Xenon porometry : a novel method for characterization of porous materials by means of ¹²⁹Xe NMR spectroscopy of xenon dissolved in a medium
1University of Oulu, Faculty of Science, Department of Physical Sciences
|Online Access:||PDF Full Text (PDF, 0.4 MB)|
|Persistent link:|| http://urn.fi/urn:isbn:9514279913
|Publish Date:|| 2006-01-31
|Thesis type:||Doctoral Dissertation
|Defence Note:||Academic Dissertation to be presented with the assent of the Faculty of Science, University of Oulu, for public discussion in Auditorium L10 (Raahensali), on February 10th, 2006, at 12 o’clock noon.
Professor Alexander Pines
Professor Roderick Wasylishen
The present thesis describes the development of a novel method, referred to as xenon porometry, for the determination of the structural properties of porous materials by means of xenon NMR spectroscopy. The method exploits the high sensitivity of the chemical shift of the ¹²⁹Xe isotope to its local environment. The purpose of the medium added to the sample is to slow down the diffusion of xenon so that the NMR signal of a xenon atom is characteristic of the properties of one pore, and the signals of all the atoms in the sample represent the distribution of the properties.
Two types of porous materials (controlled pore glasses and silica gels) with well-known properties and three different media (acetonitrile, cyclohexane, and naphthalene) were used in the studies. The behavior of the medium and dissolved xenon at different temperatures around the melting point of the medium was explained. By varying the pore size of the material, three different correlations that make it possible to measure the pore sizes of unknown materials were experimentally determined. The chemical shift of xenon inside pockets built up in the pores during solidification of the medium turned out to be especially sensitive to pore size, and this correlation makes it possible to determine the pore size distribution. The curious behavior of the chemical shift as a function of pore size was explained by using a model based on the fast exchange between xenon adsorbed on the walls of the pockets and free xenon in the middle of the pockets. It was also proved that the porosity of the materials can be determined by comparing the intensities of two signals originating from xenon dissolved in a liquid medium.
A comparison of the xenon porometry method with other methods used for pore size characterization leads to the following conclusions: The range of applications of the method is relatively wide, the measurements are fast and easy to do, the analysis of the spectra is simple on the basis of the information presented in this thesis, and the properties of the materials can be extracted from the spectral data with basic mathematical conversions. Because there are several different types of correlations available in the same spectra that represent the properties of the porous material, the complementary information of all the correlations make it possible to obtain a picture of the structures of very complex systems.
Report series in physical sciences
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