Silanols silanes

Immobilization of the aminosilane molecule changes its interaction characteristics. Because the surface silanols are more acidic than silane silanols, the interaction with the surface silanols is thermodynamically favoured over intramolecular interaction. Kelly and Leyden10 measured the enthalpy of adsorption of the aminosilane molecules. Their results indicate that interaction with the surface involves more proton transfer than in the closed form dissolved molecules. 

Therefore, it can be concluded that the polymerization takes place at the silica surface, i.e. after adsorption of the aminosilane molecules. The surface effect can be explained by the interaction of the silane NH2 group with the substrate surface. As shown above, in water solvent the hydrolyzed aminosilane molecules are stabilized by internal hydrogen bonding of the amino group to the silane hydroxyls. When the amino group is H-bonded to a surface hydroxyl group this stabilization disappears and the silane silanols can condense to form a siloxane linkage. When the reaction is performed with hydrated silica in a dry solvent (sample 1), the hydrolysis only takes place at the silica surface and can immediately be followed by the condensation reaction. In both cases, structures of type I are formed. 

In evaluating the data for the vacuum curing only, contributions from oligomerization and hydrogen bonding of silane silanols are ruled out. Additional information is obtained in comparing the data for APTS modified silica to those measured on silica modified with APDMS. The APDMS molecule has only two ethoxy groups and therefore can only form two chemical bonds to the silica surface. 

We have shown how hydroxyl groups on the silica surface act as active sites in the modification reaction. The amount of hydroxyls is controlled by the thermal pretreatment of the substrate. APTS molecules are physisorbed to the surface by hydrogen bonding of the amine group to a surface hydroxyl (H). Chemisorption of APTS to the silica surface, in dry conditions, involves the formation of siloxane bonds with release of ethanol (I). Water causes the hydrolysis of the ethoxy groups of the APTS, with formation of silane silanols. These silanols are more reactive than the original alkoxy groups. Siloxane bonds with other silane molecules or with the silica surface are formed with release of water (J). 

Physisorption may involve the amine-side as well as the silicon-side of the silane molecule. Silanol groups (on the surface as well as on the silane) therefore are very important in the reaction sequence. In this paragraph we aim to clarify the role of both surface and silane silanols. A clear distinction between both types is possible with the complementary use of solid-state NMR and FTIR, and with conversion of the surface hydroxyls to deuteroxyls (K) prior to modification with the silane. 

From both experiments, it may be concluded that silane silanols are stable, when the silica surface has a low density of silane molecules. This low density may arise from a low initial surface hydroxyl density, as in the 973K sample, or from a low loading of silane molecules, as in the latter two samples. 

The 3731 cm 1 peak is also observed in the spectrum of silica modified with APTS in aqueous conditions (figure 9.37 c). Under these circumstances a multi-layer polymerized coating is formed, after hydrolysis of the silane molecules. However, some uncondensed hydrolyzed silane silanols remain at the edge of the coating layer. 

Regeneration of the metallation agent dicobaltoctacarbonyl from HCo(CO)4 which is formed in the hydrolysis steps of Eqs. 4 and 6 suggests the possibility of a catalytic pathway for silane/silanol transformations (Scheme 2). 

Scheme 2. Catalytic pathway for silane/silanol transformations.

Nowadays, the use of the reflection electron microscope (REM) or, recently, the tunnel electron microscope, as well as secondary ion mass spectrometry (SIMS), AES, electron-dispersive X-ray spectrometry, impedance spectroscopy, and so on, are yielding substantial increases in the knowledge of corrosion reactions in coatings and at their interface with metal or other substrates. As far as zinc or zinc-coated surfaces are concerned, problems of interfacial and intercoat adhesion, differential diffusion phenomena and electrolytic cell behavior on the substrate, and interreactions of zinc with conversion coatings (chromates, phosphates, silanes, silanols, etc.) have been analyzed, leading toward spectacular improvements in, for example, paint adhesion, absorption of conversion coatings and, in general, the protective action inside films as well as on their substrates. 

The thin-layer chromatography of silanes, silanols and siloxanes has been described in detail [697-699], also the paper chromatography of these compounds [700]. 

P. Fierens, G. Vandendunghen, W. Segers, and R. van Elsuwe, React. Kinet. Catal. Letters, 1978, 8, 179 Chem. Abs., 1978, 89, 163 633). Silane-silanol condensation. 

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