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Water in nanopores: polarity determines spatial distribution

How do guest molecules act in the pores of porous materials? A group of scientists around CUI member Prof. Michael Fröba from the Institute of Inorganic and Applied Chemistry at Universität Hamburg tried to find answers to this highly relevant question for science and technology. In a study published in the scientific journal Angewandte Chemie International Edition, the scientists report their analysis of spatially confined water molecules: they found out that not only the pore size matters, but that surface interactions resulting from different polarities are highly effective.

Schematic representation of the pore-filling modes. Illustration: Benedikt Mietner

“The role of spatially confined water molecules is especially relevant for biochemical processes, for catalysis, and energy storage,” Benedikt Mietner says, who is the main author of the study. The reason is that the spatial confinement influences the physical properties of liquids, depending on the topology, the size, and the surface polarity of the pores. The influence of the pore size on the melting and freezing points of confined water has been studied extensively. In contrast, very few studies have been made concerning the effects of the surface polarity, which may be due to the lack of highly defined porous materials.

Direct view on the interface

Periodic mesoporous organosilicas (PMOs) fill this gap perfectly because they combine a highly ordered pore structure with a variable surface. The scientists used multidimensional solid-state NMR spectroscopy to analyze the PMOs, which enabled a direct view on the interface of water and the pore-wall surface at the molecular level. Thus they determined the spatial distribution of water inside the pores and found out that depending on the alternating surface polarity, water molecules either interact with only the inorganic silicate layer or with both silicate and organic layers.

Mietner: “We could show that the spatial distribution of water molecules in the pores of PMOs strongly depends on the polarity of the organic unit. We are very happy about our results and the classification as a ‘very important paper’.” The group’s study helps to understand confinement effects resulting from various surfaces more deeply. Text: CUI

 

Citation:
Mietner J. B., Brieler F. J., Lee Y. J., Fröba M.
„Properties of Water Confined in Periodic Mesoporous Organosilicas: Nanoimprinting the Local Structure“
Angewandte Chemie, Int. Ed., Volume 56, Issue 40 (2017), page 12348
DOI: 10.1002/anie.201705707