Mesophases bicontinuous

In the case of a balanced relation between the different molecular parts a SmA phase is observed. Excessive space demand of one of these parts results in the formation of a columnar mesophase or, in extreme situations, even in discontinuous cubic mesophases. Bicontinuous cubic mesophases may occur in the area between the SmA and the columnar regions. 

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... 

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]. 

Cr Cub, Cubv d E G HT Iso Isore l LamN LaniSm/col Lamsm/dis LC LT M N/N Rp Rh Rsi SmA Crystalline solid Spheroidic (micellar) cubic phase Bicontinuous cubic phase Layer periodicity Crystalline E phase Glassy state High temperature phase Isotropic liquid Re-entrant isotropic phase Molecular length Laminated nematic phase Correlated laminated smectic phase Non-correlated laminated smectic phase Liquid crystal/Liquid crystalline Low temperature phase Unknown mesophase Nematic phase/Chiral nematic Phase Perfluoroalkyl chain Alkyl chain Carbosilane chain Smectic A phase (nontilted smectic phase)... 

While there have been efforts to polymerize other surfactant mesophases and metastable phases, bicontinuous cubic phases have only very recently been the subject of polymerization work. Through the use of polymerizable surfactants, and aqueous monomers, in particular acrylamide, polymerization reactions have been performed in vesicles (4-8). surfactant foams ), inverted micellar solutions (10). hexagonal phase liquid crystals (111, and bicontinuous microemulsions (121. In the latter two cases rearrangement of the microstructure occured during polymerization, which in the case of bicontinuous microemulsions seems inevitable b ause microemulsions are of low viscosity and continually rearranging on the timescale of microseconds due to thermal disruption (131. In contrast, bicontinuous cubic phases are extremely viscous in genei, and although the components display self-diffusion rates comparable to those... 

In addition to the cubic and/or inverse cubic forms described previously, further transitional forms exist between the lamellar phase and the hexagonal meso-phase (cubic, type II) or inverse hexagonal mesophase (cubic, type III). In contrast to the discontinuous phases of types I and IV, cubic mesophases of type II and type III belong to the bicontinuous phases (Fig. 4F). A range of lyotropic mesophases are possible, depending on the mesogen concentration, the lipophilic or hydrophilic characteristics of the solvent... 

Figure 4.11 Plot of the approximate compositions for which surfactant/water mixtures can form monolayers versus the surfactant parameter of the surfactant. This plot is for chain lengths of 14A, which corresponds to hydrocarbons made up of about 12 carbon atoms. The notation for various mesophases is as follows Vi, V2 are bicontinuous cubic phases (the former containing two interpenetrating hydrophobic diain networks in a polar continuum, the latter polar networks in a hydrophobic continuum). Hi and H2 denote normal and reversed hexagonal phases. La denotes the lamellar phase, and Li and L2 denote isotropic micellar and reversed micellar phases (made up of spherical micelles).

A central issue in the field of surfactant self-assembly is the structure of the liquid crystalline mesophases denoted bicontinuous cubic, and "intermediate" phases (i.e. rhombohedral, monoclinic and tetragonal phases). Cubic phases were detected by Luzzati et al. and Fontell in the 1960 s, although they were believed to be rare in comparison with the classical lamellar, hexagonal and micellar mesophases. It is now clear that these phases are ubiquitous in surfactant and Upid systems. Further, a number of cubic phases can occur within the same system, as the temperature or concentration is varied. Luzzati s group also discovered a number of crystalline mesophases in soaps and lipids, of tetragonal and rhombohedral symmetries (the so-called "T" and "R" phases). More recently, Tiddy et al. have detected systematic replacement of cubic mesophases by "intermediate" T and R phases as the surfactant architecture is varied [22-24]. The most detailed mesophase study to date has revealed the presence of monoclinic. 

The relative stability of mesh and IPMS structures is still unclear. For example, the Ri mesophase (of rhombohedral symmetry) in the SDS-water system transforms continuously into the neighbouring bicontinuous cubic phase (Fig. 4.14) [20]. This suggests that this mesophase is a hyperbolic (reversed) bilayer Ijring on a rhombohedral IPMS. Indeed, the rhombohedral rPD surface is only marginally less homogeneous than its cubic counterparts, the P- and D-svu-faces. 

This (local) double twist configuration clearly involves a hyperbolic deformation of the imaginary layers. In contrast to the hyperbolic layers found in bicontinuous bilayer lyotropic mesophases, the molecules within these chiral thermotropic mesophases are oriented parallel to the layers, to achieve nonzero average twist. The magnitude of this twist is deternuned by the direction along which the molecules lie (relative to the principal directions on the surface), and a function of the local curvatures of the layers (K1-K2), cf. eq. 1.4. Just as the molecular shape of (achiral) surfactant molecules determines the membrane curvatures, the chirality of these molecules induces a preferred curvature-orientation relation, via the geodesic torsion of the layer. 

The rod-coil approach as a means to manipulate supramolecular structure as a function of rod volume fraction was reported to be extended to main chain multiblock copolymer systems, which generate bicontinuous cubic and hexagonal columnar mesophases depending on the rod-to-coil volume fraction [95,96]. For example, rod-coil multiblock copolymer (28) based on short length of coil (rod volume fraction, /ro(j = 0.38) exhibits a bicontinuous cubic mesophase, while copolymer (29) based on higher coil volume fraction (/i-od = 0.29) shows a hexagonal columnar mesophase. A notable feature... 

Figure 1.4 T(7)-sections through the phase prism of the systems H20-n-octane-C6E2, C8E3, Q0E4 and C12E5 at an oil/(water + oil) volume fraction of = 0.5. In order to determine the respective X-point the phase boundaries are measured only for surfactant mass fractions 7 > 7. An increase of both the hydrophobic chain length / and the size of the hydrophilic head group j shifts the X-point to lower values of 7, i.e. the efficiency increases. Simultaneously the stability range of the bicontinuous one phase microemulsion shrinks dramatically due to the increased extension of the lamellar mesophase (La). (From Ref. [26], reprinted with permission of Elsevier.)...

The ideas of the relevance of phase diagrams and thermodynamic stability as well as the bicontinuous structure were certainly not accepted immediately and many publications until well into the 1990s caused confusion as some authors still took droplet structures for granted. A title for a paper [31] in Nature as late as 1986 entitled Occurrence of liquid-crystalline mesophases in microemulsion dispersions illustrates both the slow acceptance and the ignorance of previous work on phase diagrams. 

Fig. 4. Schematic drawing of lipid-water mesophases (Lc, lamellar crystalline Lps Pp., lamellar gel L , lamellar liquid-crystalline Qn, Qn°, Qn , inverse bicontinuous cubics Hu, inverse hexagonal). The cubic phases are represented by the G, D, and P minimal surfaces, which locate the midplanes of fluid hpid bilayers.

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