Transport of multicomponent, multivalent electrolyte solutions across nanocapillaries

Abstract

Electrokinetic transport of aqueous solutions containing multiple ionic species in surface charge governed nanofluidic flows has seen limited investigation with most experimental and modeling efforts emphasizing symmetric, monovalent electrolytes. In this work, numerical models coupling steady-state Poisson–Nernst–Planck and Stokes equations along with experimental investigations were developed to characterize electrokinetic transport of potassium phosphate buffer, containing K⁺, H₂PO₄ ⁻, and HPO₄ ²⁻ across positively charged nanocapillary array membranes with 10 nm diameter nanocapillaries, sandwiched between a source and permeate reservoir. While systematically increasing phosphate buffer concentration from 0.2 to 10 mM, 0.14 mM of methylene blue (MB) dye was added to the source reservoir to study the dominating transport mechanism under a potential bias (0–0.75 V). Experiments provided validation of numerical results that elucidate fundamental transport mechanisms as a function of ion type, buffer concentration, and externally applied potential. The nanocapillary exhibits permselectivity toward anions at lower buffer concentrations (0.2, 1 mM) and was more selective for HPO₄ ²⁻ in comparison with H₂PO₄ ⁻. Transport of K⁺, H₂PO₄ ⁻, and HPO₄ ²⁻ was dominated by electromigration, with negligible effects of diffusion and convection at all buffer concentrations. However, transport of MB⁺ was dominated by diffusion at 0.2 mM buffer concentration under all potential bias conditions. Significant effects of electromigration appeared at high potential biases (0.5–0.75 V) for 1 and 10 mM bulk buffer concentrations. Additionally, in the multicomponent ion system, at all concentrations, the vast majority of the current was carried by the phosphate buffer ions and not the MB ions.