Ion transport and dehydration in sub-nanoscale pores

Ions in solution develop tightly bound layers of water -- hydration layers -- which stabilize disassociation and enable ionic currents to flow. Sub-nanometer pores in a membrane enable ions to pass provided that they shed their hydration shell. This process has an associated large energy penalty that is predicted to give rise to "quantized" steps in the ionic conductance.\footnote{ Zwolak, M., Lagerqvist, J. \& Di~Ventra, M. Quantized ionic conductance in nanopores. \emph{Phys. Rev. Lett.} \textbf{103}, 128102 (2009). }\textsuperscript{,}\footnote{ Zwolak, M., Wilson, J. \& Di~Ventra, M. Dehydration and ionic conductance quantization in nanopores. \emph{J. Phys.: Condens. Matter} \textbf{22}, 454126 (2010).} Using all-atom molecular dynamics simulation, we demonstrate that the ionic current begins to show nonlinear behavior as the radius of the pore is reduced to the sub-nanometer scale. This nonlinear behavior is seen as a sharp rise in the pore resistance and excess noise in the current. Our work sheds light on basic mechanism of ion transport through sub-nanoscale pores.