Mesh phases

Layered tin sulfide mesostructures were synthesized using a cationic surfactant as template, and tin chloride and sodium sulfide as sources of tin and sulfide [36], The structure was composed of Sn2S64 dimers charge-balanced by dodecylammonium cations. A mesostructured tin sulfide mesh phase was synthesized by reacting SnCl4, (NH4)2S and hexadecylamine (HDA) under aqueous basic conditions at 150°C [37], The structure was found to be... 

Exotic phases are found at compositions between lamellae and hexagonal cylinders (see Figs. 12-19 and 12-20). Some examples of the morphologies of these phases are shown in Fig. 12-22 these include cubic strut phases, tetragonal and rhombohedral mesh phases, and rectangular ribbon phases. 

Figure 12.22 Types of phases that occur between hexagonal and lamellar phases (a) Strut phase, the left image is the minimal P surface over which surfactant is draped, and the right is the topology of the two strut networks on either side of the minimal surface (b) tetragonal mesh phase, (c) ribbon phases containing cylinders with ellipsoidal cross sections.

The other phases are less exotic. The mesh phases consists of lamellae with ordered holes, while ribbon phases are deformed cylinders on a rectangular lattice (see Fig. 12-22). These phases can are usually type I phases with the tails inside the deformed cylinders or inside the hole-filled lamellae, but they could also be inverse, type II, phases. Type II mesh and ribbon phases seem not to have been reported much type II strut phases are common for two-tailed lipids, such as those in cell membranes. In fact, type II strut phases evidently serve biological functions, since they have been found to exist in cellular structures such as the endoplasmic reticulum and the mitochondrion (Seddon 1996). 

The receptor mechanism involved in cell uptake of lipids from blood in the form of LDL (low-density lipoprotein) has been revealed by the work of Brown and Goldstein [71]. When the lipoprotein is boimd to the receptor, a "coated" pit deepens and buds off to form a "coated" vesicle inside the cell (endocytosis). The "coat" represents a general principle of cell transport that is known mainly from the work by Bretscher [72]. The protein forming this coat is clathrin, which aggregates into planar network (or basket-like cages, morphologically similar to "mesh" phases of lipids or surfactants in water, cf. Chapter 4) and the lipid bilayer can span these nets, like a soap film spanning a wire net. 

Another group of archaebacteria is methanogens, which produce methane from carbon dioxide emd hydrogen. These bacteria are killed by oxygen and therefore occur only in anaerobic environment. In the earliest stage of evolution, when there was no oxygen atmosphere (more than 2 billion years ago), these bacteria could have existed everywhere. Their cell walls are periodic open structures ("mesh" phases) of self-assembled S-proteins, discussed in section 4.13. 

Figure 12. Schematic representations of (a) centered rectangular phase, (b) representation of six connected rhombohedral mesh phase.

The first identification of intermediate mesh phase structures was by Luzzati [74] from the measurements by Spegt and Skoulios [70-73] in anhydrous soap melts. It was not until the work of Kekicheff and others [62, 76-81] on sodium dodecyl sulfate (SDS)/water and on lithium perfluoroocta-... 

Many intermediate phase regions are bounded by lamellar phases which contain water filled defects the nonuniform curvature is retained although there is no longer any ordering of the defects within the bilayer and there are no correlations between the bilayers. These defected lamellar phases may also be regarded as random mesh phases. They have been seen in the SDS/water... 

Figure 13. The X-ray scattering from a 52 wt% CigEOg/ water sample, (a) Lamellar phase (b) random mesh phase (c) rhombohedral mesh intermediate phase (d) Ia3d cubic phase and (e) hexagonal-f-gel two phase region.

Between 5=1/2 and 2/3 are mesh phases that include regular tetragonal and hexagonal mesophases and also the defective lamellar phase [295]. For a surfactant parameter from 2/3 to 1, continuous mesh phases or cubic phases are found. Finally, there is the L phase at a surfactant parameter of 1. Indeed, in the system Ci6(EO)6-water, the surfactant parameter is calculated [142] to be 0.65 (Fig. 29), as was found in the L phase of the cesium penta-decafluorooctanoate-water system [265]. In the system C3o(EO)9-water, the surfactant parameter is calculated [262] to be 0.592. 

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