A groundbreaking discovery in nanofluidics was the observation of the tremendously enhanced water permeability of carbon nanotubes, those iconic objects of nanosciences. The origin of this phenomenon is still a subject of controversy. One of the proposed explanations involves dramatic modifications of the H-bond network of nanoconfined water with respect to that of bulk water. Infrared spectroscopy is an ideal technique to follow modifications of this network through the inter- and intramolecular bonds of water molecules. Here we report the first infrared study of water uptake at controlled vapor pressure in single walled carbon nanotubes with diameters ranging from 0.7 to 2.1 nm. It reveals a predominant contribution of loose H bonds even for fully hydrated states, irrespective of the nanotube size. Our results show that, while the dominating loosely bond signature is attributed to a one-dimensional chain structure for small diameter nanotubes, this feature also results from a water layer with "free" OH (dangling) bonds facing the nanotube wall for larger diameter nanotubes. These experimental findings provide a solid reference for further modeling of water behavior in hydrophobic nanochannels.

Water−ions interactions and spatial confinement largely determine the properties of hydrogen-bonded nanomaterials. Hydrated acidic polymers possess outstanding proton-conducting properties due to the interconnected H-bond network that forms inside hydrophilic channels upon water loading. We report here the first far-infrared (FIR) coupled to mid-infrared (MIR) kinetics study of the hydration mechanism in benchmark perfluorinated sulfonic acid (PFSA) membranes, e.g., Nafion. The hydration process was followed in situ, starting from a well-prepared dry state, within unprecedented continuous control of the relative humidity. A step-by-step mechanism involving two hydration thresholds, at respectively λ = 1 and λ = 3 water molecules per ionic group, is assessed. The molecular environment of water molecules, protonic species, and polar groups are thoroughly described along the various states of the polymer membrane, i.e., dry (λ ≈ 0), fully ionized (λ = 1), interacting (λ = 1−3), and H-bonded (λ > 3). This unique extended set of IR data provides a comprehensive picture of the complex chemical transformations upon loading water into proton-conducting membranes, giving insights into the state of confined water in charged nanochannels and its role in driving key functional properties as ionic conduction.

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