Emulsions, Microemulsions, and Lyotropic Liquid Crystals - Surface and Colloid Chemistry: Principles and Applications - page 190

Discontinuous cubic phase (micellar phase)
Hexagonal columnar phase (middle phase)
Bicontinuous cubic phase
Lamellar phase
Bicontinuous cubic phase
Reverse hexagonal columnar phase
Inverse cubic phase (inverse micellar phase)
These structures are extensively described in the current literature (Fanum, 2008;
Friberg, 1976; Birdi, 2002; Holmberg, 2004; Somasundaran, 2006). Even within the
same phases, their self-assembled structures are tunable by the concentration: for
example, in lamellar phases, the layer distances increase with the solvent volume.
Lamellar structures are found in systems such as the common hand soap, which
consists of ca. 0% soap + 20% water. The layers of soap molecules are separated by
a region of water (including, salts etc.) as a kind of sandwich. The x-ray diffraction
analysis shows this structure very clearly. Since lyotropic liquid crystals rely on a
subtle balance of intermolecular interactions, it is more difficult to analyze their
structures and properties than those of thermotropic liquid crystals. Similar phases
and characteristics can be observed in immiscible diblock copolymers.
As mentioned earlier, surfactants aggregate to form micelles, which may vary in
size (i.e., number of monomers per micelle) from a few to over a thousand monomers.
However, surfactants can form, besides simple micellar aggregates (i.e., spherical
or ellipsoidal), many other structures also when mixed with other substances. The
curved micelle aggregates are known to change to planar interfaces when additives,
the so-called cosurfactants, are added. A reported recipe consists of
Surfactant (cetylpyridinium bromide [CPBr]) + hexanol + salt + water
The addition of salts to micelles gives large micelles that turn into cylindrical shapes.
However, the addition of cosurfactant produces the liquid crystal phase . As a conse-
quence, these micellar systems with added cosurfactant are found to undergo several
macroscopic phase transitions in dilute solutions. These transitions are as follows:
Surfactant Micelles (Spherical … Cylinderical)
Surfactants + Cosurfactant … Liquid crystal
Further, the extensive change of ionic charges (if present) have been found to be very
prominent. The simple picture that bilayers in aqueous medium are principally sta-
bilized by the competition between hydration forces - van der Waals- electrostatic
interactions seem to be the most plausible basis.
The hydration forces arise from the hydrogen bond formation between the polar
groups of the surfactant and the water molecule. Van der Waals forces are attraction
forces between all molecules (these are short range).
Metallotropic liquid crystals: LC phases can also be based on low-melting inor-
ganic phases such as ZnCl 2 that have a structure formed of linked tetrahedra and