Silicas formation mechanism

Fig. 3.4 Schematic drawings illustrating the nucleation of silica precursor and possible mechanisms of reactions leading to silica formation. (A) The precursor hydrolysis. THEOS nucleates on a polysaccharide macromolecule through hydrogen bonds that are formed with hydroxy groups in the biomacromolecule. Per-...

Numerous techniques have been applied for the characterization of StOber silica particles. The primary characterization is with respect to particle size, and mostly transmission electron microscopy has been used to determine the size distribution as well as shape and any kind of aggregation behavior. Figure 2.1.7 shows a typical example. As is obvious from the micrograph, the StOber silica particles attract a great deal of attention due to their extreme uniformity. The spread (standard distribution) of the particle size distribution (number) can be as small as 1%. For particle sizes below SO nm the particle size distribution becomes wider and the particle shape is not as perfectly spherical as for all larger particles. Recently, high-resolution transmission electron microscopy (TEM) has also revealed the microporous substructure within the particles (see Fig. 2.1.8) (51), which is further discussed in the section about particle formation mechanisms. 

Several growth and formation mechanisms have been proposed for the formation of monodispersed Stober silica particles. Silica in general is an extremely well-studied system, and there are numerous publications with respect to the hydrolysis and condensation reaction. At present there are two major formation mechanisms that have been used to explain the formation of Stober silica particles. 

This chapter focuses on silica synthesis via the microemulsion-mediated alkoxide sol-gel process. The discussion begins with a brief introduction to the general principles underlying microemulsion-mediated silica synthesis. This is followed by a consideration of the main microemulsion characteristics believed to control particle formation. Included here is the influence of reactants and reaction products on the stability of the single-phase water-in-oil microemulsion region. This is an important issue since microemulsion-mediated synthesis relies on the availability of surfactant/ oil/water formulations that give stable microemulsions. Next is presented a survey of the available experimental results, with emphasis on synthesis protocols and particle characteristics. The kinetics of alkoxide hydrolysis in the microemulsion environment is then examined and its relationship to silica-particle formation mechanisms is discussed. Finally, some brief comments are offered concerning future directions of the microemulsion-based alkoxide sol-gel process for silica. 

The parameters of the pore structure, such as surface area, pore volume, and mean pore diameter, can generally be used for a formal description of the porous systems, irrespective of their chemical composition and their origin, and for a more detailed study of the pore formation mechanism, the geometric aspects of pore structure are important. This picture, however, oversimplifies the situation because it provides a pore uniformity that is far from reality. Thorough attempts have been made to achieve the mathematical description of porous matter. Researchers discussed the cause of porosity in various materials and concluded that there are two main types of material based on pore structure that can be classified as corpuscular and spongy systems. In the case of the silica matrices obtained with TEOS and other precursors, the porous structure seems to be of the corpuscular type, in which the pores consist of the interstices between discrete particles of the solid material. In such a system, the pore structure depends on the pores mutual arrangements, and the dimensions of the pores are controlled by the size of the interparticle volumes (1). 

Following similar principles of combining the aggregate-forming properties of bifunctional molecules with low cost and low toxicity, a mesoporous silica material with a three-dimensional worm-like pore system was reported. Triethanol amine (TEA) was used as the SDA and TEOS as the silica source in this mesoporous silica, TUD-1.[48] The formation mechanism is depicted in Figure 1.3(a). The properties of the material can be easily tuned by modifications in the synthesis procedure, for example, the pore size of the material was found to be proportional to... 

Ion formation mechanisms for silica gel matrices have never been studied for those elements that are not readily reducible to the metal. The solvation/desol-vation mechanism hypothesized previously may have a role in enhancing ion emission from these materials, but it would not be expected that an alkaline earth element could exist in the zero oxidation state in these glass matrices, which are oxide based. The species in the molten glass would be expected to be in the standard +2 oxidation state, but the experimentally observed species is +1. Indeed, there has never been a +2 species reported from thermal ionization, so there is the question of how the +2 species in the molten glass is converted to and emitted as a +1 ion. 

Hybrid silica materials were prepared via a sol-gel pathway at pH 9. The influence of anionic surfactant (SDS) was studied by comparing templated materials (TbSn series) with hybrid materials obtained without surfactant (Tbn series). Two mechanisms of mesostructure formation can be considered as represented on Fig. 2. The pka of aminopropyl chain is about 10.6 in the reaction mixture propyl-amines are partially protonated. Electrostatic interactions between dodecylsulfate anion and NH and sodium cation neutralization may then occur, resulting in the condensation of the silica structure around surfactant micelles and aminopropyl groups at the surface of the pores. The other mechanism is SDS chains complex-ation by P-CD cavity, which wonld result in P-CD gronps located at the surface of the pores and aminopropyl less accessible, due to steric hindrance caused by P-CD bulky groups. A complete characterization of the prodncts and adsorption capacities will help nnderstanding the formation mechanism of mesoporons hybrid silica. 

Pinnavaia etal. used neutral alkylamines as templates to form disordered mesoporous silica, named hexagonal molecular sieves (HMS). The S°I° formation mechanism was proposed between neutral amine micelles (S°) and neutral inorganic precursors (1°). The interactions between S° and 1° were assumed to be hydrogen bonding. The resulting HMS has a worm-like pore structure, with thicker framework walls and smaller X-ray scattering domain sizes compared to M41S. 

At high TPAOH content both the high availability of OH and the formation mechanism do not allow formation of large, ramified silico-aluminate oligomers favoring the growth in sheets, i.e. ERS-8 type silica-aluminas. 

Diatoms were also investigated for the mechanism of silicon transport that is an integral part of the silicification process. As the environmental concentrations of dissolved silicon are rather low, diatoms must have an efficient transport system. Orthosilicic acid must not only be transported into the cell, but also transported intracellularly into the SDV where silica formation occurs. A protein of the diatom C. fusiformis was characterized that transports silicon from seawater into the cell. This discovery was accomplished by cloning and characterizing the DNA that codes for this protein. However, silicon transporter proteins of this particular type are not necessarily involved in intracellular transport. 

Figure 1. Schematic pathway for preparing surfactant-templated mesoporous silicas, illustrating a formation mechanism based on preformed liquid crystal (LC) mesophase (route A) or a cooperative process (route B). Reprinted from [20], Copyright (2008) WILEY-VCH Verlag GmbH Co.

Grosso, D. Balkenenda, A.R. Albouy, P.A. Ayral, A. Amenitsch, H. Babonneau, F. Two-dimensional hexagonal mesoporous silica thin films prepared from block copolymers detailed characterization and formation mechanism. Chem. Mater. 2001, B (5), 1848-1856. 

The presence of pyrroles in roasted coffee was studied by Tressl et al. (1981a) who added a list of 27 coffee components. After a special separation process involving distillation -extraction of freshly roasted coffees, adsorption chromatography on silica gel, and capillary GC/MS, more than 20 /V-alkylpyrroles and 15 furfurylpyrroles were identified and (semi)quantified. Robusta coffees contain higher amounts of alkylpyrroles and lower amounts of furfurylpyrroles than arabica varieties. The authors proposed a possible formation mechanism by reaction of furfurylamine with sugar fragmentation products. 

Hollow silicon carbide (SiC) spheres have been synthesized by a microwave heating and carbothermal reduction method with carbon spheres as template and fly ash (a solid waste from coal-fired power plant) as silica source. X-ray diffraction and scanning electron microscope were employed to characterize the morphology, structure of the products. The results show that hollow spheres prepared at 1300 "C under argon atmosphere have a hollow core and SiC shell structure. The shell of a hollow SiC sphere is composed of a lot of irregular SiC nanowires with 5-20 pm in length and 50-500 nm in diameter which belongs to the p-SiC. Moreover, the formation mechanism of the hollow SiC spheres is also discussed. 

Mesostructured thin silica films can be prepared as freestanding films or supported by a variety of different substrates. In the following, a short description of the different synthesis methods is given the synthesis and the formation mechanism of mesoporous silica films has been reviewed in detail elsewhere. " ... 

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