From entrance pairing and interfacial clustering to collective diffusion: structural origins of ion transport in angstrom-scale graphene slits

Abstract

When confinement approaches the scale of hydration shells, water and ions can cease to behave as continuous media and instead assemble into discrete, cooperative motifs. Experiments using graphene and other atomically thin channels have revealed quantized conductance, inverted selectivity, and nonlinear ionic responses—signatures of transport governed by molecular geometry and interfacial correlations rather than continuum electrostatics. However, the microscopic principles linking local solvation structure to collective ionic motion remain poorly resolved. Here, molecular dynamics simulations of aqueous electrolytes confined within sub-nanometre graphene slits were performed to uncover the structural origins of angstrom-scale transport, from cation–anion pairing at the channel entrance to collective interfacial cluster diffusion inside the slits. At the entrance, ion-pair formation lowers the effective entry barrier by mitigating electrostatic repulsion from surface charges while reducing desolvation penalties through shared hydration shells. Once inside, ions reorganise into dynamic cation–anion clusters whose topology and size govern their cooperative diffusivity, giving rise to enhanced transport compared with bulk electrolytes. These results reveal a continuous mechanistic pathway—from entrance pairing and interfacial clustering, to in-slit collective motion—that bridges molecular solvation geometry with emergent transport behaviour. By tracing ionic mobility back to its structural determinants, this work could advance our understanding and engineering of ionic and molecular flows in angstrom-scale channels.