Surfactants cooperative self-assembly

However, there are different ways to describe the details of the cooperative self-assembly of the surfactant and the silicate species. The two mechanisms proposed by Davis[61,66] and Stucky[8] are most noteworthy. 

The synthesis of mesoporous silica films typically begins with the preparation of precursor solutions. These solutions contain a silica source (typically an alkoxide, although chloride and colloidal precursors can be used), a surfactant molecule used to template the mesostructure, an acid or base catalyst, and solvents. The nanoscale structure is then formed by a cooperative self-assembly of monomeric or partially... 

The chemistry involved in the formation of mesoporous silica thin films is qualitatively well understood. However, specific reaction mechanisms of the individual steps are still debated. In addition, owing to the complexity of the sol-gel reaction pathways and cooperative self-assembly, full kinetic models have not been developed. From the time of mixing, hydrolysis reactions, condensation reactions, protonation and deprotonation, dynamic exchange with solution nucleophiles, complexation with solution ions and surfactants, and self-assembly, all occur in parallel and are discussed here. Although the sol-gel reactions involved may be acid or base catalyzed, mesoporous silica film formation is carried out under acidic conditions, as silica species are metastable and the relative rates of hydrolysis and condensation reactions lead to interconnected structures as opposed to the stable sols produced at higher pH. Silicon alkoxides are the primary silica source (tetramethyl orthosilicate, tetraethyl orthosilicate, tetrapropyl orthosilicate, etc.) and are abbreviated TMOS, TEOS, and TPOS, respectively. Starting from the alkoxide, Si(OR)4, in ROH and H2O solution, some of the general reactions are ... 

Fig. 2 A schematic illustrating various routes used to synthesize mesostructured materials. Route I A cooperative self-assembly route relying on the interaction between the surfactant molecules and the inorganic precursors. Route II TLCT route. The inorganic framework condenses around preformed surfactant micelles in this case.

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. 

The cooperative self-assembly route is the most commonly used synthesis procedure for surfactant templated materials. It uses aqueous solutions at a much lower initial surfactant concentration than for the true liquid templating route, reducing the required amounts of expensive surfactant template. In these solutions, the surfactants are at a high enough concentration to form micelles, which may be spherical, elliptical, rod-like or vesicular, but do not form the ordered aggregates found in the final templated silica-surfactant composites. The solutions are in thermodynamic equilibrium so are stable at a given temperature until the silica precursors are added. Once added, a series of interactions between the inorganic species and the surfactant micelles occur, which involve simultaneous association of all components, hence the name, cooperative self-assembly. The result of the interactions is formation of the silica-surfactant composite, usually a precipitate, with an ordered nanoscale structure similar to those found in concentrated surfactant solutions. 

Studies of the mechanism of formation for cooperative self-assembly divide into investigations on three types of system, depending on pH and the type of surfactant template. Most work has been done on formation in acidic systems since the reaction here is slower and more accessible to standard time-resolved measurement techniques. Within the acidic syntheses work has divided into that concerned with small molecule surfactants and with polymeric (usually Pluronic ) surfactants. For alkaline and neutral preparations the rapid precipitation has made measurement more difficult, requiring stop flow or other fast mixing devices to study... 

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. 

Functionalization of the matrix allows incorporation of a variety of catalytic activities into the material. Recently, procedures were developed to add functional groups that are electrostatically or hydrophobically attractive to the ammonium surfactant head groups and are able to compete with silicate anions during self-assembly. This has led to a class of mesoporous materials that are functionalized only on the inside of the pores. Highly selective polymerization and cooperative catalytic systems have been developed from these materials.3 Furthermore, by incorporating caps onto the pores, chemical reagents can be stored in the channels,... 

The complex formation of PECs and PE-surfs is closely linked to self-assembly processes. A major difference between PECs and PE-surfs can be found in their solid-state structures. PE-surfs show typically highly ordered mesophases in the solid state [15] which is in contrast to the ladder and scrambled-egg structures of PECs [2]. Reasons for the high ordering of PE-surfs are i) cooperative binding phenomena of the surfactant molecules onto the polyelectrolyte chains [16-18] and ii) the amphiphilicity of the surfactant molecules. A further result of the cooperative zipper mechanism between a polyelectrolyte and oppositely charged surfactant molecules is a 1 1 stoichiometry. The amphiphilicity of surfactants favors a microphase separation in PE-surfs that results in periodic nanostructures with repeat units of 1 to 10 nm. By contrast, structures of PECs normally display no such periodic nanostructures. 

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