Concentration-Driven Disruption of Single-File Water

Strong hydrogen bonding is known to entail some of the spectacular physical properties of liquid water, including fast diffusion under nanoconfinement. Similar to biological channels, the single-file or collective motion of interconnected water molecules has been observed in nanotubes and laminar structures, exhibiting tremendous potential for energy-efficient separation applications. Desalination, breaking azeotropes, and dehumidification have been all addressed with the membranes enabling selective transport of water while most attention has been paid to the fabrication and morphological characteristics of the respective microporous materials. However, the performance of membrane processes also depends on the properties of the chemical systems to be treated, often facing problems under realistic conditions such as concentration polarization. In this study, adsorption controlled permeation is employed to explore the interfacial behavior of water-alcohol mixtures in nanostructured membranes as a function of concentration. The permeation rate of water is found to sink manifold as the molar fraction of isopropanol molecules increases, indicating breakdown of the single-file mechanism. A phenomenological model is devised to account for intermolecular interactions in the binary liquid-liquid mixture whereas kinetic simulations agree well with the experimental data. The results point to the fundamental limitations of water-selective conduits for dehydrating organic solvents.