Unit 2 - Topical formulations 2 Flashcards
What is the function of an absorption base as an ointment base?
Absorb water (from skin or air) to form water-in-oil emulsions (cf. cream)
How are absorption bases made?
Anhydrous vehicles composed of a hydrocarbon base and an emulsifier
- sterols
- sorbitan monosterate
How do absorption bases as an ointment base work?
Deposits a greasy, occlusive film on the skin surface
- less suppression of trans-epidermal water loss than hydrocarbons
Which structure in the skin is targeted by absorption bases as an ointment base?
They hydrate the stratum corneum (w/o emulsion)
What is the opposite of an absorption base?
Emulsifying base (o/w emulsion)
What are the three types of emulsifying base?
- anionic (negatively charged)
- cationic (positively charged)
- non-ionic
Why do emulsifying bases as an ointment base contain surfactants?
May help with spreading
What are the advantages of emulsifying bases as an ointment base?
Mix with aqueous secretions and readily wash off skin
- useful for scalp treatments
- get a much more even layer
What are water soluble bases as an ointment base prepared from?
Polyethylene glycols
- high molecular weight
- low molecular weight (less viscous)
What are the advantages of water soluble bases as an ointment base?
- non-occlusive (water soluble)
- do not stain clothing
What are the disadvantages of water soluble bases as an ointment base?
Incompatible with many chemicals
- limits what you can add to it
What can be added to water soluble bases as an ointment base?
Lignocaine as a topical anaesthetic
- can be used to incorporate drugs dissolved in water
How are ointments prepared?
Insoluble ingredients must be finely powdered (but do not grind fine powders!)
Medicament incorporated into base by geometric dilution
What is a paste?
Ointment containing 20 - 60% powder dispersed in base
- soft paraffins
- glycerol
- emulsifying ointments
What are the disadvantages of pastes?
Absorb noxious chemicals (e.g. ammonia from bacteria)
Why are pastes useful for localised drug action?
Pastes spread less well
Reduced irritancy
Reduced staining
What are the advantages of pastes?
Less greasy than ointments
Give some uses for pastes
Sun and wind filters (e.g. cricket players noses)
Which regions of the body are pastes unsuitable for?
Hairy areas - pull on hairs as hard to spread
Widespread areas - hard to spread (imagine hard butter on bread)
What can pastes be used to treat?
Splinter removal (magnesium sulphate) Plaque psoriasis
What can affect the stability of a cream?
Anything which affects the emulsifying agent
What forms the basis of many o/w creams?
Emulsifying waxes and ointments
What is needed to make a cream?
- emulsifying wax (contains the emulsifying agent)
- oily phase (to produce an emulsifying ointment)
- water (to produce the cream)
Which oil-soluble alcohol do all emulsifiers contain?
Cetostearyl alcohol
What substances are mixed together to form stable, complex condensed films at the o/w interface in emulsions?
- surfactants
- oil-soluble alcohols (cetostearyl alcohols)
What contributes to the emulsion stability via electron repulsion?
Charge
- stops droplets coalescing
How do non-ionic surfactants retard coalescence of droplets in emulsions?
Steric hindrance - physically get in the way through repulsion of hydrated ethylene oxide chains
What makes up emulsifying wax BP?
Sodium lauryl sulphate + Cetostearyl alcohol
What makes up emulsifying ointment BP?
Emulsifying wax BP
White soft paraffin
Liquid paraffin
What makes up aqueous cream BP?
Emulsifying ointment BP
Water
Which phase forms the dispersed phase and which forms the continuous phase in an emulsion?
- phase that persists in the droplet phase the longest
- phase in which the emulsifying agent is more soluble tends to form the continuous phase
- function of phase concentration and interfacial tension
What lowers the o/w interfacial tension enhancing stability (which determines shelf life)?
The adsorption of a surfactant
What determines whether the surfactant will stabilise an emulsion?
The type of film (barrier) formed at the interface
Describe hydrophilic creams
- non occlusive/slightly occlusive
- ‘vanishing creams’ - thin oily film (heat from skin evaporates water)
- promote percutaneous absorption (via concentration gradient - concentration increases as water evaporates)
What type of cream is a hydrophilic cream?
Oil-in-water cream
Oil - dispersed phase
Water - continuous phase
What is added to a hydrophilic cream to reduce drug precipitation?
Propylene glycol
Why is aqueous cream an anionic cream?
It contains sodium lauryl sulphate, which is negatively charged (lauryl sulphate)
Describe the emulsifying system in hydrophobic creams
Oil-soluble (low HLB)
How is Wool Alcohols Ointment (WAO) BP made?
Wool alcohols
Hard paraffin
White soft paraffin
Liquid paraffin
How is Oily Cream BP (AKA Hydrous Ointment BP) made?
Wool Alcohols Ointment BP
Water
What does Wool Alcohols Ointment act as?
Absorption base AND emulsifier
Which type of surfactant should be used with an anionic drug?
Anionic surfactant
Which type of surfactant should be used with a cationic drug?
Cationic surfactant
Why can creams support the growth of microorganisms?
They contain water
What type of water must be used to make topical creams?
Purified water
- freshly boiled and cooled
What needs to be added to a cream to prevent microbe growth?
Preservative
