Emulsions, Microemulsions, and Lyotropic Liquid Crystals - Surface and Colloid Chemistry: Principles and Applications - page 189

of LC, the pitch is on the same order as the wavelength of visible light, which causes
these systems to exhibit unique optical properties , such as selective reflection.
9.4.1.2 lyotropic liquid crystals
As compared to the cholesteric LC, the lyotropic LC consists of two or more compo-
nents that exhibit liquid-crystalline properties (dependent on concentration, temper-
ature, and pressure). In the lyotropic phases, solvent molecules fill the space around
the compounds (such as soaps) to provide fluidity to the system. In contrast to ther-
motropic liquid crystals, these lyotropics have another degree of freedom of concen-
tration that enables them to induce a variety of different phases. A typical lyotropic
liquid crystal is surfactant-water-long-chain alcohol.
A compound that has two immiscible hydrophilic and hydrophobic parts within
the same molecule is called an amphiphilic molecule (as mentioned earlier). Many
amphiphilic molecules show lyotropic liquid-crystalline phase sequences, depend-
ing on the volume balances between the hydrophilic part and the hydrophobic part.
These structures are formed through the microphase segregation of two incompat-
ible components on a nanometer scale. Hand soap is an everyday example of a lyo-
tropic liquid crystal (80% soap + 20% water).
The content of water or other solvent molecules changes the self-assembled
structures. At very low amphiphile concentration, the molecules will be dispersed
randomly without any ordering. At slightly higher (but still low) concentration,
amphiphilic molecules will spontaneously assemble into micelles or vesicles. This is
done so as to “hide” the hydrophobic tail of the amphiphile inside the micelle core,
exposing a hydrophilic (water-soluble) surface to aqueous solution. However, these
spherical objects do not order themselves in solution. At higher concentration, the
assemblies will become ordered. A typical phase is a hexagonal columnar phase, in
which the amphiphiles form long cylinders (again with a hydrophilic surface) that
arrange themselves into a roughly hexagonal lattice. This is called the middle soap
phase. At still higher concentration, a lamellar phase (neat soap phase) may form,
wherein extended sheets of amphiphiles are separated by thin layers of water. For
some systems, a cubic (also called viscous isotropic ) phase may exist between the
hexagonal and lamellar phases, wherein spheres are formed that create a dense cubic
lattice. These spheres may also be connected to one another, forming a bicontinuous
cubic phase.
The aggregates created by amphiphiles are usually spherical (as in the case of
micelles), but may also be disc-like (bicelles), rodlike, or biaxial (all three micelle
axes are distinct) (Zana, 2008). These anisotropic self-assembled nanostructures
can then order themselves in much the same way as liquid crystals do, forming
large-scale versions of all the thermotropic phases (such as a nematic phase of
rod-shaped micelles).
For some systems, at high concentration, inverse phases are observed. That is,
one may generate an inverse hexagonal columnar phase (columns of water encapsu-
lated by amphiphiles), or an inverse micellar phase (a bulk LC sample with spherical
water cavities).
A generic progression of phases, going from low to high amphiphile concentra-
tion, is