Stability (Macro)

Question 1

Physical stability

Answer 1

• Lyophobic systems have a poor interaction with the solvent
• Suspensions are ‘coarse’ i.e. contain larger particles
• Large particles sediment
• Stokes’ law governs the sedimentation

Question 2

Problems of physical instability

Answer 2

• Clotting in blood
• The medicines has not been fully distributed as its settled

Question 3

Aggregation

Answer 3

Particles in groups

Question 4

Coagulation

Answer 4

Closely aggregated and difficult to redisperse

Question 5

Flocculation

Answer 5

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

Question 6

Sedimentation

Answer 6

Process of settling or being deposited as a sediment

Question 7

Caking

Answer 7

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

Question 8

Ostwald ripening

Answer 8

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

Question 9

Stability

Answer 9

• Kinetic properties:
  
  • Brownian motion and diffusion
  • Sedimentation
  • Viscosity
• Size/shape
• Electrical properties

Question 10

Fick’s first law

Answer 10

• Particles diffuse from a high concentration to a low concentration
  𝑑𝑚/𝑑𝑡=−𝐷𝐴 𝑑𝐶/𝑑𝑥

Question 11

Stokes Law

Answer 11

• Enables you to find what the diffusion coefficent
• 𝐷=(𝑘_𝐵 𝑇)/6𝜋𝜂𝑟

Question 12

Sedimentation

Answer 12

• The rate of sedimentation is dependant on the combined forces of gravity and drag
• Particle falling under the forces of gravity according to Stokes’ law
• 𝑉=(2𝑟^2×(𝜌−𝜌_o)×𝑔)/(9𝜂_o )

Question 13

Viscosity

Answer 13

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

Question 14

Factors influencing the rheology of suspensions

Answer 14

• High volume fractions, f
• Particle size
• Particle size distribution
• Particle shape - flakes cohesive
• Electrostatic interactions - 2 particles come together reduce attraction repels
• Steric hindrance - polymers on surface of particle prefer to be free

Question 15

Shape of particles and stability

Answer 15

• Many sphere but could be sphereoid geometric
• Effect interaction if more flake stay stuck due to high surface area

Question 16

Prevention of sedimentation

Answer 16

• Reduce particle size
• Increase viscosity
• Increase density difference between 2 phases
• Collide less frequently

Question 17

Electical properties of particles

Answer 17

• Surface aquires charge
• Ion dissolution and ioniasation
• Unequal absorption

Question 18

Electrical double layer

Answer 18

• Stern layer lots of positive charge surrounding negative particle with negative charge
• Diffuse layer measure zeta potential negative also positive

Question 19

Zeta potentail

Answer 19

• Positive means magnitude is low there prone aggregate
• > 30mV means more prone to being dispersed

Question 20

Factors that effect Zeta potential

Answer 20

• Number of carboxylic groups higher will ahere to surface of the particle
• increase in charge increases the pKa value

Question 21

DLVO

Answer 21

Vt = Va + Vr
- Va = vander waals forces
- Vr = Electrical repulsion

Question 22

Electrostatic repulsion

Answer 22

• Particle with surface positive charge has layer of negative ions attracted to the surface in the stern layer