Transport Properties and Electrokinetic Characterization of an Amphoteric Nanofilter

Abstract

Transport properties of a tubular nanofilter with amphoteric properties have been investigated by means of the SEDE (steric, electric, and dielectric exclusion) homogeneous model. Within the scope of this 1D model, the separation of solutes results from transport effects (described by means of extended Nernst-Planck equations) and interfacial phenomena including steric hindrance, the Donnan effect, and dielectric exclusion (expressed in terms of (i) the Born dielectric effect, which is connected to the lowering of the dielectric constant of a solution inside nanodimensional pores, and (ii) the interaction between ions and the polarization charges induced at the dielectric boundary between the pore walls and the pore-filling solution). The effective volume charge density of the membrane has been determined from tangential streaming potential experiments coupled with conductance experiments in a potassium chloride solution at various pH values ranging from 2 to 11. The inferred values have been used in the SEDE model to compute the ion rejection rates with the dielectric constant of the solution inside the pores as a single adjustable parameter. The model provides a relatively good description of experimental data even at extreme pH values for which a ternary system has been considered (K+, Cl-, and H+ or OH- depending on the pH). The fit to experimental data at the membrane isoelectric point indicates that the confinement effect decreases the dielectric constant inside the pores only slightly (with respect to its bulk value). However, the (pH-dependent) ionization of surface sites has been found to lead to a substantial lowering of the dielectric constant inside the pores.