Membrane Charge Density Research Articles

Characterization of the surface charge property and porosity of track-etched polymer membranes.

As an important property of porous membranes, the surface charge property determines many ionic behaviors of nanopores, such as ionic conductance and selectivity. Based on the dependence of electric double layers on bulk concentrations, ionic conductance through nanopores at high and low concentrations is governed by the bulk conductance and surface charge density, respectively. Here, through the investigation of ionic conductance inside track-etched single polyethylene terephthalate (PET) nanopores under various concentrations, the surface charge density of PET membranes is extracted as ∼-0.021C/m2 at pH 10 over measurements with 40 PET nanopores. Simulations show that surface roughness can cause underestimation in surface charge density due to the inhibited electroosmotic flow. Then, the averaged pore size and porosity of track-etched multipore PET membranes are characterized by the developed ionic conductance method. Through coupled theoretical predictions in ionic conductance under high and low concentrations, the averaged pore size and porosity of porous membranes can be obtained simultaneously. Our method provides a simple and precise way to characterize the pore size and porosity of multipore membranes, especially for those with sub-100nm pores and low porosities.

Global optimization for accurate and efficient parameter estimation in nanofiltration

One of the most well-established frameworks for modeling multicomponent transport in nanofiltration (NF) is the Donnan-Steric Pore Model with Dielectric Exclusion (DSPM-DE). Conventional DSPM-DE characterizes transport across NF membranes through four governing membrane parameters: (1) pore radius; (2) effective membrane thickness; (3) membrane charge density; and (4) the dielectric constant inside the membrane pores. The process for quantifying these parameters is typically sequential. First, neutral solute experiments are performed to determine pore radius and effective membrane thickness. Next, charged species experiments are conducted, and the data is used to regress out the remaining parameters. The resulting regressions are often performed using local search algorithms that can struggle to provide low residuals with robust fits. In addition, this two-step approach tends to: (1) require a substantial number of charged and uncharged solute experiments; and (2) introduce assumed relationships between pore size and water flux, such as the Hagen-Poiseuille equation, which may not be representative of transport through complex pore networks. To address these issues, we propose the use of metaheuristic global optimization techniques supplemented with gradient-free local search and maximum likelihood estimation to simultaneously regress all four membrane parameters directly from charged species experiments. We validate our approach against eight independent datasets across diverse input salinities, compositions, and membranes.

Transport properties of graphene oxide nanofiltration membranes: Electrokinetic modeling and experimental validation

AbstractThere is a need for developing reliable models for water and solute transport in graphene oxide (GO) membranes for advancing their emerging industrial water processing applications. In this direction, we develop predictive transport models for GO and reduced‐GO (rGO) membranes over a wide solute concentration range (0.01–0.5 M) and compositions, based on the extended Nernst–Planck transport equations, Donnan equilibrium condition, and solute adsorption models. Some model parameters are obtained by fitting experimental permeation data for water and unary (single‐component) aqueous solutions. The model is validated by predicting experimental permeation behavior in binary solutions, which display very different characteristics. Sensitivity analysis of salt rejections as a function of membrane design parameters (pore size and membrane charge density) allows us to infer design targets to achieve high salt rejections. Such models will be useful in accelerating structure‐separation property relationships of GO membranes and for separation process design and optimization.

Effect of Dielectric Saturation on Ion Activity Coefficients in Ion Exchange Membranes.

Polymeric ion exchange membranes are used in water purification processes to separate ions from water. The distribution and transport of ionic species through these membranes depend on a variety of factors, including membrane charge density, morphology, chemical structure, and the specific ionic species present in the fluid. The electrical potential distribution between membranes and solutions is typically described using models based on Donnan theory. An extension of the original theory is proposed to account for the nonideal behavior of ions both in the fluid and in the membrane as well to provide a more robust description of interactions of solutes with fixed charge groups on the polymer backbone. In this study, the variation in dielectric permittivity in the membrane medium with electric field strength is taken into account in a model based on Gouy–Chapman double-layer theory to provide a more accurate description of ion activity coefficients in an ion exchange membrane. A semianalytical model is presented that accounts for the variation in dielectric permittivity of water in a charged polymer membrane. A comparison of this model with Manning’s counterion condensation model clearly demonstrates that by incorporating changes in water dielectric permittivity with electric field strength, much better agreement with experiments can be obtained over a range of salt concentrations for different ions.

Anomalous thermo-osmotic conversion performance of ionic covalent-organic-framework membranes in response to charge variations

Increasing the charge density of ionic membranes is believed to be beneficial for generating high output osmotic energy. Herein, we systematically investigated how the membrane charge populations affect permselectivity by decoupling their effects from the impact of the pore structure using a multivariate strategy for constructing covalent-organic-framework membranes. The thermo-osmotic energy conversion efficiency is improved by increasing the membrane charge density, affording 210 W m−2 with a temperature gradient of 40 K. However, this enhancement occurs only within a narrow window, and subsequently, the efficiency plateaued beyond a threshold density (0.04 C m−2). The complex interplay between pore-pore interactions in response to charge variations for ion transport across the upscaled nanoporous membranes helps explain the obtained results. This study has far-reaching implications for the rational design of ionic membranes to augment energy extraction rather than intuitively focusing on achieving high densities.

Nanofiltration of complex mixtures: The effect of the adsorption of divalent ions on membrane retention

In nanofiltration (NF), solution and membrane chemistry determine the membrane charge density. In multi-component electrolytes solutions, it is important to consider the interaction between ions and NF membranes. For instance, in the presence of divalent ions, charge reversal can take place due to the interaction of counterions with the charged functional groups of the membrane. This paper presents a model based on state-of-the-art mean-field theory that includes the effect of divalent counterions on the membrane charge density. By using a Langmuir equation with two model parameters, we consider the adsorption of divalent ions (e.g., Ca2+ and Mg2+) in the polyamide active layer of the membrane and calculate the effective membrane charge density. Two different sets of experimental data from literature are used to evaluate the model. Contrary to the statement of a recent study on NF, we show that mean-field theory can predict the rejection of all types of ions in a multi-component electrolyte solution when the effect of divalent counterions on the membrane charge density is included. Moreover, the results in this study reinforce the idea that adsorption of counterions plays a fundamental role in the performance of nanofiltration.

Dialysis membranes as liquid junction materials: Simplified model based on the phase boundary potential

We report here on the use of regenerated cellulose-based dialysis membranes as liquid junction materials to be used in electrochemical reference electrodes targeted towards environmental analysis. With high bridge electrolyte concentrations (above 100 mM), the response fitted the predictions based on the Henderson equation but an undesirable net flux of ions escaping the membrane (For 1 M KCl: J = 14.2 ± 0.8 mmol·m−2·s−1) was observed. The use of less concentrated bridge electrolyte (10 mM) resulted in deviations of up to 26 mV between experimental data and theoretical predictions. Due to the failure to estimate the liquid junction potential arising at the interface with the classical Henderson equation, a simple calculation method, providing a better fit towards experimental data, was developed and tested. The proposed model is based on partial Donnan exclusion generated by the charged surface of the membrane in aqueous samples. The validation of the new model was achieved through a comparison with the Nernst-Poisson-Planck (NPP) simulation method normally used to calculate the potential across permselective membranes. This setup was tested with water samples from the Arve river, where the bridge concentration was closely matched to the sample. Deviations between the new model and experimental data stayed below 0.5 mV, while they were above 6 mV for the Henderson equation. Because of the simple mathematical treatment, the new model may be used to estimate the charge density of such membranes.

Membrane patterning through horizontally aligned microchannels developed by sulfated chopped carbon fiber for facile permeability of blood plasma components in low-density lipoprotein apheresis

Low-density lipoprotein (LDL) apheresis via membrane filtration process is an extracorporeal blood purification technique using for the treatment of patients suffering from familial hypercholesterolemia (FH) and severe coronary artery disease (CAD). As much as the importance of LDL separation, the maximum permeability of beneficial blood plasma components through the membrane is regarded as an important issue in this method. Herein, patterned membranes are produced by the effect of horizontally aligned microchannels that formed by the incorporation of sulfated chopped carbon fiber (SCCF) in the membrane structure. Characterization studies verified the binding of carrageenan as a sulfated polysaccharide on the surface of chopped carbon fiber (CCF). Transmission and reflective optical micrographs demonstrated the orientation of SCCFs in a horizontally aligned direction in the membrane structure. Surface analyses including water contact angle and zeta potential revealed the improvement of hydrophilicity and negative charge density of membranes containing SCCF, respectively. Moreover, membrane surface roughness was enhanced with the addition of SCCF that led to a better selectivity. With the enhancement of SCCF concentration, the ratio of molecular weight cut-off to molecular weight retention onset (MWCO/MWRO) that is a marker of membrane selectivity was decreased effectively. The morphology study displayed the formation of an interfacial distance between SCCF and the polymeric matrix that produced horizontally parallel microchannels in the membrane structure. Blood plasma filtration and cardiovascular disease (CVD) risk factors verified that the M−SCCF3 (containing 0.7 wt% SCCF) due to having desired morphology and characteristics had the best performance for the LDL separation, and permeability of beneficial plasma components like high-density lipoprotein (HDL), and albumin protein. Furthermore, cell cytotoxicity analysis revealed no cytotoxicity in membranes which endorsed desired interaction of developed membranes with L-929 cells as the representation of physiological ambient. Also, reduction of protein adsorption on the membrane surface, and prolongation of coagulation times endorsed the hemocompatibility of membranes. The overall results certified that the M−SCCF3 due to having the best performance and desired structural properties was the best membrane for use in LDL apheresis.

Potentiometric Determination of Fixed Charge Density and Antibacterial Activity of Barium Molybdate Model Membrane

A barium molybdate parchment-supported model membrane was prepared by the ion interaction method. The electrochemical properties of the membrane were studied using Teorell-Meyer-Sievers, Kobatake and Nagasawa approaches. These properties were used to determine important membrane parameters like fixed charge density, transport number, mobility ratio and permselectivity. The experimental and theoretical membrane potential values were used to calculate charge density. These membrane potential values were compared with those from different approaches of thermodynamics of irreversible processes. The fixed charge density values of the prepared membrane under different electrolytic environments were found to follow the order: LiCl > NaCl > KCl. The membrane was characterized by different instrumental techniques such as SEM, TEM, FTIR, TGA/DTA, XRD and EDX analysis. The antibacterial activity of the model membrane was also studied against Gram-negative and Gram-positive bacteria using the disc diffusion method.

Analysis of Ionic Domains on a Proton Exchange Membrane Using a Numerical Approximation Model Based on Electrostatic Force Microscopy.

Understanding the ionic channel network of proton exchange membranes that dictate fuel cell performance is crucial when developing proton exchange membrane fuel cells. However, it is difficult to characterize this network because of the complicated nanostructure and structure changes that depend on water uptake. Electrostatic force microscopy (EFM) can map surface charge distribution with nano-spatial resolution by measuring the electrostatic force between a vibrating conductive tip and a charged surface under an applied voltage. Herein, the ionic channel network of a proton exchange membrane is analyzed using EFM. A mathematical approximation model of the ionic channel network is derived from the principle of EFM. This model focusses on free charge movement on the membrane based on the force gradient variation between the tip and the membrane surface. To verify the numerical approximation model, the phase lag of dry and wet Nafion is measured with stepwise changes to the bias voltage. Based on the model, the variations in the ionic channel network of Nafion with different amounts of water uptake are analyzed numerically. The mean surface charge density of both membranes, which is related to the ionic channel network, is calculated using the model. The difference between the mean surface charge of the dry and wet membranes is consistent with the variation in their proton conductivity.

Thermoelectric Response of Ion‐Selective Membranes: Modelling and Experimental Studies

AbstractThermoelectricity of ion‐selective membrane refers to the phenomenon that the membrane potential is generated in the presence of a temperature gradient, which plays a crucial role in low‐grade heat energy harvesting. Here, a new thermoelectric model is proposed, in which the entire membrane is divided into the intra‐membrane region and two diffusion boundaries. Simulated results suggest the thermally inducted membrane potential is composed of the diffusion potential inside membrane and two Donnan potentials at the membrane boundaries. The effects of the membrane charge density, electrolyte concentration, and solution temperature are theoretically explored in detail. This model is confirmed experimentally with cation‐selective hybrid nanochannels and a commercial cation‐selective polymer membrane. We believe the new thermoelectric model is beneficial to understanding the mechanism of thermoelectric response and promoting practical applications of thermal energy conversion.

Positively charged poly (N-vinyl imidazole) gel-filled loose nanofiltration membranes: Performances and modelling analysis

Loose nanofiltration (NF) membrane with pore radius approaching 1 nm and positive charges can be valuable alternative to commercial available negatively charged polyamide NF membrane in specific applications, such as removal of hardness and heavy metal ions and selective separation of dyes/salts mixture. In this work, positively charged poly(N-vinyl imidazole) (PVI) gel-filled loose NF membranes were developed by asymmetric gel-filling upon an asymmetric ultrafiltration (UF) support via UV-initiated surface grafting polymerization and post-quaternization. Compared to the polyamide NF membranes, these membranes displayed a comparable salt rejection ability due to their stronger Donnan exclusion effect. A rigorous three-parameter transport model (TPM) considering steric, Donnan and dielectric exclusion on ions partitioning via effective pore radius (rp), membrane charge density (Xd) and averaged pore dielectric constant (εp), was used to analyze the membrane parameters. Then the gel conformation model (GM) treating confined gel as semi-dilute polymer solution with a characteristic correlation length (ξ) was applied to analyze these gel composites. The results suggest that the gel polymer volume fraction (φ) is decisive in the gel mesh size (viewed as membrane pore size) and gel permeability. Surface porosity of the asymmetric UF base is a limiting factor in membrane permeability. The joint modelling analysis can clarify the membrane structure-properties relationship, performance limits and potentials of the gel-filled NF membrane and hence facilitate structure design and optimization.

Thermodynamic Characterization of Red Blood Cell Suspension and Band 3 Protein Oxy-Deoxygenating Functionality: Comparative Study

Abstract The importance of studying storage alterations in erythrocytes is highlighted by the need to understand changes that could potentially serve to optimize the storage system. With this aim, the non-equilibrium thermodynamic theory with internal variables was introduced, and perturbing the erythrocyte samples with a harmonic electric field some functions of the theory have been determined varying in the perturbation frequency. A frequency has been noted that acts as a separator element between two states showing a different entropy production above and below this frequency. In stored red blood cells compared to fresh ones, the increase in entropy production measured shows a greater state of disorder in the latter. Further alterations have been highlighted on the surface charge density of the stored erythrocyte membrane and on the speed of anionic kinetics. All these observations highlight the image of membrane structural and functional alterations of the stored erythrocytes and lead to the elaboration of a technique able to correlate a specific perturbation frequency with the aging time of red blood cells.

Novel crosslinked brominated polyphenylene oxide composite nanofiltration membranes with organic solvent permeability and swelling property

：Organic solvent nanofiltration (OSN) membranes have gained increasing attention for their wide applications in organic phase system. However, the fabrication method rather than the swelling behavior receives more attention during the OSN testing. In this work, a thin-film composite membrane with an internal cross-linking network by reacting 4,4-bipyridine with brominated poly (2,6-dimethyl-1, 4-phenylene oxide) (BPPO) has been prepared for organic solvent nanofiltration (OSN). The bipyridine groups substituted on the polyphenylene oxide polymer backbone act as cross-linkers and cationic channels for remarkable chemical stability and high solvent permeability. The permeability of the cross-linked BPPO (CBPPO) thin-film membranes exhibited high related to the viscosity and solubility parameter of solvents. The stability of CBPPO membrane was studied via various organic solvent immersing experiments. The result demonstrated that the composite membrane swelling changed to an extent of no more than 2.01%. It can be attributed to the low conformational flexibility of polymer chains with a high crosslinking degree. The ethanol permeation and dye rejection were compared before and after soaking. The calculation of the MWCO and membrane potential of modified CBPPO membranes showed that the swelling behavior of the CBPPO composite membranes is mainly affected by the membrane charge densities rather than the pore size.

Fluorescence cross-correlation spectroscopy as a valuable tool to characterize cationic liposome-DNA nanoparticle assembly.

The development of nonviral gene delivery vehicles for therapeutic applications requires methods capable of quantifying the association between the genes and their carrier counterparts. Here we investigate the potential of fluorescence cross-correlation spectroscopy (FCCS) to characterize and optimize the assembly of nonviral cationic liposome (CL)-DNA complexes based on a CL formulation consisting of the cationic lipid DOTAP and zwitterionic lipid DOPC. We use a DNA plasmid for lipoplex loading encoding the Oct4 gene, critically involved in reprogramming somatic cells into induced pluripotent stem cells. We demonstrate that FCCS is able to quantitatively determine the extent of the association between DNA and the liposomes and assess its loading capacity. We also establish that the cationic lipid fraction, being proportional to the liposome membrane charge density, as well as charge ratio between the CLs and anionic DNA play an important role in the degree of interaction between the liposomes and DNA.

Effect of pH on Total Volume Membrane Charge Density in the Nanofiltration of Aqueous Solutions of Nitrate Salts of Heavy Metals.

The separation efficiencies of aqueous solutions containing nitric salts of Zn, Cu, Fe or Pb at various pH in process of nanofiltration have been investigated experimentally. These results were used to obtain the total volume membrane charge densities, through mathematical modelling based on the Donnan–Steric partitioning Model. The experimentally obtained retention values of individual heavy metal ions varied between 36% (Zn2+ at pH = 2), 57% (Pb2+ at pH = 2), 80% (Fe3+ at pH = 9), and up to 97% (Cu2+ at pH = 9). The mathematical modelling allowed for fitting the total volume membrane charge density (Xd), which yielded values ranging from −451.90 to +900.16 mol/m3 for different non-symmetric ions. This study presents the application of nanofiltration (NF) modelling, including a consideration of each ion present in the NF system—even those originating from solutions used to adjust the pH values of the feed.

Surface functionalization of reverse osmosis membranes with sulfonic groups for simultaneous mitigation of silica scaling and organic fouling

Organic fouling and inorganic scaling are the main hurdles for efficient operation of reverse osmosis (RO) technology in a wide range of applications. This study demonstrates dual-functionality surface modification of thin-film composite (TFC) RO membranes to simultaneously impart anti-scaling and anti-fouling properties. Two different grafting approaches were adapted to functionalize the membrane surface with sulfonic groups: (i) non-specific grafting of vinyl sulfonic acid (VSA) via redox radical initiation polymerization and (ii) covalent bonding of hydroxylamide-O-sulfonic acid (HOSA) to the native carboxylic groups of the polyamide layer via carbodiimide mediated reaction. Both approaches to graft sulfonic groups were effective in increasing surface wettability and negative charge density of the TFC-RO membranes without significant alteration of water and salt permeabilities. Importantly, we verified through surface elemental analysis that covalently bound HOSA effectively covers the native carboxylic groups of the PA layer. Both the VSA and HOSA membranes exhibited lower flux decline during silica scaling and organic (alginate) fouling relative to the control unmodified membrane, demonstrating the unique versatility of sulfonic groups to endow the TFC-RO membranes with dual functionality to resist scaling and fouling. In particular, the HOSA membrane showed excellent physical cleaning efficiencies with water flux recoveries of 92.5 ± 1.0% and 88.4 ± 6.4% for silica scaling and alginate fouling, respectively. Additional results from silica nucleation experiments and atomic force measurements provided insights into the mechanisms of improved resistance to silica scaling and organic fouling imparted by the surface-functionalized sulfonic groups. Our study highlights the promise of controlled functionalization of sulfonic groups on the polyamide layer of TFC membranes to enhance the applications of RO technology in treatment and reuse of waters with high scaling and fouling potential.

Modeling permeate pH in RO membranes by the extended Donnan steric partitioning pore model

We investigate permeate pH across an RO membrane by modeling transport of ions including protons and hydroxide ions using the extended Donnan steric partitioning pore model (ext-DSP model). We show that membrane charge density plays an important role in determining permeate pH, while the effect of flow rate and salt concentration are less significant. Depending on the polarity of the membrane charge, permeate pH may be lower or higher than feed pH, which indicates the necessity of a reliable membrane charging model. A general multi-site Langmuir charging model is formulated to characterize the membrane charging mechanism and constrain the model parameters. Besides, a simple formula of permeate pH based on the trans-membrane electrostatic potential difference is shown to be a good approximation to the full model prediction.

Electrostatic effects on the electrical tension-induced irreversible pore formation in giant unilamellar vesicles

Irreversible electroporation (IRE) is a new technique in which a series of short pulses with high frequency electrical energy is applied on the targeted regions of cells or vesicles for their destruction or rupture formation. IRE induces lateral tension in the membranes of vesicles. We have investigated the electrostatic interaction effects on the constant electrical tension-induced rate constant of irreversible pore formation in the membranes of giant unilamellar vesicles (GUVs). The electrostatic interaction has been varied by changing the salt concentration in buffer and the surface charge density of membranes. The membranes of GUVs are synthesized by a mixture of negatively charged lipid dioleoylphosphatidylglycerol (DOPG) and neutral lipid dioleoylphosphatidylcholine (DOPC) using the natural swelling method. The rate constant of pore formation increases with the decrease of salt concentration in buffer along with the increase of surface charge density of membranes. The tension dependent probability of pore formation and the rate constant of pore formation are fitted to the theoretical equation, and obtained the line tension of membranes. The decrease in energy barrier of a prepore due to electrostatic interaction is the key factor causing an increase of rate constant of pore formation.

Deformation and poration of giant unilamellar vesicles induced by anionic nanoparticles

The interaction of anionic magnetite nanoparticles (MNPs) of size 18 nm with negatively charged giant unilamellar vesicles (GUVs) formed from a mixture of neutral dioleoylphosphatidylcholine (DOPC) and negatively charged dioleoylphosphatidylglycerol (DOPG) lipids has been investigated. It has been obtained that NPs induces the deformation of spherical GUVs. The reaction of other GUVs on NPs consists in the appearance of pores in their membranes. We focused the effect of electrostatics on the interaction of charged membranes with MNPs. To study the influence of the surface charge of GUVs on the processes under consideration, we varied the fraction of DOPG in the vesicles from 0 to 100%. We examined the influence of salt concentration in the range of 50−300 mM NaCl concentration. To describe the degree of deformation, a special parameter compactness was introduced. The pore formation in the membranes of GUVs was investigated by the leakage of sucrose. The compactness increases with time and also NPs concentration. The fraction of deformed GUVs increases with the increase of surface charge density of membranes as well as the decrease of salt concentration in buffer. The value of compactness for neutral membrane is 1.25 times higher than that of charged ones. The fraction of deformed GUVs become constant after 20 min, however it increases with NPs concentration. The time taken for stochastic pore formation is less for charged membrane than neutral one. The physical mechanism explaining the experimental results obtained in these investigations.