Georg Rudakoff

Solubility of silica gel in water

The solubility of silica gel in distilled water and dilute and alkaline solutions has been investigated. The solubility has been studied as a function of the pH of the gel preparation, pHs, and of the pH of the dissolution media, pHL. A kinetic model for the dissolution process has been developed that permits the determination of the rate constant as well as the equilibrium value of the solubility of the silica gel. The model explains the dependence of the rate constants on the concentration of catalytically active hydroxyl ions as an adsorption of these ions on the silica gel surface. The solubility of silica gel increases with increasing pHL. The solubility dependence on pHS is influenced both by the specific surface area of the gel and gel time. The structure of the gels is determined by the gel time. High solubilities can be achieved by a large specific surface area and a small gel time.

Ammoniation and nitridation of highly disperse silica

Infrared spectroscopy, X-ray scattering intensity measurements and chemical analysis of the N content have been used to study the reaction of flowing NH 3 with highly disperse silica (aerosil) at elevated temperatures. During interaction with flowing NH 3 (ammoniation) at temperatures ≤ 723 K, only SiNH 2 groups were formed by substitution of SiOH groups on aerosil. At temperatures higher than 773 K under flowing NH 3 , both SiNH 2 groups and Si-NH-Si groups (silazane bridges) were formed, which could be identified clearly in the IR spectra. Silazane bridges appear at temperatures ≥ 1073 K, replacing not only surface siloxane bridges but also siloxane bridges in the bulk. With increasing reaction time, nitrogen contents up to 25 mass% were introduced into the SiO 2 lattice by a reaction of aerosil with flowing NH 3 at 1373 K. X-ray scattering intensity measurements have shown that the incorporation of nitrogen in the aerosil structure gives rise to a drastic rearrangement of the amorphous SiO 2 structure. Both the primary structure order and the mid-range order of the aerosil were strongly changed after high temperature ammoniation. The treatment of aerosil in flowing N 2 at 1373 K results in formation of cristobalite.

Vibrational relaxation of liquid benzene. A temperature-dependent Raman lineshape study of the selectively deuterated molecules

Abstract The halfwidths of the symmetric CH and CD stretching modes of liquid C 6 H 6 , C 6 D 6 , sym-C 6 D 3 H 3 , p -C 6 H 2 D 4 , p -C 6 H 4 D 2 , C 6 HD 5 , and C 6 H 5 D are reported. Their temperature dependence is measured in nearly the whole liquid range. There is no coincidence with present relaxation theories.

Vibrational relaxation in liquid hydrogen and deuterium bromide—The influence of self and distinct correlation functions on the isotropic and anisotropic Raman line shape

Both, the isotropic and anisotropic Raman line shapes of orthobaric liquid hydrogen and deuterium bromide are measured at 295 K and close to the triple point (185 K). Concentration dependent studies in HBr/DBr mixtures allow an experimental separation of the resonant transfer mechanism from other line broadening mechanisms. It is found that the anisotropic spectra are nearly unaffected by isotopic dilution, whereas the isotropic ones show a blue shift and a narrowing with increasing isotopic dilution. The overall contribution of the transfer mechansim to the line broadening becomes most pronounced at the lower temperature. The results are discussed in terms of a recent Raman line shape theory of Bratos and Tarjus.

A model for viscosity in the early stages of the sol-gel transformation

The sol-gel transformation of silica in aqueous solution was studied by viscosity measurements. The first two steps of this process are nucleation and growth of particles. These early stages can be described theoretically by the theory of nucleation and condensation in a closed isochoric system. For this purpose, the Helmholtz free energy surface of sol formation was calculated as a function of particle size and sol particle concentration. The sol formation proceeds along the bottom of the valley of this free energy surface. A rate equation for the process can be derived. Good agreement between experimental results and the theoretical model was generally found. Experiments were carried out at different temperatures and pH values. The activation energy of the condensation of silica was determined to be 4.7 kJ/mol and the order of the reaction of the hydroxyl ion concentration was found to be 0.7.

The slip-model for molecular reorientation and the temperature dependence of nuclear relaxation times in neat toluene and chlorobenzene

Abstract The temperature dependence of deuterium nuclear relaxation times in selectively deuterated toluene and chlorobenzene is studied. It is shown that the temperature behaviour of molecular reorientation times can be described by the method of Pecora et al. within the limit of rotational diffusion.

Reactivity of plasma-treated SiO2 against O2: An EPR study

Abstract During plasma treated of SiO 2 (dehydrated at 700°C in high vacuum) electrons become trapped at the surface and in the bulk. The captured electrons from the well known E i centre, an F-like electron centre with the unpaired electron mainly in an s-type orbital, and CO 2 − which comes from carbon impurities. Adsorption of O 2 (partial pressure 5 × 10 -3 torr ⩽ po 2 ⩽ 10 −1 torr) gives O 2 − species that depend on the adsorption temperature, characterized by the following g -values: g 1 = 2.069, g 2 = 2.000m T ads = 300 K; g 1 = 2.039, g 2 = 2.007, g 3 = 2.002, T ads = 77 K.

Ermittlung der anisotropen brown'schen rotation monosubstituierter diphenyle in flüssiger phase durch 13CTfl-messungen

Zusammenfassung Es werden Messungen der longitudinalen 13 C-Relaxationszeiten protonierter Kohlenstoffatome in monosubstituierten Diphenylen beschrieben. Mit Hilfe des Woessnerschen Formalismus konnten die verschiedenen T 1 -Werte als Funktion der anisotropen Brownschen Rotation dargestellt werden. Zwei Rotationsdiffusionskoeffizienten der “overall”-Bewegung wurden aus den T 1 -Werten ermittelt. Es wurde festgestellt, daβ die Anisotropie der Rotation wesentlich uber dem theoretischen, durch die Tragheitsverhaltnisse bedingten, Wert liegt. Als Ursache dafur wurde das verstarkte Wirken des “frictional effects” auf die C xy -Rotation erkannt. Die intramolekulare Rotation der Phenylgruppe konnte mit zwei verschiedenen Bewegungsmodellen bestimmt werden. Es gelang, die Rotationsbarriere dieser Bewegung unter Anwendung von T 1 -Daten von Quadrupolkernen in guter Ubereinstimmung mit quantenmechanischen Werten zu ermitteln. Die Abhangigkeit der Rotationsbarriere vom Hammettschen σ m -Parameter wurde nachgewiesen.

On the determination of kinetic parameters of sol formation by rheological measurements

Abstract The sol—gel transformation of silica in aqueous solution was studied by viscosity measurements. Both the temperature and the pH value are found to have a great influence on the rate of sol formation. A theoretical model is suggested for the evaluation of experimentally determined viscosity—time data. The model is shown to correlate with the experiment in all the cases studied. It allows calculation of the kinetic parameters of the polymerisation of silica. Estimated values of the energy of activation of polycondensation, and of the order of reaction with respect to the hydroxyl ion concentration, are of the same order of magnitude as those reported in the literature. Our results, however, permit the testing of an evaluation method which can determine both quantities, i.e. the energy of activation and the order of reaction, simultaneously in one procedure.

The Influence of the Choice of a Suitable Dividing Surface on the Results of Surface Charge Density Calculations

The model of site dissociation of acid surface groups is used in connection with the GouyChapman model of the electrical double-layer for the calculation of theoretical values of the surface charge density of spherical particles in aqueous electrolyte solutions. The influence of a parameter on the particle size dependence of the surface charge density is discussed, is the distance between the plane of the location of the surface charge and the equimolecular dividing surface of the monolayer adsorption of gases.