Silica system

Other examples, concerning the water-silica system, are referred to later in the Chapter. 

ACM-silica system has been synthesized from four different solvents whose characteristics are described in Table 3.2. Three different solvents have been employed in studying ENR-sUica system and the solvent characteristics are also denoted in the same table (Table 3.2). 

Characteristics of Different Solvents Used in Sol-Gel Reaction for ACM-Silica and ENR-Silica Systems... 

Tian et al. [56] have studied poly(G-caprolactone)-silica and Sengupta et al. [57] have investigated nylon 66-silica hybrid systems and have observed that the phase separation started when Si/H20 mole ratio is increased above 2 and the resultant hybrid films become opaque. Gao [11] has reported similar observations on sol-gel-derived ionomeric polyethylene-silica system. A wide range of literatures is not available on this topic of mbber-silica hybrid nanocomposites, though Bandyopadhyay et al. [34,35] have reported the hybrid formation with different TEOS/H2O mole ratios from ACM and ENR and also demonstrated detailed structure-property correlation in these systems. The hybrids have been prepared with 1 1, 1 2, 1 4, 1 6, 1 8, and 1 10 TEOS/H2O mole ratios. Figure 3.14 shows the morphology of the ACM-silica hybrid composites prepared from different TEOS/H2O mole ratios. 

The model in Figure 3.20a applies to ACM-silica, while Figure 3.20b suits the ENR-silica system. Kraus constant, C, determined from the slope of the plots in Figure 3.19, quantifies the mbber-silica interaction in these systems. CACM/siUca is 1-85 and CENR/siika is 2.30 and these values are significantly higher than the reinforcing black-filled mbber composites [65]. Hydrogen-bonded interaction between the SiOH and the vicinal diols in ENR is responsible for this, whereas dipolar interaction between ester and SiOH in ACM-silica only results in weaker adsorption of the mbber over the filler surfaces. 

The colloid probe technique was first applied to the investigation of surfactant adsorption by Rutland and Senden [83]. They investigated the effect of a nonionic surfactant petakis(oxyethylene) dodecyl ether at various concentrations for a silica-silica system. In the absence of surfactant they observed a repulsive interaction at small separation, which inhibited adhesive contact. For a concentration of 2 X 10 M they found a normalized adhesive force of 19 mN/m, which is small compared to similar measurements with SEA and is probably caused by sufactant adsorption s disrupting the hydration force. The adhesive force decreased with time, suggesting that the hydrophobic attraction was being screened by further surfactant adsorption. Thus the authors concluded that adsorption occurs through... 

IR spectroscopy of adsorbed carbon monoxide has been used extensively to characterize the diluted, reduced Cr/silica system [48-54,60,76,77]. CO is an excellent probe molecule for Cr(ll) sites because its interaction is normally rather strong. The interaction of CO with a transition metal ion can be separated into electrostatic, covalent a-dative, and 7r-back donation contributions. The first two cause a blue shift of the vco (with respect to that of the molecule in the gas phase, 2143 cm ), while the last causes a red shift [83-89]. From a measurement of the vco of a given Cr(II) carbonyl complex, information is thus obtained on the nature of the Cr(II)- CO bond. 

For the Ti(OiPr)4/silica system, the advantage of MCM-41 (a mesoporous silica) over an amorphous silica is not evident either in terms of activity or selectivity for the epoxidation of cyclohexene with H202 in tert-butyl-alcohol.148 Nevertheless, deactivation of the catalysts seems slower, although the selectivity of the recovered catalysts is also lower (allylic oxidation epoxidation = 1 1). Treatment of these solids with tartaric acid improves the properties of the Ti/silica system, but not of the Ti/MCM-41 system, although NMR,149 EXAFS,150 and IR151 data suggest that the same titanium species are present on both supports. 

In the flocculated state for all suspensions, immediately after mixing, no transmission of light could be detected through the cuvette used and the sedimentation behaviour was different. Thus, instead of the parameters used for the pyrogenic silica systems we used the following characteristics for the precipitated silica mixtures to describe the flocculation and sedimentation behaviour ... 

Sol-gel reversibility, of hydrogels, 13 730 Sol—gel silica—PDMS hybrids, 23 81 Sol—gel silica system, chemical additions to, 23 62... 

Burnett, D.G. and Douglas, R.W. (1970). Liquid-liquid phase separation in the soda-lime-silica system. Physics and Chemistry of Glasses 11 125-135. 

In the original James and Healy model, the magnitude of the adjustable chemical term usually swamped the other two terms. By using a much smaller value of the solvation energy as suggested by Levine [27], it was found that the adjustable chemical term could be eliminated in the simulation of a number of metal cation/silica systems [26],... 

Examination of the effect of temperature (220°C-270°C) and the total heating time (20-45 minutes) on the thermal desorption process led us to adopt 2TO°C for 45 minutes as the optimum conditions for further studies. Assessment of the commercially available desorption systems in which there is flash heating of the sample, have shown that significant losses of some important compounds (p-ciesol, indole) occur on the surfaces of the metal connectors and sample transfer lines. It was to overcome these problems that the all silica system (Figure 2) was developed. 

Sitter, K.D., Andersson, A., D Haen, J., Leysen, R., Mullens, S., Maurer, F.H.J., and Vankelecom, I.F.). (2008) Silica filled poly(4-methyl-2-pentyne) nanocomposite membranes similarities and differences with poly(1 -trimethylsilyl-1 -propyne)-silica systems./. Membr. Sd., 321 (2), 284-292. 

Figure 7.9A shows the NaAlSi04-Si02 (nepheline-silica) system, after Schairer and Bowen (1956). Let us first examine the Si02-rich side of the join. At P = 1 bar, the pure component Si02 crystallizes in the cristobalite form (Cr) at r = 1713 °C (cf figure 2.6). At P = 1470 °C, there is a phase transition to tridymite (Tr), which does not appreciably affect the form of the liquidus curve, which reaches the eutectic point at P = 1062 °C. 

Although the NBS-silica system has useful potential for bromination of electron rich heterocycles, it has limited application to non-acdvated aromatics and sometimes meets problems with polybromination even for the activated heterocycles. Thus, it was of interest to investigate the potential of different brominating agents and different solids. Bromination of anisole with... 

Enthalpies of Mixing of (Metal Oxide + Silica) Systems The thermodynamics of mixing of molten metal oxides (MO ) with silica (Si02) can be... 

In the following a few pertinent data on mineral equilibria in carbonate, phosphate and silica systems are summarized. The subject has been extensively reviewed elsewhere113. ... 

Among other N20 reactions, one may mention the ODH of ethylbenzene to styrene over metal-modified mesoporous silica systems [157]. Fe-modified catalysts showed the best performance, providing 90% selectivity at 30% conversion. 

The first question in the study of the properties of binary catalysts such as the nickel-silica system concerns its texture. A priori, we can hazard a reasonable guess as to the building system that we are going to encounter. It is well known that silica forms a porous system and one may therefore... 

Antibiotics containing acidic functionalities have also been successfully chromatographed with aqueous silica systems [317,321,322]. 

Giacomelli CE, Norde W (2001) The adsorption-desorption cycle. Reversibility of the BSA-silica system. J Colloids Interf Sci 233(2) 234-240... 

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