The document discusses the electrical double layer (EDL) that occurs between ions or molecules in solution and an electrode surface. The EDL consists of two layers - the diffuse double layer and the Stern layer. The Stern layer contains ions that are tightly bound to the surface due to chemical interactions, while the diffuse double layer contains loosely associated counterions. Several models of the EDL are described, including the Helmholtz model, Gouy-Chapman model, and Gouy-Chapman Stern model. Applications of the EDL concept include zeta potential measurement and the DLVO theory of colloidal stability.

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
as object. 9/5/2017 4

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
29/01/2017 10.30 A.M
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