Silica reaction with silanes

Modification of the surfaces of coUoidal silica spheres with silane coupling agents enables transfer of the particles to nonpolar solvents. With 3-methacryloxypropyltri-methoxysilane bonded to the surface, the particles have been transferred from water to the polymerizable monomer, methyl acrylate. Electrostatic repulsion due to a low level of residual charge on the particle surfaces cause the dilute dispersions of particles to form a non-close packed colloidal crystalline array (CCA). Polymerization of the methyl acrylate with 200 nm diameter silica spheres in a CC fixes the positions of the spheres in a plastic film by the reactions shown in Figure 11.14. The difriaction... 

The surface of silica particles has many exposed silanol —OH groups which are known to imdergo facile reaction with silanes contained —Si(OR), —Si(Cl), and —Si(H) units, and with acyl or sul-fonyl acid chloride functional groups [31,32]. Thus, preparation of the desired derivatized silica particles was easily accomplished by refluxing sihca in a nonhydroxyhc solvent with excess ti receptor reagents such as 3,5- or 2,4-dinitrobenzoyl or dinitrobenzenesulfonyl chloride. This... 

Reactions with silanes are complex, and a range of possible reaction products which could be obtained from the reaction of dimethyldihalogenosilane with a silica surface is illustrated in Figure 2. 

Because the surface of the silica gel plays an active role in RP-HPLC since 30-50% of it is available to the analyte, the type of silica gel (or other matrix) used is an important variable in the chromatographic process. In addition, one can see that for a simple RP-phase such as the Cjg-phase, the modification is by no means constant. Each manufacturer has essentially its own special recipe , e.g., reaction of silica gel with silanes with or without catalyst, traces of water in the toluene-based reaction medium, purity of the silanes used, reaction temperature, mono-, di- or trifunctional bonding with the alkyl chain, etc. Considering the large number of possible combinations of different silica gels and different types of chemistry of the manufacturers, the large number and the diversity of commercially available RP Cjg columns and even RP columns can readily be appreciated. 

Silica reaction with TESPT Silanated silica... 

High operation temperature (limited by scorch phenomena) in order to raise the speed of the silane reaction with silica and the diffusion coefficient of ethanol... 

Covalent immobilization methods of NAs to a silica surface require its chemical modification. Functionally inert surface silanols (Si - OH) need to be transformed into reactive species to which the NAs can be attached irreversibly. To date, the main method for the attachment of biological moieties to silica surfaces has involved substrate reaction with organofunctional silanes of the general structure (RO)3Si(CH2)X, followed by the covalent attachment of the biological molecule to the newly introduced fimctional group on the surface [31,32]. Examples of organofimctional silanes used this way include (3-glycidoxypropyl)trimethoxysilane, (3-aminopropyl)triethoxysilane,... 

The preferentially employed approach for the fabrication of inorganic (silica) monolithic materials is acid-catalyzed sol-gel process, which comprises hydrolysis of alkoxysilanes as well as silanol condensation under release of alcohol or water [84-86], whereas the most commonly used alkoxy-silane precursors are TMOS and tetraethoxysilane (TEOS). Beside these classical silanes, mixtures of polyethoxysiloxane, methyltriethoxysilane, aminopropyltriehtoxysilane, A-octyltriethoxysilane with TMOS and TEOS have been employed for monolith fabrication in various ratios [87]. Comparable to free radical polymerization of vinyl compounds (see Section 1.2.1.5), polycondensation reactions of silanes are exothermic, and the growing polymer species becomes insoluble and precipitates... 

The different types of admixtures, known to reduce alkali-aggregate reactions, can be divided into two groups those that are effective in reducing the expansion due to the alkali-silica reaction, and those that lower expansions resulting from the alkali-carbonate reaction. For the alkali-silica reaction, reductions in the expansion of mortar specimens have been obtained with soluble salts of lithium, barium and sodium, proteinaceous air-entraining agents, aluminum powder, CUSO4, sodium silicofluoride, alkyl alkoxy silane,... 

Silica is reduced to silicon at 1300—1400°C by hydrogen, carbon, and a variety of metallic elements. Gaseous silicon monoxide is also formed. At pressures of >40 MPa (400 atm), in the presence of aluminum and aluminum halides, silica can be converted to silane in high yields by reaction with hydrogen (15). Silicon itself is not hydrogenated under these conditions. The formation of silicon by reduction of silica with carbon is important in the technical preparation of the element and its alloys and in the preparation of silicon carbide in the electric furnace. Reduction with lithium and sodium occurs at 200—250°C, with the formation of metal oxide and silicate. At 800—900°C, silica is reduced by calcium, magnesium, and aluminum. Other metals reported to reduce silica to the element include manganese, iron, niobium, uranium, lanthanum, cerium, and neodymium (16). 

The surface of silica is highly reactive and hydroxyl groups exchange hydrogen for deuterium with D20 [14—16] but not with D2. They can be replaced by Cl from Cl2 or CC14 [16] and they react with silanes and aluminium chloride [15,19], Surface alcoholates are formed when silica is contacted with primary or secondary alcohols [20] either by the reaction with hydroxyl groups... 

In addition to pure thermal stability, if the polymer is to be heated in air, one must also consider oxidative stability. In the carbon series oxidation always leads to more stable species and tends to occur, blit in the silicon series there is a much higher tendency towards reaction with oxygen. This is the principal reason for the low utility of the silane polymer. Finally, a third factor in polymer stability is the ease of attack by solvents, acids, bases, etc. This is largely determined by the ionic nature of the bonds involved. The silica based polymers should be more susceptible to such attack than carbon, since they have a higher percent of ionic nature. 

The effect of the concentration of water on the hydrolysis and condensation reactions of silanes has been investigated by Kang et al. [12] in conjunction with recent studies of the adsorption of silane coupling agents on silica surfaces from acetone-water mixtures as a function of the solvent composition [14]. Adsorption isotherms were determined by infrared (IR) spectroscopy and hydrolysis was followed by NMR. 

Silica gel was silanized with y-APTS followed by reaction with glu-taraldehyde solution, and then lipase was immobilized by CB according to the procedure described by Soares et al. (11). The enzyme (0.3 g) was... 

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