Cubic mesophase

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. 

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]. 

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 . 

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]. 

Winsor, P. A. Non-amphiphilic cubic mesophases plastic crystals . Chapter 2.2 in Ref. 15b... 

Relationship of Cubic Mesophases ("Plastic Crystals ) Formed by Non-Amphiphilic Globular Molecules to Cubic Mesophases of the Amphiphilic Series... 

The cubic amphiphilic mesophases (Sic, Vi, and V2) from their interposition in the succession of mesophases Sic, Mi, Vi, G, V2, and M2, have generally been termed liquid crystalline like the optically anisotropic amphiphilic mesophases Mi, G, and M2. The cubic mesophases formed by non-amphiphilic globular molecules have however usually been termed plastic crystals. This nomenclature has obscured the fact that these plastic crystals are fundamentally liquid crystals rather than solid cyrstals and bear a relationship to the optically anisotropic non-amphiphilic smectic and nematic liquid crystals similar to that born by the amphiphilic cubic mesophases to the optically anisotropic neat (G) and middle (Mi and M2) liquid crystalline phases. 

Amphiphilic Cubic Mesophases Viscous Isotropic Phases. In Figure 1, at positions where a high concentration of globular micellar forms might be expected, the cubic viscous isotropic mesophases Sic, Vi, and V2 are encountered. This is further illustrated by Figure 5. These mesophases, because of their inclusion within the general sequence of amphiphilic mesophases ... 

Non-Amphiphilic Cubic Mesophases Plastic Crystals. The cubic plastic crystals of the non-amphiphilic series, to which the amphiphilic... 

Table II. Crystalline Forms of Some Non-Amphiphilic Plastic Crystals (23,24) and of Some Amphiphilic Cubic Mesophases...

One feature of the non-amphiphilic cubic mesophases is that they frequently show mutual miscibility even when constituted from dissimilar molecules. Such miscibility, which contrasts with the immiscibility between dissimilarly constituted solid crystals, is also found between nematic mesophases, between corresponding smectic polymorphs, and, of course, between amorphous liquids. This miscibility is important in its implication that in the cubic mesophases of the amphiphilic series there could well be an equilibrium of related globular micellar forms (Figures 1 and 5) rather than a single clearly defined form. 

An important characteristic of the cubic mesophases, either non-amphiphilic or amphiphilic, is that because of the fairly free thermal rotational motions of their constituent units, they typically give high resolution NMR spectra. In this respect they behave like amorphous liquids and quite differently from conventional solids or from mesophases such as Mi, M2, or G in which rotation of the units is more severely restricted. 

Formation of the Vi and V2 Mesophases and of the Non-Amphiphilic Cubic Mesophase "Smectic D . In Figures 1 and 5, the formation of the amphiphilic cubic mesophases Vi and V2 is attributed to transitional globular micellar forms which arise intermediate between the indefinitely extended fibrous (M) and lamellar (G) forms which constitute the middle and neat mesophases, respectively. With a few non-amphiphilic mesogens (19, 20, 21, 22) a cubic mesophase "smectic D is found intermediate in the thermal succession of mesophases between smectic A... 

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

In order fo furfher invesfigafe fhe influence of fhe sfrucfure of the mesogenic units on the mesomorphic behavior of the dendrimers, bulky units have been introduced at the periphery of the third and fourth generations of amino-terminated PAMAM and PPI (Fig. 39, L3, Le, L9), wifh the aim of achieving cubic mesophases, since the increased volume around the dendritic core could in principle force the dendrimer to adopt a globular conformation [199,222],... 

What I have said about the pure amorphous phase is not, logically speaking, contradicted by the paper by Pechhold and Grossman, because according to their theory the polymer melt is not an amorphous phase but rather a remarkable kind of cubic mesophase. To justify that they have to use a modification of the rules of statistical mechanics, the cluster-entropy-hypothesis, and how they can justify that I do not know. No doubt we shall come to that in the discussion. For myself I will take one valuable point from that paper, a warning to show what bizarrely different models can be deemed consistent with the same diffraction evidence. 

At higher temperatures and water concentrations, the system may shift into the cubic mesophase structure (see Figure 15). The water is present as spheres totally surrounded by monoglyceride. This phase has a high viscosity and is sometimes called viscous isotropic in the hterature the two terms refer to the same structure. In the presence of more water than can be accommodated in the internal spherical phase, one obtains a mixture of lumps of this cubic structure dispersed in excess water. With a saturated monoglyceride such as GMS, the lamellar structure is the main mesophase found under practical conditions, while with unsaturated monoglycerides this cubic phase is the predominant one at lower temperatures. At lower water concentrations, the spherical water micelles are farther apart, so the viscosity of the mixture becomes lower, approaching that of melted pure surfactant. This is the fluid isotropic mesophase, sometimes referred to as the L2 phase. 

FIGURE 25.20 Water-hexadecyltrimethylammonium bromide. I micellar solution Ha two-dimensional hexagonal mesophase Qa bicontinnons cubic mesophase La lamellar mesophase. (From Wamheim, T., Jonsson, A., and Sjoberg, M., Progr. Colloid Polym. ScL, 82, 271, 1990.)... 

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