Chemistry Reference
In-Depth Information
1.1
INTRODUCTION
The term
lyotropic liquid crystals
generally refers to systems in which any form
of liquid crystallinity is induced or affected by the presence of a solvent. The
simplest form of lyotropic liquid crystal is the nematic phase, which is given
by the nonisotropic orientation of rigid particles or molecules, called meso-
gens. In what follows we shall focus on lyotropic liquid crystals of superior
order and, in particular, in those observed in surfactants (surface-active agents)
in the presence of water, with special emphasis on lipid-water systems. Disre-
garding other forms of aggregation of lipids in water to consider only those
characterized by a periodic order, lipids most frequently found to form lyo-
tropic liquid crystals are neutral lipids such as monoglycerides or phospho-
lipids (Krog, 1990). These two systems have in common one or two fatty acid
tails and a polar head with total neutral charge (in the case of phospholipids
most often the positive and negative charges are present in stoichiometric
ratio; one talks then about zwitterionic lipids). These structured fl uids are
periodically organized on the nanometer length scale and have been known
for half a century since the pioneering work of Luzzati and Husson (1962).
One particular feature that is breathing new life into these systems is the fact
that, when redispersed in excess water, some specifi c liquid crystalline struc-
tures can be maintained. This makes them particularly suitable for food, phar-
maceutics, and cosmetic applications (Fong et al., 2009; Mezzenga et al., 2005a;
Mohammady et al., 2009; Yaghmur and Glatter, 2009). Additionally, in their
bulk form, they have been used as nanoreactors to run and control regioselec-
tive reactions (Garti et al., 2005; Vauthey et al., 2000) due to the environment
constituted by a very large lipid-water interfacial area. The most frequently
found structures in lipid-water lyotropic liquid crystals are the isotropic fl uid
(L
II
), the lamellar phases (L
α
or L
C
depending on whether the alkyl tail is
amorphous or has crystallized), inverted columnar hexagonal cylinders (H
II
),
and bicontinuous double gyroid (Ia3d), double diamond (Pn3m), and primitive
(Im3m) cubic phases (de Campo et al., 2004; Mezzenga et al., 2005b; Qiu and
Caffrey, 2000). The presence of charges on the lipid-water interface tends to
disrupt the interface, and, thus, lyotropic liquid crystals, with a great nonzero
total charge, are rarely found. Nonetheless, cationic and anionic surfactants
have been used as doping agents for these systems, to engineer other phases
or to displace boundaries in the phase diagram (Borne et al., 2001). Finally, a
new class of lyotropic liquid crystals has recently emerged, that is, the macro-
molecular amphiphilic systems: the most common, Pluronics, is formed by a
diblock copolymer with one hydrophilic (generally polyethyleneoxide, PEO)
and one hydrophobic (generally polypropyleneoxide, PPO) blocks, so that in
the presence of water a strong partitioning effect leading to self-assembly and
microphase segregation of the two blocks is found.
Figure 1.1 shows a typical example of a phase diagram for a commercial
form of monoglyceride, monolinolein, in the presence of water. Remarkably,
several transitions are found within a few degrees of temperature or small
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