InterPore2026

Periodic Mesoporous Organosilicas as Host Materials for Studying Surface Chemistry and Pore Size Effects on the Properties of Nanoconfined Water

20 May 2026, 12:20

15m

Oral Presentation (MS13) Fluids in Nanoporous Media MS13

Speaker

Prof. Michael Fröba (University of Hamburg)

Description

Water is undoubtedly the most important substance on earth. It is ubiquitous in nature and a necessary liquid for the emergence of life. Although by far the most classic liquid encountered in everyday life, water presents many unusual physical properties, which are not yet fully understood. A large number of studies have highlighted the crucial role of hydrogen-bonding interactions between water molecules in determining the peculiar liquid structure and physicochemical properties of water. In most frequent situations, water is found as spatially confined or in an interfacial state rather than forming a bulk phase. From a fundamental point of view, confining water at the nanoscale in prototypical porous solids has turned out to be particularly adequate in order to better understand the unusual behavior of interfacial water.
Among several types of confinement, including clays or zeolites, the mesoporous SBA-15 and MCM-41 silicas are particularly suited hosts due to their well-defined porous geometry formed by ordered cylindrical channels. While MCM-41 and SBA-15 silica provided an adjustable pore size and can address the geometrical aspect of the nanoconfinement, the evaluation of the effect of surface interaction on the water properties is limited due to the unchanged chemical composition. In order to extend current knowledge, which has so far been based on a few studies on grafted silicas, we are contemplating new opportunities offered by the molecular scale imprint of the water−surface interaction. Periodic mesoporous organosilicas (PMOs) are particularly well-suited, though barely used in water studies so far. In contrast to the MCM-41 silica the PMOs can contain organic bridging units within the quasi-crystalline pore walls and therefore a periodically modulated surface polarity [1]. The chemistry of these bridging units can vary from hydrophilic to hydrophobic and can also contain surface ionic charge with localized cations and exchangeable anions. Unlike post-synthetically surface-grafted nanoporous silicas, PMOs allow a stoichiometric control of a periodically alternating surface chemistry along the pore channel.
Here, we present new insights into how surface chemistry and pore size influence the properties of nanoconfined water. We studied water in PMOs with pore diameters in the range of 2-5 nm. In these materials, the molecular mobility of water as well as its melting point and the properties of the non-freezable water layer (so-called t-layer) are influenced by the polarity of the organic moiety [2-6].
Surface hydrophilicity has little effect on melting point depression in larger pores but becomes increasingly influential as pore size decreases. In hydrophobic PMOs, water exhibited larger melting point depression, lower specific enthalpies, and thicker t-layers with lower average density than in hydrophilic ones. In contrast, charged PMOs behaved differently: despite higher hydrophilicity, confined water exhibited a larger melting temperature depression, lower specific enthalpy, larger critical pore radius, and comparatively thicker t-layers, likely due to higher disorder of the hydrogen-bonding network close to the surface [4,6]. Moreover, the t-layer density did not follow a simple trend based solely on hydrophilicity. These results highlight the complex interplay between pore size, surface chemistry, and interfacial water behavior.