Addressing Challenges in Ion-Selectivity Characterization in Nanopores.

Ion selectivity is the basis for designing smart nanopore/channel-based devices, e.g., ion separators and biosensors.

Computational Design of an Electro-Membrane Microfluidic-Diode System.

This study uses computational design to explore the performance of a novel electro-membrane microfluidic diode consisting of physically conjugated nanoporous and micro-perforated ion-exchange layers. Previously, such structures have been demonstrated to exhibit asymmetric electroosmosis, but the model was unrealistic in several important respects. This numerical study investigates two quantitative measures of performance (linear velocity of net flow and efficiency) as functions of such principal system parameters as perforation size and spacing, the thickness of the nanoporous layer and the zeta potential of the pore surface.

Kinematic and volumetric analysis of coupled transmembrane fluxes of binary electrolyte solution components.

The paper deals with relationships between the individual transmembrane fluxes of binary electrolyte solution components and the experimentally measurable quantities describing rates of transfer processes, namely, the electric current, the transmembrane volume flow and the rates of concentration changes in the solutions adjacent to the membrane. Also, we collected and rigorously defined the kinetic coefficients describing the membrane selective and electrokinetic properties. A set of useful relationships between these coefficients is derived.

Streaming Potential with Ideally Polarizable Electron-Conducting Substrates.

With nonconducting substrates, streaming potential in sufficiently broad (vs Debye screening length) capillaries is well known to be a linear function of applied pressure (and coordinate along the capillary). This study for the first time explores streaming potential with ideally polarizable electron-conducting substrates and shows it to be a nonlinear function of both coordinate and applied pressure. Experimental manifestations can be primarily expected for streaming potentials arising along thin porous electron-conducting films experiencing solvent evaporation from the film side surface.

Ion Separations Based on Spontaneously Arising Streaming Potentials in Rotating Isoporous Membranes.

Highly selective ion separations are vital for producing pure salts, and membrane-based separations are promising alternatives to conventional ion-separation techniques. Our previous work demonstrated that simple pressure-driven flow through negatively charged isoporous membranes can separate Li and K with selectivities as high as 70 in dilute solutions. The separation mechanism relies on spontaneously arising streaming potentials that induce electromigration, which opposes advection and separates cations based on differences in their electrophoretic mobilities.

Evaporation-driven electrokinetic energy conversion: Critical review, parametric analysis and perspectives.

Energy harvesting from evaporation has become a "hot" topic in the last couple of years. Researchers have speculated on several possible mechanisms. Electrokinetic energy conversion is the least hypothetical one.

Nanofiltration of Multi-Ion Solutions: Quantitative Control of Concentration Polarization and Interpretation by Solution-Diffusion-Electro-Migration Model.

For effective use of advanced engineering models of nanofiltration quality of experimental input is crucial, especially in electrolyte mixtures where simultaneous rejections of various ions may be very different. In particular, this concerns the quantitative control of concentration polarization (CP). This work used a rotating disklike membrane test cell with equally accessible membrane surface, so the CP extent was the same over the membrane surface.

Analysis of Membrane Transport Equations for Reverse Electrodialysis (RED) Using Irreversible Thermodynamics.

Reverse electrodialysis (RED) is an electro-membrane process for the conversion of mixing energy into electricity. One important problem researchers' face when modeling the RED process is the choice of the proper membrane transport equations. In this study, using experimental data that describe the membrane Nafion 120 in contact with NaCl aqueous solutions, the linear transport equation of irreversible thermodynamics was applied to calculate the power density of the RED system.

Flow through negatively charged, nanoporous membranes separates Li and K due to induced electromigration.

Flow through negatively charged nanopores separates Li and K with selectivities of up to 70 and Li passages from 20% to above 100%. Remarkably, both the Li/K selectivity and Li passage initially increase with flow rate, breaking the permeability/selectivity trade-off. Modelling demonstrates that flow through the membranes creates electric fields that retard transport of cations.

A Limiting Case of Constant Counterion Electrochemical Potentials in the Membrane for Examining Ion Transfer at Ion-Exchange Membranes and Patches.

Ion passage through ion-exchange membranes is vital in electrodialysis desalination, batteries and fuel cells, and water splitting. Simplified models of ion transport through such membranes frequently assume complete exclusion of co-ions (ions with the same sign of charge as the fixed charge in the membrane) from the membrane. However, a second assumption of constant counterion electrochemical potentials across the membrane leads to simple analytical expressions for ion fluxes and transmembrane potentials.

Modelling nanofiltration of electrolyte solutions.

This review critically examines current models for nanofiltration (NF) of electrolyte solutions. We start from linear irreversible thermodynamics, we derive a basic equation set for ion transfer in terms of gradients of ion electrochemical potentials and transmembrane volume flux. These equations are extended to the case of significant differences of thermodynamic forces across the membrane (continuous version of irreversible thermodynamics) and solved in quadratures for single salts and trace ions added to single salts in the case of macroscopically-homogeneous membranes.

High Selectivities among Monovalent Cations in Dialysis through Cation-Exchange Membranes Coated with Polyelectrolyte Multilayers.

Cation-exchange membranes allow preferential passage of cations over anions, but they show minimal selectivity among cations, which limits their use in ion separations. Recent studies show that modification of cation-exchange membranes with polyelectrolyte multilayers leads to exceptional monovalent/divalent cation electrodialysis selectivities, but no studies report high selectivity among monovalent ions. This work demonstrates that adsorption of protonated poly(allylamine) (PAH)/poly(4-styrenesulfonate) (PSS) multilayers on Nafion membranes leads to high K/Li selectivities in Donnan dialysis, where K and Li ions in a source phase pass through the membrane and exchange with Na ions in a receiving phase.

Current-Induced Concentration Polarization of Nanoporous Media: Role of Electroosmosis.

Current-induced concentration polarization of nanoporous media is explored theoretically by using approach of local thermodynamic equilibrium within nanopore cross-sections. The problem is solved in quadratures in terms of irreversible thermodynamics. The phenomenological coefficients are further specified by using capillary space-charge model for straight slit-like and cylindrical capillaries.

"Breakthrough" osmosis and unusually high power densities in Pressure-Retarded Osmosis in non-ideally semi-permeable supported membranes.

Osmosis is the movement of solvent across a membrane induced by a solute-concentration gradient. It is very important for cell biology. Recently, it has started finding technological applications in the emerging processes of Forward Osmosis and Pressure-Retarded Osmosis.

Deviations from Electroneutrality in Membrane Barrier Layers: A Possible Mechanism Underlying High Salt Rejections.

Reverse osmosis and nanofiltration (NF) employ composite membranes whose ultrathin barrier layers are significantly more permeable to water than to salts. Although solution-diffusion models of salt transport through barrier layers typically assume ubiquitous electroneutrality, in the case of ultrathin selective skins and low ion partition coefficients, space-charge regions may occupy a significant fraction of the membrane barrier layer. This work investigates the implications of these deviations from electroneutrality on salt transport.

Coating of Nafion membranes with polyelectrolyte multilayers to achieve high monovalent/divalent cation electrodialysis selectivities.

Electrodialysis (ED) membranes typically exhibit modest selectivities between monovalent and divalent ions. This paper reports a dramatic enhancement of the monovalent/divalent cation selectivities of Nafion 115 membranes through coating with multilayer poly(4-styrenesulfonate) (PSS)/protonated poly(allylamine) (PAH) films. Remarkably, K(+)/Mg(2+) ED selectivities reach values >1000, and similar monovalent/divalent cation selectivities occur with feed solutions containing K(+) and Ca(2+).

Mechano-chemical effects in weakly charged porous media.

The paper is concerned with mechano-chemical effects, namely, osmosis and pressure-driven separation of ions that can be observed when a charged porous medium is placed between two electrolyte solutions. The study is focused on porous systems with low equilibrium interfacial potentials (about 30 mV or lower). At such low potentials, osmosis and pressure-driven separation of ions noticeably manifest themselves provided that the ions in the electrolyte solutions have different diffusion coefficients.

Electrokinetics in undeveloped flows.

For the correct interpretation of results of tangential electrokinetic measurements with porous materials, in particular, composite/asymmetric membranes on porous supports, it is necessary to have the data available for various channel heights. In some kinds of equipment, the variation of channel height is technically possible only for a range of relatively large heights. This communication shows that under these conditions, the fluid flow can become undeveloped and the conventional approaches to the interpretation of electrokinetic measurements should be modified accordingly.

Separation of ions using polyelectrolyte-modified nanoporous track-etched membranes.

Selective ion exclusion from charged nanopores in track-etched membranes allows separation of ions with different charges or mobilities. This study examines pressure-driven transport of dissolved ions through track-etched membranes modified by adsorption of poly(styrene sulfonate) (PSS)/protonated poly(allylamine) (PAH) films. For nominal 30 nm pores modified with a single layer of PSS, Br(-)/SO4(2-) selectivities are ∼3.

Fundamentals of selective ion transport through multilayer polyelectrolyte membranes.

Membranes composed of multilayer poly(4-styrenesulfonate) (PSS)/protonated poly(allylamine) (PAH) films on porous alumina supports exhibit high monovalent/divalent cation selectivities. Remarkably, the diffusion dialysis K(+)/Mg(2+) selectivity is >350. However, in nanofiltration this selectivity is only 16, suggesting some convective ion transport through film imperfections.

Over-limiting currents and deionization "shocks" in current-induced polarization: local-equilibrium analysis.

The problem is considered theoretically of dynamics of current-induced concentration polarization of interfaces between ideally perm-selective and non-ideally perm-selective ("leaky") ion-exchange media in binary electrolyte solutions under galvanostatic conditions and at negligible volume flow. In contrast to the previous studies, the analysis is systematically carried out in terms of local thermodynamic equilibrium in the approximation of local electric neutrality in virtual solution. For macroscopically homogeneous media, this enables one to obtain model-independent results in quadratures for the stationary state as well as an approximate scaling-form solution for the transient response to the step-wise increase in electric-current density.