Analysis of Hydrophobicity in Micropores of Metal-Organic Framework Materials

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 surface and structural hydrophobicity, with quantitative assessment delivered by analysis of a data set covering 26 materials. Application of a modified Kelvin analysis (MKA) indicates that bulk water properties and macroscopic contact angles can characterize the equilibrium behavior of water in molecular level micropores of nanostructured MOF materials. Additionally, the concepts of nanocapillarity and nanowetting are established and prove capable of further characterizing the hydrophobicity of these materials. Finally, the methods and analyses applied to these phenomena in MOF micropores could foreseeably aid in screening studies, be used to assess the presence of cooperative, wetting, and condensation-like mechanisms and be further employed to estimate contact angles and scrutinize phenomena associated with hydrophobicity in other microporous, nanostructured materials.