Mg²⁺-Channel-Inspired Nanopores for Mg²⁺/Li⁺ Separation: The Effect of Coordination on the Ionic Hydration Microstructures

The separation behaviors of Mg²⁺ and Li⁺ were investigated using molecular dynamics. Two functionalized graphene nanopore models (i.e., co-5 and coo-5) inspired by the characteristic structural features of Mg²⁺ channels were used. Both nanopores exhibited a higher preference to Mg²⁺ than to Li⁺, and the selectivity ratios were higher for coo-5 than for co-5 under all the studied transmembrane voltages. An evaluation of the effect of coordination on the ionic hydration microstructures for both nanopores showed that the positioning of the modified groups could better fit a hydrated Mg²⁺ than a hydrated Li⁺, as if Mg²⁺ was not dehydrated according to hydrogen bond analysis of the ionic hydration shells. This condition led to a lower resistance for Mg²⁺ than for Li⁺ when traveling through the nanopores. Moreover, a distinct increase in hydrogen bonds occurred with coo-5 compared with co-5 for hydrated Li⁺, which made it more difficult for Li⁺ to pass through coo-5. Thus, a higher Mg²⁺/Li⁺ selectivity was found in for coo-5 than for co-5. These findings provide some design principles for developing artificial Mg²⁺ channels, which have potential applications as Mg²⁺ sensors and novel devices for Mg²⁺/Li⁺ separation.