Influence of osmosis, screening charges, and electrical neutrality of nanopores on object translocation

It has been known for a long time that osmosis transfers water from low to high salt concentrations, but few simulation studies have been conducted in confined spaces. This study evaluated the osmotic axial pressure at charged disk surfaces within charged cylindrical nanopores, how osmosis influences other axial pressures, and what controls it. The screening charges on the disk surfaces determine the osmotic axial pressures and critically influence the other axial pressures. External factors (electric field and ion concentrations) and internal (disk and pore wall charges) control the screening charges. The external electric field influenced the Coulomb and dielectric axial pressures more by altering the disk screening charges than by acting directly on the fixed charges. In contrast, the pore wall fixed charges influenced the Coulomb and dielectric axial pressures exclusively by altering the disk screening charges, and their influence extended into the pore center (i.e., well beyond the length of the pore wall diffuse double layer). Both pressures increased near the pore center, especially for larger disks, indicating greater screening charge accumulation around larger disks. The axial osmotic pressure was constant along the disk surface, almost to the tip, and was disk size-independent, owing to the radially constant and disk size-independent screening charges at the disk surfaces. Finally, the axial osmotic pressure at the disk surfaces (with dielectric and fluidic pressures) typically counter-balanced the Coulomb pressure that drives the disk translocation through the nanopore, reducing its speed. The disk translocation direction may reverse at low external ion concentrations.