Presentation on theme: "Colloid Stability ? A state of subdivision in which the particles, droplets, or bubbles dispersed in another phase have at least one dimension between."— Presentation transcript:
Colloid Stability ?
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
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
Surface atoms [in %] in dependence on the particle size [in nm] % nm
Colloidal systems have large surface areas surface atoms become dominant
Colloid stability Colloidal gold: stabilized against coagulation ! Creme: stabilized against coagulation ! Milk: stabilized against coagulation !
Particle – Particle interactions Interaction Energy ( V tot ) – Distance of Separation (d) Relationship d
V tot (d) = V attr (d) + V rep (d) - Van der Waals attraction - Electrostatic repulsion - Steric repulsion
DLVO - Theory 1940 – Derjaguin; Landau; Verwey; Overbeek Long range attractive van der Waals forces Long range repulsive electrostatic forces
DLVO – Theory Van der Waals attractive energy a) between two plates: b) between two spheres:
Gouy Chapman model planar double layer Ions as point charges
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
Solution of the P-B equation x ekx x xd xd For small potentials (< 25 mV) : Integrable form
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
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
Electrostatic stabilization stabilized against coagulation Kinetically stable state energetic metastable state in the secondary minimum with an energy barrier
Critical coagulation concentration (CCC) The energy barrier disappears by adding a critical amount of low molecular salts
DLVO – Theory (CCC) V tot / dd = 0 V tot = 0 for two spheres:
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
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
Steric stabilization What will be happen when we add polymers to a colloidal dispersion ?