Silica, surface OH groups

This is a simple second order equation which usually applies to description of dissociative chemisorption from the gas phase on the homogeneous surface. However, Eq. (43a) describes well only an initial part of kinetic chemisorption isotherm for interaction of trimethylchlorosilane on dehydrated pyrogenic silica [113] and mixed alumina-silica and titania-silica [114] surface. At the same time, whole kinetic chemisorption isotherms are described using equations derived for the heterogeneous surface. Thus, high fractional reaction orders in respect to the silica surface OH groups obtained by Hertl and Hair for chemisorption of silanes and siloxanes [106,107] and in other studies [113,114] may be explained by heterogeneity of the oxides surface. 

It follows from Table 6 that the Eg values, calculated for a dissociative addition to siloxane bonds of silica surface at the interaction with (CH3)3SiX (X = N3, NCS and NCO) compounds, are by 10 to 20 kJ-mole lower than the activation energies of reactions of these reactants with the silica surface OH groups. The lowest Eg value in the Aji process is displayed by the reaction with X = NCS that features the weakest Si X bond and a substituent with the highest electron acceptor ability, while the reactions with X = NCO and X = N3 occur to the close activation energies, which, probably, is related to the close strength of Si-X bond in these reactants. 

As shown previously, the silica surface OH groups manifested as nucleophiles at the interaction with organosilicon compounds. On the hypothesis that reaction mechanism is invariant in going from the silica surface to parent and mixed Ti or A1 oxides, then one would expect the decrease of chemisorption activation energy with enhancement of the surface sites donor ability parameter. 

As it follows from the quantum chemical calculations, two minima on the potential curves for the interaction of organic molecules with the silica surface OH group are due to formation of linear hydrogen bonded and following cyclic donor-acceptor complexes prior to the transition state for proton transfer in these reactions. In case of large molecules interaction, the second minimum often missing from the potential curve and the donor-acceptor complex is consistent with the transition state of proton transfer reaction. 

Simple linear relationships developed between the quantum chemical indexes of organic molecules (their maximum charges and energies of the frontier orbitals) and their empirical donor acceptor parameters as well as the heats of the hydrogen-bonded complexes formation with silica surface OH groups or the chemisorption activation energies on this surface may be used for estimation of the complexes stability and reactivity of organic compounds towards Bronsted sites of other oxide surfaces. The electron donor ability of... 

Fig. 62 Deflection AFM images, obtained in a contact mode, for the 83PU-17PHEMA-silica nanocomposites with 3 wt% silica [268]. Samples differed by a type of the functional cover of silica surface -OH groups (initial cover), -NH2 or -CH=CH2 groups. Most typical and relatively rare nanostructure images are shown on the left and on the right, respectively...

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