disperse systems

• define colloidal dispersions

  a mixture in which one substance which has fine particles (dispersed phase) is mixed into another substance (dispersion medium)
• what is the range of size for colloidal dispersions?

  ranges within 1nm to 1mm
• examples of types of colloids...

  emulsions, suspensions and aerosols
• colloids can be divided into two categories, what are they?

  lyophilic colloids and lyophobic colloids
• lyophilic colloids...

  • the affinity for dispersion medium is high - easily/uniformly dispersed throughout the medium without the need for additional stabilisers + less likely to aggregate
  • spontaneous formation of colloidal dispersion
  • when the dispersion medium is water = hydrophilic
  • tends to form a solution
• examples of lyophilic colloids...

  natural polymers (acacia, tragacanth, methylcellulose) proteins (albumin)
• lyophobic colloids...

  • preparation requires:
  • the breaking of large particles of colloidal dimensions
  • aggregation of smaller particles e.g. when you have iodide and silver
  • need to provide energy to break large particles
  • when the dispersion medium is water = hydrophobic
• what are the properties of colloids?

  • they are polydisperse
  • exists in several shapes
  • it shows the Tyndall effect
  • don't settle down when left undisturbed for some time
  • cannot be separated by the filtration process
  • colloidal particles show Brownian movement
• what is the Tyndall effect?

  This means that colloid-dispersed particles can scatter a beam of light.
• what is Brownian motion?

  the random motion by small particles suspended in the fluid.
• frequent encounters between colloids can lead to what?

  • Coagulation - permeant contact between particles
  • Aggregation - next step after coagulation; insoluble aggregates = lose solubility = and can result in sedimentation at the bottom.
  • Flocculated particles - temporary contact
  • Dispersed particles - rebound and remain free as single particles
• what is the DLVO theory?

  Explains the interactions between colloidal particles and the coagulation and stabilisation. Sums together the molecular forces of attraction and repulsion and seeing what the net affect is.
• the DLVO theory combines Van der Waals forces of attraction and the force of repulsion that is defined on the electrical double layer theory.
• electrical double-layer concept...

  explains the distribution of electrical charges near the interface between colloidal particles and the surrounding fluid
• what are the layers that consist of the electrical double layer?

  • Stern layer and the diffuse layer
• The net charge within the diffuse layer counteracts the charge on the colloidal particle's surface, creating an overall electrically neutral region beyond a certain distance from the surface. This creates a buffering zone where the charge declines over distance.
• what does the creation of the buffer zone do?

  The buffering zone created by the electrical double layer helps maintain the stability of colloidal dispersions by minimising interactions between particles and facilitating their dispersion within the medium.
• role of the electrical double layer...

  plays a critical role in stabilising colloidal systems by providing a mechanism for repulsion between similarly charged particles, preventing their aggregation or coagulation
• describe this image...

  As the distance between two particles gets smaller, the greater the repulsive force between the particles and the stronger the forces of attraction.

  After a certain distance, repulsive forced disappears.

  
• when two colloidal particles get together in solution - they both exhibit forces of attraction and forces of repulsion for each other
• what does the black curve represent?

  The net effect curve - the result of the attractive and repulsive forces cancelling one other.

  
• which state out of the primary minimum and secondary minimum do you not want to achieve?

  The primary minimum - as at this state the attractive force is greater than the repulsive force and so the particles are attracted to one another and coagulate (become unstable). Colloids cannot separate after it has coagulated.
• what are the ways you can control the stability of lyophobic colloidal systems?

  1. Controlling the primary maximum via repulsion forces
  2. Controlling secondary minimum
  3. Adding electrolytes
• how do you control the stability in lyophobic systems by controlling the primary maximum?

  Want to have a high primary maximum as it helps stabilise the formulation. This is done by having strong repulsion forces.

  • primary maximum >> thermal energy (kBT) - system is stable ( if two particles come close together they can repel and separate.
  • primary maximum << thermal energy - system is unstable (they will be in the primary minimum and aggregate)
• primary maximums...

  A) stable

  B) unstable

  
• • Primary maximum >> thermal energy (kBT): the system is stable. Not sufficient energy to overcome the maximum: no close contact.
  • Primary maximum << thermal energy (kBT): the system is unstable. Irreversible changes possibly leading to coagulation or precipitation
• how do you control the stability in lyophobic systems by controlling the secondary minimum?

  • Secondary minimum << thermal energy (kBT): the particles always repel each other. no close contact so won’t aggregate. Seen in most particles in a suspension.
  • Secondary minimum is moderately deep: the system gives rise to loose, easily reversible form of flocculation (important for emulsions and suspensions)
• secondary minimums...

  A) repel

  B) flocculation

  
• what kind of secondary minimum is important for emulsions and suspensions?

  One that is moderately deep.
• how does adding electrolytes affect lyophobic systems?

  Adding electrolytes compresses the double layer, and reduces the repulsion forces thus reducing the primary maximum and may cause instability. Lower the repulsion forces, and decrease the primary maximum to the point where the primary minimum is reached.
• ways to control the stability of lyophilic systems...

  • steric stabilisation
  • pH adjustments
• controlling stability of lyophilic systems...

  • solutions of macromolecules (like proteins) use a combination of electric double-layer interaction and salvation to control.
  • unaffected by small amounts of added electrolytes
  • high amounts of electrolytes = water salvation is lost leading to salting-out effects
• steric stabilisation for lyophilic systems...

  involves the use of polymer chains or surfactants adsorbed onto the surface of colloidal particles. these molecules create a steric barrier that prevents close approach and aggregation of particles by providing a repulsive force between them. stabilising the colloids. but there is a loss of hydration.
• steric sterilisation is effective in preventing what?

  particle flocculation and sedimentation
• Stroke law...

  Influences the stability of colloidal systems:

  • Sizes, density and viscosity affects the velocity of movement of the disperse phase thus its stability
  • The larger the colloid more likely it’s de-stabilised. Solid is much more likely to sediment.
• summary of properties of lyophobic and lyophilic sols -

  ...

  A) sensitive

  B) aggregation

  C) electrolyte

  D) repulsion

  E) low

  F) symmetric

  G) stable

  H) high

  I) pI

  J) solvation

  K) negative

  L) asymmetric