Silica siloxane groups

The surface of silica is covered by a layer of acidic silanol and siloxane groups. This highly polar and hydrophilic character of the filler surface results in a low compatibihty with the rather apolar polymer. Besides, highly attractive forces between silica particles result in strong agglomeration forces. The formation of a hydrophobic shell around the silica particle by the sUica-sUane reaction prevents the formation of a filler-filler network by reduction of the specific surface energy [3]. 

An understanding of the surface chemistry of silica is required to interpret its chromatographic properties. The silica surface consists of a network of silanol groups, some of which may. be hydrogen bonded to water, and siloxane groups, as shown in Figure 4.2. A fully hydroxylat silica surface contains about 8... 

Our interest in polyethylene glycols centered on a simple scheme to immobilize these materials onto metal oxide surfaces. The surface of silica gel contains both silanol-OH groups and -0-strained siloxane groups(29). A simple synthetic pathway to produce covalently bonded glycols was proposed where reaction(30) would occur between the OH group of the glycol and the surface of a refractory oxide. 

The surface chemistry of amorphous silica is somewhat less complicated than that of carbon. Generally, only two kinds of end groups are possible on the surface silanol groups and siloxane groups ... 

Based on this approach Schouten et al. [254] attached a silane-functionalized styrene derivative (4-trichlorosilylstyrene) on colloidal silica as well as on flat glass substrates and silicon wafers and added a five-fold excess BuLi to create the active surface sites for LASIP in toluene as the solvent. With THF as the reaction medium, the BuLi was found to react not only with the vinyl groups of the styrene derivative but also with the siloxane groups of the substrate. It was found that even under optimized reaction conditions, LASIP from silica and especially from flat surfaces could not be performed in a reproducible manner. Free silanol groups at the surface as well as the ever-present impurities adsorbed on silica, impaired the anionic polymerization. However, living anionic polymerization behavior was found and the polymer load increased linearly with the polymerization time. Polystyrene homopolymer brushes as well as block copolymers of poly(styrene-f)lock-MMA) and poly(styrene-block-isoprene) could be prepared. 

We have also observed that when the pore size become smaller (less than 2 nm), the quality of the monolayer degrades (Figure lc). The 2,Si peaks corresponding to the siloxane groups are broad, indicative of polymeric siloxanes with heterogeneous chemical environment. At the same time, the peak corresponding to the bulk silicon (from mesoporous silica) has a pronounced Q, component, suggesting the siloxanes are not chemically bonded to the substrate. 

The effect of drying temperature on the available surface hydroxyl concentration of a silica is shown in Figure 8. A constant slurry concentration of 4% silica/toluene was used in these experiments. These results are comparable with other data (20, 62), and they indicate that the surface hydroxyl group concentration is reduced with increasing drying temperatures. The reaction of metal hydrocarbyl compounds with strained siloxane groups is illustrated in Reaction 4. 

The rehydroxylation process of silica proceeds in two steps. In a first step, water molecules preadsorb on the hydrophilic silanol sites. In a second step, this preadsorbed water causes a bond breaking of a siloxane group, yielding two new silanols. 

This table shows that a reaction of TCS with siloxane bridges does not occur at reaction temperatures of 423 K. Apparently, this reaction temperature is too low to induce a cleavage of the siloxane bridge. Reaction does occur, however, when the temperature is raised to 623 K. When the Cl content is converted to the amount per nm2 and divided by 3, the amount of TCS that has actually reacted with the siloxane bridges can be calculated (table 9.8, last column). For a silica gel, pretreated at 973 K, approximately 0.3 siloxane groups per nm2 react with trichlorosilane at 623 K. 

Besides reacting with the hydroxylic surface groups, BX3-molecules are known to react with siloxane groups. The reaction of BX3 with siloxane groups was first witnessed by Morrow and Devi in 1971.14 Treating totally dehydroxylated (1523 K) samples with BF3 they found IR-bands, characteristic for the monodentate surface groups. Furthermore they reported the formation of new silica bands at 888 and 908 cm"1 due... 

It is interesting to note however that reaction occurs with siloxane groups at room temperature for silica samples pretreated at high temperatures. If the so formed monodentate (see reaction (C)) groups are close enough to a residual SiOH group, reaction may occur with the formation of bidentate groups on the surface. This... 

The basic skeleton o7 tEe modified sorbents consists of a surface-active silica gel. Modification is in the form of a surface reaction with specially active silanes at the silanol groups of the silica gel. Following elimination of these silanol groups, new siloxane groupings are formed, on which aliphatic hydrocarbon groups are chemically bound by silicon-carbon bonds to the silica gel skeleton. Pre-coated plates with these modified silica gels are designated ... 

Figure 2.30 is the first example we consider. Here, the adsorbent Is hydrophilic silica (TK 900, Degussa), containing sllanol and siloxane groups on the surface. The pH was not checked It could have exerted some influence because an electrical double layer will develop at the silica-water Interface the state of hydration of the Interface Is also pH-dependent. The Isotherm clearly exhibits different regimes, with almost vertical parts and shorter or longer plateaus. So much fine structure Is not always found for instance with C Eg instead of C12E5 the short first plateau Is absent (not shown in fig. 2.30). 

It is known that surface of silica is covered with silanol or siloxane groups [1,2]. The Si-OH shows a specific interaction with water (hydrophilic), by means of the localised electron density of water molecule and the positive charge of OH groups. On the other hand, Si-O-Si are hydrophobic due to the non-polarity of these bonding [10]. Thus, it is reasonable to think that depending on the amount of silanol groups in the surface of silica, the amount of water adsorbed at very low relative pressures would be different. 

Chemically, each silicon atom is surrounded by four oxygen atoms in the form of a tetrahedron. The surface of silica contains (a) siloxane groups (Si — O — Si ), (b) silanol groups (Si —OH), (c) water hydrogen-bonded to the silanol groups, and (d) nonsorbed capillary or bulk water. The silanol groups (about 8 Limol/m ) represent adsorption-active surface centers... 

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