Pharmaceutical Manufacturing Handbook: Production and

776 MICROEMULSIONS AS DRUG DELIVERY SYSTEMS
5.10.5.1 Polarized Light Microscopy
When a mixture of oil, water, and surfactant(s) is examined under a polarized light
microscope, the textures observed depend on the nature of the surfactant aggregate
formed and the relative ratio of the comprising constituents. If the resulting aggregates
are anisotropic, they tend to show strong birefringence and characteristic
textures could be viewed when examined using a polarized light microscope. On the
other hand, if the resulting aggregates are isotropic (as with ME systems or coarse
emulsions), then polarized light microscopy would be of less value in disclosing
structural information. Thermotropic and lyotropic liquid crystalline (LC) systems
such as lamellar, hexagonal, and reverse hexagonal mesophases are anisotropic and
exhibit characteristic textures when viewed under a polarizing light microscope. The
only type of LC systems that is isotropic and would not display birefringence when
viewed under a cross - polarizer is the cubic mesophase.
Polarized light microscopy is a simple technique to learn and use, readily available,
and of great value to differentiate between various anisotropic LC systems. It
is also of value to formulation scientists investigating amphiphile – oil – water mixtures
with emphasis on colloidal systems in general and MEs in particular. This is
mainly due to the fact that many LC systems may appear transparent to the naked
eye and can be easily misinterpreted as isotropic ME systems. Thus it becomes
essential when investigating systems of amphiphile – oil – water to confi rm fi ndings
based on visual appearance with polarized light microscopic examination.
5.10.5.2
Transmission Electron Microscopy
Transmission electron microscopy was one of the earliest techniques used to investigate
MEs [38, 39] . Freeze fracture along with replication is a sophisticated sample
preparation method for TEM that requires careful attention to a variety of details
to avoid formation of sample artefacts. The technique involves rapidly freezing the
sample by immersing it in a cryogen (slush nitrogen, propane, ethane, and freons).
For systems with volatile ingredients the freezing must be achieved rapidly to avoid
phase separation or crystallization. Thus, high cooling rate and adequate environmental
control of the samples before freezing are critical to prevent loss of volatile
components. The frozen sample is then transferred to a vacuum chamber and split
under vacuum by a fracturing device. The fractured surface is then shadowed with
metal (usually platinum) deposited from one side. The shadowed surface is then
coated with a layer of carbon that is directly deposited from above the specimen.
The carbon layer is transparent when examined and forms a supportive backing for
the shadowing metal deposited on the fractured surface. The specimen is then
removed from the vacuum and treated with solvents of different polarities, leaving
the metal carbon fi lm as a replica of the fracture surface.
Although freeze - fracture TEM provides direct visualization of ME structures, it
is not currently in wide use probably due to the experimental diffi culties associated
with the technique. The points to consider when preparing conventional TEM replicas
are the physical and chemical sample properties, freezing, cleaving, etching,
replication, cleaning, and mounting steps of the procedure.
Jahn and Strey [40] investigated systems with varying water - to - oil ratios at constant
amphiphile concentration. The TEM images support the notion of a bicontinu-