Armin Mozhdehei1, Philip Lenz2,3, Stella Gries4,8, Sophia-Marie Meinert2, Ronan Lefort1, Jean-Marc Zanotti5, Quentin Berrod6, Markus Appel7, Mark Busch4,8, Patrick Huber4,8, Michael Fröba2,3, Denis Morineau1
1 Institute of Physics of Rennes, CNRS-University of Rennes, UMR 6251, F-35042 Rennes, France.
2 Institute of Inorganic and Applied Chemistry, University of Hamburg, 20146 Hamburg, Germany.
3 The Hamburg Centre for Ultrafast Imaging, Luruper Chaussee 149, 22761 Hamburg, Germany
4 Institute for Materials and X-ray Physics, Hamburg University of Technology, Hamburg 21073, Germany
5 Université Paris-Saclay, Laboratoire Léon Brillouin, CEA, CNRS, F-91191 Gif-sur-Yvette, France
6 Univ. Grenoble Alpes, CNRS, CEA, Grenoble INP, IRIG, SyMMES, F-38000 Grenoble, France
7 Institut Laue-Langevin, F-38042 Grenoble, France
8 Centre for X-ray and Nano Science CXNS, Deutsches Elektronen-Synchrotron DESY, 22603 Hamburg, Germany.
Water is the most abundant liquid on Earth. It is essential to the emergence of life, omnipresent in natural environments (biological, geological) and ubiquitous in many technological processes. Among liquids, water is a fascinating system due to its many unusual fundamental properties, the study of which constitutes per se an entire field of scientific research.
We are seeing increasing interest in water-based technologies, such as catalysis, desalination, osmotic energy harvesting and environmental remediation, which rely on nanoporous systems. When water is incorporated into such nanoporous solids, a higher level of complexity is attained due to the predominance of spatial restriction and interfacial interactions.
Geometric confinement reshapes the extent of the hydrogen bond network, and the presence of a liquid-solid interface breaks the translational symmetry of the bulk liquid phase. A possible illustration of this phenomenon is the liquid properties being spatially heterogeneous. Hence, distinct structure and dynamics can be observed in the layers adjacent to the surface of the pores, while a more bulk-like behavior is retrieved at larger distances and, eventually, at the center of the pores.
Under these circumstances, the nature of the water-pore interaction obviously plays a central role. Surface functional groups can act as preferential adsorption sites for water molecules, disrupting their ability to form water-water H-bonds, and reducing their mobility. A large proportion of mesoporous solids are formed from oxide materials that include surface hydroxyls, such as silanol groups for silica, which explains why this phenomenon has been frequently observed in the literature. It was also shown that this moderate slowing of interfacial water dynamics can be adjusted according to the hydrophilic character of the porous matrix, which depends on the surface chemistry of the pore.
Figure 1: (a) Illustration of the water dynamics probed by QENS. Illustration of the trajectory of water in (b) neutral DVP-PMO and (c) charged DVMeP-PMO.
Building on these foundational studies, the present work addresses the impact of longer-ranged interactions, such as electrostatic forces, that occur in charged porous media. To explore this in a controlled way, we employed two different periodic mesoporous organosilicas (PMOs) with comparable porous geometry (pore diameter 3.5nm), but different surface chemistries as illustrated in Fig. 1. The first one, termed DVP-PMO, presents an essentially neutral pore surface, while the second one, termed DVMeP-PMO, presents a positively charged pore surface induced by the presence of methylpyridinium cations (see red regions in Fig. 1c).
Figure 2: Evolution of the half-width at half-maximum of the sharp Lorentzian (ΓT) as a function of Q2, obtained from the fitting of QENS spectra recorded on SHARP for water confined in (a) neutral DVP-PMO and (b) charged DVMeP-PMO at four different temperatures. The fits using the jump-diffusion model are shown as solid lines.
The dynamic structure factor of the two water-filled PMOs was acquired on SHARP time-of-flight instrument at various temperature, and successfully modelled by the combination of a fast local rotation and a slower jump translation diffusion (see Fig. 2). The diffusion coefficient DT for water confined within neutral PMOs is only slightly smaller than that of bulk water (25% reduction at 300 K). This moderated effect of confinement aligns with previous observations made for nonionic porous materials of similar size. A starkly different observation was made for water within the ionically charged DVMeP-PMO, where the diffusion coefficient is reduced by a factor of 4, and, more strikingly, the residence time is 1 order of magnitude longer than for bulk water.
Complementary inelastic fixed window scans measured on IN16B backscattering spectrometer for water adsorbed in ionically charged DVMeP-PMO loaded at two different relative humidity (33% and 75% RH), indicate that the huge impact of the ionic interaction on the water dynamics is not restricted to the adsorbed molecules located at the pore surface but affects the water molecules present in the entire pore volume.
This research was carried out as part of the DFG-ANR collaborative project (ANR-23-CE29-0028) for which we express our gratitude. D.M. and A.M. acknowledge the funding by ANR-22-CE50-0002.
Reference: J. Phys. Chem. C 2025, 129, 40, 18311–18324
