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Colloids

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

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Colloids

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

  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. • Or go directly to • http://sst.tees.ac.uk/external/U0000504/Notes/mscnotes/

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