Evaluation of Hydrophobicity and Angstrofluidic Properties of Water in Microporous Metal-Organic Framework Materials by Unified Isotherm Analysis

The fundamental, angstrofluidic features of confined water and corresponding observations of hydrophobicity or hydrophilicity 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 the features of confined water are currently absent from fundamental analyses. Proposed herein are novel metrics that are demonstrably capable of relating hydrophobic and hydrophilic isotherm types beyond the problematic classic definition of hydrophobicity. They are 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, a continuous transition from hydrophobic to hydrophilic behavior is uniquely captured by simplification of unified isotherm analysis which also explicitly links nine distinct adsorption equilibrium models. Subsequent application for characterization of water adsorption in 26 ostensibly hydrophobic, microporous, Metal-Organic Framework (MOF) materials demonstrates the utility of 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 analysis proves capable of verifying some widely accepted protocols for nominally stepwise isotherms and “rules of thumb” involving inflection point locations which are predominantly determined by the angstrofluidic properties of water. The novel deliverables of this study broadly offer means to successfully capture interactions that underpin hydrophobic observations, discriminate foundational features of confined water and advance fundamental descriptions of angstrofluidic phenomena.