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

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

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

Types of colloid

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.

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

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

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.

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.

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

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

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

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.

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

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

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

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

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.