Analysis of Hydrophobicity in Micropores of Metal-Organic Framework Materials

Abstract

Water molecules in hydrophobic micropores of Metal Organic Framework (MOF) materials show disparate behavior in adsorption equilibrium. Ranging from behavior indicative of large, Type V uptakes occurring at low relative pressure, to Type III equilibrium where little water is adsorbed, this phenomenon provides a challenge for a priori prediction. In an effort to elucidate adsorption mechanisms, this study advances a quantitative description of hydrophobicity through novel application of Ising model analysis (IMA). It is applied to scrutinize hydrophobicity indicated by Type III adsorption and displayed by micropores that are too large to induce a Type V water adsorption equilibrium. Two types of hydrophobicity are identified and partitioned into structural hydrophobicity and surface hydrophobicity. Additionally, through analysis of a data set for 25 materials, the utility of the concepts of nanocapillarity and nanowetting in a modified Kelvin analysis (MKA) are established for water in MOF systems. The method and analysis are applied to these phenomena in MOF micropores and could foreseeably aid in screening studies, be used to assess the presence of cooperative, wetting, and condensation-like mechanisms and be further employed to assess hydrophobicity in other microporous materials.