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

Fundamental understanding of water adsorption mechanisms in microporous materials is challenged by disparate scales of confinement in spacings of molecular dimensions. Spanning both continuum and molecular levels, classical definitions of hydrophilicity and hydrophobicity reach inherent limitations under extreme confinement. Replacing these classical definitions requires merging discrete clustering and condensation phenomena, both of which are governed by distinct analyses that appear incapable of unification. This creates a dearth of robust quantitative tools for fundamental assessment. Currently, there is a critical need for benchmarks that are capable of unifying disparate length scales, equilibrium isotherm types and contact angle assessment. To this end, the study herein provides a unified model to explain wide ranging hydrophilic and hydrophobic 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. Application of the analysis to structurally realistic micropores of functionalized and non-functionalized graphitic carbons and nanotubes produces assessments comparable with equivalent microporous metal organic framework (MOF) materials. The analysis successfully connects classical hydrophobic and hydrophilic features at both the molecular and continuum levels, 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, extraction of fundamental angstrofluidic features permits elucidation of conditions for clustering and condensation, prewetting and wetting, as well as complete or incomplete micropore filling. Through elucidation of angstrofluidic, hydrophilic and hydrophobic features of water in carbon, this study resolves the perplexing and apparently contradictory behavior of water interaction at molecular and continuum levels.