Simple Model for Hydrophilic, Hydrophobic, and Angstrofluidic Features of Water Confined in Micropores: Effects of Pore Size and Surface Functionality on Mapping of Equilibrium Isotherm Types

Hydrophilic and hydrophobic properties exhibited by microporous materials are foundational to a wide range of mature and emerging technologies. However, robust descriptions of fundamental underpinnings are confounded by conditions of extreme confinement intrinsic to pores of molecular dimensions. Inherent limitations imposed by the failure to capture multiscale behavior of water create a dearth of simplified benchmarks for characterization. Broad analytical assessments that are capable of bridging disparate length scales, unifying equilibrium isotherm types, and identifying interaction mechanisms remain elusive. To this end, the study herein provides a unified model and benchmark to explain wide-ranging observations of water in carbon micropores. Based on Ising-Model-Modified-Kelvin analysis (IMMKA), a set of simple, analytic, governing relations are offered to quantify water adsorption equilibrium. The analysis successfully connects classical hydrophobic and hydrophilic features with isotherm types under stringent conditions of chemical and mechanical equilibrium. Affinity- and hydrophobicity-transforming effects of micropore size and functional site density are captured by the analysis that is vetted by blind and global isotherm prediction. Moreover, the fluidic features of water in angstrom-sized pores (angstrofluidic features) are partitioned and characterized by application to slit pores and nanotubes. Furthermore, extraction of fundamental equilibrium properties permits consideration of conditions for clustering and condensation, prewetting and wetting, and complete or incomplete micropore filling. Through elucidation of angstrofluidic, hydrophilic, and hydrophobic features, this study resolves the perplexing nature of the interaction of water with microporous carbon.