Chemistry Reference
In-Depth Information
often critical to technological applications, there exists a considerable body of
LLC research focused on understanding phase behavior.
Phase behavior data is most commonly reported in the form of the
temperature-binary composition (i.e., water-amphiphile) phase diagram
(Laughlin, 1994). Phase diagrams are generated by producing specifi c compo-
sitions and carefully analyzing the phases that result without causing compo-
sitional alterations. Classically, this is achieved by sealing precise compositions
within glass ampoules.
LLC phases manifest themselves in different regions of the phase diagram
depending on the amphiphile being studied, and in certain cases they share a
phase boundary with excess water, which results in the LLCs being “dilutable
in excess water,” an important property when it comes to producing LLC
dispersions, which are discussed later.
There are several characterization techniques that can be employed in
determining temperature-composition phase diagrams of LLC systems. These
range from extremely “low tech” (e.g., direct visual observation) to extremely
“high tech” (employing, e.g., synchrotron X-ray radiation). We fi rst introduce
optical microscopy, which allows the simple and direct observation of various
LLC properties as temperature is varied and is often the fi rst technique applied
in qualitative studies of LLC phase behavior. When applied in combination
with spectroscopic measurements, as employed in diffusive interfacial trans-
port (DIT) - near - infrared microspectroscopy, quantitative information regard-
ing the position of compositional phase boundaries can also be obtained.
Rheological measurements of LLC phases are discussed in Section 4.2.2. Scat-
tering techniques (particularly X-ray scattering) have become a powerful tool
in determining LLC nanostructure and are discussed in Section 4.2.3. Pulsed
fi eld gradient nuclear magnetic resonance (NMR) is a complementary tech-
nique to small-angle X-ray scattering (SAXS) which will be introduced in
Section 4.2.4. Recently, analysis of free volume obtained from position anni-
hilation lifetime spectroscopy (PALS) has also been found to show consistency
with phase and rheological behavior, and this technique is briefl y discussed in
Section 4.2.5. Electron microscopy techniques, which allow the direct observa-
tion of phase nanostructure, are also discussed in Section 4.2.6.
4.2.1
Polarized Optical Microscopy
Friedrich Reinitzer discovered liquid crystals in 1888 and observed their ability
to rotate the polarization of light (Reinitzer, 1888). Since then, polarized light
microscopy has been a standard technique for observing liquid crystalline
phases, including lyotropic liquid crystals.
The observation of LLCs using light and polarized light microscopy is a
simple technique for basic phase behavior characterization. Birefringent
anisotropic phases (such as the lamellar and hexagonal phases) are readily
detected under cross-polarized light (Laughlin, 1994; Rosevear, 1968), with
specifi c optical textures allowing such phases to be distinguished from each
other, as shown in Figure 4.2.
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