PHA 327 - Exam 2 - Emulsions 5
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What type of emulsions are Hydrophilic colloids usually used for?
Are hydrophilic colloids usually used with other emulsifying agents?
Yes, though they can be used alone, they are usually used with other or thickening agents
When applied to water what happens to hydrophilic colloids?
They swell or dissolve in water
What does a particulate film produced by finely divided solids prevent in an emulsion?
Coalescence of particles
What subgroups are included in Finely Divided Solids?
- Non-swellable clays
- Polar and non-polar inorganic solids
Non-swellable clays and polar and non-polar inorganic solids are in what group of Emulsifiers?
Finely divided solids
Polar solids make better emulsifiers for what type of emulsion?
Why do polar solids make better emulsifiers for O/W emulsions?
They are wetted better by water
Non-polar solids make better emulsifiers for what type of emulsion?
W/O (in the absence of wetting agents)
What are the three problems commonly encountered with emulsions?
- Creaming or sedimentation
- Phase inversion
- Aggregation and Coalescence
What is creaming of an emulsion?
The upward movement of dispersed particles in an emulsion
What is sedimentation?
The downward movement of dispersed particles
Do creaming and sedimentation represent the breakdown of the emulsion?
No, they maintain their individual particles and only need mild agitation to re-disperse
How do you fix an emulsion that has creamed?
How can you determine the rate that creaming or sedimentation will occur in an emulsion?
Using Stoke’s equation
What is phase inversion?
- When an O/W emulsion becomes a W/O emulsion
- Vice versa
What is the term for the situation where an O/W emulsion becomes a W/O emulsion or vice versa?
What is stok’s equation?
V = d2(ps-po)g/18n
V = velocity of sedimentation(cm/sec)
- D= diameter of particles
- ps =density of dispersed phase(g/cm3)
- po= density of dispersion medium (g/cm3)
- g = acceleration due to gravity (981 cm/sec2)
- n = viscosity of dispersion medium (poise)
Why does creaming and sedimentation occur?
- Due to density differences between the internal and the external phase
- Internal phase is less dense =creaming
- Internal phase is more dense =sedimentation
Why is creaming undesirable?
- Increases the chance of coalescence
- Is inelegant
- May lead to variable dosing due to lack of uniformity
How can you decrease the chance of creaming?
- Reduce particle size
- Increase the viscosity of the dispersion medium
- Adjust the density differences between the two phases
How can you increase the viscosity of the dispersion phase to prevent creaming?
Add thickening agents
What is aggregation of an emulsion?
Dispersed droplets come together but do not fuse
What is it called when dispersed droplets come together but do not fuse?
What is Coalescence?
The complete fusion of dispersion droplets and the separation into two immiscible liquids
What is the term for the complete fusion of dispersion droplets?
Coalescence is preceded by what?
Aggregation can lead to what?
Is aggregation reversible?
To some extent, yes
Is Coalescence reversible?
What is Coalescence dependent on?
- The structural properties of the interfacial film
Coalescence and aggregation can occur when what things happen?
- Addition of emulsifier incompatible substances
- Bacterial contamination
- Too high internal phase volume
- Freezing of aqueous phase
- High temperatures
What emulsion issue is dependent on the structural components of the interfacial film?
Aggregation is dependent on what property?
- The electrical potential of the dispersion droplets
- Additional of emulsifier incompatible substances
What emulsion issue is dependent on the electrical charge of the dispersion droplets?
Why do aggregation and coalescence increase with increasing temperature?
- Increase the number of collisions
- May coagulate the emulsifier
What can increase the number of collisions in an emulsion; coagulate the emulsifier and increase aggregation and coalescence?
How does freezing of an aqueous phase effect an emulsion?
- Ice causes pressure on oil droplets
- Salts may precipitate and affect the electrical barrier
What can cause unusual pressures on the oil droplets in an emulsion and cause concentration of salts, disrupting the electrical barrier?
Freezing of the aqueous phase
What three things can cause inversion of an emulsion?
- Addition of an electrolyte
- Change in phase volumes
- Cooling of an emulsion that was prepared by heating and mixing
Addition of an electrolyte, changes in phase volumes and cooling of an emulsion that was created by heating and mixing are all causes of what emulsion issue?
What is flocculation?
Aggregation of dispersion droplets
What is breaking of an emulsion?
Complete phase separation
Name the four mechanical emulsifiers used in large scale industrial emulsification:
- Colloid mills
- Mechanical stirrers
Colloid mills, Ultrasonifiers, Homogenizers and Mechanical stirrers are all used for what?
Industrial large scale emulsification
For extemporaneous compounding of emulsions what tool would you use?
- Mortar and pestle with a rough inner surface
- Hand homogenizers
- Electric mixers
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