Highly Selective Current-Induced Accumulation of Trace Ions at Micro-/NanoPorous Interfaces
During application of an electrical current through a composite micro-/nano-porous membrane, trace-ion accumulation in the microporous layer should vary greatly with the ion diffusion coefficient to allow selective accumulation of specific ions. In this study, we examine the theoretical enrichment factors in the microporous region as a function of current density and trace-ion diffusion coefficient to demonstrate the promise of such micro-/nano-porous interfaces for ion separations. Trace-ion accumulation relies on charged nanopores that exclude coions (ions with the same charge as the nanopore walls)and give rise (under current) to depletion of the dominant (high-concentration) salt in the region next to the nanopores. This depletion gives rise to high local electric fields. Trace ions accumulate in this region because convection carries them toward the nanoporous interface, whereas electromigration in the opposite direction is strongest next to the interface. For ions whose diffusion coefficients are half of the corresponding value for the dominant salt, calculated enrichment factors reach 150. In contrast depletion occurs for ions with the same diffusion coefficient as the dominant salt. Even higher enrichment factors appear for divalent ions due to their relatively low diffusion coefficients. Lastly, with monovalent coions whose diffusion coefficients differ by only 2%, calculated selectivity factors reach values >1.2 (>1.8 for divalent coions), depending on the magnitudes of the diffusion coefficients. Such selectivities may prove useful in separating very similar species such as isotopes or equally charged peptides.