Modification of silica particles

Surface Modification of Silica Particles and Silica Glass Beads... 

Chloride microtraps were produced through the surface modification of silica particles by using known procedures. 3-Glydoxypropyltrimethoxysilane is hydrolyzed on silica particles starting from the hydroxyl groups resulting in the modification of their surfeces. This process leads to the formation of amine groups that act as chloride traps. Complete description of the fabrication procedure is described in the literature [1]. 

The use of silica particles in bioapplications began with the publication by Stober et al. in 1968 on the preparation of monodisperse nanoparticles and microparticles from a silica alkoxide monomer (e.g., tetraethyl orthosilicate or TEOS). Subsequently, in the 1970s, silane modification techniques provided silica surface treatments that eliminated the nonspecific binding potential of raw silica for biomolecules (Regnier and Noel, 1976). Derivatization of silica with hydrophilic, hydroxylic silane compounds thoroughly passivated the surface and made possible the use of both porous and nonporous silica particles in all areas of bioapplications (Schiel et al., 2006). 

Surface functionalization of silica particles or fluorescent silica particles typically is done using functional alkyl silanes. The process may be used to add a reactive group to the surface of the particles for spontaneous coupling to biomolecules or it may be used to add the appropriate nucleophilic group to the surface, such as an amine or a carboxylate. Silane modification chemistry is discussed in more detail in Chapter 13. 

After the catalytic runs no modification of mean particle size is observed for this last system. Conversly, Ru CO) deposited on silica-alumina is readily decomposed at 200°C to metallic particles of 1 nm mean size which are also catalysts for the F-T synthesis. The catalytic activity at 200°C is C i one tenth of the Y zeolite supported ones and methane is practically the only hydrocarbon formed. Electron microscopy examination of the catalyst after reaction reveals a drastic sintering of the... 

In chemical modifications of inorganic particles, the adsorbed water sometimes affects the reaction of the hydroxyl group with the modifier. According to Fripiat et al. (6), evacuation at 200°C and 10-7 mmHg for 24 h still leaves about 0.2 mmol/ g adsorbed water on silica (Aerosil 200). 

Modification of silica gel with volatile or gaseous compounds is performed in the vapour phase. Industrial-scale reactors and laboratory scale gas adsorption apparatus have been used. In the industrial field, fluidized bed and fluid mill reactors are of main importance. In fluidized bed reactors,82 the particles undergo constant agitation due to a turbulent gas stream. Therefore, temperatures are uniform and easy to control. Reagents are introduced in the system as gases. Mass transport in the gas phase is much faster than in solution. Furthermore, gaseous phase separations require fewer procedural steps than solution phase procedures, and may also be more cost-effective, due to independence from the use and disposal of non-aqueous solvents. All these advantages make the fluidized bed reactors preferential for controlled-process industrial modifications. 

The number of microscratches and remaining silica particles for the modified slurry is much less than for unmodified slurry. PVP, which modifies the silica particles and plays a preventive role in dissolving silicon ions, is thought to improve the suspension stability. Due to the surface modification, microscratches on the silicon wafer are decreased, as improved suspension stability prevented the undesirable agglomeration, hi addition, as the reactivity of silicon ion with the silicon wafer is much higher than that of silica particles, the number of... 

Dispersion stabilization with nanoparticles is also known. A recent example of a dispersion stabilized by nanoparticles was published by Tohver et al. This group used zirconia particles to stabilize an aqueous colloidal system of larger silica particles. The dispersion was stabilized by electrostatic stabilization and thus is essentially applicable only to aqueous systems. Surface modification of the particles changes the stabilization mechanism to steric stabilization, and dispersions in both aqueous and nonaqueous systems have been demonstrated. 

All of these mechanisms which affect crosslink density were confirmed by experimental studies. The classic case of a reactive particle filler is silica filled polysiloxane (Figure 6.25). Silica particles have numerous OH groups which react with the crosslinking component of polysiloxane. Modification of silica by silanes reduces reinforcement. 

In addition, I will discuss modification of the surfaces of silica particles. 

This type of surface modification, if adequate functional groups are used, can be useful in incorporating silica particles into polymeric matrices (e.g., into rubber for tires) or in increasing the hydrolytic stability of high-surface-area silica (e.g., that used for membranes). Surface modification of silica is a very important principle and is widely commercialized. 

The amount of Si ions dissolution is found to be dependent on surface modification, which was confirmed by induchvely coupled plasma-atomic emission spectrometer (ICP-AES) analysis. Table 2.2 shows the dissolution amount of Si ions with and without surface modification of fumed silica slurry. Without surface modification, the amount of Si dissoluhon was 1.370 0.002 mol/L, whereas surfaces modified with poly(vinylpyrrolidone) (PVP) polymer yielded a dissoluhon of 0.070 0.001 mol/L, almost 20 hmes less than the unmodified surface. Figure 2.6 represents the electro-kinetic behavior of silica characterized by electrosonic amplitude (ESA) with and without surface modification. When PVP polymer modified the silica surface, d5mamic mobility of silica particles showed a reduchon from -9 to -7 mobility units (10 m /Vxs). Dynamic mobility of silica particles lacking this passivation layer shows that silica suspensions exhibit negative surface potentials at pH values above 3.5, and reach a maximum potential at pH 9.0. However, beyond pH 9.0, the electrokinetic potential decreases with an increasing suspension pH. This effect is attributed to a compression of the electrical double layer due to the dissolution of Si ions, which resulted in an increase of ionic silicate species in solution and the presence of alkali ionic species. When the silica surface was modified by... 

Figure 11.14 Sol-gel synthesis of colloidal silica spheres, modification of surface with methacrylate groups, and copolymerization with methyl acrylate to form a CCA of silica particles in a polyfmethyl acrylate) film. DMPA, 2,2-dimethoxy-2-phenylacetonephenone. (Reproduced with permission from J. M. Jethmalani and W. T. Ford, Chem. Mater. 1996, 8, 2138.)...

If neither solvent density nor viscosity are important in column packing, then what is The missing factor is the interaction of the particles with each other, which is mediated by the slurry solvent. This is the factor that had been recognized by Kirkland (4) when he developed the packing technique using a stabilized aqueous suspension of silica particles. The technique as implemented by Kirkland, however, works only for unmodified silica, and cannot be used for bonded phases without modification of the procedure. But the underlying principle is correct. 

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