Selective water transport through glycine-functionalized heptazine nanopores: A molecular dynamics study

Achieving high water permeability alongside strong ion rejection in ultrathin nanoporous membranes remains a central challenge for next-generation desalination. Here, we use pressure-driven nonequilibrium molecular dynamics to examine a glycine-functionalized heptazine membrane whose pore-edge modification reduces the effective aperture to approximately 5 Å. Under a 1500 atm transmembrane load, the simulations show sustained water permeation ( k H 2 O = 21.4 ± 0.7 ns −1 ) with no Na + or Cl − crossing events during 10 ns of production. This observation is reported as a finite-time lower bound on ion exclusion rather than as proof of absolute rejection over experimental timescales. Spatial density and occupancy analyses reveal that water is funneled through a localized pore-centered corridor while ions are excluded from the membrane interior, accumulating on the feed side in a concentration-polarization layer ( C ∕ C bulk ≈45). Confinement within the glycine-modified channel suppresses hydrogen-bond connectivity ( n HB drops from ∼3.5 to ∼0.7), induces layered interfacial hydration, and imposes a modest orientational bias. Occupancy-derived profiles, obtained by a formal Boltzmann-like transformation of the observed nonequilibrium density, indicate a modest relative penalty for water entry (∼+1.3 mol −1 ) but a dramatically larger effective penalty for ions (≫10 mol −1 ), reflecting combined steric narrowing and hydration-mediated exclusion. These results demonstrate that glycine functionalization transforms a wider heptazine pore into a hydration-sensitive selective nanochannel, supporting the design principle that pore chemistry and pore geometry must be engineered together.