When Does Water Become Confined? Insights from Surface Vibrational Spectroscopy

Abstract:

Water confined between surfaces underpins technologies ranging from batteries and fuel cells to desalination membranes and nanofluidic devices. Yet even a basic question remains unresolved: when does water actually become confined? Proximity to a surface changes water's molecular environment, but this alone does not establish when confinement produces behavior distinct from that at an isolated interface.

Despite its technological importance, the crossover from interfacial to genuinely confined water remains poorly understood.Across three studies, we use surface-specific vibrational and femtosecond spectroscopy to disentangle the effects of confinement, surface charge, and ionic motion. Surprisingly, we find that true confinement emerges only at separations below approximately one nanometer: above this scale, the observed water response can be understood largely in terms of individual interfaces rather than a distinct confinement-induced state. Only at angstrom-scale separations does confinement fundamentally reshape the hydrogen-bond network.

[1] Surface chemistry adds another essential dimension: even nominally neutral hexagonal boron nitride spontaneously acquires charge upon contact with water, highlighting that confinement cannot be understood independently of interfacial electrostatics.

[2] Finally, we show that ionic rearrangements within the electric double layer occur on tens-of-picoseconds timescales, faster than diffusion-limited expectations but consistent with classical Debye theory.

[3]Together, these results distinguish the consequences of being near a surface from those of being truly confined. They establish a molecular basis for understanding when confinement matters and how it couples to the charges and dynamics that govern aqueous technologies.[1] Y. Wang et al., Nature Commun. 16, 7288 (2025).
[2] Y. Wang et al., J. Am. Chem. Soc. 147, 30107 (2025). [3] A. Greco et al., Science 388, 405 (2025).

Departament: https://www.mpip-mainz.mpg.de/en/bonn

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