Part 4

Question 1

What is a colloid?

Answer 1

• solid or fluid particles 1nm- 1µm

Question 2

What is a colloidal dispersion?

Answer 2

homogeneous suspensions of solid or liquid particles in a fluid/solid

Question 3

Describe the properties of a colloidal dispersion

Answer 3

• 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

Question 4

Describe the model of Brownian motion

Answer 4

• 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

Question 5

What is diffusion?

Answer 5

• a balance of thermal energy and friction

- independent of other molecules

Question 6

What is the ballistic regime and when does it happen?

Answer 6

• at short time scales

- mean square displacement becomes proportional to t squared

Question 7

What can be calculated from the curve of mean displacement?

Answer 7

• the mass of a particle

Question 8

Why do colloids aggregate and what is this process called?

Answer 8

• to reduce their interfacial energies
• called coagulation
• interactions between colloids are a balance of attractive and repulsive forces

Question 9

Describe the Van der Waals forces between colloids

Answer 9

• different to interactions between atoms
• depend on colloid geometry and medium they are dispersed in
• strong
• changing surface type changes the VdW strength

Question 10

What happens when a Van der Waals interaction is repulsive?

Answer 10

• lead to quantum levitation

- used in nano devices with ultra low static friction

Question 11

Describe the Electrostatic forces in colloids

Answer 11

• 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

Question 12

How does salt affect electrostatic interaction

Answer 12

• adding salt to the solution strongly screens the potential

Question 13

What happens when two charged surfaces approach each other?

Answer 13

• excess osmotic pressure between the two surfaces changes

- repulsive interaction between the two surfaces

Question 14

what is osmotic pressure?

Answer 14

a measure of the concentration of ions within a solution

Question 15

What is DLVO theory?

Answer 15

the theory combines electrostatic and vdW forced

Question 16

What happens a high Debye length?

Answer 16

• the plot of interaction energy vs separation distance has a primary minimum and maximum
• stable colloidal dispersion

Question 17

What happens at intermediate Debye lengths?

Answer 17

• the plot has a primary maximum and a primary and secondary minimum
• stable or unstable colloidal dispersions (flocculation)

Question 18

What happens at low Debye length?

Answer 18

• only a primary minimum

- unstable dispersion and coagulation

Question 19

How can be control stability?

Answer 19

• adding salt

Question 20

When is coagulation prevented

Answer 20

• when temp> 20kT

- primary maximum is kinetic barrier

Question 21

How do sterics stabilise colloids?

Answer 21

• 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

Question 22

Why is electrostatic stabilisation hard?

Answer 22

• requires polar solvents and has charged particles are very sensitive to salt

Question 23

What happens when a small amount of free polymer is added to a colloidal dispersion?

Answer 23

coagulation is induced

Question 24

Explain the depletion interaction in colloidal dispersions

Answer 24

• 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