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Xenon porometry


The NMR resonance frequency of the 129Xe isotope of a xenon atom is extremely sensitive to its local environment, and this sensitivity has been widely utilized in studies of porous materials. However, due to the fast diffusion of the absorbed gas, the observed signal is an average of the properties of the large amount of pores sampled by the atoms during the time scale of the NMR measurement.


In our studies, we immersed the porous material in a liquid or solid medium. Because the medium slows down the diffusion of xenon, the signal of a particular xenon atom is characteristic of the very local properties of the material, and the signals of all the xenon atoms in the different parts of the sample represent the distribution of certain properties of the sample. We refer this method to as xenon porometry. [1-5]


The experiments show that the method provides three different, novel ways of determining pore sizes: two on the basis of the chemical shifts of the signals, and one by means of the phase transition temperatures revealed by the spectra. In addition, the porosity of the sample can be determined by means of the intensities of two signals observed from the liquid medium.


We have shown that in certain cases the correlation between the chemical shift and pore size makes it possible to determine the pore size distribution in a very simple way by converting the chemical shift scale of the spectrum into a pore radius scale. Below is an example of the determination of pore size distribution by means of the signal arising from the pockets inside the solidified medium. This signal has turned out to be especially sensitive to the pore size.



Determination of the pore size distribution

Determination of pore size distribution by xenon porometry using a solid medium. The porous material is immersed in a liquid medium, and xenon gas is added to the sample (a). Dissolved xenon atoms (dots) diffuse inside the pores (b). During freezing, empty pockets build up in the pores due to contraction of the medium and xenon squeezes out from the solidifying medium into the pockets (c). The chemical shift of xenon inside the pocket depends on the pore size, and the distribution of the signals observed from the different pores represents the pore size distribution (d). Using the determined correlation (e), the pore size distribution can be obtained by converting the chemical shifts to pore radii (f).


References

[1] PhD thesis: Ville-Veikko Telkki, Xenon porometry, a novel method for characterization of porous materials by means of 129Xe NMR spectroscopy of xenon dissolved in a medium, University of Oulu, Report Series in Physical Sciences, Report No. 37, 2006.

[2] Ville-Veikko Telkki, Juhani Lounila, and Jukka Jokisaari, Behavior of Acetonitrile Confined to Mesoporous Silica Gels as Studied by 129Xe NMR: A Novel Method for Determining the Pore Sizes, J. Phys. Chem. B, 109, 757 (2005).

[3] Ville-Veikko Telkki, Juhani Lounila, and Jukka Jokisaari, Determination of Pore Sizes and Volumes of Porous Materials by 129Xe NMR of Xenon Gas Dissolved in Medium, J. Phys. Chem. B, 109, 24343 (2005).

[4] Ville-Veikko Telkki, Juhani Lounila, and Jukka Jokisaari, Xenon porometry at room temperature, J. Chem. Phys., 124, 034711 (2006).

[5] Ville-Veikko Telkki, Juhani Lounila, and Jukka Jokisaari, Influence of Diffusion on Pore Size Distributions Determined by Xenon Porometry, Phys. Chem. Chem. Phys., 8, 2072 (2006).

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