Distribution of silanol groups

Figure 8 (a) Schematic diagram showing distribution of fillers in different parts of anionic elastomer [27]. (b) Proposed structural model showing interaction of silanol groups on silica surface with carboxylale groups [27]. 

With regard to a solubility equilibrium, the fact that vitreous silica behaves like a precipitate of polymeric silicic acid must be caused by the similarity between polymeric silicic acid and the hydrated surface of vitreous silica. Both forms can release silicic acid by hydrolysis and desorption, and likewise both forms are able to adsorb and condense silicic acid by means of silanol groups randomly distributed on their surfaces. Thus, in order to explain equal final states, the only assumption necessary is that the condensates will not attain the degree of dehydration of the bulk of the vitreous silica. The resulting equilibrium then relates to the two-phase system silicic acid—polymeric precipitate, and strictly speaking, this system is in a supersaturated state with respect to vitreous silica, which can be considered as an aged form of silica gel. 

In the absence of L H, the condensation of silanol groups is slow and inefficient, since after three days the molar masses slightly shifted (note that the reactions qnoted in Table 1 were carried out only during 24h, unless otherwise stated). In the presence of L H, the reaction is much faster and larger molar masses are reached, which seems to indicate that the SiH/SiOH condensation is competitive with the SiH/SiOMe one. Snch result may explain the double distribution found in some rnns of Table 1, where both fast L H addition and fair content of water present the best conditions for this co-condensation reaction to occur. 

Fumed silica is a highly dispersed silicon dioxide of large industrial importance and a wide spectrum of applications. Due to its production in a flame process fumed silica exhibits a smooth and nonporous particle surface. Additionally to its high surface area fumed silica bears isolated and statistically distributed surface silanol groups that render this product hydrophilic. A most important technical reaction, therefore, is the silylation and hydrophobization of the hydrophilic surface. 

Normally, impregnation provides the efficient incorporation of metal compounds inside the pores, but the particle growth is not controlled (particle size distribution is broad and particles are located statistically) if no special conditions are applied (see above). On the other hand, particle size is often restricted by the pore size. If silanol groups of the mesoporous siHca are involved in the interaction with metal compounds as discussed in the last example, additional control can be expected although the amount of silanol groups in the calcined silica is rather low. 

Deuterium-exchange measurements have shown that various types of amorphous dispersed silica contain not only surface hydroxyl groups but also structurally bound water inside the silica skeleton and fine ultramicropores. According to infrared spectral measurements [53], such bound water consists of silanol groups inside the silica sample (the adsorption band of stretching vibrations is about 3650 cm ). The distribution of OH groups between the surface and the bulk of the sample depends on a number of factors, but mainly on the method of preparation of the silica sample and its subsequent treatment. 

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