Emulsion Dosage Forms Flashcards
What are emulsions and creams?
Dispersions of 1 liquid into another
What are the 3 types of emulsions in pharmaceuticals?
Oil in water (o/w)
Water in oil (w/o)
Multiple emulsions (e.g. o/w/o)
Describe oil in water (o/w) creams
More comfortable and cosmetically acceptable as they are less greasy and more easily washed off using water
Describe water in oil (w/o) creams
More difficult to handle
Many drugs incorporated into creams are hydrophobic - will be released more readily from this type of cream
They are also more moisturising - provide an oily barrier which reduces water loss from the stratum corneum
Define: Flocculation
The process by which individual particles aggregate into clot-like masses or precipitate into small lumps - of the dispersed phase
Define: Coalescence
The process by which 2 or more particles merge during contact to form a single particle - of the dispersed phase
(Groups of particles from flocculation forming fewer but larger masses)
Irreversible
Define: Rapid creaming
The migration of a dispersed phase of an emulsion under the influence of buoyancy
The particles float upwards (or sink) depending on their mass and the viscosity of the continuous phase
This is reversible - by shaking
Can happen at any stage of the separation of an emulsion
Define: Cracking/breaking
The complete separation and fusion of the dispersed phase
Irreversible
List 5 ways of avoiding creaming
- Reduced droplet size
- Increase viscosity
- Reduced density difference (between the 2 phases)
- Disperse phase concentration
- Prevent flocculation and coalescence
How do surfactants affect the stability of emulsions?
Surfactants aid dispersal and reduce tendency for coalescence
How do charged surfactants prevent flocculation and coalescence?
Charged surfactants increase surface charge = repulsive interactions between droplets
Phospholipids surround the oil droplet with the charged hydrophobic heads pointing outwards
Prevents flocculation and coalescence
How do non-ionic surfactants prevent flocculation and coalescence?
Non-ionic surfactants create a solvated layer leading to steric repulsion (due to tails)
Head of phospholipid in aqueous droplet, tails pointing outward
They lack toxicity and lower sensitivity to additives
What is the best kind of surfactant?
Mixed surfactant = mixture of those with high/low HLB numbers give more stable emulsions
What is an HLB number?
Hydrophile Lyophile Balance
Determines whether a surfactant is predominantly hydrophobic or hydrophilic
Decided based on the polar/non-polar functional groups in the surfactant
High HLB number (18) = hydrophilic, water-soluble
What does a high HLB number imply?
That the surfactant is hydrophilic and water-soluble
e.g. solubilising agents and detergents
18
What does a low HLB number imply?
That the surfactant is hydrophobic and oil-soluble
e.g. anti-foaming agents
3 or 0
What does a medium HLB number imply?
That the surfactant is water dispersible
e.g. wetting/spreading agents
9
Name a method used to decrease the size of droplets
Passage through colloid mill
Which type of emulsion should only be used for oral and IV preparations?
Oil in water
In which situations are oil in water emulsions best?
Oral
IV
Topical - but water in oil more hydrating
In which situations are water in oil emulsions best?
Topical - more greasy formulation but more hydrating than o/w
Not for use in oral or IV preparations
How do viscosity modifiers affect the stability of emulsions?
Can decrease the rate of creaming by addition of hydrophilic colloids (polymers)
e.g. methylcellulose
Define: Colloid/Colloidal dispersions
A solution which contains particles between 1-1000 nanometers in diameter
But are able to stay evenly distributed throughout the solution
What is a possible issue with preservatives in an emulsion?
They may partition into the oil phase when they are required in the aqueous phase
What is the purpose of including humectants in an emulsion?
Humectants help to retain moisture on the skin
List 3 tests used to identify the emulsion type
- Miscibility test - o/w emulsion will be miscible in water
- Conductivity - aqueous continuous phases readily conduct electricity
- Staining test - use of water-soluble or oil-soluble dyes
List 3 emulsifying agents
Surfactants
Hydrophobic colloids
Finely divided solids
How do surfactants act as emulsifying agents?
Adsorb at interface between immiscible liquids = barrier to coalescence
Lower the surface tension by increasing surface area
Use of a mix of oil & water soluble surfactants
Non-ionic surfactants = less toxic and less sensitive to electrolytes/pH
How do hydrophilic colloids act as emulsifying agents?
Polymers
Form viscoelastic layer = multi-layer of interacting polymers (network)
This provides mechanical robustness to prevent coalescence
Don’t significantly reduce surface tension
How do finely divided solids work as emulsifying agents?
They adhere to interface
Need to preferably wetted to 1 phase
Can act to provide mechanical robustness to prevent coalescence
What type of emulsion has the greater viscosity?
Water in oil emulsion exhibits a higher apparent viscosity
Define: Rheological properties
Those relating to flow
List 4 factors which affect the rheological properties
- Dispersed phase concentration = increases viscosity
- Radius of dispersed phase = small size increases viscosity
- Viscosity of continuous phase
- Surfactant emulsifier
How can the rate of creaming or coalescence be measured?
- Ratio of volumes - creamed:total volume
- Changes in size of globules
- Viscosity changes
Used to investigate properties of the emulsion
What are microemulsions?
Isotropic liquid mixtures, thermodynamically stable
Contain high percentage of oil, water and 15-25% emulsifier
Can form spontaneously - swollen micelle
