W5 Stability (Macro)

Question 1

Aggregation

Answer 1

Particles in groups

Question 2

Coagulation

Answer 2

Closely aggregated and difficult to redisperse

Question 3

Flocculation

Answer 3

Aggregates have an open structure with particles a small distance apart, attracted by weak forces to form flocs or flakes

Question 4

Sedimentation

Answer 4

Process of settling or being deposited as a sediment

Question 5

Caking

Answer 5

Deflocculated particles (fine separate particles) form cakes which are difficult to re-suspend

Question 6

Ostwald ripening

Answer 6

Dissolution of small crystals or sol particles and the re-deposition of dissolved species on the surfaces of larger crystals or sol particles

Question 7

What equation calculates rate of sedimentation?

Answer 7

Stokes’ law

Question 8

What is viscosity?

Answer 8

Related to molecular weight of suspended particles/suspending agents
* Resistance to flow under an applied stress

Question 9

Factors influencing the rheology of
suspensions

Answer 9

a) High volume fractions, f
b) Particle size
c) Particle size distribution
d) Particle shape
e) Electrostatic interactions
f) Steric hindrance

Question 10

How can sedimentation be prevented?

Answer 10

Refer to stokes equation (top line/bottom line)
- Decrease particle size and density difference between 2 phases
- Increase viscosity of continuous phase

Question 11

Why does sedimentation need to be prevented?

Answer 11

Uneven distribution of particles

Question 12

Electrical properties

Answer 12

Most surfaces acquire charge
* Various charging mechanisms
* Ion dissolution
* (Ca2+10(PO43-)6(OH-)2) (solid) ⇌ 10Ca2+ (aq) + 6PO43- (aq) + 2OH- (aq)
* Ionisation
* Citrate COO-
* Polystyrene latex COO-
* Amino acids and proteins COO- and NH3+
* Unequal ion adsorption

• Electrical double layer of ions
• Stern layer
• Diffuse layer
• Zeta potential = magnitude and
  type (+ or –) of the electrical
  potential at the slipping plane
• Low zeta potential (0 to 5 mV) are
  prone to aggregate
• Zeta potential > 30 mV tend to
  remain dispersed

Question 13

Factors affecting zeta potential

Answer 13

• Ion concentration- charge of ions determines magnitude
• pH of continuous phase- Alters the ionisation of ionic species in the continuous phase and the surface
  charge of ionisable groups

Question 14

A weak force of attraction is also known as:

Answer 14

Flocculation

Question 15

What is DLVO?

Answer 15

Quantitative approach to the stability of lyophobic systems
* Assumes the only interactions involved are
* Van der Waals forces of attraction (VA)
* Electrostatic repulsive forces (VR)

Question 16

What is an anticoagulant?

Answer 16

A medication that helps prevent blood clots