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Colloidal Aggregation

Colloidal Aggregation. FDSC400. Goals. Aggregation rate Interaction potentials Electrostatic repulsion Van der Waals attraction DVLO theory Steric repulsion. Second order reaction (fast kinetics):. Aggregation Rate. or. 2P P 2. Smoulokowski Kinetics.

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Colloidal Aggregation

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  1. Colloidal Aggregation FDSC400

  2. Goals • Aggregation rate • Interaction potentials • Electrostatic repulsion • Van der Waals attraction • DVLO theory • Steric repulsion

  3. Second order reaction (fast kinetics): Aggregation Rate or 2P P2

  4. Smoulokowski Kinetics • Gives the rate of collision of freely diffusing particles • We know the diffusion coefficient of spheres • Combining D = diffusion coefficient, r = radius, h = continuous phase viscosity

  5. Slow Kinetics • Smoulokowski kinetics assumes each collision leads to a droplet aggregation. • In fact only a tiny proportion of collisions are reactive DG 2P G kslow=kfast/W P2 Function of energy barrier

  6. An Interaction Potential • A plot showing energy to move a particle from an infinite distance to a given distance from a second particle. • The pair of particles will try to find the optimum separation to minimize energy but can be blocked by a significant (>2-3 kT) energy barrier.

  7. Van der Waals Attraction Hamaker constant ~5e-21 J • Always attractive • Very short range

  8. Surface Charge y=y0e-kh

  9. Effective Charge Surface charge Effective charge at distance h y=y0e-kh Distance from surface Reciprocal Debye length – increases with ionic strength Low ionic strength=long range High ionic strength=short range

  10. Electrostatic Repulsion • Repulsive or attractive depending on sign of charges • Magnitude depends on magnitude of the charge • Gets weaker with distance but reasonably long range • Short range at high I

  11. DVLO Theory • VDVLO=VVdW+Velectrostatic • The height of the barrier increases with surface potential • Its width increases with decreasing I

  12. Steric Repulsion Droplets approach each other Protein layers overlap Proteins repel each other mechanically & by osmotic dehydration What happens when protein molecules on different droplets are reactive?

  13. Dispersed Systems-summary- • Types of dispersed system • Surface tension • Surface active materials • Properties of emulsions • Mechanisms of emulsion destabilization (stabilization) • Foams • Aggregation kinetics • DVLO theory and modifications

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