1. An overview http://sst.tees.ac.uk/external/U0000504/Notes/mscnotes/
Colloids
An overview

2. What is a colloid? A colloid is a suspension of particles in a medium
The particles may be solid, liquid or gas and are called the disperse phase
The medium may also be solid liquid or gas and is called the continuous phase

3. Types of colloid

4. Why don’t colloids break down?
They do. Colloids are inherently unstable
Gravitational forces and attractive forces on the surface of the particles will cause the two phases to separate
This is prevented by Brownian motion and charges on the surface of the particles which keep the particles suspended.

5. Stokes’ Law A particle in a fluid may descend or rise
The rate at which it does depends on a balance between gravity, buoyancy and friction
This results in a constant terminal velocity, the value of which is given by Stokes Law

6. Brownian Motion
Brownian is the result of the continuous buffeting of the colloidal particles by the molecules of the continuous phase.
It gives rise to the diffusion where the particles will migrate from a region of high concentration to one of low concentration
Diffusion is defined by Fick’s law which is on the right

7. Kinetic Stabilisation
Kinetic stabilisation is the result of a combination of Stokes law and Fick’s law effects
Particles will tend to settle as a result of Stokes’ law
This sets up a concentration gradient which causes diffusion in the opposite direction to settling
If the two are in equilibrium the colloid will be stable.

8. Molecular attraction
Colloidal particles tend to be attracted to one another as a result of Van der Waal’s forces between the molecules on the particle surface
This results in agglomeration of particles and the break up of the colloid.
Van der Waal’s forces are inversely proportional to the square of the distance between colloidal particles.

9. Electric double layer
Some colloidal particle have charges on their surface
If the continuous phase is an electrolyte, ions are attracted to the particle surface
This results in a charged layer near the particle surface which decays exponentially from the surface
This is known as the electrical double layer

10. Surface Tension The molecules of a liquid will attract each other
In the body of the liquid this attraction is equal all round
At the surface, the attraction is unbalanced
This imbalance of attractive forces is called surface tension

11. Emulsifiers An emulsifier molecule comprises two sections
A hydrophyllic (Water loving head) and a hydrophobic (Water hating) tail
Such molecules are called surfactants
They form a layer on the droplet surface

12. Emulsifiers - continued
The tail of the molecule is in the oil and the head in the water
“Spare” molecules cluster in the form of micelles
This aids the break up of droplets into smaller ones
Charges on the surface of the emulsifier keep the droplets apart.

13. Stearic repulsion
This is a result of macromolecules adhering to the particle surface
The shape and conformation of the molecule prevents aggregation of particles

14. Breakdown of colloids
The breakdown of a colloid is the result of particles coming together to form larger particles. There are three basic forms
Flocculation
Coagulation
Coalescence

15. Breakdown of electrostatically stabilised colloids
The DVLO theory explains the breakdown of such colloids
It is based on a balance between the Van der Waal’s forces and the repulsive forces. Thus
Eint = Eatt + Erep
Adding electrolyte tends to reduce double layer thickness, such that Eatt becomes dominant

16. Bridging and Depletion flocculation
Bridging flocculation is the result of macromolecules becoming attached to two particles
Depletion flocculation is the result of an osmotic effect where macromolecules act as a semi-permeable membrane

17. Coalescence
Coalesence is the merging of two liquid droplets into a single droplet
The process involves expulsion of the continuous phase from between the droplets
The ease with which this occurs depends on surface tension and continuous phase viscosity.

18. For more information This lecture has only been an overview.
More information is available via the Module website.
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