Highly Selective Current‐Induced Accumulation of Trace Ions at Micro‐/NanoPorous Interfaces

Abstract

During application of electrical current through composite micro-/nanoporous membranes, trace-ion accumulation in the microporous layer should vary greatly with the ion diffusion coefficient to allow selective accumulation of specific ions. This study examines the theoretical enrichment factors in the microporous region as a function of current density and trace-ion diffusion coefficient. Trace-ion accumulation relies on charged nanopores that exclude coions, which gives rise to depletion of the dominant salt and high local electric fields in the region next to the nanopores. 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. With monovalent coions having 2% difference in diffusion coefficients, calculated accumulation selectivity factors reach >1.2. Such selectivities may prove useful in separating very similar species such as isotopes or equally charged peptides.

Current‐Induced Ion Concentration Polarization at a Perfect Ion‐Exchange Patch in an Infinite Insulating Wall

“…Wang and coworkers reported as much as a million‐fold preconcentration factor for proteins . Our recent computer simulations suggest that ion accumulation near a related nanoporous interface can occur with order of magnitude selectivity for ions with different (but relatively close) mobilities …”

“…This leads to Eq. (11), which shows that the normal derivative of the dimensionless counter ion electrochemical potential is proportional to the normal derivative of dimensionless salt concentration.…”

“…[10] Our recent computer simulations suggest that ion accumulation near a related nanoporous interface can occur with order of magnitude selectivity for ions with different (but relatively close) mobilities. [11] In research to simplify the construction of preconcentration devices, the Crooks group expanded on their prior expertise with bipolar electrodes [12][13] to create enrichment zones with only a single channel that spans a bipolar electrode (Figure 4A). [14][15][16][17][18] Because of the potential profile in the solution above the electrode, the electrode minus solution potential drop is more negative at the right side of the electrode than at the left side.…”

Osmotic Pressure and Diffusion of Ions in Charged Nanopores

“…In "symmetrical" long straight capillaries, eq 12 can be easily solved in quadratures for an arbitrary right-hand side being a function of only one crosssectional coordinate (e.g., radial position). For cylindrical pores, 32 this results in…”

“…In “symmetrical” long straight capillaries, eq can be easily solved in quadratures for an arbitrary right-hand side being a function of only one cross-sectional coordinate (e.g., radial position). For cylindrical pores, this results in The space-charge model postulates ion partitioning due to electrostatic interactions with fixed charges on the nanopore walls. The Poisson–Boltzmann (PB) equation for the quasi-equilibrium dimensionless electrostatic potential, ψ, describes these interactions where κ is the reciprocal Debye screening length defined as is the ionic strength in the virtual solution.…”