Exploring Impacts of Surface Coatings to Modify Water Uptake and Selectivity within Metal–Organic Nanotubes

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.