Electrical double layer theory

Electrical double layer theory under surface and colloidal chemistry

1. Electrical Double Layer Theory M.K.C.L. Chathushani 12/AS/091 Dept : of Physical Sciences & Technology

2. Content Introduction Electrical Double Layer Layers of the double layer Theories based on double layer Applications References 2 2/15/2017 2

3. Introduction 2/15/2017 3

4. Introduction A ll the molecules or particles carry out a electric charge due the their properties . E lectrochemistry and the surface and colloidal chemistry can be combine with this phenomena . 2/15/2017 4

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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. 2/15/2017 6

7. Electrical Double Layer 2/15/2017 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. C harge 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. 2/15/2017 8

9. EDL is a transition region between two phases consists of, An inner monomolecular layer An outer diffuse region A layer intermediate between inner molecular layer and the outer diffuse layer 2/15/2017 9

10. Structure of double layer Has 03 structures. 2/15/2017 10

11. Helmholtz model D escribed by the Helmholtz in 1879 . D escribed that the charge separation at the interface between metallic electrolyte and an electrolyte solution. T he charge of the surface of the metal was neutralized by the opposite sign of the electrolyte. 2/15/2017 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 ε - Permittivity of vacuum ε r - Relative permittivity of the medium. 2/15/2017 12

13. Considering the ions are point charges. 2 E lectrical double layer act as a capacitor . 3 2/15/2017 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. 2/15/2017 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 2/15/2017 15

16. Gouy- Chapman Stern model In 1924 Stern developed this method. C ombined 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. 2/15/2017 16

17. Layers of the EDL 2/15/2017 17

18. Layers of EDL M ostly used the Gouy chapman – stern model . Two layers can be described . 2/15/2017 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 . O ccurs due to the absorbing and coulomb interaction. 2/15/2017 19

20. Diffuse Layer O ccurs next to the stern layer . O ccurs in between the stern layer and the bulk . B oth positive and negative charges can be seen . Boundary  Slipping plane 2/15/2017 20

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22. Theories based on EDL 2/15/2017 22

23. The nature and behavior of the every system is controlled by two parameters . 2/15/2017 23

24. Application of EDL 2/15/2017 24

25. Zeta potential T erm that used in colloidal dispersion for electro kinetic potential. U sually 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. 2/15/2017 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. 2/15/2017 26

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28. Effect of zeta potential and the suspension particles 2/15/2017 28

29. DLVO Theory W as named by the scientists named as D erjaguin , L andau, V erwey, and O verbeek . V ery important for suspension of solid. 2/15/2017 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. 2/15/2017 30

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32. V T = V R + V A Where, V A = S um of the Vander Waals attractive V R = E lectrical double layer repulsive (V R ) forces V T = T otal energy of the double layer The electrostatic repulsive forces Vander waal forces 2/15/2017 32

33. Applications of EDL 2/15/2017 33

34. Application of the EDL U ses of zeta potential is to study colloid-electrolyte interactions . Intravenous Fat Emulsions Drug Targeting and Delivery Systems To make the EDLC 2/15/2017 34

35. References http://www.ceb.cam.ac.uk/research/groups/rg-eme/teaching-notes/the-electrical-double-layer , 31/01/2017, 18.00-18.20 http://glossary.periodni.com/glossary.php?en=electrical+double+layer , 31/01/2017 ,18.30-18.50 https://web.nmsu.edu/~snsm/classes/chem435/Lab14/double_layer.html ,01/02/2017 , 15.30-16.00 2/15/2017 35

36. Thank you 2/15/2017 36