Silica phases properties

The reactivity of VPI-8 has not been reported yet but the molecular sieve should have some ion-exchange properties with Zn2+ in the framework. Additionally, VPI-8 appears to be quite thermally stable much like the other high silica phases discussed so far. 

The most common approach to chromatographic stationary-phase characterization is in terms of bulk-phase properties, such as percent carbon loading onto the silica substrate. This property together with the surface area of the substrate and the molecular characteristics of the bonded silane can be used to calculate the bonding density (A) of the chromatographic sorbent [60] ... 

Since pure mesoporous silica phases does not show any catalytic activity many successful attempts have been made to vary the inorganic composition towards transition metal oxides or metal chalcogenides [5-12], In particular the semiconducting properties of the latter offer a great range of possible applications in materials chemistry. 

Many reviews have been written on the preparation, physico-chemical properties and application of silica in modem separation science [5-7]. RP LC with silica based bonded stationary phases is utilised for the majority of LC separations in laboratories world-wide. Their ubiquity derives from their versatility, in that generally a wide range of both ionic and non-ionic analyte species can be separated with these columns by careful selection of the stationary phase and mobile phase properties. 

To further explore the influence of silica material properties (morphology, surface area, silanol concentration, and surface treatment) on the silica flame-retardant properties, various types of silicas (silica gel, fumed silicas, and fused silica) were investigated.50 51 Material properties of the various silicas are summarized in Table 8.6. These different types of silicas were added to polypropylene and polyethylene oxide to determine their flame-retardant effectiveness and mechanisms. Polypropylene was chosen as a non-char-forming thermoplastic, and polyethylene oxide was chosen as a polar slightly char-forming thermoplastic. Flammability properties were measured in the cone calorimeter and the mass loss rate was measured in the radiative gasification device in nitrogen to exclude any gas phase oxidation reactions. 

The IUPAC Compendium of Chemical Technology defines Normal Phase as an elution procedure in which the stationary phase is more polar than the mobile phase . In practice, the most widely used stationary phases for preparative HPLC are based on silica and the polarity of the underlying silyl ether and silanol provides the required hydrophilic surface. Amino and cyano bonded silica are also commonly used in normal phase mode though the latter also has some reversed phase properties. The predominant mechanism of interaction is hydrogen bonding. However, the silanol is mildly acidic so the silica surface will also have mild cation exchange properties. 

The dependence of EOF mobility, pE0F, on mobile-phase properties has been explored theoretically, and confirmed in a number of experiments on silica and bonded silica columns [49]. Comparisons have also been made with the results... 

Utilizing this principle, Heck reactions were performed [57]. The silica gel was derivatized with a CgHiy-trimethoxysilane to obtain the desired reversed phase properties, followed by treatment with palladium acetate and triph-enylphosphine in cyclohexane. After removal of the solvent, an air-stable catalyst supported on reversed-phase silica gel (RPSG) was formed. This supported catalyst was employed with variable success in Heck C-C coupling reactions, as depicted in Scheme 16. 

This is a fully porous, spherical stationary phase with unique reversed-phase properties but it can also be used in the normal-phase mode. PGC has a crystalline graphite surface. It is chemically stable from lOM acid to 10 M alkali and can withstand high temperatures. The selectivity (i.e. the elution order) is different from silica-based reversed phases. PGC is recommended for the separation of highly polar and ionized compounds as well as of... 

Physical adsorption and chemical immobilization techniques were used for the modification of silica gel surface by 1-aminoanthraquinone to produce three modified silica gel phases. These modified phases were studied for their metal sorption and extraction properties. The chemically modified silica phases were applied for selective extraction and preconcentration of Cu(II) and Cr(III) from seawater samples. ... 

Thus, investigations of a variety of mixed nanooxides using several methods reveal that the surface properties of SA, ST, and AST materials (such as content of SiOH, Si(OH)2, (-0)4Si, A1(V1), A1(V), and Al(IV), intensity of the FTIR bands of surface hydroxyls, heat of immersion in water, desorption temperature, and content of desorbed water) are complex functions of the specific surface area, total and surface content of all oxide components, composition of the surface (both oxide phase distributions and distributions of the Ti and A1 atoms in the silica phase and vice versa), and treatment temperature (differently affecting SA, ST, and AST samples because ternary oxides can lose hydroxyl coverage at lower temperatures than binary oxides). Many of the mentioned properties very clearly correlate with the surface content of alumina and/or titania in mixed oxides. Despite these differences, nanooxides are morphologically similar even at different specific surface areas because of features of HT synthesis of these oxides. 

Diol silica phases were among the first bonded silica gek to be developed in order to overcome the disturbing adsorption properties of underivatized siUca. Chemically bonded diol phases exhibit high polarity and hydrogen bonding... 

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