Impact of asymmetry soft layers and ion partitioning on ionic current rectification in bipolar nanochannels

Abstract

When both positive and negative charges are present at the nanochannel surface, the system is called a bipolar nanochannel, or bipolar diode. Bipolar nanochannels have a much higher degree of rectification than unipolar nanochannels with asymmetric geometries because, in the backward mode, pass on an ionic current close to zero. In modeling bipolar soft nanochannels, it is usually assumed that the properties of the soft layer (PEL) and electrolyte are the same, which is not valid for soft layers with high charge densities. In the present work, the effect of considering different properties for the soft layer and electrolyte on the ion current rectification in cylindrical bipolar soft nanochannels was studied. To this end, by utilizing a numerical computational procedure using the finite element method (FEM), Poisson-Nernst-Planck (PNP) and Navier-Stokes (NS) equations were solved for steady-state conditions by considering different permittivities, diffusion coefficients, and dynamic viscosities for PEL and electrolyte. The model findings, which were verified through comparing with the existing theoretical data, revealed that considering differences in the properties of the soft layer and electrolyte led to a significant improvement in the rectification factor. For instance, at the bulk concentration of c0=10mM, the rectification factor of 695 for a bipolar cylindrical soft nanochannel at ηε=ημ=1,ηD=2 reached a value of 13430 at ηε=ηD=0.6,ημ=2. For a similar system, at a bulk concentration of c0=1mM and a symmetric soft layer Rs,p=Rs,n=5nm the value of Rf was 1822, and it reached a value of nearly twice, i.e. 3535, where an asymmetric soft layer Rs,p=3nm<Rs,n=7nm was applied.