Condensation rate concentration dependence

Control of pH is critical in the determination of the Si/Al ratio. As the pH increases, the ability of the silicate to condense decreases because of a decrease in the amount of Si-CCspecies relative to Si-OH. The anionic form is necessary in order for the initial nucleophilic attack to take place. In contrast, the condensation rate of Al(OH) 4 remains constant and so aluminium-rich zeolites crystallise preferentially at high pH and vice versa. Zeolite synthesis also depends on a wide range of experimental parameters, including concentrations and degree of supersaturation, the source of the framework materials, solvent... 

The condensation rate depends on the structure of the zirconocene, the temperature, the Al Zr ratio, and the concentration. Methane production is much faster with MAO than with the weaker Lewis acid trimethylalu-minum. More than 2 mol of methane is eliminated per mole of zirconium in 15 min when high Al Zr ratios are applied (Fig. 9). This is complicated by... 

One important feature of derivative spectra is that peak heights are usually directly proportional to concentration. This is more desirable than the logarithmic relationship in direct absorption spectroscopy. Another important feature is that the sensitivity to concentration depends on the rate of change in molar absorptivity at a particular wavelength, dejclX, rather than on the absolute magnitude of e itself. Thus, very sensitive analyses are possible for compounds that have sharp absorption peaks. Since absorption spectra are broadened in condensed phases, derivative spectroscopy finds particular application in gas analysis where absorption peaks are much sharper. 

Membrane-penetrant systems which show pressure (concentration) dependent permeation rates have been shown to develop highly non-linear concentration and activity profiles within the membranes [17]. Time is required to establish these profiles. In addition, water and other condensible vapors create large swelling gradients from the fully swollen wet (feed) side to unswollen dry (permeate) side of the membranes. The relaxation of the resulting stresses may control the time to attain steady-state flux [18]. 

In addition to ethylene and propylene oxide, a variety of other cyclic ethers have also been copolymerized with MA. Monomers such as cyclohexene oxide,piperylene dimer mono and diepoxide, epichlorohydrin, " " 3,3,3-trichloropropylene oxide, " tetrahydrofuran, " " and ethylene carbonate or ethylene sulfite " have received attention. Condensation reactions between allyl glycidyl ether and MA are reported to be highly useful for preparing plastics with remarkable hardness, high heat distortion, and brilliant clarity.The cyclohexene oxide copolymerizations were second order in MA, with an activation energy of 13.8 kcal/mol. For the epichlorohydrin system the rate was dependent on the temperature and proportional to the catalyst concentration, with an activation energy of 14.5 kcal/mol. 

This reaction pertains above the isoelectric point of silica (>pH 2-4.5, depending on the extent of condensation of the silicate species), where surface silanols may be deprotonated depending on their acidity. The acidity of a silanol depends on the other substituents on the silicon atom. When basic OR and OH are replaced with OSi, the reduced electron density on Si increases the acidity of the protons on the remaining silanols [66]. Therefore, Iler s mechanism favors reactions between larger, more highly condensed species, which contain acidic silanols, and smaller, less weakly branched species. The condensation rate is maximized near neutral pH where significant concentrations of both protonated and deprotonated silanols exist. A minimum rate is observed near the isoelectric point. (See Fig. 18b.)... 

Mutually Transparent Precursors. According to equation 3, we expect the conditions for mutual transparency to be fuMlled in general when D < LS. In addition, since equation 3 assumes an infinite condensation rate, mutual transparency may also be realized under conditions where D somewhat exceeds L5 if the condensation rate is low. The ellipsometry results obtained for both the A2(fresh) and A2(aged) series of sols show low volume percent porosities and essentially no dependence of porosity on sol age. These observations are completely consistent with the idea of mutual transparency the ability of the pol3nners to freely interpenetrate as they are concentrated on the substrate surface should lead to dense pacing and should effectively hide any differences due to size and fractal dimension of the individual precursor species. For example, the differences noted in the scattering behavior of the A2(fresh) and A2(aged) sols are not manifested as differences in porosity in the ellipsometry data (Table I). 

Commercial condensed phosphoric acids are mixtures of linear polyphosphoric acids made by the thermal process either direcdy or as a by-product of heat recovery. Wet-process acid may also be concentrated to - 70% P2O5 by evaporation. Liaear phosphoric acids are strongly hygroscopic and undergo viscosity changes and hydrolysis to less complex forms when exposed to moist air. Upon dissolution ia excess water, hydrolytic degradation to phosphoric acid occurs the hydrolysis rate is highly temperature-dependent. At 25°C, the half-life for the formation of phosphoric acid from the condensed forms is several days, whereas at 100°C the half-life is a matter of minutes. 

Ethanol removed by the vapor stream can be recovered by condensation, vapor recompression, or scmbbiag. Ia the first two methods, the coaceatratioa of the recovered ethanol depends on the relative humidity of the sweep stream and the ratio of sweep and permeation rates. In scmbbiag, the rate of water deflvery to the Hquid-gas coatactor affects the ethanol concentration ia the recovered stream. 

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