Colloidal Dispersed Fomrulations 2 W2 Flashcards
Dispersions can be maintained by controlling what 2 aspects
Particle motions and dynamics
Particle-particle interactions
Particle motions and dynamics
Control Particle Size and Density:
- Smaller particles with a lower density can help reduc the particle settling velocities and stabilise the suspensions.
Adjust Viscosity of the Continuous Phase:
- The viscosity of the continuous phase can be increased to slow down particle movement, inhibit aggregation, and decrease sedimentation.
Particle-particle interactions
Control Interactions between Particles:
- Self- assembly, film coating and spray drying can be used to modify the surfaces of particles and reduce the attractive interactions between them.
Induce Flocculation:
- By manipulating the continuous phase, flocs can be created, ensuring that particles remain in weak attractive interactions with each other.
What causes particles to move
All bodies are subject to forces that act on them and can make them move:
o For large bodies, a significant one is gravity;
o For small bodies (from a few hundred nanometres down to the molecular level), different forces dominate, that together give rise to so-called Brownian motion
What 2 equations are we going to focus on
o Stokes-Einstein-Sutherland diffusion equation
o Stokes’ Sedimentation rate equation.
How can we describe the motion of small particles
For small bodies different force dominate that together give rise to the Brownian motion
The Stokes-Einstein-Sutherland diffusion equation relates the diffusion
that arises from this motion to the energy of the particles and the viscosity
of the medium they are moving in
What is the The Stokes-Einstein-Sutherland diffusion equation
D = KbT / 6πna
- D is the diffusion coefficient (m2/s),
- Kb is Boltzmann’s constant (kB = 1.38 ×10-23 J.K-1),
- T is temperature (K)
- η is the viscosity of the medium (Pa.s)
-a is the (spherical) particle’s radius (m).
What does the Stokes-Einstein-Sutherland diffusion equation tell us
KbT shows the energy
6πna shows the viscosity x particle radius
o The diffusion coefficient itself tells us the size of the area a particle of a given size can explore per second.
o A large particle, at a low temperature, in a very viscous medium, will move only within a small area
o A small particle, at a higher temperature, in a less viscous medium, will be able to explore a larger area.
Particles that dont move very far and aren’t being affected by gravity …
Won’t collide and interact with other particles very often
Small particles in a viscous medium will remain…
Stable in suspension
How can we describe the effect of gravity on larger particles?
o For larger particles (larger than a few hundred nanometres), gravity becomes significant, and particles are subject to Sedimentation.
The Stokes Sedimentation equation describes how the rate of sedimentationdepends on the density of the particles relative to that of the medium, and the
viscosity of the medium they are moving in
What is the The Stokes Sedimentation equation
V = 2𝑎^2g(Pd - Pc) / 9n
- v is the sedimentation rate, or velocity, (m/s),
- ρd is density of dispersed phase components
- ρc is density of continuous phase components
- η is the viscosity of the medium (Pa.s)
- a is the (spherical) particle’s radius (m)
- g is the acceleration due to gravity (m/s2).
What does the Stokes Sedimentation equation tell us
2𝑎^2g(Pd - Pc) shows us the radius squared x acceleration x differences in densities
9n show us the viscosity
o The sedimentation rate tells us how quickly a particle will fall in the continuous medium.
o A large, dense particle, in a low density and low viscosity medium, will fall quickly
o A small, less dense particle, in a dense, viscous medium, will fall more slowly and stay suspended for longer.
Two processes that describe the motion of particles.
Diffusion and sedimentation
Large particles diffuse ……. But sediment ……
Less
More rapidly
