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Colloids An overview

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1 Colloids An overview

2 What is a colloid? b A colloid is a suspension of particles in a medium b The particles may be solid, liquid or gas and are called the disperse phase b 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? b They do. Colloids are inherently unstable b Gravitational forces and attractive forces on the surface of the particles will cause the two phases to separate b This is prevented by Brownian motion and charges on the surface of the particles which keep the particles suspended.

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

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

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

8 Molecular attraction b 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 b This results in agglomeration of particles and the break up of the colloid. b Van der Waal’s forces are inversely proportional to the square of the distance between colloidal particles.

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

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

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

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

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

14 Breakdown of colloids b 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 b The DVLO theory explains the breakdown of such colloids b It is based on a balance between the Van der Waal’s forces and the repulsive forces. Thus  E int = E att + E rep b Adding electrolyte tends to reduce double layer thickness, such that E att becomes dominant

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

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

18 For more information b This lecture has only been an overview. b More information is available via the Module website. b Or go directly to  mscnotes/

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