed surface charge
• Preferential adsorption of ions.
• Dissociation of surface charged
species.
• Isomorphs replacements.
• Charge crystal replacement.
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6. Electrical double layer consist of follows
• Diffuse double layer.
• Stern layer (tightly bound layer).
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7. Diffuse double layer
• This layer is loosely associated with the
layer.
• This layer called as bulk liquid layer.
• There are excess of negative ion after
uniform distribution.
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8. Stern layer
• This is the inner region.
• Here adsorbed ion due to chemical
interaction.
• Cations are adsorbed by the negative
surface.
• Counter ions is positive charged cations.
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9. Model of electrical double layer
1. Helmholtz model
2. Gouy-Chapman model
3. Gouy-Chapman stern model
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10. Helmholtz model
• Introduced by the Helmholtz in
1879.
• Describe about charge separation
between solid surface and
electrolyte solution.
• He proposed that surface charge
is balances by a layer of
oppositely charge ions.
• Counter ions are cations.
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11. • Potential of Helmholtz layer is
described by the Poisson’s
equation.
• When consider point charge
equation can rewrite as:
𝜕
𝜕𝑥2
2
=0
• Potential of capacitors:
𝐶 𝐻 =
𝜀0 𝜀 𝑟
𝑙
l-thickness of double layer
φ- Electric potential
Ρ- Charge density
x- Distance from the electrode
εr- Permittivity of vacuum
ε0- Relative permittivity of the
medium
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12. Gouy-Chapman model
• Here assumes ions are point
charges.
• Ions don’t interact with each other.
• Assume diffuse layer starts at some
distance from the surface.
• Counter ions are cations.
• Those are affect for thickness of
double layer.
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13. • Concentration of counter ion follow the Boltzmann distribution.
• Counter ion concentration decrease.
• Bulk solution ion concentration increases.
• Exponential potential decrease.
Where,
no- bulk concentration
z- Charge on the ion
e- Charge on a proton
k- Boltzmann constant
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14. Gouy-Chapman stern model
• Stern said ions have finite size.
• Better than Helmholtz model.
• There is stern layer due to surface adsorbs
the ions.
• counter ions are anions.
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15. How potential does vary with distance in layer?
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16. Application of EDL
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17. Application of the electrical double layer
• EDL can be used in many application.
• Following concepts are based on the EDL.
•Zeta potential
•DLVO theory
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18. Zeta Potential
• It is a parameter of electrochemical equilibrium on interface.
• It is depend on:
• Properties of liquid and surface.
• Electrostatic repulsion between particles.
• High zeta potential value – stronger repulsion, the more stable colloidal
system.
• Example-fat droplet in milk has high zeta potential. Because prevent
against coalescence. In cheese formation adds acid to prevent from
coalescence.
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19. • Zeta potential measure the
effectiveness of surface
charge of the electrical double
layer.
• Zeta potential uses to estimate
of Stern potential and the
main characteristic of the
electrostatic repulsion
preventing particles
aggregation
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20. DLVOTheory
• DLVO- Derjaguin, Landau, Verwey
and Overbeek.
• Explain the stability of colloidal
suspension.
• Describe of electrostatic repulsion
and Vander Waals attraction.
• Energy needs to overcome the
repulsive force.
• Van der Waals force is between
molecules in each colloidal
application.
Repulsive
force
Attractive
force
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22. References
• http://nptel.ac.in/courses/105103025/module2/lec7/1.html 29/01/2017 10.30
A.M
• http://www.dispersion.com/zeta-potential-short-tutorial 29/01/2017 10.30
A.M
• Derjaguin ; B.V. Landau;Theory of the stability of strongly charged lyophobic
sols and the adhesion of strongly charged particles in solution of electrolytes;
Acta Phys. Chim; USSR, 14, 733 (1941)
• http://onlinelibrary.wiley.com/doi/10.1002/bip.1977.360160704/abstract
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