Emulsions, Microemulsions, and Lyotropic Liquid Crystals - Surface and Colloid Chemistry: Principles and Applications - page 191

easily form glasses. The addition of long-chain soap-like molecules leads to a series
of new phases that show a variety of liquid crystalline behavior both as a function
of the inorganic-organic composition ratio and temperature. This class of materials
has been named metallotropic.
9.4.1.3 biological liquid crystals
The effect of temperature on biological living systems needs a few remarks. The
human body temperature is made up of molecules and cell structures so as to func-
tion at optimum at 37°C. If the temperature changes a few degrees (plus or minus)
from 37°C, then the effect is considerable but not lethal. This is because the sys-
tem shows LC behavior. In other words, biological reactions can go on functioning
(though with some restrictions) even if the temperature is 36°C or 38°C.
Lyotropic liquid-crystalline nanostructures are abundant in living systems.
Accordingly, lyotropic LC have been of much interest in such fields as biomimetic
chemistry. In fact, biological membranes and cell membranes are a form of LC.
Their constituent rod-like molecules (e.g., phospholipids) are organized perpendicu-
larly to the membrane surface; yet, the membrane is fluid and elastic. The constituent
molecules can flow in plane quite easily but tend not to leave the membrane, and
can flip from one side of the membrane to the other with some difficulty. These LC
membrane phases can also host important proteins such as receptors freely “float-
ing” inside, or partly outside, the membrane.
Many other biological structures exhibit LC behavior. For instance, the concen-
trated protein solution that is extruded by a spider to generate silk is actually an LC
phase. The precise ordering of molecules in silk is critical to its renowned strength.
DNA and many polypeptides can also form LC phases. Since biological mesogens
are usually chiral, chirality often plays a role in these phases.
9.4.1.3.1 Theory of Liquid Crystals Formation
Microscopic theoretical treatment of fluid phases can become quite involved owing
to the high material density, which means that strong interactions, hard-core repul-
sions, and many-body correlations cannot be ignored. In the case of LC, anisotropy
in all of these interactions further complicates analysis. There are a number of fairly
simple theories, however, that can at least predict the general behavior of the phase
transitions in LC systems.
Additionally, the description of LC involves an analysis of order. In particular,
a sharp drop of the order parameter to B is observed when a transition takes place
from the LC phase into the isotropic phase. The order parameter can be measured
experimentally in a number of ways, such as diamagnetism, birefringence, Raman
scattering, NMR, and EPR.
A very simple model that predicts lyotropic phase transitions is the hard-rod
model proposed by Onsager (Friberg, 1976). This theory considers the volume
excluded from the center-of-mass of one idealized cylinder as it approaches another.
Specifically, if the cylinders are oriented parallel to one another, there is very little
volume that is excluded from the center-of-mass of the approaching cylinder (it can
come quite close to the other cylinder). If, however, the cylinders are at some angle
to one another, then there is a large volume surrounding the cylinder where the