Surface silica type

Schematic representation of the kaolinite structure. It reveals the 1 1 structure due to the alternation of silica-type (black) and gibbsite-type layers (white). Furthermore, the edge surface exposes aluminol and siianol groups.

In bulk coating processes, bulk materials are joined to the substrate either by a surface melt process or by attachment of the solid material. An example of the latter is the application of heat-resistant tiles of silica-type material to the aluminum alloy skin of a space shutde vehicle, enabling the vehicle to withstand the reentry heat. 

Silica type Surface area (m2 g ) Pore volume (ccg ) Activity (gg hr-1)... 

Keywords a-diketone, silica gel surface, Norrish type I reaction, photoreaction... 

Whereas the surface area of a crystalline silica is in fact the external surface area, the surface area and the pore size distribution of an amorphous silica are actually determined by the dimensions of the silica spheres (primary particles) that build up the network. For non-aggregated spherical particles, this relationship is very straightforward. In this silica type, the primary particles are not clustered and Sheinfain s3 globular theory can be applied. The globular theory predicts an inverse relationship between surface area and the primary particle size by the following equation ... 

The different reaction parameters to be used for this series of substrates, compared to the low pretreatment temperature range, indicate a difference in deuteration process. For the high temperature pretreated samples, only isolated and geminal hydroxyls are present on the silica surface. These types of silanols have a low water physisorption ability. Therefore, instead of the formation of a hydration layer prior to H/D exchange, direct exchange from vapour phase has to occur. This is favoured at higher temperatures. 

Based upon more than 100 samples, with a specific surface area varying from 5 to 1000 m2/g, Zhuravlev found that the silanol number for a fully hydroxylated silica amounts 4.6 0.5 OH/nm2. This constant is claimed to be independent of the origin and structural characteristics (specific surface area, type of pores, pore size distribution,. ..) of the sample. 

The effect of solvent type and aminosilane concentration has been evaluated. The third component in the reaction system is the silica substrate. The surface of the silica gel carries the active sites for adsorption. The concentration of these sites varies with varying silica type, its specific surface area and pretreatment temperature. Additionally, surface adsorbed water has a clear effect on the reaction mechanism. Isotherm data, reported in the previous paragraph, only accounted for fully hydrated or fully dehydrated silica. The effect of the available surface area and silanol number remains to be assessed. Information on these parameters allows the correlation of data from studies in which different silica types have been used. In this part the effect of these parameters in the loading step is discussed. Silica structural effects on the ultimate coating, after curing, are evaluated in the next paragraph. 

Contrary to carbon-black-filled conventional rubbers, which form a semi-rigid interface at the carbon black surface, PDMS chain units at the silica surface are not rigidly linked to the silica surface. Two types of dynamic processes are thought to occur at the interface relatively fast anisotropic reorientation of chain units in the interfacial layer and slow adsorption-desorption of chain units (Figure 10.13) [108, 113]. 

A high degree of hydrophobic character is an almost unique characteristic of silicon-rich or pure-silica-type microporous crystals. In contrast to the surface of crystalline or amorphous oxides decorated with coordinatively unsaturated atoms (in activated form), the silicon-rich zeolites offer a well-defined, coordinatively saturated sur ce. Such surfrces, based on the strong covalent character of the silicon-oxygen bond and the absence of hydrophilic centers, display a strong hydrophobic character unmatched by the coordinativeiy unsaturated, imperfect surfaces. Also, hydrophobic zeolite crystals have been reported to suppress the water affinity of transition metal cations contained in the zeolite pores. This property permits the adsorption of reactants such as carbon monoxide or hydrocarbons in the presence of water. 

Scheme 1. Arrangement of silanol groups on the dehydrated fumed silica surface and types of some heterolytic reactions with its participation.

Figure 2. Charging behaviour of silica type (panel a) and gibbsite type (panel b) surfaces.

Some results for silica type surfaces in the absence of s.a. are shown in Fig. 5. 

Equations (70) and (71) show that even in the absence of s.a. t/>s is not only dependent on logKH — pH, but also on This is clearly expressed in panel (c) of Fig. 5. The surface potential of silica type surfaces is, in general, strongly deviating from the Nernst equation according to which the potential is independent of Cs and linearly dependent on pH. 

In the surface charge modeling presented in the next sections, silica was selected as an example of silica-type surface. This does not imply that the present author subscribes to the opinion that silica cannot carry positive surface charge. In view of different contradictory clues this remains an open question. 

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