Introduction of MicroEmulsions

By: Pharma Tips | Views: 6843 | Date: 02-Jun-2011

The term “microemulsion” was first introduced by Hoar and Schulman (1943) todescribe a clear solution obtained when normal O/W coarse emulsions were titratedwith medium-chain length alcohols.


The term “microemulsion” was first introduced by Hoar and Schulman (1943) to
describe a clear solution obtained when normal O/W coarse emulsions were titrated
with medium-chain length alcohols. Since then, there has been much dispute about
the relationship of these systems to solubilized systems (i.e. micellar solutions,
surfactant-free solutions) and to emulsions. Danielson and Lindman (1981) define
microemulsion as a system of water, oil, and amphiphile which is (an) optically
isotropic and thermodynamically stable liquid solution.

The main difference between normal coarse emulsions and microemulsions
lies in the droplet size of the dispersed phase. Microemulsions have droplets
 typically in the size range 10–100 nm and because of this small size range,
they produce only a weak scattering of visible light and hence, they appear transparent.
The features that distinguish microemulsion systems from emulsions are shown in Table 1.3.

Microemulsions are thermodynamically stable systems. The driving force for
their thermodynamic stability is the ultralow interfacial tension (10−2–10−4m Nm−1).
When the interfacial tension is this low, the interaction energy between droplets has
been shown to be negligible and a negative free energy formation is achieved making
the dispersion thermodynamically stable. The large interfacial tension between
oil in water, which is typically about 50 m Nm−1, is reduced by employing surfactants.
However, it is generally not possible to achieve the required interfacial tension
with the use of a single surfactant. Amphiphiles such as medium-chain length
alcohols are added as cosurfactants to achieve the desired interfacial tension. Due
to their amphiphilic nature, they partition between the aqueous and oil phase
thereby altering the solubility properties of these phases. In addition, by interacting
with surfactant monolayers at the interface, they affect their packing, which in turn
can influence the curvature of the interface and interfacial free energy.

Depending upon the phase volume ratio and the nature of the surfactant used, a
microemulsion can be one of the three types: O/W, bicontinous, and W/O.

An O/W microemulsion is formed when the concentration of oil is low and a W/O microemulsion
is formed when the concentration of water is low. In conditions where the
volumes of oil and water are equal, a bicontinuous microemulsion is formed in
which both oil and water exist as a continuous phase. A wide variety of internal
structures exists within microemulsion systems. They may be spherical, spheroid,
or cylindrical rod-shaped micelles and may exist in cubic, hexagonal, or lamellar
phases. The relative amounts of aqueous phase, oil phase, and surfactant required
to form a microemulsion can be determined with the aid of triangular/ternary phase
diagrams. For example, in Fig. 1.5, each corner of the triangle represents 100% of
one of the components. Moving away from that corner reduces the volume fraction
of that specific component and any point on one of the axes corresponds to a mixture
of two of those components in a defined ratio. Any point inside the triangle
represents a mixture of all the three components in a defined ratio. A review written

Table 1.3 Differences between microemulsions and emulsions
       
        Microemulsions
                                                        Emulsions
 Thermodynamically stable                                             Thermodynamically unstable
 Optically transparent                                                     Cloudy colloidal systems
 Interfacial tension 10−2–10−4 m Nm−1                           Interfacial tension 20–50 m Nm−1
 May be single or multiple phase                                     Multiple phase only
 Require no energy in their formation                                External energy required for formation

May be single or multiple phase Multiple phase only
Require no energy in their formation External energy required for formation
by Forster et al. (1995) describes in detail the physical meaning of the phase behavior
of ternary oil/surfactant/water systems, and the concepts related to microemulsion
formation and the influence of additives on those microstructures.
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