Impacts of the shape of soft nanochannels on their ion selectivity and current rectification

Abstract

The influence of channel geometry on the ion selectivity and ionic current rectification (ICR) of soft nanochannels was numerically investigated. The nanochannels coated with polyelectrolyte layers (PELs) are termed as soft nanochannels. Two categories of channels, asymmetric (trumpet, conical, bullet, and cigar-like) and symmetric (cylindrical), were considered in this study. When PEL is dense, the ionic partitioning effect can not be ignored. To this end, through adopting a numerical approach using the finite element method (FEM), 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 model findings were verified by comparing them with the existing simulation data. For the cylindrical type nanochannels, the results indicate that the rectification factor is approximately equal to unity, and the ion selectivity at positive voltages is approximately equal to that at negative voltages. For other geometries, the ion selectivity factor of the channel was STrumpet>SConical>SBullet>SCigar. On the other hand, at low bulk concentrations, the rectification factor was RfCigar>RfBullet>RfConical>RfTrumpet, while at high bulk concentrations, it was RfBullet>RfCigar>RfConical>RfTrumpet. Considering a charge density of 80mol/m3 and a bulk concentration of 20mM, we demonstrate that the rectification factors for the bullet and trumpet types nanochannels, from 3.35and0.56 by ignoring the ion partitioning effect, can reach the values of 5.78 and 0.83 by considering the ion partitioning effect, respectively.