emulsions

• name some examples of pharmaceutical emulsions...

  Emulsions and cream, total parenteral nutrition (TPN), oral administration of therapeutic agents
• define emulsion

  An emulsion is a fluid obtained through the inter-dispersion of two or more in principle immiscible liquids, typically an OIL (i.e. a hydrophobic liquid) and WATER. One liquid will be dispersed as globules in another.
• the types of emulsion...

  oil in water (o/w)

  water in oil (w/o)

  multiple emulsions (w/o/w)

  and micro emulsions
• to form a dispersed system where the colloids are hydrophobic what do you need to provide?

  energy - e.g. shaking or any mechanical mechanism
• what is a regular emulsion?

  when there is lots of water, little oil and the right amount of surfactant. after shaking there will be droplets of oil surrounded by surfactants or any amphiphilic molecules in water
• in a regular emulsion which part is the continuous phase?

  the water
• what is an inverted emulsion?

  relatively low amount of water, a large amount of oil and the right amount of surfactant. after shaking there'll be droplets of water in oil.
• in an inverted emulsion, what part is the continuous phase?

  the oil
• why are emulsions needed for pharmaceutical use?

  to help deliver drugs that are not only poorly soluble but hydrophobic as well
• what is the characteristic of emulsions?

  they are inherently unstable - this means that the physical stability (presence of droplets) is essential
• what are the formulation considerations for emulsions?

  • Flow properties are important: an emulsion or a cream needs to be removed easily from the container or spread on the affected area (e.g. skin) - needs the right viscosity
  • The formulation must be aesthetically and texturally pleasing.
• the appearance of emulsions is affected by what?

  the size of the droplets
• course emulsions -> droplet size 10-100um appears to be what colour?

  white (when there's a high concentration of droplets or milky in appearance
• fine emulsions -> droplet size: 0.1-5um will appear to look like what?

  cloudy appearance (small droplets diffract less light so looks more transparent)
• micro-emulsions → droplet size: 0.01-0.1um will appear to look like what?

  will be transparent
• what are the four different tests that can be used to determine the type of emulsion?

  Phase dilution

  Dye solubilisation

  Conductivity

  Filter paper wetting
• what is the phase dilution test?

  • Adding drops of emulsion to water results in
  • (i) dilution if it's an o/w emulsion as droplets disperse in water or in
  • (ii) an oily continuous phase that does not disperse for w/o emulsions (so there's phase separation)
• how does the dye solubilisation test determine the type of emulsion?

  the use of an oil-soluble dye will lead to

  • coloured droplets for an o/w emulsion
  • clear droplets for a w/o emulsion
• how does conductivity determine the type of emulsion?

  water has a higher conductivity than oil thus conductivity of o/w is higher than w/o emulsions
• how does filter paper wetting determine the type of emulsion?

  o/w emulsions spread rapidly while w/o spread very slowly
• what is the internal phase fraction?

  the volume (or height) of the internal phase divided by the total volume of the emulsion and expressed as a percentage.

  • also called oil fraction/percentage

  
• emulsification requires work...

  • Formation of an emulsion is not spontaneous.
  • 2 immiscible liquids suggest high surface tension
  • Produce work to increase the surface area between the 2 phases.
  • Emulsifying agents may be applied to reduce the work
• there are a few types of instability - name 3 types

  creaming, coalescence and phase separation
• what is the work applied to oil-water mixtures?

  The deformation of the L/L interface by shear forces.

  Large droplets are disrupted again by shear forces to smaller ones.
• what is the creaming of an emulsion?

  Results in a lack of uniformity of drug distribution which leads to variable dosage. Emulsion in some cases and be shaken to separate the particles but this leads to flocculation (where you get associated particles together)
• what are the 2 types of creaming?

  • Upward creaming: The dispersed phase is less dense than the continuous phase. Normally observed in o/w emulsions. The velocity of sedimentation is negative.
  • Downward creaming: The dispersed phase more dense than the continuous phase. The velocity of sedimentation is positive. Normally observed in w/o emulsions.
• creaming is influenced by what?

  • Particles size,
  • Viscosity of the continuous phase,
  • Difference of density between dispersed and continuous phases.
• what is coalescence?

  The merging or joining together of separate entities or particles into one. If a droplet comes in contact with another droplet coalescence occurs and results in a larger droplet and a reduction in the surface tension and surface area decreases.
• what is the reason for coalescence in emulsions?

  Insufficient amount of surfactants or polymers in the emulsions or incompatibilities between them.
• what is the breaking of the emulsion or phase separation?

  Breaking is the destruction of thee film surrounding particles (e.g. surfactants)

  The separation of oil and the aqueous phase is called the breaking of the emulsion.

  Complete separation is an irreversible process
• what is the critical value of the internal phase fraction for o/w and w/o emulsions?

  O/W emulsions, the critical value is 74%

  W/O emulsions, the critical value is approx 40%
• what does the critical value refer to?

  the maximum internal phase fraction at which the emulsion can remain stable in the type of emulsion. so if the internal phase fraction exceeds the critical value the emulsion becomes more unstable and may undergo phase separation.
• in practice, stable o/w emulsions are obtained at about 50% why is that?

  because the lower the volume of the internal phase the lower the probability of droplets coalescence.
• how are emulsions stabilised?

  by adsorbing surfactants or polymers on the droplet surface
• charge surfactants adsorbing to droplets leads to what?

  charged stable droplets
• non-ionic surfactants stabilise by forming a hydrophilic layer on the oil droplet
• overall a hydrophobic droplet is converted to a hydrophilic droplet
• surfactant at the surface of an aqueous droplet (w/o emulsion) are stabilised by steric repulsive forces
• what are other emulsion stabilisers?

  macromolecules and solid particles
• how to macromolecules stabilise emulsions?

  adsorb at the surface and prevent coalescence by steric hindrance