It has already been noted that on purely thermodynamic grounds, emulsions are physically unstable. A reduction of the interfacial area by coalescence reduces the system’s energy, and this process is thermodynamically favoured. However, thermodynamic stability of emulsions differs from pharmaceutical stability as defined by the formulator or the consumer. Acceptable stability in a pharmaceutical dosage form does not require thermodynamic stability. If an emulsion creams up (rises) or creams down (sediments), it may still be pharmaceutically acceptable as long as it can be reconstituted by a modest amount of shaking. Similar considerations apply to cosmetic emulsions; however, in the latter, creaming is usually unacceptable because any unsightly separation makes the product cosmetically inelegant. It is important, therefore, to remember that the standard of stability depends to a large extent on the observer, since subjective observations or opinions by themselves do not suffice to define such a parameter as acceptable stability.
- Coalescence and breaking
- Ostwald ripening
- Phase inversion
Flocculation: Flocculation is described as reversible aggregation of droplets of the internal phase in the form of three-dimensional clusters. In flocculated emulsion, the globules do not coalesce and can be easily redispersed upon shaking. The reversibility of this type of aggregation depends on the strength of the interaction between particles as determined by the chemical nature of the emulsifier, the phase volume ratio and the concentration of dissolved substances, especially electrolytes and ionic emulsifiers. In the absence of a mechanical barrier at the interface (weak interfacial films due to insufficient amounts of emulsifier), emulsion droplets aggregate and coalesce rapidly. In other words, flocculation differs from coalescence primarily by the fact that the interfacial film and the individual droplets remain intact. Flocculation and emulsion rheology are closely related. The viscosity of an emulsion depends to a large extent on flocculation, which restricts the movement of particles and can produce a fairly rigid network. Agitation of an emulsion breaks the particle–particle interactions with a resulting drop of viscosity, i.e. shear thinning.
Creaming: Under the influence of gravity, the dispersed droplets or floccules tend to rise (upward creaming) or sediment (downward creaming), depending on the differences in specific gravities betweenthe phases, to form a layer of more concentrated emulsion, the cream. Generally, a creamed emulsion can be restored to its original state by gentle shaking. The process of creaming, which inevitably occurs if there is a density difference between the phases, should not be confused with flocculation, which is due to particle interactions resulting from the balance of attractive and repulsive forces. Most oils are less dense than water so that the oil droplets in o/w emulsions rise to the surface to form an upper layer of cream. In w/o emulsions, the cream results from sedimentation of water droplets and forms the lower layer. The Stokes’ equation is very useful in understanding the process of creaming:
Rate of creaming = d2(Ƿs-Ƿo)g/18ƞ
where d is the diameter of the particles of dispersed phase (cm), Ƿs the density of the dispersion medium (g/cm3), Ƿo the density of the dispersed phase (g/cm3), g the acceleration due to gravity (cm/s2) and ƞ the viscosity of the dispersion medium (poise).
Coalescence and breaking: Coalescence is a growth process during which the emulsified particles join to form larger particles. It is an irreversible phenomenon that occurs due to the rupture of the interfacial film surrounding the dispersed globules. Coalescence is not the only mechanism by which dispersed phase droplets increase in size. If the emulsion is polydispersed and there is significant miscibility between the oil and water phases, then
Ostwald ripening, where droplet sizes increase due to large droplets growing at the expense of smaller ones, may also occur. This destabilizing process occurs when small emulsion droplets (less than 1 μm) have higher solubilities than do larger droplets (i.e. the bulk material) and consequently are thermodynamically unstable. Any evidence for the formation of larger droplets by merger of smaller droplets suggests that the emulsion will eventually separate completely or break. The major factor that prevents coalescence in emulsions is the mechanical strength of the interfacial barrier. Thus, good shelf life and absence of coalescence can be achieved by the formation of a thick interfacial film. Hence, various natural gums and proteins are useful as auxiliary emulsifiers when used at low levels, but can even be used as primary emulsifiers at higher concentrations.
Phase inversion: An o/w emulsion prepared with a monovalent water-soluble soap (sodium stearate) can be inversed to the w/o type by adding calcium chloride due to the formation of divalent soap(calcium stearate). Inversion may also be produced by alterations in the phase-volume ratio. For example, if an o/w emulsifier is mixed with oil and a little quantity of water, a w/o emulsion is produced by agitation. Since the water volume is less, it forms a w/o emulsion. But when more water is added slowly, phase inversion occurs and an o/w emulsion is produced. Inversion has also been observed when an emulsion, which has been prepared by heating and mixing the two phases, is cooled. It is due to the temperature-dependent changes in solubility of the emulsifying agents. Phase inversion can be prevented by choosing proper emulsifying agents in suitable concentrations. Wherever possible, it is better to ensure that the internal phase does not exceed 74% of the total volume of the emulsion.
Multiple choice questions (MCQs)
1.What is the emulsifier present in milk that makes it stable?
c)Lactic acid bacillus
2.o/w microemulsion containing hydrophilic surfactants produces
c)Milky white emulsion
d)Untense white emulsion
3.Events that are likely to occur sequentially, in physical unstability are
a)Flocculation, creaming, breaking and coalasence
b)Flocculation, creaming, coalescence and breaking
c)Breaking, coalescence, flocculation and creaming
d)Coalascence, flocculation, creaming and breaking
4.Upward creaming means ____ rate of sedimentation
5.Emulsifier used to stabilize the w/o emulsion
6.An emulsifier is considered to be ideal, if it is soluble in
c)Both a and b
d)None of the above
7.Emulsion made with tweens are
8.Emulsion containing more than two phases are called as
d)None of the above
9.Emulsion is a
b)Thermodynamically unstable preparation
d)Both b and c
10.The coalescence rate of o/w emulsion is _____ than w/o emulsion
d)None of the above
11.Auxillary emulsifying agents are used to stabilize the emulsion. They act on the principle
a)Adjusting the HLB value
b)Strengthening the non polar tails of the emulsifier
c)Strengthening the polar tails of the emulsifier
d)Thickening the continuous phase
12.Downward creaming means ____ rate of sedimentation
13.A mixture of span 20 and tween 20 forms ____ type of emulsion
14.Which of the following is/are theories of emulsification?
a)Monomolecular adsorption theory
b)Multimolecular adsorption theory
c)Solid particle adsorption theory
d)All of the above
15.Which of the following is not the cause of instability in an emulsion?
- b)Transparent emulsion
- b)Flocculation, creaming, coalescence and breaking
- b)Span 20
- c)Both a and b
- b)Multiple emulsion
- d)Both b and c
- d)Thickening the continuous phase
- a) Negative
- d)All of the above
1. GAURAV KUMAR JAIN – THEORY & PRACTICE OF PHYSICAL PHARMACY, 1st editio 2012 Elsevier, page no. 240-244.
2. Martins Physical Pharmacy, 6th edition 2011, page no. 768-771.