What is a colloid?
• solid or fluid particles 1nm- 1µm
What is a colloidal dispersion?
homogeneous suspensions of solid or liquid particles in a fluid/solid
Describe the properties of a colloidal dispersion
- very high interfacial area
- can form crystals
- large enough to approach bulk properties
- small enough that thermal energy dominates, which leads to Brownian motion
- exhibit non newtonian eggects
Describe the model of Brownian motion
- the net force on a colloidal particle is equivalent to a random walk along a surface
- diffusion coefficient for colloid is much smaller than that of atoms/molecules
- effective model for one colloid at infinite dilution
- ignores mass dependence of diffusion rate
What is diffusion?
- a balance of thermal energy and friction
- independent of other molecules
What is the ballistic regime and when does it happen?
- at short time scales
- mean square displacement becomes proportional to t squared
What can be calculated from the curve of mean displacement?
- the mass of a particle
Why do colloids aggregate and what is this process called?
- to reduce their interfacial energies
- called coagulation
- interactions between colloids are a balance of attractive and repulsive forces
Describe the Van der Waals forces between colloids
- different to interactions between atoms
- depend on colloid geometry and medium they are dispersed in
- strong
- changing surface type changes the VdW strength
What happens when a Van der Waals interaction is repulsive?
- lead to quantum levitation
- used in nano devices with ultra low static friction
Describe the Electrostatic forces in colloids
- charged colloids in water have counter ions
- the counter ions screen electrostatic forces
- solvent is assumed to be a a continuum dielectric with no structure
- can be predicted using the poisson Boltzmann equation
How does salt affect electrostatic interaction
- adding salt to the solution strongly screens the potential
What happens when two charged surfaces approach each other?
- excess osmotic pressure between the two surfaces changes
- repulsive interaction between the two surfaces
what is osmotic pressure?
a measure of the concentration of ions within a solution
What is DLVO theory?
the theory combines electrostatic and vdW forced
What happens a high Debye length?
- the plot of interaction energy vs separation distance has a primary minimum and maximum
- stable colloidal dispersion
What happens at intermediate Debye lengths?
- the plot has a primary maximum and a primary and secondary minimum
- stable or unstable colloidal dispersions (flocculation)
What happens at low Debye length?
- only a primary minimum
- unstable dispersion and coagulation
How can be control stability?
- adding salt
When is coagulation prevented
- when temp> 20kT
- primary maximum is kinetic barrier
How do sterics stabilise colloids?
- as particles approach each other there is an increase osmotic pressure in the inter particle region
- This is an entropic force as the number of configurations for polymers decreases with interparticle distance
- Effective steric stabilisation requires: high surface coverage, strong adsorption (low free polymer concentration) and good solvent conditions X < 0.5
- Chemical approaches: copolymers and surfactants
Why is electrostatic stabilisation hard?
- requires polar solvents and has charged particles are very sensitive to salt
What happens when a small amount of free polymer is added to a colloidal dispersion?
coagulation is induced
Explain the depletion interaction in colloidal dispersions
- Polymers are spherical and polymer radius> colloidal radius
- When two colloids approach there is a depletion region that can’t be reached by the polymer.
- In the depletion region the osmotic pressure is lower than in the bulk solution leading to attraction between the two colloids
- It is an entropic force
- The strength of the depletion force increases linearly with the concentration of polymer and temperature
- The polymer/colloid size ratio also affects the strength of the depletion interaction
- The larger a polymer is, the greater the depletion zone and therefore the greater the attractive force
- As the polymer to size ratio increases so does the range of the interaction
- Affects the phase coexistence diagams
