Effect of charge density distribution of polyelectrolyte layer on electroosmotic flow and ion selectivity in a conical soft nanochannel

Abstract

Electroosmotic flow (EOF) and ion transport through a conical nanochannel coated with polyelectrolyte layer (PEL) are numerically investigated. The diffuse character is defined by an ununiform distribution of the polymer segment density. Soft step (Type I), exponential (Type II), and sigmoidal (Type III) functions were considered to describe the charge density distribution of PEL. A nonlinear model based on the Poisson-Nernst-Planck and the Navier–Stokes equations is used. The equations were solved using the finite element method. At applied voltages of ±1V, the electroosmotic velocity within diffuse soft nanochannels follows the order of uTypeI>uTypeII>uTypeIII. The soft step function leads to more ion selectivity. Considering a charge density of 100 mol/m3 and a salt concentration of 100 mM, we demonstrate that the rectification factors for the conical nanochannels, from 3 by considering the uniform PEL, can reach the value of 18 by considering the soft step function for PEL.