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

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.