(339c) Gravimetric Analysis Under Process-Near Conditions at Nanogramme Resolution: New Sorption and Solvation Apparatus for Very Small Sample Quantities

With the development and commercialization of the Magnetic Suspension Balance (MSB) at Ruhr-University and Rubotherm GmbH at the end of the 1980s [1], the gates were opened for the development of a wide range of apparatuses for the analysis of weight changes in fluids at process-near conditions. Applications are well known and vary from generation of reference density data of pure fluids and mixtures [2], thermogravimetric analysis [3] to adsorption and absorption [4]. The main limiting factor in the conventional MSB has often been the configuration, which has been limited to a bottom-loading device, as the sample has necessarily been suspended by an electromagnetic coupling from a laboratory microbalance. Recently, based on a patented technology developed at ETH Zürich, Rubotherm GmbH has developed a new Nano Magnetic Suspension Balance (NMSB) where the magnetic levitation coils have the dual function of suspending and weighing the suspended body enclosed under high pressure. The electric suspension current is directly proportional to the weight of the suspended body, and is measured at high resolution, thus dispensing with the hitherto required laboratory microbalance. The new balance is able to measure weight changes down to 10^-8 g, where conventional MSBs are limited by the laboratory microbalance to a highest mass resolution of 10^â??6 g. Additionally, the new NMSB permits free configuration of the associated apparatus, either as a top or bottom loading balance.

A new apparatus has been developed within the RESOLV cluster of excellence (funded by the German Research Foundation) which takes advantage of the new NMSB, regarding the higher resolution as well as the free choice of measuring configuration. To realize this, a flexible interface has been devised to permit conversion of the same balance between a top and bottom loading configuration, depending on the measurement to be performed. The top loading configuration is particularly advantageous for thermogravimetric analysis with elevated temperatures, as the high temperature low density area is situated above the NMSB itself which is in the low temperature, high density area of the pressure cell. This configuration is used for thermogravimetric analysis and adsorption quantification on small sample quantities of porous materials; here we have obtained high quality results of N₂ and CO₂ adsorption on metal organic framework materials with sample quantities in the range of 5 mg (many 100s of factors smaller than i.e. volumetric methods). The bottom loading configuration is somewhat simplified and is used for experiments with high dead weights but which require the high resolution of the new NMSB. Here we are investigating weight changes in moistening and drying processes on two dimensional model surfaces simulating new coatings against biofouling [5]. These samples combine high weight (of the wafer substrate) with very low specific surface area, resulting in a small measurable weight difference. The flexible new apparatus is connected to a gas dosing system comprising a component for static dosing of dry gases and a component for dosing binary gas mixtures under flow conditions mixed with saturated vapours. Results of these dynamic mixture measurements will also be presented.

[1] H. W. Lösch, PhD Thesis, Ruhr-University Bochum (1987)

[2] W. Wagner, R. Kleinrahm, Densimeters for very accurate density measurments of fluids of large ranges of temperature, pressure and density, Metrologia, 41 (2004) 24-39

[3] C. Seibel, T. M. Fieback, New sample carrier systems for thermogravimetric analysis under forced flow conditions and their influence on microkinetic results, Rev. Sci. Instr., 86 (2015) 95104

[4] H. W. Lösch, R. Kleinrahm, W. Wagner, Neue Magnetschwebewaagen für gravimetrische Messungen in der Verfahrenstechnik, Chem. Ing. Tech., 66 (1994), 1055-1058

[5] A. Rosenhahn, S. Schilp, H. J. Kreuzer, M. Grunze, The role of â??inertâ?? chemistry in marine biofouling prevention, Phys. Chem. Chem. Phys. (2010) 4275-4286