Polymerization, silicate

A fascinating area is micellar autocatalysis reactions in which surfactant micelles catalyse the reaction by which the surfactant itself is synthesized. Thus synthesis of dimethyldoceylamino oxide (reaction between dimethyl dodecyl amine and H2O2) benefits from this strategy. Here an aqueous phase can be used and an organic solvent can be avoided. Synthesis of mesoporous molecular sieves benefit through micellar catalysis and silicate polymerization rates have been increased by a factor 2000 in the presence of cetyltrimethyl ammonium chloride (Rathman, 1996). 

For moderate anodic potentials the dissolution rate becomes enhanced and the ratio of H2/Si is reduced due to the contribution of the electrochemical reaction path [Pa6]. At the electrode surface Si-H as well as Si-OH groups are present. For higher concentrations of the silicate monomer produced by reaction (4.3) silicate polymerization takes place [Ni6]. Passivation takes place for more anodic potentials, due to formation of Si-O-Si bonds according to reaction (4.2). 

Minerals and mineral series with the same basic chemical units, such as the silicate polymerized ions, and very similar crystal structures are related and referred to collectively as mineral groups. The amphiboles are a group composed of several mineral series, two of which were cited in the preceding examples. The several series that make up the amphibole group reflect the changes in the size and location of cations associated with the polymerized silicate chains. Because several amphibole species occur in fibrous fonn, we discuss this group in much greater detail, and include an idealized crystal structure. 

Si MAS NMR spectra of the uncalcined MCM-41 samples synthesized normally and with TPA+ and Na+ are shown in Fig 4. It was observed that the ratio of Q4/Q3 peaks was higher in samples synthesized with additional cations. The effect was most pronounced with TPA" as the additional cation. The higher Q4/Q3 ratio indicates that the silicate polymerization during the formation of the mesostructure was enhanced by the presence of the additional cations, Upon calcination, the free silanol groups are forced to condense to form Si-O-Si bond and 29Si MAS NMR of the samples showed predominantly Q4 peak. However, these... 

In a different study, Chen and co-workers optimized the silicate polymerization method [131]. In their approach, the outlet frit is prepared by first filling the column with a sodium silicate solution. Then, the portion of the column at which the frit is desired is brought in contact to a heating element for a few seconds and the frit is... 

R. Ryoo and J.M. Kim, Structural Order in Mcm-41 Controlled by Shifting Silicate Polymerization Equilibrium. J. Chem. Soc. Chem. Commun., 1995, 711-712. 

Whether a ligand increases or decreases Al solubility depends on the particular Al-ligand complex and its tendency to remain in solution or precipitate. Ligands that increase the overall solubility of Al include F", oxalate ", citrate ", fulvic acid, and silicate(monomeric). Ligands that decrease the overall solubility of Al include phosphate, sulfate, silicate(polymeric), and hydroxyl. Figure 5.9 summarizes the influence on Al of common soluble ligands that may be encountered in soil solution. 

The reactions occurring for multivalent metal cations occur over many condensation steps and may not obey the kinetic rules outlined above. Silicate polymerization is known to depend on the presence of oligomeric ion species. - - Gibbsite (formally Al(OH)3) is known not to precipitate sufficiently rapidly from supersaturated solutions, but needs seeding for particle growth Moreover, depending on the temperature and the composition of the solution, different solid species may form. This is illustrated in Figure 8.21. ... 

Prepolymerization of Silicate. From phosphate-buffered mixtures of identical chemical composition, different zeolites crystallize depending on the degree of prepolymerization of the silicate. Polymeric silicates tend to give ZK-15 or zeolites of chabazite structure [low-silica ZK-14 (17)] in the same pH range where metasilicate causes zeolite Y to crystallize (Table IV). Thus, either the depolymerization of waterglass is slower than generally thought or the precursors formed with partially depolymerized silicate are stabilized sufficiently to prevent further rapid depolymerization. 

Conversion to silicic acids The general method has been described (10, 19). The sample of concentrated silicate solution (about 5 M Si02) in a syringe was injected suddenly into the vortex of a cooled (0-5 C) dilute solution of H2S0 in a blender. The volume and concentration of acid was such as to result in a sol at pH 1.7 and at a concentration of not over 100 mM Si02(6000 ppm). This requires enough acid to convert the alkali to NaHSO and leave 0.02 N free acid. Unlike monomer which is most stable at about pH 3, silicic acid from these commerical silicates polymerizes least rapidly at pH... 

In the in situ polymerization method, the layered silicate is swollen within the liquid monomer or a monomer solution so the polymer formation can occur between the layered silicates. Polymerization can be initiated by either heat or radiation, by the... 

Rgure 1.11 Porod plots of scattering data of silicates polymerized under a variety of conditions (from a study by Schaefer and Keefer ) (a) two-step acid-catalyzed tetraethoxysilane (TEOS) system (b) two-step acid-and base-catalyzed TEOS system (c) one-step base-catalyzed system TEOS system (W = 1) (d) one-step base-catalyzed system TEOS system (VP = 2) (e) aqueous silicate system. LUDOX . VP is the water/ silica ratio. 

It also is interesting to recall that nucleation in these systems stops abruptly in spite of the fact that ample reagents appear to be present between the two existing growth centers. This observation can be interpreted to mean that the transfer of material to the crystal, by reaction at the surface, is sufficiently rapid to prohibit silicate polymerization in the bulk which would be necessary for further nucleation. In fact, a net depolymerization must be occiur ing in the bulk phase during crystal growth to prohibit further nucleation. However, if the growing centers are physically removed from the system, as in [25], nucleation may recommence. 

Silica, in its various forms, can cause serious problems for an RO system. Silica scaling and fouling can occur via a number of ways and is not well understood due to the number of different mechanisms that can take place. The concentration of silica, the speciation of silica, and temperature, pH, and the general chemistry of the water all affect the chemistry of silica scaling and fouling. In general, the silica issues that affect RO systems can be summarized as deposition of silicates, polymerization of silicic acid to amorphous silica, and the accumulation of amorphous colloidal particles. To understand the potential problems, it is first necessary to understand a little about the chemistry of silica. 

Another important sol-gel filler additive to BEMFC membranes is mesoporons silicate nanoparticles. Valle et al. [141] reported a general pathway for the production of hierarchically structured transparent composites starting with a mixture of tetraalkoxysilane, as silica precursor, a surfactant as template-forming agent, a solvent, water, and a hydrophobic polymer (e.g., BVDF). The precursors force the silicate polymerization to occur in isolated mesoporons spheres entrapped within... 

General Trends presents an overview of silicate polymerization in alkoxide-based systems. 

The following subsections present structural information obtained for silicate systems from the Guinier and Porod regions of small-angle scattering. In the following section this information is rationalized on the basis of severai kinetic growth processes pertinent to silicate polymerization. 

By increase of the degree of silicate polymerization of HCP, i.e. to change the c-s-f (constituent, structure and interface) on atom-molecular level, we obtained PSC (polymerized silicate cement) of very high compressive strength of 344 MPa with a brittleness index of 0.63 (as brittle as a cement mortar of about mark 200)(4). 

GPC method By using GPC method the degree of silicate polymerization of TMS-derivatized samples is determined. The detail procedure was reported in Ref. 1. Because the resolution of GPC is not infinite, just as one of other chromatography, the peaks in graph are always widened. This phenomenon is usually shown by Tung eq. as follows(7) ... 

Because the degree of silicate polymerization in CSH gel of PSC material increases, its intrinsic properties are improved. Comparing with ordinary low porosity HCP, the optimum compacting pressure of PSC material may be reduced that the more hydrates are produced. 

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