Emulsions Flashcards
What is an emulsion?
A fluid obtained throught the interdispersion of two or more immiscible liquids, typically an oil and water
What are the types of emulsions?
Oil in water (o/w)
Water in oil (w/o)
Multple emulsions (w/o/w)
Microemulsions
Are emulsions stable?
No.
The presence of droplets is essential for stability
How does the size of droplets affect the appearance of an emulsion?
Coarse emulsions - droplet size: 10-100um = milky
Fine emulsions - droplet size: 0.1-5um = cloudy
Microemulsions - droplet size: 0.01-0.1um = transparent
What are the four ways used to determine the type of an emulsion?
PHASE DILUTION - adding drops of emulsion to water. Emulsion will only mix with a liquid that is miscible with its continuous phase
DYE SOLUBILISATION - use of an oil soluble dye and microscope - o/w are paler than w/o
CONDUCTIVITY - systems with an aq continuous phase will conduct electricity (o/w) whereas oily continuous phase will not (w/o)
FILTER PAPER WETTING - blue to pink when exposed to stable o/w emulsion
What is the definition of ‘the fraction of internal phase’?
vol of internal phase/total vol of emulsion (as a %)
Is emulsification a spontaneous process?
No - energy (work) needs to be put in.
What are the types of instability of emulsions?
Creaming - happens due to different densities of the two liquids. Oil less dense than water so oil droplets at top.
Creaming -> flocculation - however this only happens when internal phase is <5% and droplet size is small.
Flocculation -> coalescence -> phase separation
Is flocculation ideal in emulsions?
No flocculation is bad as it means that the two droplets are close together which leads to coalescence and larger droplets. This leads to separation and therefore no longer an emulsion
How is coalescence related to surface tension?
Two droplets come into close contact with each other and leads to coalescence of droplets and a larger droplet is formed. This results is REDUCED surface tension.
What is the critical value of the fraction of internal phase for an o/w emulsion?
74%
What is the critical value of the fraction of the internal phase of an w/o emulsion?
40%
How are emulsions stabilised?
Adsorbing surfactants or polymers on the surface of droplets
What is the effect of adding a surfactant to an emulsion?
Charged surfactants lead to charged stable droplets
Non ionic surfactants stabilise emulsions by the formation of a hydrophilic layer on the oil droplet
On addition of a surfactant a hydrophobic droplet is converted into a hydrophilic droplet
Surfactant at the surface of an aq droplet (w/o) are stabilised by steric repulsive forces
What is the hydrophile lipophile balance (HLB)?
It is related to the volume ratio between the hydrophilic and lipophilic part of the surfactant.
As a general rule: the higher the hydrophilicity of a surfactant, the higher the HLB value
