PH2107 - Emulsions 2 Flashcards
What are the four types of instability that can occur with emulsions?
Flocculated
Creamed
Phase inverted
Cracked
What is a flocculated emulsion?
Globules associate
What is a creamed emulsion?
Flocced globules rise to the surface
What is a cracked emulsion?
Flocced globules completely separate and merge to form an oil layer
What is an inverted emulsion?
Emulsion flips from o/w to w/o, or from w/o to o/w
what are some different methods for emulsifying
High shear stirring
(heat/melting required if viscous)
High pressure homogenization
force the premix through a narrow orifice or valve at high
pressures (typically 10–100 MPa). Forcing the emulsion
through a valve at high pressure creates turbulence and
very high shear forces, thus reducing droplet size.
Microfluidics/ultrasonication
The resulting emulsion is temporary,
without an emulsifier
What is the difference between creaming in w/o emulsion compared to o/w emulsion?
It is upside down
- in o/w emulsion creaming occurs on top
- in w/o emulsion creaming occurs on the bottom
What is the difference between cracking in w/o emulsion compared to o/w emulsion?
It is upside down
- in o/w emulsion cracking occurs on top
- in w/o emulsion cracking occurs on bottom
Why is creaming and cracking inverted depending on the structure of the emulsion?
Density - oil floats on water
What causes flocculation in emulsions?
Due to interaction of VA and VR
- can be primary minimum or secondary minimum phenomenon
- if primary minimum, it is extremely difficult to break up floccs
Why is flocculation in emulsions a reversible state?
Globules remain separate
What causes cracking in emulsions?
Closeness of flocculated globules any weakness in the interfacial film may result in irreversible coalescence (cracking)
What causes emulsion creaming?
Globule flocculation
- dispersed phase rises to the top (o/w)
- settles to the bottom (w/o) of the emulsion
Why can creaming in emulsions be redispersed?
Each globule is still surrounded by its protective emulsifier
Why is creaming likely to progress to cracking?
Globules in cream layer are close to each other and may overcome repulsion
What is emulsion cracking?
Complete and irreversible separation of the emulsion into its two constituent oil and water phases
Why can’t a cracked emulsion be re-dispersed?
The emulsified layer surrounding the globules has been destroyed
What causes an emulsion to crack?
- pH change
- improper storage
- chemical incompatibility (bad formulation)
What causes phase inversion of emulsions?
- heat
- chemical instability
- phase volume ratio (PVR) > 60-70% dispersed phase
Why is phase inversion of emulsions sometimes useful?
Sometimes induced in manufacturing as a means of obtaining a finer product
What requirements are needed for emulsion stability?
- globules to retain their original character
- size
- polydispersity index
- globules must remain uniformly distributed throughout continuous phase over shelf life
- no settling
- no agglomeration
What are the problems with phase separation (cracking) in parenteral emulsions?
- modulated rate of drug release
- dosage irregularity (potentially fatal)
What is emulsification?
The process of dispersing one liquid (often containing the bioactive compounds, drug) in a second immiscible liquid as small globules
What are the possible methods of emulsification?
- high shear stirring
- high pressure homogenisation
- microfluidics/ultrasonication
How does high pressure homogenisation work?
Force the premix through a narrow orifice or valve at high pressures (typically 10 - 100 MPa)
- forcing the emulsion through a valve at high pressure creates turbulence and very high shear forces, thus reducing droplet size
What is needed to create a permanent emulsion following emulsification?
An emulsifier
What is mayonnaise made from?
- oil and water
What is added to oil and water in mayonnaise as an emulsifier?
Egg yolk (lecithin)
What are lecithins?
Phosphatidyl cholines
Ionic emulsifiers
- also used to form micelles and liposomes
What are the advantages of using lecithins compared with synthetic alternatives?
Totally biodegradable and metabolised
- integral part of biological membranes making it virtually non-toxic
What is the structure of lecithins?
Same zwitterion headgroup but vary with alkyl chain esterifying groups
