Condensation of silica

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.

Between 5 and 23 minutes, a continuous evolution is observed, characterised by the increase of the intensity at low q and by the q"1 slope in a log-log plot that is the signature of ID objects (figure 3). This corresponds to the transformation of the shape of the micelles from spherical to cylindrical and their continuous growth in length up to few tens of nanometers. The increase of turbidity in solution is related to the growth of the micelles, and coincides with the beginning of the condensation of silica. 

The pH value will control the condensation of silica, so the pH value should be adjusted to form monomer then oligomer of silica in order to obtain its condensation and polymerization around the micelles of surfactant Under acidic conditions, silica source such as... 

A dielectric oxide layer (eg. silica) is useful as a shell material because of the stability it lends to the core and its optical transparency. The classic method of Stober for solution deposition of silica are adaptable for coating of nanocrystals with silica shells.111 This method relies on the pH and the concentration of the solution to control the rate of deposition. The natural affinity of silica to oxidic layers has been exploited to obtain silica coating on a family of iron oxide nanoparticlcs including hematite and magnetite111 Such a deposition process is not readily extendable to grow shell layers on metals. The most successful method for silica encapsulation of metal nanoparlides is that due to Mulvaney and co-workers.114 In this method, the surface of the nanoparticles is functionalized with aminopropyltrimethylsilane. a bifunctional molecule with a pendant silane group which is available for condensation of silica. The next step involves the slow deposition of silica in water followed by the fast deposition of silica in ethanol. Fig. 13 shows the TEM images... 

EPR spectrum of CAT 16 appears only when the molar ratio of Si/CTAC exceeds 6.3. This result indicates that condensation of silica on the outer surface of the micelles to form the pore walls of MCM-41 sets in only after a certain concentration limit is reached. 

A major difference between silicate polymers and most organic polymers is that the condensation of silica is reversible. Amorphous Si02 has a finite... 

Figure 1.2 Condensation of silica and gibbsite layers to give the kaolin layer structure fnm = A X 10 J...

The effect of temperature on the condensation of silica has been mentioned before (e.g., stabilization of mesoporous silica). The effect of the temperature on the formation of the mesostructure can be understood by considering the g parameter of quaternary ammonium surfactants. As the mixture gel is heated, the conformational disorder of the surfactant tail increases, increasing the surfactant molecular volume and as a result the g value. At the same time, the repulsion of the charged headgroups is also increased with heating, which leads to an increase in the headgroup area a0 value. 

Careful control of the surfactant-water content and the rate of condensation of silica at high alkalinity resulted in hollow tubules 0.3 to 3 pm in diameter.[292] The wall of the tubules consisted of coaxial cylindrical pores, nanometers in size, that are characteristic of those of MCM-41. The formation of this higher-order structure may take place through a liquid-crystal-phase transformation mechanism involving an anisotropic membrane-to-tubule phase change. 

Figure 3.9. Hypothetical reaction process illustrating transfonnation of an initially precipitated Ni-Al LDH into a phyllosilicate-like phase during aging. The initial step involves the exchange of dissolved silica for nitrate within the LDH interlayer followed by polymerization and condensation of silica onto the octahedral Ni-Al layer. The resulting solid possesses structural features common to 1 1 and 2 1 phyllosilicates. (From Ford et al., 2001.)...

We determined that vicinal dihydroxy organic reagents stabilize the soluble forms of silica. The stability of monosilicic acid solution is determined by the structure of the stabilizer ethylene glycol and humic acids do not stabilize soluble forms of silica enough, but glycerin and catechol stabilize the silica solution when they are added at 5 - 7% to the solution. This fact is connected with the formation of hydrogen bonds and stable penta- and hexacoordinated compounds, preventing the processes of poly condensation of silica in solution. 

Figure 4. The pH dependence of growth and condensation of silica particles during gel formation. [Adapted from (13).]...

Chapter 48 The Chemistry of Hydrolysis and Condensation of Silica Sol-Gel Precursors. 637... 

The main investigations are now directed toward a) the modification of micro-polarity by co-condensation of silica precursors with organosilanes (CH3Si(OR)3 [25-28] or C6H5Si(OR)3 [29]... 

The first materials synthesized by Mobil Oil researchers [1,2] were based on silica or aluminosilica materials of so-called M41S family including hexa-gonally arranged MCM-41, cubic MCM-48, and lamellar MCM-50. The synthesis is schematically depicted in Figure 1. In water solution surfactant micelles are formed, their size can control in some respect the final dimensions of Ihe forms. Silica precursors surround the surface of the micelles, which is followed by condensation of silica building blocks into the more compact entities. Removal of surfactants carried out usually at temperatures higher than 500°C provides open structure with accessible mesopores. 

Figure 1.20 Proposed model of PLL-induced condensation of silica platelets. Reprinted with permission from Ref [83] 2005, American Chemical Society.

Beyond fibers and fiber mats, electrospinning can be fruitfully employed to design hybrid membranes [68,69]. Especially, the condensation of silica precursors tetraethyl orthosilicate and a sulfonated chlorosilane during electrospinning in the presence of a fluorinated polymer has been used to allow intimate mixing of a siliceous inorganic framework and an organic polymeric component [68]. 

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