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Abstract Mechanisms of uptake in metal–organic materials are complex and are dependent on the chemistry of the pore space and material interface. In the current study, the importance of the material surface is evaluated on the water uptake of a metal–organic nanotube (UMONT) crystalline solid. This material has previously demonstrated selective water uptake and reported isotherms suggested a two‐step adsorption process that involved initial surface adsorption followed by pore filling. The proposed mechanism and importance of surface chemistry for water adsorption are tested by altering the surface of the UMONT with more hydrophobic surface coatings. Crystals of UMONT are coated with ammonium trifluoroacetate (ATFA), polyvinylidene fluoride (PVDF), and polyacrylonitrile (PAN), and the water adsorption behavior is analyzed through batch and flow‐through experiments. Uptake experiments reveal that ATFA significantly decreased the water uptake compared to observed in pristine UMONT while polymer coatings do not impact the adsorption behavior as significantly. In addition, ATFA disrupts the water selectivity of the UMONT material, allowing both ethanol and methanol to be detected in the system. These results indicate that changing the surface layer from a hydrophilic to hydrophobic with a chemisorbed monolayer will disturb the two‐step mechanism and the water uptake properties of the material. 

Abstract Hybrid materials, such as metal organic nanotubes (MONTs) can possess nanoconfined water molecules within their pore space and the overall behavior of the water within the material may be tuned based upon interactions with the inner channel walls. We have previously developed a range of methods (electron density mapping, kinetic models, and water interaction enthalpies) to evaluate water behavior under nanoconfinement using a uranium‐based metal organic nanotube (UMONT) but have not explored their applicability across a range of materials. In the current study, we test our methodologies on two additional MONT materials (LaMONTandCu‐LaMONT) to determine if the techniques can be utilized in other systems to predict behavior within complex hybrid materials. In addition, we explored how to use Hirshfeld surface maps generated by the CrystalExplorer software in the visualization and prediction of water behavior within complex hybrid materials. 

Abstract Water‐mediated proton conductivity in nanoporous materials is influenced by channel water ordering and the hydrophobicity/hydrophilicity of interior walls, making metal‐organic nanotubes (MONTs) useful systems for exploring these relationships due to their high crystallinity and tunable hydrophobicity. In the current study, electrochemical impedance spectroscopy is utilized to explore the proton conductivity on two metal organic nanotubes (UMONT and Cu‐LaMONT) with weak hydrophobic behavior that possess extended water networks within the 1‐D channels. Measurements performed at 95% RH and 20 °C indicate values of 1.63 × 10⁻⁴S cm⁻¹for UMONT and 3.80 × 10⁻⁴S cm⁻¹for Cu‐LaMONT, which is lower than values for walls with acidic, hydrophilic functional groups or nanotubular materials with strictly hydrophobic behavior. Proton conductivity decreases sharply with lower humidity, with Cu‐LaMONT being more sensitive to humidity changes. At low temperatures, UMONT outperforms LaMONT due to its well‐established hydrogen bonding network and hydrophobic interior. The anisotropic nature of proton conduction is also confirmed through pelletized powder sample analysis, emphasizing that the conductivity occurs through the water networks located within the 1‐D MONT channels. These findings emphasize the importance of understanding water–pore interactions and the resulting proton conductivity mechanisms to understand complex systems and design advanced materials. 

Through a combination of many analytical approaches, we show that a metal organic nanotube (UMON) displays selectivity for H 2 O over all types of heavy water (D 2 O, HDO, HTO). Water adsorption experiments combined with vibrational and radiochemical analyses reveal significant differences in uptake and suggest that surface adsorption processes may be a key driver in water uptake for this material.