Evidence of ion transport through surface conduction in alkylsilane-functionalized nanoporous ceramic membranes

Alkylsilane-modified nanoporous ceramic membranes exclude water from their pores yet exhibit transmembrane electrical conductivity in aqueous electrolyte solutions. That effect was studied using impedance spectroscopy and (29)Si NMR. Anodic aluminum oxide membranes with alkylsilane-functionalized pores exhibited a transmembrane electrical resistance that increased with the length of the hydrocarbon chain. Microstructural studies revealed that the conduction was due primarily to a small number of "hydrophilically defective" pores in membranes modified by long-chain alkylsilanes and both hydrophilic defects and surface conduction in pores modified by short-chain alkylsilanes. Hydroxyl groups in short-chain alkylsilane layers act as "water wires" to enable surface ion transport. The local concentration of hydroxyl groups decreased with alkylsilane chain length, explaining the resistance trend. This constitutes the first direct evidence that alkylsilane functionalization affects electrical as well as wetting properties.     

Tangential flow streaming potential measurements: Hydrodynamic cell characterization and zeta potentials of carboxylated polysulfone membranes

Computational fluid dynamics calculations were carried out to ensure that a self-made tangential flow mode streaming potential measurement cell meets the hydrodynamic stipulations of laminar, steady and established electrolyte flow necessary for reproducible electrokinetic measurements. The calculations show that the cell design meets all of these conditions.Six carboxylated polysulfones with a range of different degrees of substitution (DS) from 0.26 to 1.74 carboxyl groups per polymer repeat unit were synthesized in a two-stage process of lithiation and carboxylation. Ultrafiltration membranes were made from both the unmodified polysulfone and these hydrophilic materials. The zeta potentials of these membrane surfaces were determined in 0.001 M KCl solution as a function of pH. The curves show the theoretically expected profiles for non-ionic and weakly acidic materials. The growing influence of the COOH dissociation on the surface charge formation is indicated by the flattening of the curves at low pH values. The magnitude of the negative zeta potentials plateau values ranged from −52 to −20 mV. While unmodified PSU has a plateau value of −52 mV this value decreases continuously with increasing DS to −20 mV for the PSU-COOH 1.74 material. It is suggested that this arises from a shift of the electrokinetic shear plane into the bulk electrolyte solution due to an extended swelling layer reflecting the enhanced hydrophilicity of these membrane surfaces.     

Uranyi ion transport through tri-n-octylamine-xylene based supported liquid membranes

Tri-n-octylamine (TOA) dissolved in xylene has been used as carrier, constituting liquid membrane supported in Celgard 2400 polypropylene microporous film for the transport of uranyl ions against their concentration gradient from aqueous acid solutions to an alkaline aqueous phase. Effect of sttrring rate, nitric acid concentration and TOA concentration in the organic membrane phase, on the flux of uranyl ions through the membrane has been studied. Viscosity and density data have been obtained to estimate diffusion coefficients and hence the permeability coefficients to compare the same with experimental values, using distribution coefficient data, measured from solvent extraction experiments and available in the literature. Analysis of the flux data has been performed to study the stoichiometry of the chemical reaction involved in complex formation reaction. The results have been compared with simple liquid-liquid extraction data.     

Zeta potential of ion-conductive membranes by streaming current measurements

Surface charge properties have a significant influence on membrane retention and fouling performance. As a key parameter describing the surface charge of membranes used in aqueous applications, zeta potential measurements on membranes of various types have attracted great attention. During the zeta potential characterization of a series of ion-conductive sulfonated poly(sulfone) membranes, it was found that the measured streaming current varied with the thickness of the sample, which is not predicted by the classical Smoluchowski equation. Moreover, for higher conductivity membranes with an increased concentration of sulfonate groups, the zeta potential tended toward zero. It was determined that the influence of membrane bulk conductance on the measured streaming current must be taken into account in order to correctly interpret the streaming current data for ion-conductive polymers and understand the relationship between membrane chemical composition and zeta potential. Extrapolating the measured streaming current to a membrane thickness of zero has proven to be a feasible method of eliminating the error associated with measuring the zeta potential on ion conductive polymer membranes. A linear resistance model is proposed to account for the observed streaming currents where the electrolyte channel is in parallel with the ion-conductive membranes.     

Size selective DNA transport through a nanoporous membrane in a PDMS microfluidic device

A microfluidic counter current dialysis device for size based purification of DNA is described. The device consists of two polydimethylsiloxane (PDMS) channels separated by a track etched polycarbonate membrane with a 50 nm pore size. Recovery of fluorescein across the membrane was compared with 10 and 80 nucleotide (nt) ssDNA to characterize the device. Recovery of all three analytes improved with decreasing flow rate. Size selectivity was observed. Greater than 2-fold selectivity between 10 nt and 80 nt ssDNA was observed at linear velocities less than 3mm s(-1). Increasing the ionic strength of the buffer increased transport across the membrane. Recovery of 80 nt ssDNA increased over 4-fold by adding 30 mM NaCl to the buffer. The effect was size dependent as 10 nt showed a smaller increase while the recovery of fluorescein was largely unaffected by increasing the ionic strength of the buffer.     

Ion transport in inhomogeneous ion exchange membranes

The effect on current efficiency produced by fluctuations in an ion-exchange membrane's fixed ion concentration (due to ion clustering) is determined by considering transport in several model membranes containing different fixed ion distributions. Electroneutrality is not assumed, so as to include nonlinear effects due to space charge in the clusters. The Nernst—Planck and Poisson equations are solved using perturbation theory, and the case of small fluctuations in fixed charge density is considered in order to obtain analytic solutions to the perturbation equations. Results show that an inhomogeneous distribution of fixed ions gives rise to increased current efficiency when the counterions and co-ions experience in the membrane unequal forces due to the potential produced by the fixed ions. The length scale over which the inhomogeneities occur is important in determining the current efficiency.     

Gas separation in nanoporous membranes formed by etching ion irradiated polymer foils

Polymer membranes with pores with radii in the range of several 10–100 nm were formed by irradiating polyimide foil with highly energetic heavy ions and etching the latent ion tracks with hypochlorite. The aerial density of the pores could be chosen up to an upper limit of 10⁸ pores cm^?2, at which too many pores start to overlap. The straight cylindrical pores were tested for their gas permeation and gas separation performance. With a gas mixture of CO and CO₂ as model system, gas chromatographic measurements showed that CO penetrates faster through the membrane than CO₂, leading to gas separation. This is possible because the mean free path of the molecules is in the order of the pore radius, which is in the transition flow region close to molecular flow conditions.     

Concentration dependence of electroosmosis and streaming potential across charged membranes

Electroosmosis and streaming potential measurements were carried out across charged membranes made of collodion and sulfonated polystyrene. Experiments were confined to the range where linear flux/force relationships hold. Saxén's relationship holds between electroosmosis and streaming potential; for porous charged membranes these exhibit an approximate inverse proportionality to ionic mobility at the limit of low electrolyte concentration. Both tend towards zero at the limit of high electrolyte concentration.     

Diffusive transport of Cu(II) ions through thin ion imprinted polymeric membranes

A novel ion imprinted polymeric membrane (IIPM) for copper (Cu) ions transport was prepared by a ion imprinting technique via cross-linking of blended chitosan (CS)/polyvinyl alcohol (PVA) using glutaraldehyde (GA) as the cross-linker and Cu ions as the template. The obtained IIPM was characterised and evaluated by scanning electron microscopy (SEM), energy dispersive X-ray (EDX) and permeation studies. Cavities of IIPM containing recognition sites for Cu ions were formed in the compact structure of the CS/PVA membrane which was prepared via the solution casting method. Under the optimum conditions, transport factor of the IIPM reached 2 when the permeation time was 18 h. Selective permeation of Cu ions versus nickel ions through the imprinted membrane was confirmed and a selectivity factor of 1.71 was obtained.     

Studies of polyamines transport through liquid membranes with D2EHPA as a carrier

The transport of polyamines through the liquid membranes with di-2-ethylhexyl phosphoric acid (D2EHPA) was investigated. The study was performed in three main steps: liquid-liquid extraction (LLE), bulk liquid membrane (BLM) extraction, and supported liquid membrane (SLM) extraction. Equilibrium distribution experiments allowed determining the extraction constants and stoichiometric coefficients for each polyamine. It turned out that one amino group binds two molecules of carrier (one D2EHPA dimer) and the extractability of polyamine rises with the increase in number of function groups in the molecule. The BLM and SLM experiments showed that despite considerable differences in distribution ratio between various polyamines the extraction efficiencies for all of them are very approximate. The smaller diamines compensate the lowest affinity to membrane phase with faster interface reaction kinetics and higher diffusivity. Finally, the SLM extraction conditions were optimized. The main parameters that influence the transport are the pH of the donor and acceptor phases. The extraction efficiencies obtained for polyamines are high (80-90%) and give hope for an application in bioanalytical chemistry.     

Effect of aging on UF membranes by a streaming potential (SP) method

In this paper we have determined, by a streaming potential (SP) method, the isoelectric point (IEP) of a new cellulose membrane, the regenerated cellulose material. This membrane is more hydrophilic than the classical cellulose acetate material and less sensitive to protein adsorption, with an IEP of 3.4.Furthermore, we have validated an SP method as a new method to control aging of porous membranes. We validate the SP technique on a surface of 30 cm² for a membrane with a molecular weight cut-off of 10 kDa.In this new field of research where nondestructive techniques are not many, we have shown, for the first time, the efficiency of coupling permeability and SP measurements to control aging of mesoporous polymer membranes.     

Highly rectified ion transport through 2D WSe2/MoS2 bi-layered membranes

Through a two-step vacuum-filtration process, WSe₂ and MoS₂ nanosheets were sequentially deposited onto a polymeric nanoporous support, forming WSe₂/MoS₂ bi-layered heterostructure. Highly rectified ion transport phenomenon is observed through the heterogeneous 2D layered membranes.     

Adduct-ion formation in trapped ion mobility spectrometry as a potential tool for studying molecular structures and conformations

Recent developments in the field of ion mobility spectrometry provide new possibilities to explore and understand gas-phase ion chemistry. In this study, hyphenated trapped ion mobility spectrometry-mass spectrometry (TIMS-MS) was applied to investigate analyte ion mobility as function of adduct ion formation for twelve pharmaceutically relevant molecules, and for tetrahydrocannabinol (THC) and its isomer cannabidiol (CBD). Samples were introduced by direct infusion and ions were generated with positive electrospray ionization (ESI+) observing protonated and sodiated ions. Measurements were performed with and without addition of cesium-, lithium-, silver- and sodium ions to the samples. For the tested compounds, metal adduct ions with the same m/z but with different mobility and collision cross section (CCSs) were observed, indicating different molecular conformations. Formation of analyte dimers was also observed, which could be associated with molecular geometry of the compounds. By optimizing the range and speed of the electric field gradient and ramp, respectively, the separation of THC and CBD was achieved by employing the adduct formation. This study demonstrates that the favorable resolution of TIMS combined with the ability to detect weakly bound counter ions is a valuable means for rapid detection, separation and structural assignment of molecular isomers and analyte conformations.     

Interaction with micelles as a driving force for uphill transport of organic cations across ion exchange membranes

Uphill transport of L-Dopa and phenylalanine (Phe) across cation-exchange membranes into micelle-containing receiver solutions is reported. With L-Dopa as the analyte in a sample solution at a pH where it is it a zwitterion, preconcentration by a factor of 3.2 +/- 0.2 (n = 5) is observed when 0.10M sodium dodecyl sulfate (SDS) is the receiver. When the SDS concentration is varied, preconcentration of L-Dopa is not observed until the critical micelle concentration is reached. Similar results were obtained with Phe as the analyte under conditions where it is protonated in both the sample and receiver. The transport is demonstrated to obey the assumptions required to quantify the results by the fixed-time kinetic method. That is, the amount of Phe transferred from a 200-ml sample across a 10-cm(2) membrane into a 5-ml receiver was directly proportional both to the dialysis time for up to 90-min and to the initial concentration of Phe in the sample when a 60-min dialysis time was used. The latter yielded a constant enrichment factor, 4.8 +/- 0.2 (n = 6), when the sample concentration of Phe was in the range 0.61 mM-6.0 muM. Means to increase the enrichments to practical values are discussed.     

Electrostatic effect of streaming potential on ion-exchange phenomena in resins for ion chromatography

Ion-exchange phenomena in the inner phase of resins for ion chromatography were studied by measuring streaming potentials in a flow system. On the basis of the values obtained, the electrostatic behaviour in connection with anion-exchange reactions in the resin phase was evaluated. The electrostatic potentials in the resin phase varied with the exchanging ion species in the solution phase under various salt conditions. On comparing retention times in ion chromatography, it was found that the electrostatic behaviour influences the selectivity coefficient of the resin.     

Measurements of transient membrane potential after current switch-off as a tool to study the electrochemical properties of supported thin nanoporous layers

We have studied the potential of chronopotentiometry after current switch-off as a tool for electrochemical characterization of thin supported nanoporous layers. Within the scope of this technique, a thin supported electrochemically active layer is polarized by direct electric current until a steady state is reached. After that, the current is switched-off in a stepwise manner, and the reading of transient membrane potential begins. A linear non-steady-state theory of the method has been developed in terms of a model-independent approach of network thermodynamics. The measurements of transient membrane potential after current switch-off have been carried out in KCl solutions of various concentrations for a commercially available nanofiltration membrane (Desal5 DK). Such membranes consist of micron-thick active (or barrier) nanoporous layers and much thicker (100-200 microm) and coarse-porous supports (the pore size usually is 0.1-5 microm). The reproducibility of the method has been found to be quite reasonable especially in not too dilute electrolyte solutions and at not too short times (> or = 10 ms). The relaxation measurements have been complemented by the measurements of the steady-state membrane potential and by sample measurements of salt rejection in the pressure-driven mode, which enabled us to carry out a self-consistent interpretation of the experimental data. This has revealed, in particular, that the ion rejection mechanism related to the fixed electric charges is not the dominant one in the case of the Desal5 DK nanofiltration membrane. Proceeding from a quantitative interpretation of relaxation patterns, we could also determine some properties of membrane support, namely, the porosity and the salt diffusivity. They have been found to have reasonable values remarkably independent of salt concentration, which confirms the self-consistency of our interpretations.     

Pore formation coupled to ion transport through lipid membranes as induced by transmembrane ionic charge imbalance: atomistic molecular dynamics study

We have employed atomic-scale molecular dynamics simulations to address ion transport through transient water pores in phospholipid membranes. The formation of a water pore is induced by a transmembrane ionic charge imbalance, which gives rise to a significant potential difference across the membrane. The subsequent transport of ions through the pore discharges the transmembrane potential and makes the water pore metastable, leading eventually to its sealing. The findings highlight the importance of ionic charge fluctuations in spontaneous pore formation and their role in ion leakage through protein-free lipid membranes.     

Ion transport and water dissociation in bipolar ion exchange membranes

This paper treats ion transport and water dissociation in “bipolar membranes”, consisting of juxtaposed cation and anion exchangers. Bipolar membranes are the close ion analogues of the semiconductor p—n junction and show similar, but not identical, current rectification behavior. The major difference between these two systems is that bipolar membranes contain a total of four mobile species. We attempt a simplified treatment which includes the flux of all four ions and find that, though the current is on the average carried by only two ions, which two they are depends on the voltage range in question. The flux of minority carriers, though relatively unimportant at small applied voltages, becomes crucial at high voltages, and at very high voltages the process of water splitting dominates. When the fluxes of all four ions are taken into consideration it is possible to predict qualitatively the experimentally observed current—voltage curves over the entire voltage range. We discuss the importance of symmetry and the restrictions of studying an idealized system. Suggestions for further work are included.