Preparation of Hydrated Silicon Dioxide for Reproducible Chemisorption Experiments

High purity fume silica has been treated following the recommended procedure, by heating at 1173 K for 120 h then by boiling in distilled water for 70 h. Finally the wet paste was frozen and liophilized. The resulting light powder, somewhat denser than the starting material, gave reproducible results in chemisorption experiments. Its surface is fully and evenly hydrated and its bulk does not contain water inclusions. After treatment with trimethyl-[dimethylamino]-silane at 453 K for 170 h such a sample was covered with a dense trimethylsiloxy layer with a surface concentration of 4·75 μmol m −2

Determination of Relative Configuration with the Aid of Gas Chromatographic Data. An Example

ISSN:0021-9665ISSN:1945-239

The Use of Hydrotropic Solutions for Counter-Current Separations

ISSN:0021-9665ISSN:1945-239

Specific Surface Area of Hydrated Silicon Dioxide Measured by Chemisorption

Various methods of specific surface area determinations have been compared: the BET-method, Sing's α s -method and the calculation from the monolayer capacity of dense chemisorbed trimethylsiloxy and (3,3-dimethylbutyl)-dimethyl-siloxy layers. It was shown that fume silicas as well as xerogels precipitated from water gave consistent results if the samples underwent a 70 h hydrothermal treatment in boiling water. The methods were calibrated on a similarly hydrated, low surface area fume silica. Slight differences in behaviour were observed between the two classes of silica. Surprisingly, the results disproved any serious dependence of the form of the nitrogen adsorption isotherm on surface curvature

Nitrogen and Argon Standard Adsorption Isotherms on Low Energy Surfaces

The following experimental approach was used to obtain nitrogen and argon standard isotherms at the boiling temperature of the adsorbates. The surface of hydrated fume silicas and silica gels of widely differing structures were covered by the densest monomolecular organosiloxy layer. The specific surface area of these surface-modified samples was obtained in two different ways. First, it was calculated by assuming the uniform globular model for the structure of the silicon dioxide starting material. Second, within a group of silicas covered by the same substituent, relative surface areas were determined by direct comparison of the adsorption isotherm of a sample with that measured on one of the silicas elected as a standard. Calculated surface areas gave excellent correlation with those obtained by direct comparison. Reference isotherms were derived by this study giving the complete information necessary for the determination of specific surface areas of powders having similar surface properties to those studied in the present work

The Influence of the Calibration Procedure of a Volumetric Adsorption Apparatus on the Form of the Isotherm and its BET Parameters

An up-to-date piston-operated automatic adsorption apparatus has been tested. Statistical analysis of the results has shown that the adsorbate must be considered as a real gas if a precision of 1% in the adsorption isotherm is to be aimed for at points around 1 bar for gases having a second virial coefficient higher than about 30 cm 3 /mol. A quadratic calibration is recommended and the influence of operational variables demonstrated on a few examples. Results can be applied, mutatis mutandis, to any volumetric adsorption apparatus. It has then been shown that the generally applied linear calibration procedure gives deformed isotherms, their evaluation resulting in BET surface areas 2–12% lower than those obtained by subtracting the non-adsorbed part of the adsorbate with the quadratic, real gas, calibration curve. Finally, it has been shown that isotherms determined by the gravimetric method are the same as those determined with the real gas calibration and give the same BET surface areas