Emulsions, creams and ointments

What is an emulsion?

A liquid medicine where one insoluble liquid is dispersed as microscopic droplets in another immiscible liquid

Why are emulsions physically unstable?

There is a thermodynamic drive for the two phases to separate

What are the main physical signs of emulsion instability?

cracking and creaming

What is creaming?

Droplets rise or settle but the emulsion can be redispersed

What is cracking?

Irreversible separation of the two phases

What is an emulsifier?

An amphiphilic substance that stabilises emulsions

What does amphiphilic mean?(in this context)

having a hydrophilic head and hydrophobic tail

How do emulsifiers stabilise emulsions?

assemble at the oil-water interface and reduce interfacial tension

Why are mixed emulsifiers better than single emulsifiers?

They form a dense interfacial film, giving better stability

Example of a mixed emulsifier

emulsifying wax BP

Why are o/w emulsions prone to chemical instability?

Large surface area makes oils susceptible to hydrolysis and oxidation

Why are antioxidants used in emulsions?

To prevent oxidation of unsaturated fatty acids

What properties should preservatives have in an emulsion?

should not partition into the oil phase or interact with the emulsifier

Why are emulsions useful for oral administration of oils?

more palatable, easier to administer and more digestible

Why are lotions useful topically?

They are runny and good for large or hairy areas

Why are creams preferred over lotions sometimes?

Less messy and easier to apply

What is a cream?

A semi-solid emulsion applied to the skin, either o/w or w/o

What can creams contain besides oil and water?

soluble ingredients in either phase and/or insoluble powders

What is an ointment?

A greasy, spreadable semi-solid with a hydrophobic base

What is a paste?

An ointment with a high powder content

Why do ointments have fewer stability issues than emulsions?

in a single-phase and contain little to no water

Common ointment base ingredients

Glycerides, hydrocarbons(paraffins), and lanolin

What is a disadvantage of lanolin?

Can cause allergic reactions

Two main steps in ointment preparation

• base preparation by fusion

• powder dispersion by trituration

What is fusion?

Melting and mixing ingredients over a steam bath

What order are ingredients added during fusion?

In decreasing order of melting point

What is trituration?

Grinding and mixing powders into the base using a tile or mortar