Emulsion is a thermodynamically unstable system consisting
of at least two immiscible phase, one of which is dispersed in the other liquid
phase, the system being stabilized by an emulsifying agent. One phase is the
disperse phase which is distributed throughout the other continuous
phase or dispersion medium. Emulsions can be liquids or semi-solids. The
latter are referred to as creams. The particle or globule size is
0.1-10μ. When the globule size is < 5μ they are called fine emulsions
but can have emulsions <10nm which are called microemulsions.
Emulsions are coarse dispersions, classified
together with suspensions, and other semisolids including ointments,
pastes, aerosols and gels.
Classification of
emulsions.
There is always a polar (e.g. aqueous) phase and a
non-polar phase (e.g. oil). It is not necessary to have oil or water to form an
emulsion. Both phases may be non-aqueous. Of importance is that the phases are
immiscible.
When oil is the disperse phase and water is the continuous
phase, the emulsion is oil-in-water (o/w). Medicinal emulsions for oral
administration are usually o/w. When the oil is the dispersion medium, the
emulsion is water-in-oil (w/o). Emulsions for external use are usually
w/o. There may be also multiple emulsions. For example when minute globules of
oil are dispersed in w/o the emulsion is o/w/o.
Determination of the
type of emulsion
a)
Dye-solubility (staining test): A small
amount of water-soluble dye (eg methylene blue) is dusted over the surface of
emulsion. The dye dissolves f the continuous phase is water (o/w emulsion), but
forms clumps on surface of w/o emulsions.
b)
Dilution test (miscibility test): Water is
added to the emulsion. If the emulsion is o/w, they mix freely. If it is w/o,
the is phase separation.
c)
Electrical conductivity test: A pair of
electrodes connected to the external source is immersed in the emulsion. O/w
conducts electricity shown by a deflection of the galvanometer. W/o does not.
d)
Paper stain: w/o stain the paper oily.
Theories of emulsion.
Emulsion can be prepared by using different emulsifying
agents, each of which has different mechanism of achieving stability. As such
there are various theories of emulsion, but each theory must be able to explain
the stability, type of emulsions, instabilities (e.g. breaking, inversions),
the role of emulsifying agent, and other physical chemical factors e.g. pH, the
role of non-ionic surfactants.
It is a fact that when two immiscible liquids are agitated
together so that one is dispersed into the other as small globules, they quickly
separate. This is because the molecules of each separate liquid have more cohesive
force between themselves than the adhesive force between each other.
When a liquid is broken into small globules the
interfacial area increases and this increase is accompanied by an increase in
the surface free energy:
ΔF = γo/wΔA Where ΔF = increase in free energy
γo/w
= oil –water interfacial tension
ΔA
= increase in surface area.
For example imagine 1cm diameter
globule with interfacial energy of 55mJ/m2
Interfacial energy = interracial
energy X energy per unit area
= πd2 γo/w
= 3.14 (10-2)255
= 0.017 mJ
If the globule is broken into
small globules of 10μ diameter, then
Volume of original globule = 4/3 πd1 3
Volume of emulsion globule 4/3 πd2 3
= d1 3
d2 3
= (10-2)3
(10-5)3
= 109 globules
the total interfacial energy is
109(πd2)
2 γo/w
=109
x 3.14 x(10-5)2 x 55 = 17.28 mJ
Total energy
input is 17.28 –0.017 =17.26 mJ
