Boost ionic selectivity by coating bullet-shaped nanochannels with dense polyelectrolyte brushes

The influence of channel geometry on the ionic selectivity and ionic current rectification of soft nanochannels was numerically investigated. The nanochannels coated with polyelectrolyte layers (PELs) are termed as soft nanochannels. The asymmetric category of nanochannels, i.e., bullet-shaped, was considered in this study. When PEL is dense, the ionic partitioning effect cannot be ignored. To this end, through adopting a numerical approach using the finite element method, Poisson–Nernst–Planck and Navier–Stokes equations were solved at steady-state conditions by considering different values of permittivity, diffusivity, and dynamic viscosity for the PEL and the electrolyte. The results show that the PEL–electrolyte property difference leads to a significant improvement of the rectification behavior, especially at low and moderate salt concentrations. This not only highlights the importance of considering different properties for the PEL and the electrolyte but also implies that the rectification behavior of soft nanochannels/nanopores may be improved considerably by utilizing denser PELs. Considering a charge density of 80 mol/m3 and a bulk concentration of 20 mM, we demonstrate that the rectification factors for the bullet nanochannels, from 3.35 by ignoring the ion partitioning effect, can reach the values of 4.88 by considering the ion partitioning effect, respectively.