Stability (Macro) Flashcards
Physical stability
- 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
Problems of physical instability
- Clotting in blood
- The medicines has not been fully distributed as its settled
Aggregation
Particles in groups
Coagulation
Closely aggregated and difficult to redisperse
Flocculation
Aggregates have an open structure with particles a small distance apart, attracted by weak forces to form flocs or flakes
Sedimentation
Process of settling or being deposited as a sediment
Caking
Deflocculated particles (fine separate particles) form cakes which are difficult to re-suspend
Ostwald ripening
Dissolution of small crystals or sol particles and the re-deposition of dissolved species on the surfaces of larger crystals or sol particles
Stability
- Kinetic properties:
- Brownian motion and diffusion
- Sedimentation
- Viscosity
- Size/shape
- Electrical properties
Fick’s first law
- Particles diffuse from a high concentration to a low concentration
𝑑𝑚/𝑑𝑡=−𝐷𝐴 𝑑𝐶/𝑑𝑥
Stokes Law
- Enables you to find what the diffusion coefficent
- 𝐷=(𝑘_𝐵 𝑇)/6𝜋𝜂𝑟
Sedimentation
- 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 )
Viscosity
- Related to molecular weight of suspended particles/suspending agents
- Resistance to flow under an applied stress
Factors influencing the rheology of suspensions
- 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
Shape of particles and stability
- Many sphere but could be sphereoid geometric
- Effect interaction if more flake stay stuck due to high surface area
Prevention of sedimentation
- Reduce particle size
- Increase viscosity
- Increase density difference between 2 phases
- Collide less frequently
Electical properties of particles
- Surface aquires charge
- Ion dissolution and ioniasation
- Unequal absorption
Electrical double layer
- Stern layer lots of positive charge surrounding negative particle with negative charge
- Diffuse layer measure zeta potential negative also positive
Zeta potentail
- Positive means magnitude is low there prone aggregate
- > 30mV means more prone to being dispersed
Factors that effect Zeta potential
- Number of carboxylic groups higher will ahere to surface of the particle
- increase in charge increases the pKa value
DLVO
Vt = Va + Vr
- Va = vander waals forces
- Vr = Electrical repulsion
Electrostatic repulsion
- Particle with surface positive charge has layer of negative ions attracted to the surface in the stern layer
