Stability Problem of Emulsion

EMULSION

Emulsion may be defined as a biphasic system consisting of two immiscible liquids usually water and oil, one of which is finely subdivided and uniformly dispersed as droplets throughout the other. Since such a system is thermodynamically, a suitable emulsifying agent is required to stabilize the system.

It has two phases:

      i. oil phase

     ii. water phase

In other way:

      i. External phase

     ii. Internal phase

The phase which makes globules or droplets is known as internal phase or disperse phase and other is external or continuous phase.

Oil can be present as internal and external phase and water also as internal or external phase.

Emulsion is normally opaque. Particle sizes of emulsion are from 0.1 to 100 μm. It can be used orally, topically and parentally.

FACTORS AFFECTING THE STABILITY OF AN EMULSION:

Particle size: Increased particle size of the internal phase causes the decreased stability of an emulsion. Smaller size of the particles of internal phase is always preferable.

Particle-particle interaction: Deflocculated particles are always preferable because the less the particle-particle interaction the more the stability of an emulsion.

Particle density: The less the particle density the more the stability of an emulsion.

Bulk phase/external phase density: the more the bulk phase density the more the stability of an emulsion.

Bulk phase viscosity: Generally the more the viscosity of bulk phases the more the stability of emulsion.

Instability of an emulsion:

Various deviations from ideal behavior of an acceptable emulsion is known as its instability.

The signs of instability are--

1.   Flocculation or coagulation

2.  Creaming

3.  Coalescence

4.  Breaking or cracking

5.  Phase inversion

6.  Deterioration by micro-organism

7.  Miscellaneous physical and chemical changes.

Flocculation:

Flocculation is the joining together of globules to form large clumps or floccules within the emulsion. In flocculation the interfacial film and the individual droplets remain intact the globules do not coalesce and may be redispersed by shaking.

Cause:

Flocculation depends on the electrical potential on the surface of the droplets. There is a high repulsive force among the dispersed globules. If the repulsive force is decreased the globules tend to aggregate. The lowering of repulsive force may be caused

i. when the used emulsifying agent is insufficient

ii. when the volume of the internal phase is high

Remedy:

The presence of high charged density on the dispersed droplets will ensure the presence of a high energy barrier and these reduce the incidence of flocculation.

Creaming:

Creaming is the rising or settling of dispersed globules to form a concentrated layer at the surface or at the bottom of the emulsion.

Cause:

Creaming is influenced by gravity on the globules. So it depends on the sedimentation or creaming rate. This rate depends

i. diameter of the dispersed globules

ii. Viscosity of the dispersion medium

iii. Density difference between two phases

Creaming of emulsion can be explained by stokes law:

Where,

V= rate of creaming

r= particle radius in cm

D =particle diameter in cm

d1=density of the internal phase

d2=density of the external phase

g= gravitational constant

η= viscosity of the external phase

From the equation we can say

i. Increase diameter of globules increases the velocity and vice versa.

ii. Increase viscosity of the dispersion medium decreases the velocity and vice versa.

iii. If d1>d2- downward creaming

•   d1<d2- upward creaming
•   d1=d2- No creaming

Remedy:

i. Reduction of the globules size by using an efficient homogenizer.

ii. Increasing the viscosity of the continuous phase by using viscosity imparting agent.

iii. By reducing the density difference between two phases.

iv. By controlling the dispersed phase concentration.

v. By storing in a cool place or low temperature.

Coalescence:

Coalescence is the fusing of the globules into larger drop or drops. In coalescence the globules lose their individuality. During fusion the interfacial film is destroyed. So coalescence is irreversible. Globules can not be redistributed by shaking.

Cause:

Coalescence depends on the structural properties of the interfacial film. So it occurs when emulsifying agent loses its activity or the amount of emulsifying is insufficient.

Remedy:

i. By adding sufficient amount of emulsifying agent and passing the product through the proper emulsifying machinery.

ii. During preparation the addition of emulsifying agent should be appropriate because the use of wrong emulsifying agent loses its activity within a short period of time.

Breaking or Cracking:

This involves coalescence of the dispersed globules and separation of the dispersed phase as a separate layer. Redispersion can not be achieved by shaking and therefore the advantages of emulsification are lost and accurate dosage form is impossible.

Cause:

Cracking may be caused any physical or biological or chemical effect that changes the nature of the interfacial film of an emulsifying agent. The physical factors causing breaking are described below.

i. Opposite types of emulsifying agent: Addition of an emulsifying agent of opposite type tends to decrease the power of emulsification which cause cracking.

Example: Soaps of monovalent metals (Sodium lauryl sulphate) produce o/w emulsion while soaps of divalent metals (Calcium lauryl sulphate) produce w/o emulsion. Addition of monovalent soap to a divalent soap emulsion or a divalent soap to a monovalent soap emulsion leads to instability or cracking.

ii. Addition of common solvent: Addition of a solvent in which both disperse and continuous phase are soluble forms a single phase system.

iii. Temperature: An increase in temperature may coagulate certain types of macromolecular emulsifier which are protein in nature cause instability of an emulsion. An increase in temperature will increase the number of collision between globules that are effective in overcoming the barrier to coalescence and instability of an emulsion.

iv. Freezing: If an emulsion is kept below 40C the aqueous phase will turn into ice. These ice crystals will press on oil phase. As a result cracking will occur.

v. Incorporation of excess disperse phase: Emulsion with a disperse phase concentration in    excess of 74% has a marked tendency to crack.

vi. Addition of insufficient amount of emulsifying agent.

Remedy:

 i.  By incorporating more emulsifying agent.

ii.  By controlling the temperature at which emulsion is kept.

iii. By controlling disperse phase concentration.

iv. By adding correct emulsifying agent.

Phase inversion:

An emulsion is said to invert when it changes from an o/w to a w/o emulsion and vice versa. Inversion is mainly brought about by chemical degradation. But there is some effect of physical factor--

 i. By changing the phase volume ratio.

ii. By cooling the emulsion specially those which are prepared by heating and mixing of two phases.

Deterioration by micro-organism:

Mold, yeast and bacteria may--

a. Bring about the decomposition of the emulsifier.

b. Contaminate the aqueous phase.

c. Destroy oil soluble vitamins.

Remedy:

Add chemical agent that will act specially as preservative. Combination of para-hydroxy benzoates 0.1% to 2% of methyl ester and 0.02% to 0.05% of propyl ester are frequently used for this purpose. Preservative should be adequately soluble in both phases. If not more than one type of preservatives should be used one for oil phase and other for aqueous phase.

The emulsifying agent and other ingredients of the formulation should not form complex with the preservatives.

Miscellaneous:

Care must be taken to protect emulsion against deterioration caused by light, temperature and freezing and thawing.