Hexagonal surfactants

Richtering and coworkers (Richtering et al. 1994 Linemann et al. 1995) have have carried out light-scattering studies and rheological studies of hexagonal surfactant phases formed... 

FIGURE 16.6 AFM images of octadecyl trimethyl ammonium bromide on mica (a) and graphite (b) surfaces, showing that graphite induces a hexagonal surfactant aggregate structure, while the mica surface renders a featureless lamellar structure. (From Liu, J.-F., and Ducker, W. A. 1999. J. Phys Chem. B 103 8558. With permission.)... 

These structures are commonly referred to as hexagonal Hquid crystals. As the surfactant concentration is further increased, the tubules expand in a second direction to form large, stacked lamellar sheets of surfactants, commonly referred to as lamellar Hquid crystals. These Hquid crystals are very important in soap making. 

The model has been successfully used to describe wetting behavior of the microemulsion at the oil-water interface [12,18-20], to investigate a few ordered phases such as lamellar, double diamond, simple cubic, hexagonal, or crystals of spherical micelles [21,22], and to study the mixtures containing surfactant in confined geometry [23]. 

The other type of porous glass that has cylindrical pores is mesoporous silicate (MPS) (14,15). The advantage of MPS is in its feasibility to make a small pore diameter, typically below 10 nm. A columnar-phase liquid crystal, formed from surfactant molecules with a long alkyl chain tail and silicate molecules, is calcined to remove hydrocarbons. At the end, a hexagonal array of straight and uniform cylindrical holes is created in a crystalline order. MPS is not available commercially either. 

An example for a partially known ternary phase diagram is the sodium octane 1 -sulfonate/ 1-decanol/water system [61]. Figure 34 shows the isotropic areas L, and L2 for the water-rich surfactant phase with solubilized alcohol and for the solvent-rich surfactant phase with solubilized water, respectively. Furthermore, the lamellar neat phase D and the anisotropic hexagonal middle phase E are indicated (for systematics, cf. Ref. 62). For the quaternary sodium octane 1-sulfonate (A)/l-butanol (B)/n-tetradecane (0)/water (W) system, the tricritical point which characterizes the transition of three coexisting phases into one liquid phase is at 40.1°C A, 0.042 (mass parts) B, 0.958 (A + B = 56 wt %) O, 0.54 W, 0.46 [63]. For both the binary phase equilibrium dodecane... 

The phase behavior of a-ester sulfonates has been studied in detail with methyl laurate and methyl palmitate [58]. In both cases, at higher temperatures, as the surfactant concentration increases, there is a transition from an isotropic solution to a hexagonal liquid crystalline phase and finally, at high surfactant concentrations, to a lamellar liquid crystal (Fig. 4). The crystal/liquid-crys-tal phase transition occurs at even higher temperatures as the chain length increases. On the other hand, chain length has practically no influence on the... 

Figure 4.4. Preparation of MTS materials. The diagram shows self assembly of the surfactant into micelles followed by condensation of silica around the micelles. The micelles arrange themselves into an approximately hexagonal array. After the formation of the silica around the micelles, the micelles are burnt out, leaving pores where the micelles were. The pores are an accnrate reflection of the size and shape of the micelles. This makes the pores uniformly sized and shaped.

Gold nanostructures of various sizes and morphologies were synthesized at room temperature using naturally occurring biodegradable plant surfactants [ 76]. The sizes and shapes (spherical, prisms, and hexagonal) of the synthesized nanopartides were dependent on the concentration of the gold ions and the type of plant surfactant used for preparation. 

In binary mixtures of water, surfactants, or lipids the most common structure is the gyroid one, G, existing usually on the phase diagram between the hexagonal and lamellar mesophases. This structure has been observed in a very large number of surfactant systems [13-16,24—27] and in the computer simulations of surfactant systems [28], The G phase is found at rather high surfactant concentrations, usually much above 50% by weight. 

The micelles are spherical, but when the concentration of surfactant increases, the shape of the ionic micelles changes following the spherical sequence cylindrical-hexagonal-laminar [22], In the case of nonionic micelles the shape... 

The thermal stability of mesoporous frameworks substantially increases with an increase in the wall thickness and pore size, which can be varied even for the same template by changing the processing conditions. Ozin et al.55 developed a way to prepare crystalline titania films with a 2D-hexagonal architecture by replacement of ethanol in the Pluronic-containing precursor solution with more hydrophobic butanol-1. The latter promotes phase separation at low surfactant-to-titania ratios, resulting in thicker pore walls, which are more compatible with the crystal growth during subsequent calcination. 

The most widely studied member of the M41S family is hexagonal MCM-41, which was first prepared by the I S+ liquid-crystal approach (168, 169). A charged surfactant like CTAB produces well-ordered mesoporous materials having ID... 

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