Colloids

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Colloids

• An overview
• http//sst.tees.ac.uk/external/U0000504/Notes/mscn
  otes/

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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

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Types of colloid
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Why dont 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.

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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

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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 Ficks law which is on
  the right

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Kinetic Stabilisation

• Kinetic stabilisation is the result of a
  combination of Stokes law and Ficks 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.

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Molecular attraction

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

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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

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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

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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

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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.

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Stearic repulsion

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

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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

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Breakdown of electrostatically stabilised colloids

• The DVLO theory explains the breakdown of such
  colloids
• It is based on a balance between the Van der
  Waals forces and the repulsive forces. Thus
• Eint Eatt Erep
• Adding electrolyte tends to reduce double layer
  thickness, such that Eatt becomes dominant

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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

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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.

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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/mscn
  otes/