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

The fundamental, angstrofluidic features of confined water and corresponding observations of hydrophobicity in microporous materials are dictated by molecular and continuum level phenomena, directed by surface and nanostructural interactions and characterized by isosteric heat of adsorption and micropore size. These features are critical to addressing the water-energy nexus and are foundational to a wide range of technologies involving biological or energy storage applications. However basic metrics for assessment of these features are currently absent from fundamental analyses. Proposed herein is a novel metric that is demonstrably capable of relating hydrophobic and hydrophilic isotherm types. It is coherently coupled with characterization methodology which not only circumvents difficulties inherent in other analyses but also partitions surface from structural hydrophobicity and is uniquely delivered in a simple, rigorous, analytic form. Moreover, transitions from hydrophobic to hydrophilic behavior are uniquely captured by unified isotherm analysis which explicitly links nine distinct adsorption equilibrium models. Subsequent simplification for characterization of water adsorption in 26 ostensibly hydrophobic, microporous, Metal-Organic Framework (MOF) materials yields Ising-Model-Modified-Kelvin-Analysis (IMMKA). Embedded in the analysis are concepts of nanocapillarity and nanowetting which directly deliver estimates of contact angle and isosteric heat that are consistent with independent assessments. The findings herein broadly offer means to successfully capture interactions that underpin hydrophobic observations, discriminate foundational features of confined water and advance fundamental descriptions of angstrofluidic phenomena.