Colloidal Aggregation FDSC 400 Goals Aggregation rate Interaction

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Colloidal Aggregation FDSC 400

Colloidal Aggregation FDSC 400

Goals • • • Aggregation rate Interaction potentials Electrostatic repulsion Van der Waals attraction

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

Aggregation Rate • Second order reaction (fast kinetics): or 2 P P 2

Aggregation Rate • Second order reaction (fast kinetics): or 2 P P 2

D = diffusion coefficient, r = radius, h = continuous phase viscosity Smoulokowski Kinetics

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

Slow Kinetics • Smoulokowski kinetics assumes each collision leads to a droplet aggregation. •

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

An Interaction Potential • A plot showing energy to move a particle from an

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 k. T) energy barrier.

Van der Waals Attraction • Always attractive • Very short range Hamaker constant ~5

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

Surface Charge y=y 0 e-kh

Surface Charge y=y 0 e-kh

Effective Charge Effective charge at distance h Surface charge y=y 0 e-kh Distance from

Effective Charge Effective charge at distance h Surface charge y=y 0 e-kh Distance from surface Reciprocal Debye length – increases with ionic strength Low ionic strength=long range High ionic strength=short range

Electrostatic Repulsion • Repulsive or attractive depending on sign of charges • Magnitude depends

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

DVLO Theory • VDVLO=VVd. W+Velectrostatic • The height of the barrier increases with surface

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

Steric Repulsion Droplets approach each other Protein layers overlap Proteins repel each other mechanically

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?

Dispersed Systems -summary • • • Types of dispersed system Surface tension Surface active

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