A note on stability of emulsions and evaluation.

Stability of Emulsions and Evaluation of Stability

1) Stability of Emulsions

There are two principal requirements for the stability of an emulsion:

i) There should not be changes in mean droplet size and size distribution of the droplets in an emulsion.

ii) The suspended droplets should remain homogeneously dispersed in the continuous phase throughout it's shelf life.

Stability/ instability of the emulsions can be seen by observing following phenomenon:

a) Flocculation:  

It is the aggregation or clumping of dispersed droplets. This aggregation is re-dispersible by shaking. As the aggregates form due to electrical charges on the droplets/ globules where interfacial film remains intact, flocculation is reversible.

Sometimes, flocculation further results into coalescence where the number of globules come together and form a large globule (interfacial film breaks).

b) Creaming :

In o/w emulsion, dispersed oil globules migrate at the top and accumulate, whereas in w/o emulsion, dispersed water globules migrate downwards and accumulate to form two distinct layers in the emulsion. 

Creaming is undesirable, as larger globules travel rapidly and coalesce easily leading the cracking of emulsion.

Rate of creaming can be reduced by:

i) reducing the globule size of dispersed phase.

ii) increasing the viscosity of system.

iii) equalising the densities of both phases (oil and water).

Viscosity of the system can be increased by:

i) homogenisation

ii) increasing the concentration of dispersed phase.

iii) increasing the concentration of emulsifying agent.

iv) addition of viscosity enhancing agents in o/w emulsions.

c) cracking: 

Cracking is an irreversible process where rupture of interfacial films of dispersed globules and their coalescence occur.

Cracking leads to irreversible separation of two phases.

Interfacial films break due to:

i) incompatibility of emulsifying agents with other components in an emulsion.

ii) actions by microbes.

iii) exposure to high and low temperatures.

Emulsions are prone to cracking if:

- the phase volume ratio is larger.

- the dispersed globules are closely packed.

d) Phase inversion:

In phase inversion, dispersed phase becomes continuous phase and continuous phase becomes dispersed phase.

o/w emulsion undergoes phase inversion to form w/o emulsion.

Phase inversion occurs due to :

i) addition of dispersed phase beyond its threshold. (Here emulsion may break completely)

ii) addition of material that changes the solubility of an emulsifying agent.

iii) exposure of an emulsion to the phase inversion temperature.

iv) incorrect mixing procedure. 

2) Evaluation of Emulsion Stability

1. Emulsion is stored for its proposed shelf life and tested. (Real time study- time consuming)

2. Accelerated stability studies are preferred where stressed conditions like temperature changes, centrifugation, agitation are applied.

3. Agitation increases the rate and decreases the time of collision of globules.

4. Centrifugation increases the rate of creaming and cracking. 

5. Exposing the formulation to low and high temperatures is carried out in accelerated stability studies.

6. Various physical parameters such as particle (globule) size, particle count, phase inversion, electrophoretic changes,etc are tested during stability study.