Liquid crystals cooperative self-assembly

Formation Mechanism of Mesostructure Liquid-crystal Template and Cooperative Self-assembly... 

Five specific routes to mesoporous templated materials exist, although only three of these are used to prepare mesoporous silicates. These are true liquid crystal templating, cooperative self-assembly in solution and... 

The three major routes are (i) true liquid crystal templating at high surfactant concentrations, which is used for the formation of monoliths, thick layers or, via electrodeposition techniques, formation of thin films (ii) cooperative self assembly at surfactant concentrations where micelles are present in solution, which can be used to make powders (with either well-defined particle shapes or random structures), fibres and thin films grown at interfaces from solution and (iii) EISA at very low surfactant concentrations, where no micelles are initially present in solution, and solutions are in general prepared in nonaqueous solvents. This route is used to prepare thin films by dip or spin coating and powders via aerosol routes. The following sections will look at the current understanding of the mechanisms involved in each route to mesoporous materials. 

All authors conclude that carbons with adjusted pore size distribution in the entire range of the nanopores can be obtained, depending the synthesis conditions and cationic surfactants used as templates. Despite this, it is an effective pathway to obtain porous carbons, even though the pore formation mechanism is not well understood. Hence, different mechanisms are used to explain its effect emerged, such as liquid crystal templating mechanism, cooperative self-assembly, electrostatic interaction between cationic surfactant molecules and the anionic RF polymer chain and micelles as nanoreactors to produce RF nanoparticles [51, 70]. In these cases, the simple mold effect from the globular form and the RF polymerization around it is insufficient to explain the structuring of the material by the template, where spherical closed pores would be expected. 

Before concluding the discussion on the LCT models, it is worth pointing out that under certain conditions a true cooperative self-assembly of inorganic ions (e.g., silicates) and surfactants is possible. For example, at low temperature and high pH, conditions that prevent condensation of silicate ions, the formation mechanism schematically represented in Fig. 5 can be operative. According to this mechanism, the formation of mesoporous materials occurs via so-called silicatrophic liquid crystals (SLC). Three steps are involved in the synthesis pathway ... 

After the discovery of the first mesoporous silicas through external templating, a lot of work has been done to understand and rationalize the formation mechanisms of these materials. Numerous research groups employed a variety of techniques (e.g., NMR spectroscopy. X-ray diffraction, cryo-TEM, electron paramagnetic resonance, and fluorescence) toward this objective. Several models have been proposed [10] and two of them are generally accepted the liquid crystal templating approach and the cooperative self-assembly approach. In both models, the interactions between the surfactant molecules and the inorganic species direct the formation of the ordered solid. 

Figure 18.1 Synthesis of periodic mesoporous materiais assisted by SDAs (A). Route 1 true liquid crystal templating mechanism. Route 2 cooperative self-assembly mechanism. (Adapted from Refs [44,45].)...

It is also meaningful to discuss the phenomenon of polymer assembly in the disclination from the viewpoint of entropy. As mentioned above, the orientational order of liquid crystals is induced by the entropy of translation, while the random coUs of polymers are due to the entropy of rotation. The coexistence of a liquid crystal and a polymer causes a conflict between the different entropies. The polymers in the liquid crystal phase thus assemble in the discUnation region, where the orientational order is lowered and the polymers can maximize their conformational entropy. A satisfactory coexistence is then self-sufficiently achieved by cooperation between the blue phase (which must have disclinations) and the polymer (which tends to be excluded from the ordered region). The elastic energy of the liquid crystal orientational field that accumulates around the disclination also plays an important role in the diffusion of the polymer. The curvature of the director increases closer to the disclination core. The elastic energy, which is proportional... 

Synthesis by Molecular Self-Assembly Liquid Crystals and Cooperative Assembly... 

Synergy in molecular recognition and self-assembly occurs in various phases in both solutions and also in solid states and liquid crystals. On-surface self-assembly possesses a special advantage in that molecules are visualized by scanning tunneling microscopy (STM). This allows us to gain detailed insight into how molecules act cooperatively with each other. 

Scheme 4.2 The templated synthesis of mesoporous silica (a) true liquid crystal templating and (b) cooperative sol-gel and self-assembly.

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