Silica Stober

We have repeated similar degrafting experiments for brush formation via ATRP. While there have been reports on degrafting using conventional radical polymerization [10,58], this discussion will be limited to brush formation by ATRP. In unpublished work [59], we immobilized an ATRP initiator, (1 l-(2-bromo-2-methyl)propionyloxy)undecyltrichlorosilane) on StOber silica and conducted a styrene polymerization. Degrafting of the PS brushes was conducted by etching of the silica cores with HE From TGA analysis of the immobilized initiator and the corresponding PS brush system, we determined that there are 4.8 initiator molecules/nm and / = 0.06. The initiator density corresponds well to the values of 2.4-5.0 reported by Patten and co-workers [56,57] for the immobilization of (2-(4-chloromethylphenyl)ethyl)dimethylethoxysilane on a similar support. 

With slight variations, the Stober silica precipitation process proceeds from the same chemicals. The starting material is TEOS, tetraethoxysilane, Si(OC2H5)4 the solvent is an alcohol (preferably ethanol) water is added and ammonia acts as the catalyst to initialiate the hydrolysis and condensation reaction. In a very schematic way the reaction could be described as follows ... 

Fig. 2.1.2 Reaction rate of the Stober silica particle growth reaction at a constant TEOS concentration of 0.2 mol/L each data triplei indicates the reaction temperature of 293, 313. and 333 K, respectively. (From Ref. 37.)...

An interesting modification of the Stober silica process has been described by Unger et al. (50). By using a mixture of TEOS and an alkyltriethoxysilane they were able to synthesize monodispersed porous silica particles. The porosity is created by the alkyl groups, which act like space holder. After calcination/burnout of the organics, a well-defined porosity is left behind in the silica particles. The materials are used for very fast high-pressure liquid chromatography. 

Numerous techniques have been applied for the characterization of StOber silica particles. The primary characterization is with respect to particle size, and mostly transmission electron microscopy has been used to determine the size distribution as well as shape and any kind of aggregation behavior. Figure 2.1.7 shows a typical example. As is obvious from the micrograph, the StOber silica particles attract a great deal of attention due to their extreme uniformity. The spread (standard distribution) of the particle size distribution (number) can be as small as 1%. For particle sizes below SO nm the particle size distribution becomes wider and the particle shape is not as perfectly spherical as for all larger particles. Recently, high-resolution transmission electron microscopy (TEM) has also revealed the microporous substructure within the particles (see Fig. 2.1.8) (51), which is further discussed in the section about particle formation mechanisms. 

Fig. 2.1.7 Transmission electron micrograph of StOber silica particles.

Fig. 2.1.8 Transmission electron micrograph showing the internal structure of Stober silica particles. (From Ref. 51.)...

Fig. 2.1.10 Adsorption of gases on/in StOber silica particles. (Courtesy of Horst Reichert, University of Mainz, Germany.)...

StOber silica particles also show a low density of the powder as precipitated. All reported literature values are at or below a density of 2.0 g cm-3, and van Helden et al. (14,15) reported values of as low as 1.61 g cm-3. These results are in accordance with the previously discussed microporosity and TEM substructure in the particles. 

Only at calcination temperatures above 800°C does the density increase to the literature value of amorphous silica of 2.2 to 2.25 g cm-1. The exact microstructure within the Stober silica particles depends very much on the specific precipitation conditions, which are discussed in more detail in section 2.1.4. 

VSi MAS-NMR experiments by van Blaaderen et al. (11), Labrosse et al. (51), Humbert (52), and Davis et al. (53) have indicated the same porous microstruclure within the Stober silica particles as observed by TEM and the surface area analysis. The publications reported high values for the Q1 and the Q2 species, which are an indication of a very open internal structure or molecular network. Q" values of approximately 65%, Q1 of 30%, and Q2 of about 5% were reported. 

Several growth and formation mechanisms have been proposed for the formation of monodispersed Stober silica particles. Silica in general is an extremely well-studied system, and there are numerous publications with respect to the hydrolysis and condensation reaction. At present there are two major formation mechanisms that have been used to explain the formation of Stober silica particles. 

Second, nucleation and growth of Stober silica particles is modeled by a controlled aggregation mechanism of subparticles, a few nanometers in size, as for example presented by Bogush and Zukoski (19). Colloidal stability, nuclei size, surface charge, and diffusion and aggregation characteristics are the important parameters in this model. 

LaMer, Monomer Addition Growth Model. Most of the recent publications (13,18,37,43-45) concerning the Stober silica precipitation describe a first-order hydrolysis of TEOS as the rate-limiting process in the silica particle precipitation. The second reaction step, the condensation reaction, was found to be faster by at least a... 

The formation of ordered sphere-packing structures was observed in certain rheological experiments as just described. Due to the extremely uniform size of the particles, an ordered dense packing structure will develop during sedimentation of the Stober silica particles (see Fig. 2.1.12) when the dispersion is either sterically or electrostatically stabilized. The gemstone opal is essentially based on this principle (80-88). A transmission election replica picture is shown in Figure 2.1.13. The uniform... 

Awaiting further Information, table 4.3 gives Indirect data, obtained in a subsequent paper Kijlstra et al. K This work concerned experiments on the electrophoretic mobility, conductivity and dielectric relaxation of (Stober) silica and haematite (a -Fe Og) sols. The reported values of are those needed... 

For gel type or precipitated silicas that may be (partially) penetrable for protons the shape of the crs(pH) curves is comparable, but the magnitude of the charge densities may be quite different [42]. Results obtained for Stober silica in the presence of simple 1-1 electrolytes suggest that in this ca.se a Donnan model (volume charge densities) is more appropriate as double layer model than a SGC model. 

These few examples do not exhaust a long list of difficulties in calculation of exact values of potential from electrophoretic or electroacoustic data, and most results reported in literature appear to be rough estimates rather than exact values, except for some results obtained with nearly spherical particles having very narrow size distributions, e.g. Stober silica. Many publications tend to overestimate the accuracy of ( potentials presented therein. For example in a recent publication a graph was presented showing the variations of the measured potential as a function of time, and all data points showed in that graph ranged from 6 to 7 mV. 

Siderite, lEP of, 208 Silica (see also Quartz Stober silica) effect on lEP of metal oxides, 75 kinetics of dissolution, 2 Silicate complexes, of Eu, 312 Silicates... 

PZC/IEP of Stober Silica Obtained by Rapid Addition of Si(EtO)4to 4.6 M Ammonia in Aqueous Ethanol at Room Temperature... 

PZC/IEP of Stober Silica Obtained by Aging of Solution of Si(EtO)4 and Ammonia in Aqueous Etbanol at 40°C... 

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