Physical stability of Emulsion and Remedies for Physical stability

Stability of Emulsion :

 A stable emulsion may be defined as one in which the disperse phase retain their initial character and remain uniformly distributed throughout the continues phase.  Stability may be defined in term of physical and chemical stability.

Physical Stability:  Physical stability is that type of stability of emulsions which is associated with the physical characters of emulsions. It may be due to creaming, aggregation or flocculation, coalescence and inversion.

Creaming and Sedimentation : Creaming is due to the upward movement of disperse phase. Creaming leads to coalescence and breakage of emulsion. Creaming occur when the density of disperse phase is less then the continues phase. Creaming is reversible phenomenon and can be easily reversed by mechanical agitation for a shorter time.

Disadvantages of Creaming: 

1) it leads to inelegant emulsion.

2) Dose disturbances may occur. 

3) creaming may cause coalescence and further breakage of emulsion.

Remedy: Stocks law define remedies both for creaming and sedimentation. Stokes law

V²=d²∆Ꝿ g/18η   

D²  is the diameter of the globules reducing the diameter of the globules V (velocity) of creaming will decrease and thus lesser chance for creaming. Small particle size of disperse phase lesser the chances of creaming.

∆Ꝿ is the difference between the density of disperse phase and disperse medium. Higher the difference the greater will be the chance of creaming.

g is the gravitational velocity.

η is viscosity------- increasing the viscosity creaming chances will be reduced. Viscosity can be increased by adding viscosity builders like Methyl cellulose in O/W emulsion and addition of soft paraffin in W/O emulsion. Storing the emulsion at low temperature (above the freezing point ) increase the viscosity of emulsion thus lower the creaming phenomenon. 

Proper ratio of Disperse phase to Disperse medium: One approach to reduce creaming is to have proper ratio of Disperse phase to disperse medium. The ratio of less then 20 % will readily lead to creaming. It is theoretically possible to have disperse phase ratio up to 74 % but in practically it is not more then 60% and some studies prefer 50% not more.

Flocculation:  Flocculation is the aggregation of globules into a loose cluster. The globules retain their individual integrity but are just in aggregated form. Flocculation enhance the chance of creaming and it is the preceding step in coalescence. Flocculation is a reversible phenomenon and can be disturbed by agitation.  Flocculation is an electrical phenomenon in which the globules stick together due to some sort of attraction between them.

Remedy : By adding possible effective surfactants possessing a suitable electric charge to prevent this phenomenon.

Coalescence:  Coalescence is the growth process in which small particle/globules of disperse phase merge together and form a large particle, which further merge and lead to separation of two phase (breaking). As said causes of coalescence are creaming and flocculation. Both of these precede the coalescence. Coalescence is the irreversible phenomenon. It is differ from flocculation in irreversibility and integrity of particle (in flocculation no merging occur) and another difference between is that coalescence dose not depends on charge.

Reasons: coalescence occur when there is the breakage of interfacial film of the emulsifiers around the dispersed globules.

Remedy: Coalescence can be overcome by adding Mix emulsifying agents which give

1)Elastic and rigid film.

2) Viscous film (Elastic and viscous film reduces the chances of Coalescence

3) cautious use of HLB scale for selection of emulsifying agents.

Inversion : Inversion is also one of the problem with the emulsion. Inversion is the process in which o/w water emulsion turns to w/o emulsion or vice versa. This phenomenon is due to

Temperature: high temperature may change the characteristic of emulsifying agent thus promote phase inversion i.e  polysorbates stabilized o/w emulsion is inverted to w/o because of the breakage of H bonds in polysorbates due to heat.  Temperature at which inversion occur is called phase inversion temperature (PIT).

Electrolyte addition: Sodium-oleate which stabilize O/W emulsion when Calcium chloride is added to it turns to W/O emulsion because calcium oleate is more soluble in oil.

Ratio of disperse phase to disperse medium: using a high ration above 60 percent of dispersed phase inverses the emulsion. Limit of disperse phase is 74 %  which is impractical.