Emulsions, Microemulsions, and Lyotropic Liquid Crystals - Surface and Colloid Chemistry: Principles and Applications - page 188

9.4.1.1 thermotropic liquid crystals
Thermotropic phases are those that occur in a certain temperature range. If the tem-
perature is raised too high, thermal motion will destroy the delicate cooperative
ordering of the LC phase, pushing the material into a conventional isotropic liquid
phase. At too low a temperature, most LC materials will form a conventional (though
anisotropic) crystal. Many thermotropic LCs exhibit a variety of phases as the tem-
perature is changed. For instance, a particular mesogen may exhibit various smectic
and nematic (and finally isotropic) phases as temperature is increased.
9.4.1.1.1 The Nematic Phase
One of the most common LC phases is the nematic, where the molecules have no
positional order but no long-range orientational order either. So, the molecules flow,
and their center of mass positions are randomly distributed as in a liquid, but they all
point in the same direction (within each domain). Most nematics are uniaxial: they
have one axis that is longer and preferred, with the other two being equivalent (can
be approximated as cylinders). Some liquid crystals are biaxial nematics, meaning
that, in addition to orienting their long axis, they also orient along a secondary axis.
Liquid crystals are a phase of matter whose order is intermediate between that
of a liquid and that of a crystal. The molecules are typically rod-shaped organic
moieties about 25 Å (2.5 nm) in length, and their ordering is a function of tem-
perature. The nematic phase, for example, is characterized by the orientational order
of the constituent molecules. The molecular orientation (and hence the material's
optical properties) can be controlled with applied electric fields. Nematics are (still)
the most commonly used phase in liquid crystal displays (LCDs), with many such
devices using twisted nematic geometry. The smectic phases, which are found at
temperatures lower than the nematic, form well-defined layers that can slide over one
another like soap. The smectics are thus positionally ordered along one direction. In
the Smectic A phase, the molecules are oriented along the layer normal, while in the
Smectic C phase they are tilted away from the layer normal. These phases, which are
liquid-like within the layers, are illustrated in the following text. There is a very large
number of different smectic phases, all characterized by different types and degrees
of positional and orientational order.
9.4.1.1.1.1 Chiral Phases The chiral nematic phase exhibits chirality ( handed-
ness ). This phase is often called the cholesteric phase because it was first observed
for cholesterol derivatives. Only chiral molecules (i.e., those that lack inversion sym-
metry) can give rise to such a phase. This phase exhibits a twisting of the molecules
along the director, with the molecular axis perpendicular to the director. The finite
twist angle between adjacent molecules is due to their asymmetric packing, which
results in longer-range chiral order. In the smectic phase, the molecules orient roughly
along the director, with a finite tilt angle, and a twist relative to other mesogens.
The chiral pitch refers to the distance (along the director) over which the mesogens
undergo a full 360° twist (the structure repeats itself every half pitch since the positive
and negative directions along the director are equivalent). The pitch may be varied
by adjusting temperature or adding other molecules to the LC fluid. For many types