Electrochemical Detection of Selective Anion Transport through Subnanopores in Liquid-Crystalline Water Treatment Membranes

Abstract

The anion-selective transport through subnanoporous liquidcrystalline (LC) water treatment membranes was quantitatively detected by the deposition and electrochemical analysis of the LC membrane on the GaN electrode. The time course of the capacitance and Warburg resistance of the LC membrane suggest that the interaction of the LC membrane with monovalent Cl − ions is distinctly different from that with SO 4 2− ions. A continuous decay in capacitance suggests the condensation of Cl − ions in subnanopores, whereas the interaction between SO 4 2− ions and the inner wall of subnanopores is much weaker. The chronoamperometry data further suggest that SO 4 2− ions are transported through subnanoporous channels 10 times faster than Cl − ions. These results, together with the previous X-ray emission spectroscopy, suggest that SO 4 2− ions, which possess similar hydrogen-bonded structures to the hydrogen-bonded networks inside the subnanopores, can exchange the associated water molecules and hop along the network of water molecules, but Cl − ions bind and accumulate inside subnanopores. The well-controlled supramolecular selfassembly of LC building blocks opens a large potential toward the fine adjustment of hydrogen-bonding networks in nanospace providing materials new functions, which cannot be realized by bulk water.