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Electrical double layer theory
- 1. M.K.C.L. Chathushani 12/AS/091 Dept: of Physical Sciences & Technology
- 2. Content 1. Introduction 2. Electrical Double Layer 3. Layers of the double layer 4. Theories based on double layer 5. Applications 6. References 29/5/2017 2
- 3. 9/5/2017 3
- 4. Introduction • All the molecules or particles carry out a electric charge due the their properties. • Electrochemistry and the surface and colloidal chemistry can be combine with this phenomena. Electrochemistry Electrode Electrolysis Electro motive force Electrolyte 9/5/2017 4
- 5. • An electrical conducting material • Divided into two parts. • Anode • Cathode Electrode • Defined as the decomposition of substance by means of the electric current. • Redox reaction was pushes to the nonspontaneous side. Electrolysis • Substance that produces electrically conducting solution when dissolved in a polar solvent. • Solution is considered as the neutral solution. Electrolyte • Describes as the EMF or cell potential. • The potential energy difference between two cells or electrodes. Electro motive force 9/5/2017 5
- 6. Electrochemistry • Study of reactions in which charged particles cross the interface between two phases of matter , such as interface between a solid and a liquid. 9/5/2017 6
- 7. 9/5/2017 7
- 8. Electrical Double Layer Theory • When electrode immersed in an electrolytic solution, charge accumulation will occur. • Particle size should be greater than 1 nm. • Charge separation always occur at the interface of the electrodes in the solution. • The excess charge on the electrode surface is accumulated by an accumulation of the excess ions of the opposite charge in the solution. 9/5/2017 8
- 9. • EDL is a transition region between two phases consists of, 1. An inner monomolecular layer 2. An outer diffuse region 3. A layer intermediate between inner molecular layer and the outer diffuse layer 9/5/2017 9
- 10. Structure of double layer • Has 03 structures. Helmholtz model Gouy – Chapman model Gouy- chapman stern model 9/5/2017 10
- 11. Helmholtz model • Described by the Helmholtz in 1879. • Described that the charge separation at the interface between metallic electrolyte and an electrolyte solution. • The charge of the surface of the metal was neutralized by the opposite sign of the electrolyte. 9/5/2017 11
- 12. • The potential in the Helmholtz layer is described by the Poisson’s equation. 1 Where, φ - Electric potential ρ - Charge density x - Distance from the electrode ε0 - Permittivity of vacuum εr - Relative permittivity of the medium. 9/5/2017 12
- 13. • Considering the ions are point charges. 2 • Electrical double layer act as a capacitor. 3 9/5/2017 13
- 14. Drawback of the model • The model does not account for the dependence of the measured capacity on potential or electrolyte concentration. • This is the neglect of interactions that occur away from the OHP. 9/5/2017 14
- 15. Gouy-Chapman Model • The thermal motion of the ions near the surface was considered. • That described that diffuse double layer has an ions which have the opposite charges with the surface. • The change in concentration of the counter ions near a charged surface follows the Boltzmann distribution. Where, no = bulk concentration z = charge on the ion e = charge on a proton k = Boltzmann constant 9/5/2017 15
- 16. Gouy- Chapman Stern model • In 1924 Stern developed this method. • Combined the two previous models by adapting the compact layer of ions used by Helmholtz and next to the diffuse layer of Gouy Chapman extending into the bulk solution. • Consider, • ions have finite size • consequently the closest approach of OHP to the electrode will vary with the ionic radius. 9/5/2017 16
- 17. 9/5/2017 17
- 18. Layers of EDL • Mostly used the Gouy chapman – stern model. • Two layers can be described. 9/5/2017 18
- 19. Stern layer • Also known as the Stationary Layer • Occurs in next to the surface of the particle. • Ions are bound to the surface very firmly. • Occurs due to the absorbing and coulomb interaction. 9/5/2017 19
- 20. Diffuse Layer •Occurs next to the stern layer. •Occurs in between the stern layer and the bulk. •Both positive and negative charges can be seen. Boundary Slipping plane 9/5/2017 20
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- 23. •The nature and behavior of the every system is controlled by two parameters. Disperse phase • Provides particles Dispersion media • Provide fluid in which particles are dispersed 9/5/2017 23
- 25. Zeta potential • Term that used in colloidal dispersion for electro kinetic potential. • Usually denoted using the Greek letter zeta (ζ). Zeta potential is the potential in the inefficient double layer at the location of the slipping plane relative to the point in the bulk away from the interface. 9/5/2017 25
- 26. • Depend on the location of the plane. • Caused by the net electrical charged contained within the region of bonded by the slipping plane. • Widely used for quantification of the magnitude of the charge. • Key indicator of the stability of colloidal dispersions. • Stern potential ≠ zeta potential • The value of zeta potential is cannot be measured directly from experimentally. 9/5/2017 26
- 27. High ζ Low ζ •will confer stability. •electrically stabilized •Attractive forces may exceed this repulsion •Tend to coagulate or flocculate. 9/5/2017 27
- 28. Effect of zeta potential and the suspension particles 9/5/2017 28
- 29. DLVO Theory • Was named by the scientists named as Derjaguin , Landau, Verwey, and Overbeek. • Very important for suspension of solid. 9/5/2017 29
- 30. Assumptions of DLVO theory • Dispersion in dilute. • Only two forces act on the dispersed particles. Those are Vanderwaals forces and electrostatic forces. • The electric charge and other properties are uniformly distributed over the solid surface. • The distribution of ions determined by the electrostatic forces, Brownian motion and the entropic dispersion. 9/5/2017 30
- 31. 9/5/2017 31
- 32. VT = VR + VA Where, VA = Sum of the Vander Waals attractive VR = Electrical double layer repulsive (VR) forces VT = Total energy of the double layer Depends on density surface charge thickness of the double layer. Depends on chemical nature size of the particle The electrostatic repulsive forces Vander waal forces 9/5/2017 32
- 33. 9/5/2017 33
- 34. Application of the EDL • Uses of zeta potential is to study colloid-electrolyte interactions. • Intravenous Fat Emulsions • Drug Targeting and Delivery Systems • To make the EDLC 9/5/2017 34
- 35. References 1. http://www.ceb.cam.ac.uk/research/groups/rg-eme/teaching- notes/the-electrical-double-layer, 31/01/2017, 18.00-18.20 2. http://glossary.periodni.com/glossary.php?en=electrical+double+l ayer , 31/01/2017 ,18.30-18.50 3. https://web.nmsu.edu/~snsm/classes/chem435/Lab14/double_la yer.html ,01/02/2017 , 15.30-16.00 9/5/2017 35
- 36. 9/5/2017 36
Editor's Notes
- Electrolyte – ex: water After dissolving separate into cations and anions When applying an electric potential to the solution, ions are stared to travel opposite directions. Then electrical current will occurs.
- s a structure that appears on the surface of an object when it is exposed to a fluid. In here object may be a solid particle, a gas bubble, a liquid droplet, or a porous body. The double layer refers to two parallel layers of charge surrounding the object.
- The electrode holds a charge density (σM) arising from either an excess (-σM) or deficiency (+σM) of electrons at the electrode surface. The charge on the electrode is balanced by redistribution of the ions in the solution by an equal but oppositely charged amount of ions. The result is two layers of opposite charge separated by some distance and it is limited to the radius d/2 of the attracted ions and a single layer of solvation around each ion. The line drawn through the center of such ions marks the boundary known as the ‘Outer Helmholtz Plane’ (OHP) and the region within it the electrical double layer. The potential in the Helmholtz layer is described by the Poisson’s equation, which relates the potential with the charge distribution.
- Gouy suggested that interfacial potential at the charged surface could be attributed to the presence of a number of ions of given sign attached to its surface, and to an equal number of ions of opposite charge in the solution. In other words, counter ions are not rigidly held, but tend to diffuse into the liquid phase until the counter potential set up by their departure restricts this tendency. The kinetic energy of the counter ions will, in part, affect the thickness of the resulting diffuse double layer. Gouy and, independently, Chapman developed theories of this so called diffuse double layer in which the change in concentration of the counter ions near a charged surface follows the Boltzmann distribution.
- Charge opposite to the surface charge
- Diffused double layer (DDL) is the result of claywaterelectrolyte interaction. Cations are held strongly on the negatively charged surface of dry finegrained soil or clays. These cations are termed as adsorbed cations. Those cations in excess of those needed to neutralize electronegativity of clay particles and associated anions are present as salt precipitates. When dry clays come in contact with water, the precipitates can go into solution. The adsorbed cations would try to diffuse away from the clay surface and tries to equalize the concentration throughout pore water. However, this movement of adsorbed cations are restricted or rather minimized by the negative surface charge of clays. The diffusion tendency of adsorbed cations and electrostatic attraction together would result in cation distribution adjacent to each clay particle in suspension.
- Also it can be defined as the potential difference between the dispersion medium and the stationary layer of fluid attached to the dispersion particle.
- At a high zeta potential will confer stability. At the solution or dispersion will resist aggregation. When the potential is small, attractive forces may exceed this repulsion and the dispersion may break and flocculate. So, colloids with high zeta potential (negative or positive) are electrically stabilized while colloids with low zeta potentials tend to coagulate or flocculate.
- The stability of a colloidal system will determined by the sumation of the attractive forces and the repulsive forces.
- Unlike a ceramic capacitor or aluminum electrolytic capacitor, the Electrical Double Layer Capacitor (EDLC) contains no conventional dielectric. Triglyceride emulsions are medical products; they are sub micron emulsions of vegetable oils in water, emulsified by phospholipids, which provide a high zeta potential, and a correspondingly long shelf life (2-3 years). The emulsions are used to feed patients intravenously who cannot be fed orally (e.g. due to gastrointestinal surgery). zeta potential – pH curve for a drug-containing emulsion that is flocculated at pH 7. Data of this type allows a rational selection of formulation pH and emulsifier to maximise zeta potential and hence emulsion stability.zeta potential – pH curve for a drug-containing emulsion that is flocculated at pH 7. Data of this type allows a rational selection of formulation p H and emulsifier to maximise zeta potential and hence emulsion stability.zeta potential – pH curve for a drug-containing emulsion that is flocculated at pH 7.