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Colloid Stability ?

Colloid Stability ?. Colloidal systems. A state of subdivision in which the particles, droplets, or bubbles dispersed in another phase have at least one dimension between 1 and 1000 nm all combinations are possible between : gas, liquid, and solid W. Ostwald.

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Colloid Stability ?

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  1. Colloid Stability ?

  2. Colloidal systems • A state of subdivision in which the particles, droplets, or bubbles dispersed in another phase have at least one dimension between 1 and 1000 nm • all combinations are possible between : • gas, liquid, and solid • W. Ostwald

  3. Surface area of colloidal systems • Cube (1cm; 1cm; 1cm) after size reduction to an edge length of 500 nm:  surface area of 60 m2 • Spinning dope (1 cm3) after spinning to a fibre with diameter of 1000 nm:  fiber length of 1273 km • 1 liter of a 0.1 M surfactant solution:  interfacial area of 40000 m2

  4. Surface atoms [in %] in dependence on the particle size [in nm] % nm

  5. Colloidal systems • have large surface areas • surface atoms become dominant

  6. Colloid stability • Colloidal gold: stabilized against coagulation ! • Creme: stabilized against coagulation ! • Milk: stabilized against coagulation !

  7. Particle – Particle interactions • Interaction Energy ( Vtot) – Distance of Separation (d) Relationship d

  8. Vtot(d)= Vattr(d)+ Vrep(d) - Van der Waals attraction - Electrostatic repulsion - Steric repulsion

  9. DLVO - Theory • 1940 – Derjaguin; Landau; Verwey; Overbeek • Long range attractive van der Waals forces • Long range repulsive electrostatic forces

  10. a) between two plates: b) between two spheres: DLVO – TheoryVan der Waals attractive energy

  11. Double layer models • Helmholtz • Gouy Chapman • Stern

  12. Gouy Chapman model • planar double layer • Ions as point charges

  13. I distribution of ions in the diffuse double layer (Boltzmann equation) II equation for the room charge density III Poisson relation Aus I, II und III folgt: Poisson – Boltzmann - relation Electrolyte theory

  14. For small potentials (< 25 mV) : Integrable form ( ) y 2 d x ( ) = k y 2 x 2 d x ( ) - k y = × y × 0 x x k e Solution of the P-B equation

  15. Resulting repulsive overlap energy • Between two plates: c° – volume concentration of the z – valent electrolyte b) Between two spheres DLVO – TheoryElectrostatic repulsive energy

  16. Vtot(d)= Vattr(d)+ Vrep(d) Vvan der Waals = - A a / 12 d Velectrost. = k e-d A – Hamaker constant a – particle radius d – distance between the particles 1/ - thickness of the double-layer

  17. Electrostatic stabilization • stabilized against coagulation  Kinetically stable state • energetic metastable state in the secondary minimum with an energy barrier

  18. Critical coagulation concentration (CCC) • The energy barrier disappears by adding a critical amount of low molecular salts

  19. Vtot / dd = 0 Vtot = 0  for two spheres: DLVO – Theory(CCC)

  20. DLVO – Theory(CCC) • For two spheres the ccc should be related to the valency (1 : 2 : 3) of the counterions as: 1000 : 16 : 1,3

  21. CCC of a colloidal dispersion as a function of the salt concentration electrolyte CCC of a Arsensulfid -Dispersion Schulze-Hardy-ratio NaCl 5,1 10-2 1000 KCl 5,0 10-2 1000 MgCl2 7,2 10-4 13 CaCl2 6,5 10-4 13 AlCl3 9,3 10-5 1,7

  22. Steric stabilization • What will be happen when we add polymers to a colloidal dispersion ?

  23. Particle – Particle interactions Polymer adsorption layer

  24. Particle – Particle interactions Overlap of the polymer adsorption layer

  25. Overlap of the adsorption layer • Osmotic repulsion • Entropic repulsion • Enthalpic repulsion

  26. Sterically stabilized systems can be controlled by • The thickness of the adsorption layer • The density of the adsorption layer • The temperature

  27. Stabilization and destabilization in dependence on the molecular weight of the added polymer

  28. Stabilization and destabilization in dependence on the polymer-concentration

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