27. Formulation considerations of Emulsions
1. Emulsions are inherently thermodynamically unstable due to the differ-
ences in the molecular forces of interaction between the molecules of the
two liquid phases.
2. The oxygen and hydrogen atoms in the water molecules in the aqueous
phase bond with surrounding water molecules through dipolar and
hydrogen-bonding interactions, whereas the carbon atoms in the oil phase
bond with the surrounding molecules predominantly through weak
hydrophobic and Van der Waals interactions.
3. Creation of surface of interaction between the two phases is
thermodynamically unfavorable.
4. Therefore, production of emulsions requires the introduction of energy into
the system.
5. This is accomplished by trituration on the small scale and homogenization
on the pilot and large scale.
6. In addition, interfacial molecules of both phases must be stabilized against
the tendency for the self-interaction of phases, which can lead to the
coalescence and collapse of the dispersed phase.
28. Minimization of interfacial free energy
Dispersion of insoluble phases results in thermodynamic instability and high
total free energy of the system. Since every system tends to spontaneously
reduce its energy to a minimum, all emulsions tend to separate into the two
insoluble phases with time.
When one liquid is broken into small globules, the interfacial area of the
globules is much greater than the minimum surface area of that liquid in a
phase-separated system. Thus, the dispersed phases tend to coalesce or
phase separate to minimize the surface of interaction between the two
phases.
This phase separation is driven by greater forces of interaction between
molecules of the same phase than between the molecules of different
phases.
A phase-separated system represents the state of minimum surface-free
energy.
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29. The surface-free energy of an emulsion is evident as the interfacial
tension between the two phases.
Addition of a surfactant to an emulsion leads to preferential
translocation of the surfactant molecules to the interface of the two
liquid phases since a surfactant is amphiphilic and molecules in both
phases form attractive interactions with the surfactant molecule.
The presence of a surfactant at the interface of dispersed-phase
globules lowers the interfacial tension.
Reduction of the interfacial tension delays the kinetics of coalescence of
the two phases.
Frequently, combinations of two or more emulsifying agents are used to
adequately reduce the interfacial tension by forming a rigid interfacial
film.
30. Size, viscosity, and density: Stoke’s law
Creaming or sedimentation of the dispersed phase in an emulsion is mathematically
modeled by Stoke’s law), which indicates that the physical stability of an emulsion can
be enhanced by
1. Decreasing the globule size of the internal phase. The dispersed globule size less
than 5 μm in diameter contributes to good physical stability and dispersion of the
emulsion.
2. Increasing the viscosity of the system. Gums and hydrophilic polymers are
frequently added to the external phase of an o/w emulsion to increase viscosity, in
addition to reducing the interfacial tension and forming a thin film at the interface.
Higher the viscosity of the continuous phase, lower the Brownian motion, collision
frequency, and energy of collisions of the dispersed-phase globules. Increasing the
viscosity can have an unwanted effect of reduction of deliverable volume from the
container because highly viscous liquids tend to adhere to the container. In addition,
the non-Newtonian viscosity behavior of the emulsion is an important consideration.
For example, shear-thinning system may be preferred, whereas a shear-thickening
system would be undesirable.
3. Reducing the density difference between the dispersed phase and the dispersion
medium. This reduces the creaming tendency by minimizing the driver for separation
and preferential accumulation of the dispersed phase in a particular direction.
31. Optimum-phase ratio
1. The ratio of volume of the disperse phase to the volume of the dispersion medium
(phase ratio) greatly influences the characteristics of an emulsion.
2. The optimum-phase volume ratio is generally obtained when the internal or dispersed
phase is about 40%–60% of the total quantity of the product.
3. A conventional emulsion containing less than 25% of the dispersed phase has a high
propensity toward creaming or sedimentation. Nevertheless, a combination of proper
emulsifiers and suitable processing technology makes it possible to prepare
emulsions with only 10% disperse phase without stability problems.
4. Such a combination of emulsifiers includes the use of a hydrophilic emulsifier in the
aqueous phase and a hydrophobic emulsifier in the oil phase. Such a combination
leads to the formation of a closely packed surfactant film at the interface.
5. For example, a combination of sodium cetyl sulfate and cholesterol leads to a
closely packed film at the interface that improves emulsion stability. On the other
hand, sodium cetyl sulfate and oleyl alcohol do not form a closely packed or
condensed film. Consequently, this combination results in a poor emulsion.
32. Zeta potential
Emulsions can be stabilized by electrostatic repulsion between the droplets.
High zeta potential on the surface of the droplets causes the dispersed-phase droplets
to repel each other and thereby resist collisions due to Brownian motion, mixing, and
gravitational forces. Thus, the droplets remain suspended for a prolonged period of
time.
For example, if negatively charged lecithin is adsorbed at the droplet surface it creates
a net negative charge and zeta potential on the dispersed-phase droplets.
Highly charged dispersed-phase droplets, however, can coalesce irreversibly when
colliding with high enough energy to overcome the repulsive forces.
Optimizing the zeta potential of the dispersed phase to facilitate flocculation can
minimize a system’s propensity toward undesirable coalescence.
The zeta potential can be optimized by the addition of positively charged electrolytes
to the outer, continuous phase. At appropriate zeta potential, the system achieves a
state where flocculation is facilitated over coalescence.
33.
34. lists the composition of two typical o/w pharmaceutical emulsions.
Examples of emulsion formulations
