  The ability of semipermeable membranes to selectively impede the transport of
undesirable solutes is key to many applications. Yet, obtaining a systematic
understanding of how membrane structure affects selectivity remains elusive due
to the insufficient spatiotemporal resolution of existing experimental
techniques, and the inaccessibility of relevant solute transport timescales to
conventional molecular simulations. Here, we utilize jumpy forward-flux
sampling to probe the transport of sodium and chloride ions through a graphitic
membrane with sub-nm pores. We find chlorides to traverse the pore at rates
over two orders of magnitude faster than sodiums. We also identify two major
impediments to the transport of both ion types. In addition to the partial
dehydration of the leading ion, its traversal induces charge anisotropy at its
rear, which exerts a net restraining force on the ion. Charge anisotropy is
therefore a crucial hidden variable controlling the kinetics of ion transport
through nanopores.
