Electrostatic-driven dehydration of ions in nanoporous membranes.

Surface charge critically affects ion-selective membrane performance, particularly in separating ions with similar size and charge, the key challenge in water treatment. Herein, we investigate the permeation of alkali chlorides (LiCl, KCl, and CsCl) through steric hindrance-free nanoporous membranes with tunable surface charge densities. Supported by molecular dynamics simulations, we confirm that electrostatic effects promote the dehydration of Cl, the counterions to the membrane charge, at the positively charged membrane surface. This dehydration leads to a great tendency of Cl to absorb to the membrane surface and be retained, compromises Cl partitioning, and impedes salt cotransport. For negatively charged membranes, Cs with its lower hydration energy undergoes greater electrostatic-driven dehydration and partition hindrance than K, resulting in selective KCl transport. Our findings provide both theoretical and experimental proofs of ionic dehydration and transport impediments driven by electrostatic interactions at charged membrane surfaces, presenting an in-depth perspective for designing ion-selective membranes to separate similar ions based on charge effects.