Chemistry Reference
In-Depth Information
reviewed in various publications cited in the Bibliography. The discussion here is
limited to a general descriptive introduction to water-based systems. The possible
role of mesophases in various surfactant applications will be mentioned in the
appropriate chapters.
5.2.3. Liquid Crystalline Phases in Simple Binary Systems
It has been recognized for many years that surfactant solutions with concentrations
well above the critical micelle concentration can exhibit physical properties that
indicate the presence of various degrees of structure above the ''simple'' micellar
level. Such structure formation may be manifested in bulk by abrupt changes in
viscosity, conductivity, and other transport phenomena, birefringence, the existence
of characteristic X-ray diffraction and radiation scattering patterns, or spectroscopic
analyses. As the surfactant concentration is increased, changes in the physical
characteristics of the solution signal corresponding changes in the nature of the
aggregated solute as illustrated in Figure 4.2.
We know that the existence of mesophases in aqueous solutions of pure or rea-
sonably homogeneous surfactants is a direct result of the nature of water-surfactant
interactions. As the amount of solvent available between the simple dilute solution
micelles decreases, interactions between adjacent structures increase to the point
that a form of ''second phase'' coalescence can be invoked, leading to formation of
larger disk-shaped or cylindrical micelles (Figure 5.3a). As the concentration process
continues, hexagonal close-packed arrays of cylinders may appear (Figure 5.3b),
producing the hexagonal or normal middle phase. The next step in the process is
the coalescence of the adjacent, mutually parallel cylinders to produce the neat
phase, characterized by lamellar bilayer structures separated by solvent phases (Fig-
ure 5.3c). Under some circumstances, especially in the presence of nonaqueous sol-
vents, a reversed hexagonal phase will appear that contains close-packed cylindrical
arrays, but with the internal region of the cylinders containing the aqueous phase
(Figure 5.3d).
In some surfactant systems, more complex phase behavior involving one or more
viscous isotropic structures will appear. Such phases usually exhibit an X-ray
pattern characteristics of a cubic lattice. Such phases are now recognized as the
primarily cubic bicontinuous phases introduced above. More recent research with
bulky surfactant molecules has led to the suggestion of ''wormlike'' or ribbon
micelles that may be best described—conceptually, at least—as ''super''aggrega-
tions of smaller micellar units or twisted hexagonal systems.
The conditions of temperature and concentration that produce the various
solution phases in surfactant-solvent systems can be determined (with a great deal
of laboratory work) by the construction of phase diagrams. The construction-
interpretation of such diagrams is a complex undertaking that is beyond the
scope of this work. However, sufficient literature exists to permit certain general-
izations that will help in understanding the activity of most reasonably simple sur-
factant systems.
