Ionic dispersion in nanofluidics

Abstract

An analytical solution for dispersion of ionic and neutral solutes in nanoscale channels is presented. Results suggest that in the presence of relatively thick electrical double layers (EDLs) characteristic of nanofluidics, the dispersion of ionic solutes differs from that of neutral solutes on which previous theory is based. Ionic dispersion for circular cross-section channels is quantified as a function of a valance parameter, the relative EDL thickness, and the form of the velocity profile. Two unique mechanisms governing ionic dispersion in both pressure- and electrokinetically driven flows are identified. The results of the analytical solution, employing the linearized form of the Poisson-Boltzmann equation, are supported and extended by the results of an independent computational model employing the nonlinear Poisson-Boltzmann equation. Applicability of the computational results is not limited by the Debye-Hückel approximation. Collectively, these results indicate that dispersion of ionic species in nanoscale channels is markedly charge dependent, and substantially deviates from that of neutral solutes in the same flow.