Impact of surface hydrophobicity and ion steric effects on the electroosmotic flow and ion selectivity of a conical nanopore

The electroosmotic flow (EOF) and ion transport within a hydrophobic conical nanopore connecting reservoirs are studied numerically. The ion transport equation is modified to consider the ion steric effect for a finite ion size. The continuum-based model compiled with the modified Poisson-Nernst-Planck and Navier-Stokes equations are adopted for analyzing the hydrodynamics and electric field. The governing equations in their full form are solved in a coupled manner through a control volume approach over a nonuniform staggered grid arrangement. Analytic solutions for the EOF in the hydrophobic nanopore with large membrane thickness is obtained by including the volume exclusion effects in the electrochemical potential of the ionic species. The analytic solutions are obtained by considering the steric interactions based on either the Bikerman model or the Carnahan Starling (CS) model. However, the numerical solutions are presented based on the CS model for steric interactions. The present numerical results for the hydrophobic nanopore with large pore length are validated with these analytic solutions and existing correlations based on experimental data. The fluid slip at the pore surface modifies the nanopores electrokinetic characteristics by modifying the convective transport of fluid and ions. We have elucidated the impact of pore hydrophobicity and ion steric interactions on the EOF, ionic current, ion concentration polarization (ICP) and ion selectivity when the pore length is considered to be in the order of the pore radius. The effect of pore length on the electrokinetics in the hydrophobic nanopore is also analyzed. Our numerical solution for large pore length merged with the analytic expression for the EOF in the hydrophobic nanopore of length much higher than the radius. Hydrophobicity creates significant enhancement in EOF and the rate of EOF augmentation is higher for the thinner Debye length. The perm selectivity performance of the nanopore is enhanced when the wall is considered to be hydrophobic. The steric interactions of ions create an enhancement in EOF, ion selectivity, and ICP. The pore hydrophobicity diminishes the impact of these steric interactions. (c) 2021 Elsevier Inc. All rights reserved.

Automated summary

This study numerically investigates the electroosmotic flow and ion transport within a hydrophobic conical nanopore, taking into account ion steric effects and fluid slip at the pore surface. The results show that the pore hydrophobicity enhances the electroosmotic flow, while ion steric interactions can enhance EOF, ion selectivity, and ion concentration polarization.