Evaluation of Hydrophobicity in Micropores of Nanostructured Metal-Organic Framework Materials from a Unified Isotherm Analysis

Hydrophobic micropores of Metal-Organic-Framework (MOF) materials show disparate behavior in water adsorption equilibrium with observations ranging from negligible, Type III to large, stepwise, Type V uptakes. Elementary understanding of this phenomenon is confounded by the lack of analytical descriptions broadly capable of elucidating the fundamental nature of hydrophobicity in nanoporous structures. This is exemplified by the absence of robust, quantitative hydrophobicity metrics that can be applied to rigorously assess the variety of observed adsorption equilibrium types including challenging, stepwise isotherms. By partitioning into surface and structural hydrophobicity, the analysis herein provides a novel fundamental metric which is shown to be capable of dictating multiple isotherm types, ranging from hydrophilic to hydrophobic. Assessment of the mathematical limits of this metric yields a simple, yet rigorous, characterization methodology which is applied to 26 ostensibly hydrophobic, microporous, MOF materials. The resulting Modified-Kelvin-Analysis (MKA) that is based on bulk fluid properties and accounts for nanocapillarity and nanowetting, is demonstrably capable of characterizing the equilibrium behavior of water in molecular level micropores. Subsequent thermodynamic assessment of the results yields validation of the proposed relationships and interpretation. Furthermore, this investigation uniquely delivers a unified isotherm analysis framework which can quantitatively relate hydrophilic and hydrophobic isotherms as well as unite nine distinct, fundamental models describing adsorption equilibrium. Finally, the methods and analyses applied to these phenomena in MOF micropores could foreseeably aid computational studies, probe surface wetting, assess contact angles and scrutinize phenomena associated with hydrophobicity in other microporous, nanostructured materials.