Electrical double layer theory

electrical double layer theory for colloidal chemistry

1. Electrical Double L ayer H.P PATHIRAGE 12/AS/073 2/9/2017 1

2. Content Introduction Application of EDL 2/9/2017 2

3. Introduction 2/9/2017 3

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. 2/9/2017 4

5. Followings are developed surface charge Preferential adsorption of ions. Dissociation of surface charged species. Isomorphs replacements. Charge crystal replacement. 2/9/2017 5

6. Electrical double layer consist of follows Diffuse double layer. Stern layer (tightly bound layer). 2/9/2017 6

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. 2/9/2017 7

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. 2/9/2017 8

9. Model of electrical double layer Helmholtz model Gouy -Chapman model Gouy -Chapman stern model 2/9/2017 9

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. 2/9/2017 10

11. Potential of Helmholtz layer is described by the Poisson’s equation. When consider point charge equation can rewrite as: = Potential of capacitors: l-thickness of double layer   φ- Electric potential Ρ- Charge density x- Distance from the electrode ε r - Permittivity of vacuum ε - Relative permittivity of the medium   2/9/2017 11

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. 2/9/2017 12

13. Concentration of counter ion follow the Boltzmann distribution . C ounter ion concentration decrease. Bulk solution ion concentration increases. Exponential potential decrease. Where, n o - bulk concentration z- Charge on the ion e- Charge on a proton k- Boltzmann constant       2/9/2017 13

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. 2/9/2017 14

15. How potential does vary with distance in layer? 2/9/2017 15

16. Application of EDL 2/9/2017 16

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 2/9/2017 17

18. Zeta Potential It is a p arameter of electrochemical equilibrium on interface. It is d epend 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. 2/9/2017 18

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 2/9/2017 19

20. DLVO Theory 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 2/9/2017 20

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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 2/9/2017 22

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