disperse systems - macro Flashcards
What are the key physical stability properties of suspensions?
- 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
in a suspension, two substances may not mix.. what is the first approach?
preventing sedimentation
in a suspension, two substances may not mix.. what is the second approach?
enabling redispersion
e.g shake the medicine for 10 seconds
What are the different types of instability phenomenon? (6 types)
- Aggregation
- Coagulation
- Flocculation
- Sedimentation
- caking
- Ostwald ripening
What is aggregation?
- Particles in groups
What is coagulation?
- Closely aggregated and difficult to redisperse
What is flocculation?
- Aggregates have an open structure with particles a
small distance apart, attracted by weak forces to
form flocs or flakes
What is sedimentation?
- Process of settling or being deposited as a sediment
What is caking?
- Deflocculated particles (fine separate particles)
form cakes which are difficult to re-suspend
What is 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
What does instability lead to?
- Physical instability results in poor dosing reproducibility
What factors affect stability?
- Kinetic properties (motion of the particles with respect to dispersion medium)
- Brownian motion and diffusion
- Sedimentation
- Viscosity
- Size/shape
- Electrical properties
What is Brownian motion and diffusion?
- Particles diffuse from a high concentration to a low concentration
- Diffusion rate is based on Fick’s first law:
𝑑𝑚/ 𝑑𝑡 = −𝐷𝐴 x 𝑑𝐶/𝑑𝑥
What is the diffusion co efficient/ stokes-einstein equation?
𝐷 = 𝑘𝐵𝑇/ 6𝜋𝜂𝑟
* kB = Boltzmann constant
* T = Absolute temperature
* η = Viscosity of medium
* r = Radius of the solute molecule
What is the rate of sedimentation equation?
𝑉 = 2𝑟2 × (𝜌 − 𝜌o) × 𝑔/9𝜂o
* v = sedimentation rate
* r = particle radius
*
P= density of the disperse phase
*
P0 = density of the continuous phase
* g = gravity
* η0 = viscosity of the continuous phase
What is sedimentation dependant on?
- 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
what are some extra rules about the sedimentation rate?
- only applies to > 0.5 μm
*If 𝜌 − 𝜌o < 0 then creaming rather than caking
What is viscosity?
- Related to molecular weight of suspended particles/suspending agents
- Resistance to flow under an applied stress
What factors affect rheology suspensions?
a) High volume fractions, f
b) Particle size
c) Particle size distribution
d) Particle shape
e) Electrostatic interactions
f) Steric hindrance
What are the shape of most molecules?
Many suspended particles are spherical
* Several measurement techniques assume a sphere
small deviation shape?
- Ellipsoidal model
large deviation shape possibilities?
- Hydroxyapatite (rod-shaped)
- Clay suspension (plate)
- Polymers in solution (coil)
How to prevent sedimentation? (relating to the equation)
- Form smaller particles
- Decrease the density difference between the two phases
- Increase viscosity of continuous phase
What are the two layers of the electrical double layer of ions?
- stern layer
- diffuse layer
