Ion-selective separation in charge-modified cation-anion dual-channel nanofluidic systems: a molecular dynamics study

In this work, molecular dynamics simulations were employed to systematically investigate the ion transport performance and the underlying permeation mechanisms in charge-modified dual-channel systems. The results demonstrate that the charge-modified dual-channel design-by offering distinct pathways for cations and anions-achieves effective ion separation and outperforms both unmodified nanochannels and conventional electrodialysis. The system achieves a Li⁺/Mg²⁺ separation factor of approximately 3.0 at 0.1 V nm^-1, coupled with high Li⁺ flux and superior selectivity. The marked difference in ion transport rates and fluxes between the channel interior and the external solution leads to Mg²⁺ accumulation and increased ion association near channel entrances. Compared to unmodified nanochannels, surface charge modification significantly reduces this ion enrichment at the channel entrance, particularly for Li⁺. By directing cations and anions through separate channels, surface charge modification mitigates ion association, thereby enhancing overall ion permeation. Concurrently, it suppresses Mg²⁺ transport, thereby enhancing the Li⁺/Mg²⁺ selectivity. Overall, this work elucidates the microscopic mechanisms by which surface charge patterning regulates near-entrance ion adsorption and electrostatic interactions to achieve highly efficient selective ion transport, providing theoretical guidance for designing high-performance ion-separation membranes for lithium extraction from brines.