Water Adsorption at Pyrogenic Silica Surfaces Modified by Phosphorus Compounds

Abstract

Water adsorbed on pyrogenic silica modified by phosphorus chlorides, which were then hydrolyzed, was studied by 1 H NMR, dielectric, and IR spectroscopies and quantum chemical ab initio and semiempirical AM1 and MNDO/H methods. Hydrophosphinic groups (HPA) bound to pyrogenic silica surfaces are stable at the adsorbed water content Cw e 12 wt %, which corresponds to water adsorption at standard conditions. Increase of the adsorbed water content leads to hydrolysis of the tSisOsP bonds via nucleophilic substitution SNi(Si) mechanism. Upon heating, the adsorbed phosphinic acid can cleave the tSisOsSit bridges and form new sites responsible for changes of structure of the adsorbed water clusters and chemical shift values for the 1 H NMR spectra. Great clusters of the adsorbed water molecules, which form at Cw > 12 wt %, are localized near ionized bound hydrophosphinic groups or adsorbed acid molecules. For air/H2O/HPA/ SiO2, the adsorption energy of water decreases if the water content increases. The dielectric and 1 H NMR spectra of H2O/HPA/SiO2 have significant distinctions in comparison with those for the parent pyrogenic silica, and these differences depend not only on the adsorbed water content but also on reaction temperature and other treatment parameters.

Surface features of P-doped silica: a comparison between IR spectroscopy and theoretical modelling

“…Since the structure of the active sites at the surface of the real sample are completely unknown, ab initio modelling by means of a cluster approach was attempted, by designing small clusters containing Si, O and P atoms. Only a few papers in the literature discuss theoretical modelling of P-doped silica, 3,9 and the adopted cluster models are quite different from those shown here. Most of the models adopted in this paper are made of a six-membered siloxane ring (with six Si atoms) to which different types of P-containing groups are bound.…”

“…P-doped silica is a very important material for fibre optic technology and microelectronics, 1,2 and also because modification of silica with P can increase its interaction with polar adsorbates. 3 Moreover, the study of P-doped silica can be useful as a starting point for the analysis of more complex materials. In fact, this article is meant to be a first attempt to model the surface of bioactive glasses, i.e.…”

Study of Hydrated Structures on the Surface of Mesoporous Silicas and Carbosils by 1H NMR Spectroscopy of Adsorbed Water

“…i. Adsorption of Water on the Mesoporous Silica Gel Surface . It has been established that the 1 H NMR spectroscopy technique can be used to study water adsorbed on the surface of nonporous silicas and that in a wide region of the hydrated surface its spectrum is observed in the form of a single signal, whose width monotonously increases while the temperature and adsorbed water concentration decrease. , The lower the concentration of adsorbed water, the larger the signal width and the weaker its dependence upon the temperature.…”

“…One may expect that the influence of the surface in the mesoporous adsorbents will affect most of the substance located in pores, and therefore, the properties of the water layers at the interface may differ significantly from the corresponding characteristics on the surface of nonporous adsorbents. We have studied earlier the structure of adsorption complexes and hydrated layers on the surface of nonporous pyrogenic silica , and synthesized composite carbon-containing silicascarbosils. , The aim of the present work is the examination of changes in hydration properties of porous silica due to its carbonization. As a main method of investigation the 1 H NMR spectroscopy of adsorbed molecules has been chosen. , Contrary to the CP-MAS 1 H NMR spectroscopy and CRAMPS (combined rotation and multiple-pulse spectroscopy), this method is characterized by a lesser resolution.…”

“…We have studied earlier the structure of adsorption complexes and hydrated layers on the surface of nonporous pyrogenic silica , and synthesized composite carbon-containing silicascarbosils. , The aim of the present work is the examination of changes in hydration properties of porous silica due to its carbonization. As a main method of investigation the 1 H NMR spectroscopy of adsorbed molecules has been chosen. , Contrary to the CP-MAS 1 H NMR spectroscopy and CRAMPS (combined rotation and multiple-pulse spectroscopy), this method is characterized by a lesser resolution. However, in combination with the bulk freezing , procedure, the above method permits the signal of molecules of a substance that has undergone perturbation originating from the adsorbent surface to be resolved from the total proton signal of solid and liquid phases.…”