Silica solution rate constants

Hurd, D. C. and Theyer, F. Changes in the physical and chemical properties of biogenic silica from the central equatorial Pacific-- . Solubility, specific surface area, and solution rate constants of acid-cleaned samples, 211 230, in COLbb, Jr., T. R. P., editor) "Analytical Methods in Oceanography," Adv. Chem. Ser. 147, 1975. 

Solution Rate Constants of the Various Forms of Silica... 

Figure 11 shows that greater free energy changes are required for both the solution and precipitation of quartz than for amorphous silica, based simply on the differences between their solubilities and solution rate constants. This may account in part for the frequent supersaturation of quartz solutions silica monomer can absorb to the quartz surface without actually becoming a part of the quartz structure. This effect is enhanced in solutions of decreasing ionic strength and pH where AG increases as 2 decreases. 

The colloidal stability of silica Suspensions in the present work was assessed by sediment volumes and from the optical coagulation rate constant. In the first method, 50 mg of silica was dispersed in 5 cm3 polymer solution (concentration 10-2 g cm 3) in a narrow tube and the sediment height found at equilibrium. Coagulation rates of the same systems were found by plotting reciprocal optical densities (500nm, 1cm cell) against time. When unstable dispersions were handled, the coagulation was followed in... 

The first term on the right side of the solution represents the extent to which the silica concentration deviates from equilibrium. Since the term appears as a negative exponential function in time, its value decays to zero (as can be seen in Fig. 26.1) at a rate that depends on the surface area and rate constant. As t becomes large, the first term disappears, leaving only the equilibrium concentration meq. 

Behymer and Hites (1985) determined the effect of different substrates on the rate of photooxidation of anthracene (25 pg/g substrate) using a rotary photoreactor. The photolytic half-lives of anthracene using silica gel, alumina, and fly ash were 1.9, 0.5, and 48 h, respectively. Anthracene (5 mg/L) in a methanol-water solution (1 1 v/v) was subjected to a high pressure mercury lamp or sunlight. Based on a rate constant of 2.3 x lO /min, the corresponding half-life is 30 min (Wang et al., 1991). 

Tris-allyl-neodymium Nd(//3-C3I Ishdioxane which performs as a single site catalyst in solution polymerization was heterogenized on various silica supports which differed in specific surface area and pore volume. The catalyst was activated by MAO. In the solution polymerization the best of the supported catalysts was 100 times more active (determined by the rate constant) than the respective unsupported catalyst [408]. 

In addition to the studies in which supported catalysts are exclusively used for gas-phase polymerizations one study is available in which the supported catalyst is optimized in a solution process prior to its application in the gas phase. Tris-allyl-neodymium [Nd(/ 3- C3H5)-dioxane] which is a known catalyst in solution BD polymerization is heterogenized on various silica supports differing in specific surface area and pore volume. The catalyst is activated by MAO. In solution polymerization the best of the supported catalysts is 100 times more active (determined by the rate constant) than the respective unsupported catalyst [408]. In addition to the polymerization in solution, the supported allyl Nd catalyst is applied for the gas-phase polymerization of BD [578,579] the performance of which is characterized by macroscopic consumption of gaseous BD and in-situ-analysis of BD insertion [580]. 

The depolymerization rate constant ki, expressed in moles cm sec , is the product of Csat (or Keq) and 2 1> 2, 22, 27). The chemical meaning of such a constant is that for a given temperature, pH, and ionic strength, ki represents the maximum solution flux per unit area which can be expected from a given silica sample. At equilibrium this flux must be equal and opposite to the product of 2 and Cboi when Csoi = Csat i-e., dC/dt = 0. 

Contrary to ion exchange, which is a fast-reversible process, the dissolution of rock minerals by alkalis is a long-term irreversible kinetic process. In alkaline solutions, soluble silica exists as several species. The exact speciation is not well established, but at lower concentrations it may be summarized by Eqs. 10.20 to 10.23. Table 10.4 summarizes the published rate constants of those equations collected by Bunge and Radke (1982). 

Monomer and excimer fluorescence decays of Py, 1Py(3)1Py and the alkylpyrenylsilanes PPS and PDS, adsorbed on silica surfaces have been reported in the literature (21, 31-43). However, whereas for inter- and intramolecular excimer formation in homogeneous solution the rate constants of excimer formation and dissociation could be determined from the fluorescence decays (11,15,23), a considerably more complex situation is encountered on the silica surfaces (c.f. Section 3.2.1). This is not surprising, as the multiple adsorption sites at the inhomogeneous surfaces ma)ce different pathways in the excimer formation process li)[Pg.61]

Heggie Ml, Jones R, Latham CD, Maynard SCP, Tole P (1992) Molectrlar diffusion of oxygen and water in crystalline and amorphous silica. Philosophical Magazine B 65 463-471 Helgeson HC, Mtrrphy WM, Aagaard P (1984) Thermodynamic and kinetic corrstraints on reaction rates among mineral and aqueous solutions. II. Rate constants, effective strrface area, and hydrolysis of feldspar. Geochim Cosmochim Acta 48 2405-2432... 

Figure 10. Silicomolybdate reaction rate constant k vs. SiOitNa O ratio of silicates used. Key A, silicic acid, 16.6 wM SiO, pH 1.7, aged 1 min at 25°C, made from sodium silicate solutions diluted from 5-1 M 5/0, aged 3 mo at 25°C and B, colloidal silica in THF extracted from A.

Constants and activation energy of quartz and amorphous silica dissolution rate are listed in Table 2.22 and correlation of their dissolution rate vs. solution pH values is displayed in Figure 2.44. 

Detailed experimental data on the rate constants associated adsorption/desorption kinetics or conformational interconversion of different forms of a protein chromatographed on -alkylsilicas are currently very sparse. The kinetics of de-naturation of several proteins on n-butyl-bonded silica surfaces have been reported. Fig. 18 for example, shows the dependence of peak area on the incubation time of lysozyme on the bonded phase surface, from which rate constants for interconversion on the stationary phase, i.e. were derived [63]. The graphical representations derived from quantitative numerical solutions of the probabihty distributions... 

The thermal cis- trans isomerization of the azobenzene ligands confined in the nanopores has a constant rate constant and exhibits faster isomerization than in solution with the exception of the bulky AzoG3 dendrimer. This result is consistent with Brinker and coworkers observation of azobenzene-modified nanoporous silica films and supports their two-rate-constant physical model. The nanoporous silica materials prepared by Brinker and coworkers have a cubic (BCC) pore structure. The azobenzene ligands positioned on the pore coimections have different local environments from those positioned on the spherical pore surfaces. Thus the azobenzene ligands isomerize at two different rates—fast and slow. The MCM-41 nanoporous materials prepared by Zink and coworkers have a hexagonal array pore structure in which all the azobenzene ligands positioned on the channel surfaces have the same local environment. Thus simple one-rate-constant first-order kinetics is sufficient to describe the isomerization process. 

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