Dehydration of the surface of pyrogenic silica by 1H nuclear magnetic resonance

The Fumed Silica Surface: A Study by NMR

“…The concentration of silanol groups on the silica surface is an issue that has been explored since the 1960's, based on a variety of methods. − The silanol concentration for Aerosil, another fumed silica, is well-known to be about 3 OH/(nm) 2 , a value obtained by quite a few investigations. − Use of silicone polymer as a 1 H NMR intensity standard in this Cab-O-Sil silica study provides another very useful way of calculating hydroxyl densities on surfaces . Absolute integrated intensities of the NMR signals for a mixture consisting of known weights of silica and silicone rubber enable one to evaluate the surface densities of hydroxyls.…”

“…Because the widespread utility of amorphous silicas, e.g., in sorption, heterogeneous catalysis, and composite materials, is largely a result of their surface properties, studies of the structures, chemistry, and properties of a variety of fumed silica surfaces have been carried out by many researchers for many years. − Careful analysis by a variety of methods has shown that the surface reactivity of a silica depends substantially on the quantity and structural environment of its surface hydroxyl groups, which in turn depend on the origin of the silica and its storage and treatment conditions − …”

“…Details of CP dynamics and relaxation properties in 29 Si CP-MAS studies, as well as high-resolution 1 H NMR studies, have provided important details on the surface structures of silica gel. − However, a comprehensive 29 Si NMR study, in which CP dynamics and/or various relaxation issues are used to elucidate surface details, has not been published on fumed silica. There have been only two published reports on fumed silica that have involved solid-state 1 H NMR techniques, , both of which described studies of the dehydration and rehydration of Aerosil silica. The results of these two studies are not in agreement with each other, and the assignments of various peaks in the 1 H NMR spectra have remained unclear.…”

Characteristics of the Hydration Layer Structure in Porous Titania−Silica Obtained by the Chemical Vapor Deposition Method

“…At the same time, the chemical shift for ice is 7 ppm, and that for liquid water is 5 ppm . For the water adsorbed on the silica (Aerosil) surface, the magnitude of chemical shift ranges from 3 to 4 ppm, when the water concentration is significantly smaller than the concentration of the surface hydroxyl groups, , to δ = 4.5−4.7 ppm for the strongly hydrated surfaces. , In the first case, there are primary complexes of water molecules bonded to the surface hydroxyl groups, and in the second, water polyassociates form.…”