Counterion Blockade\nin a Heterogeneously Charged Single-File\nWater Channel

The Possion–Nernst–Planck theories fail\nto describe\nthe ionic transport in Angstrom channels, where conduction deviates\nfrom Ohm’s law, which is attributed to the dehydration/self-energy\nbarrier and dissociation of Bjerrum ion pairs in previous work. Here,\nwe find that the cations can be strongly bound to the surface charge,\nwhich blocks the ionic transport in a single-file water channel, causing\nnonlinear current–voltage curves. The presence of free ions\nsignificantly increases the probability of bound ions being released,\nresulting in an ionic current. We find that ionic conduction gradually\nbecomes Ohmic as the surface charge density increases, but the conduction\namplitude decreases due to the increased friction from the bound ions.\nWe rationalize the ionic transport using 1D Kramers’ escape\ntheory framework, which describes nonlinear ionic current and the\nimpact of surface charge density on the <i>I</i>–<i>V</i> curves. Our results show that the strong Coulomb interaction\nbetween the counterion and surface charge may cause ionic blockade\nin Angstrom channels.