Emulsions, Microemulsions, and Lyotropic Liquid Crystals - Surface and Colloid Chemistry: Principles and Applications - page 187

Solid … Melting Point … Liquid
However, if an LC substance is heated, it will show more than one melting point.
Thus, liquid crystals are substances that exhibit a phase of matter that has proper-
ties between those of a conventional liquid and a solid crystal. For instance, an LC
may flow like a liquid but have the molecules in the liquid arranged and/or oriented
in a crystal-like way. There are many different types of LC phases that can be dis-
tinguished based on their different optical properties (such as birefringence). When
viewed under a microscope using a polarized light source, different liquid crystal
phases will appear to have a distinct texture. Each “patch” in the texture corresponds
to a domain where the LC molecules are oriented in a different direction. Within a
domain, however, the molecules are well ordered. Liquid crystal materials may not
always be in an LC phase (just as water is not always in the liquid phase; it may also
be found in the solid or gas phase).
Liquid crystals can be divided into the following types:
Thermotropic LCs
Lyotropic
Thermotropic LCs exhibit the phase transition into the LC phase as temperature
is changed, whereas lyotropic LCs exhibit it as a function of concentration of the
mesogen in a solvent (typically water) as well as temperature.
9.4.1 l I q u I d c r y S T a l p h a S e S
The various LC phases (called mesophases) can be characterized by the type of
molecular ordering that is present. One is positional order (whether or not mole-
cules are arranged in any sort of ordered lattice), and another is orientational order
(whether or not molecules are mostly pointing in the same direction); moreover, order
can be either short-range (only between molecules close to each other) or long-range
(extending to larger, sometimes macroscopic, dimensions). Most thermotropic LCs
will have an isotropic phase at high temperatures. That is, heating will induce in
them a conventional liquid phase characterized by random and isotropic molecular
ordering (a little to no long-range order) and fluid-like flow behavior. Under other
conditions (for instance, lower temperature), an LC might inhabit one or more phases
with significant anisotropic orientational structure and long-range orientational order
while still having the ability to flow.
The ordering of liquid crystalline phases is extensive on the molecular scale. In
fact, the self-assembly characteristic as possessed by lipids (amphiphiles) is the basic
building feature in LCs. This order extends up to the entire domain size, which may
be on the order of micrometers (μm), but usually does not extend to the macroscopic
scale as often occurs in classical crystalline solids. However, some techniques (such
as the use of boundaries or an applied electric field) can be used to enforce a single
ordered domain in a macroscopic LC sample. The ordering in an LC might extend
along only one dimension, with the material being essentially disordered in the other
two directions.