Atomistic insights into the effects of carbonyl oxygens in functionalized graphene nanopores on Ca²⁺/Na⁺ sieving

Residual Ca²⁺ decreases the efficiency and increases the power consumption of the chlor-alkali industry. However, Ca²⁺ and Na⁺ sieving is challenging due to the similar ionic radii of these cations. Inspired by the presence of carbonyl oxygens in key selective filters of biological Ca²⁺ and Na⁺ channels, we used molecular dynamics to investigate the effects of carbonyl oxygen atoms in modified graphene nanopores of various sizes (characteristic diameters: 0.57–1.50 nm) on Ca²⁺/Na⁺ sieving. The results demonstrated that selectivity is closely associated with the different roles of the carbonyl oxygen atoms. In small nanopores, Ca²⁺ sheds increased numbers of water molecules due to the predominant steric effect of carbonyl oxygen atoms. Thus, Ca²⁺ must overcome a higher energy barrier than Na⁺. This requirement prevents the passage of Ca²⁺. In large nanopores, carbonyl oxygen atoms do preferentially substitute water molecules outside the first hydration shell of Ca²⁺ compared with those outside the first hydration shell of Na⁺, thereby hindering Na⁺ departure from the nanopore. These findings provide useful guidance for the further development of Ca²⁺ separation materials as sensors and ion separators.