Cubic Mesophases

In addition to the cubic and/or inverse cubic forms described above, further transitional forms exist between the lamellar phase and the hexagonal mesophase (cubic, type II) or inverse hexagonal mesophase (cubic, type III) [6]. In contrast to the discontinuous phases of types I and IV, cubic mesophases of type II and III belong to the bieontinuous phases (Fig. 4f). A range of lyotropic mesophases are possible, depending on the mesogen concentration, the lipophilic or hydrophilic characteristics of the solvent, and the molecule itself [6]. 

Keywords Columnar mesophase Cubic mesophase Dendrimer Liquid crystal Metallomesogen Micro-segregation Organic semiconductor Perfluorinated molecule Polyphilic molecule Self-assembly... 

The lamellar structure does not possess any intrinsic curvature and is considered as the midpoint of an ideal, symmetrical LLC phase progression (Fig. 12.1) [4, 5]. The current chapter is mainly focused on the inverted (W/O) mesophases (cubic and hexagonal), representing an important class of nanostructures for potential pharmaceutical applications. 

B. Donnio, D. N. Bruce, H. Delacroix, T. Gulik-Krzywicki. Freeze-fracture electron microscopy of thermotropic cubic and columnar mesophases. Liq... 

Using the so-called "block copolymers (a block of Na A-monomers at one end is covalently bonded to a block of Nb B-monomers) one can also realize the analogy of order-disorder phenomena in metallic alloys with polymers one observes transitions from the disordered melt to mesophases with various types of long range order (lamellar, hexagonal, cubic, etc ). We shall not consider these phenomena here further, however... 

Figure 8.9 Schematic representation of a conical monodendron self-assembled into a supramolecular spherical micellar dendrimer, and then into an fm3m cubic mesophase.

Coco, S., Cordovdla, C., Donnio, B., Espinet, P., Garda-Casas, M.J. and Gudlon, D. (2008) Self-Organization of Dendritic Supermolecules, Based on Isocyanide-Gold(I), -Copper(l), -Palladium(II), and -Platinum(ll) Complexes, into Micellar Cubic Mesophases. Chemistry - A European Journal, 14, 3544-3552. 

The family of primary silver thiolate compounds AgSC H2n+1 ( = 4, 6, 8, 10, 12, 16, or 18), which in the solid state consists of 2x,[AgSR] layers, behaves as thermotropic liquid crystals. On heating, they display successively lamellar (smectic A), cubic, and micellar mesophases.969... 

Figures 9a-c represent transmission electron micrographs of different lyotropic liquid crystals after freeze fracture without etching. The layer structure of the lamellar mesophase including confocal domains, hexagonal arrangement of rodlike micelles within the hexagonal mesophase, as well as close-packed spherical micelles within the cubic liquid crystal can be clearly seen.

Mesophase with a three-dimensional spatial distribution of helical director axes leading to frustrated structures with defects arranged on a lattice with cubic symmetry and lattice constants of the order of the wavelength of visible light. 

Mesophase with an overall three-dimensional order of cubic symmetry in which each micellar unit cell contains several hundred molecules in random configurations, as in a liquid. 

Note 2 A cubic mesophase is optically isotropic it may be distinguished from an isotropic liquid or a homeotropic phase by the fact that the optically-black isotropic phase or homeotropic phase nucleates in the bireffingent SmC phase in straight-edged squares, rhombi, hexagons and rectangles. 

Note 3 A cubic mesophase may be formed by rod-like molecules with strong, specific intermolecular interactions, such as hydrogen bonding, between them. However, they are also found in polycatenary compounds where there are no specific, strong interactions. 

Note 4 Cubic mesophases have long been known in thermotropic salt-like compounds and in lyotropic liquid-crystals. 

Noted There are several types of thermotropic and lyotropic cubic mesophases, with different symmetry and miscibility properties when the space groups of these are known, they should be included in parentheses after the term Cub . 

Example The following compound exhibits a crystalline phase (Cr), smectic SmC, cubic (Cub), smectic SmA mesophases, and an isotropic (I) phase, with transitions at the specified temperatures ... 

At higher concentrations, micelles assemble in turn, to form hexagonal or cubic phases while longer chains or multi-chain compounds afford lamellar phases in which the amphiphilic derivative is arranged in parallel bilayers, separated by water. The succession of mesophases depending on temperature and concentration of the amphiphile can be visualized in a phase diagram (Fig. 3 c). 

Cyclic carbohydrates with two alkyl chains (e.g. 1,2-dialkyl (or 1,2-diacyl) glycerol 8 a (sug=Glcp, Galp) present structural similarities with glycerophospho-lipids. They form complex mesophases such as bicontinuous cubic phases, inverted hexagonal phases or myelin figures [58-61]. Other dialkyl derivatives... 

Phenyl-functionalized silicate mesophases with hexagonal or cubic symmetries influence of synthesis parameters. 

Most of the one-pot syntheses of organically functionalized mesoporous silicates have been done under basic conditions. Only hexagonal phases (2d, p6m) were reported so far, except one very recent example of a phenyl-functionalized cubic phase [17], analogous to the bicontinuous MCM-48 phase (la3d) [8]. The cubic phase prepared under acidic conditions from PTES and TEOS is indeed related to a different type of cubic mesophases, micellar mesophases, reported in the literature for various surfactant/solvent systems [23] as well as for lipid-containing systems [24]. 

These micellar cubic mesophases require large surface curvature and low charge density. Their formation is thus favored by the use of surfactant molecules with large polar head group, and acidic conditions under which the charge density at the silicate/surfactant is always limited. The fact that this phase can be prepared with CTAB when PTES is present, suggests the existence of specific interactions between the phenyl groups and the polar head of the surfactant molecules. It was indeed reported that benzene molecules are preferably located at the hydrophilic-hydrophobic interface [29]. 

---