  Partitioning and transport of water and small solutes into and through
nanopores is important to a variety of chemical and biological processes and
applications. Here we study the partitioning of positive ions of increasing
size into the pore interior using extensive molecular dynamics simulations.
Despite the simplicity of the simulation system, the results provide new and
useful insights on both the pore hydration and ion partitioning. For low
negative charge densities on the pore, the driving force for partitioning of
positive ions into the pore is weak, and no partitioning is observed.
Increasing the pore charge gradually leads to partitioning of positive ions
into the pore. Interestingly, over a range of intermediate negative charge
densities, nanopores display both thermodynamic as well as kinetic selectivity
toward partitioning of the larger K+ and Cs+ ions into their interior over the
smaller Na+ ions. The variation of thermodynamic driving force and the average
partitioning time with the pore charge density together suggest the presence of
free energy barriers in the partitioning process. We discuss the role of ion
hydration in the bulk and in the pore interior as well as of the pore hydration
in determining the barrier heights for ion partitioning and the observed
thermodynamic and kinetic selectivity.
