Evaluation of Hydrophobic, Hydrophilic, and Water Adsorption Properties of Microporous Metal–Organic Framework Materials by Unified Isotherm Analysis

Abstract

Global challenges posed by freshwater scarcity and the water–energy nexus drive demand for novel macromolecular design of tailored nanostructures endowed with a variety of hydrophilic and hydrophobic features. Offering potential to meet this demand, metal–organic framework (MOF) materials are synthesized from coordinated formations that create versatile reticular structures with variable water adsorption affinities. However, advances in the fundamental understanding of water interactions within these structures are impeded by the failure of classical analyses to identify mechanisms of interaction, connect fundamental isotherm types, and provide appropriate benchmarks for assessment. Proposed herein is a novel definition of hydrophobicity that is coherently coupled with a unified isotherm analysis to connect a wide array of rigorous equilibrium isotherm types bounded by asymptotic limits dictated by hydrophilic rectangular Type I and hydrophobic stepwise Type V (unimodal Type VI). Moreover, distinct forms of hydrophobicity, associated benchmarks and discrete classes of material behavior are introduced to characterize hydrophobic to hydrophilic transitions. Simplification of the analysis for application to microporous MOF materials displaying nominally stepwise water adsorption isotherms yields Ising-Model-Modified-Kelvin-Analysis (IMMKA) that is delivered in a simple, rigorous, analytic form. Mechanisms of pore filling are characterized, allowing verification of widely accepted protocols and “rules of thumb” for the prediction of micropore filling pressures. The analysis successfully captures the foundational features of water in extreme confinement, including the role of pore morphology in directing the molecular and fluidic interactions that underpin the pseudocondensation mechanism of water in micropores.