Chemistry Reference
In-Depth Information
Figure 8.4
Schematic presentation of H
II
mesophase showing the packing of water-
fi lled rods surrounded by lipid layers.
and a wide range of pressures (1-3000 bars). The new inverse micellar phase
consists of an hexagonal close packing (hcp) of identical, spherical inverse
micelles, with two lattice parameters,
a
and
c
, that were found to be
a
=
71.5 Å
and
c
116.5 Å (Fig. 8.5 ). This is the fi rst new inverse LLC phase, which was
reported for two decades, and it is the only known phase whose structure
consists of a close packing of identical inverse micelles.
=
8.2
PHASE BEHAVIOR
Most of the common surfactants form direct (type I) phases, where the inter-
face bends away from the polar solvent toward the tail region. However, most
biological amphiphiles (such as phospholipids) form type II LLC, where the
interface curves toward the polar region. Only a few synthetic amphiphiles
can mimic the behavior of biological lipids and form inverted mesophases.
The unsaturated monoglycerides (monoolein and monolinolein) belong to
this category (Drummond and Fong, 2000). GMO, monoolein, is a polar lipid
that is commonly used as a food emulsifi er. Presently, GMO is the preferred
amphiphile for formulating LC phases for scientifi c research and drug
delivery. This is a nontoxic, biocompatible, and biodegradable lipid, which
possesses low water solubility, but in excess water it swells and forms several
LC phases.
Monoolein-water is one of the most extensively investigated types of lipid-
based mesophases (Drummond and Fong, 2000). The phase diagram of this
system (Fig. 8.6) revealed complex structural behavior. At room temperature
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