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Colloid Stability ?

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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 Colloidal systems

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Surface area of colloidal systems Cube (1cm; 1cm; 1cm) after size reduction to an edge length of 500 nm: surface area of 60 m 2 Spinning dope (1 cm 3 ) 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 m 2

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

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Colloidal systems have large surface areas surface atoms become dominant

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Colloid stability Colloidal gold: stabilized against coagulation ! Creme: stabilized against coagulation ! Milk: stabilized against coagulation !

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Particle – Particle interactions Interaction Energy ( V tot ) – Distance of Separation (d) Relationship d

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V tot (d) = V attr (d) + V rep (d) - Van der Waals attraction - Electrostatic repulsion - Steric repulsion

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

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DLVO – Theory Van der Waals attractive energy a) between two plates: b) between two spheres:

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Double layer models Helmholtz Gouy Chapman Stern

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Gouy Chapman model planar double layer Ions as point charges

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Electrolyte theory 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

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Solution of the P-B equation x ekx x xd xd For small potentials (< 25 mV) : Integrable form

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DLVO – Theory Electrostatic repulsive energy Resulting repulsive overlap energy a)Between two plates: c° – volume concentration of the z – valent electrolyte b) Between two spheres

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V tot (d) = V attr (d) + V rep (d) V van der Waals = - A a / 12 d V electrost. = k e - d A – Hamaker constant a – particle radius d – distance between the particles 1/ - thickness of the double-layer

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

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

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DLVO – Theory (CCC) V tot / dd = 0 V tot = 0 for two spheres:

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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

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CCC of a colloidal dispersion as a function of the salt concentration AlCl 3 CaCl 2 MgCl 2 KCl NaCl electrolyte 1,79, , , , , Schulze-Hardy-ratioCCC of a Arsensulfid -Dispersion

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

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Particle – Particle interactions Polymer adsorption layer

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Particle – Particle interactions Overlap of the polymer adsorption layer

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Overlap of the adsorption layer Osmotic repulsion Entropic repulsion Enthalpic repulsion

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

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

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Stabilization and destabilization in dependence on the polymer-concentration

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