Symmetric Membranes Research Articles

Cellulose acetate membranes fabricated by a combined vapor-induced/wet phase separation method: morphology and performance evaluation

Cellulose acetate (CA) microfiltration membranes were fabricated by a combined vapor-induced and wet phase separation technique. A systematic morphology study was performed on the membranes prepared under different fabrication conditions and dope solution compositions. The results showed that symmetric CA membranes could be obtained by controlling relative humidity, temperature and exposure time during vapor-induced phase separation step. While the surface morphology and pore size of the membranes could be mainly tailored by adjusting the dope solution composition. The performance study of the membranes showed that for operation at low transmembrane pressures, only symmetric membranes fabricated by vapor-induced phase separation process have permeability which can be reached 100 L/m2h using polyvinylpyrrolidone with an average molecular weight of 30,000 (PVPK30) or polyethylene glycol with an average molecular weight of 400 (PEG400) as additives. Bacteria removal performance of the membranes showed that samples prepared at optimized solution compositions of CA/PVP and CA/PEG fabricated at constant vapor-induced phase separation (VIPS) condition of 5 min exposure time, 45 °C and 55% relative humidity can completely filtrate the E. coli bacteria. The prepared CA membranes showed no cytotoxicity in mouse L929 fibroblast cells, confirming their biocompatible nature for possible water treatment or biomedical applications.

Vertical Routing of Spinning-Dipole Radiation from a Chiral Metasurface

We propose a perfect photonic router based on a specially designed chiral bi-metasurface membrane for spin-polarized point light sources. Due to the mirror symmetry breaking in the chiral metamembrane, the radiation power flux of the clockwise and counterclockwise spinning dipoles to the opposite sides of the slab becomes different. We show that spinning dipoles in the specially designed chiral D$_4$-symmetrical bi-metasurface membrane can emit light either upwards or downwards depending on their rotation direction. We attribute this phenomenon to the Fano-resonance effect which is a result of the guided modes coupling with the far field. We show the advantage of D$_4$-symmetrical structures for the achievement of 100\% routing efficiency. This phenomenon can find applications in spintronics for spin-selective inter-chip coupling or as a measurement tool of spin polarization in memory cells.

Polyhydroxyalkanoate (PHA) based microfiltration membranes: Tailoring the structure by the non-solvent induced phase separation (NIPS) process

Within the purpose of developing more sustainable membrane systems, it is herein proposed to implement a biobased and biodegradable material, the poly(hydroxybutyrate-co-hydroxyvalerate) (PHBHV), to replace conventional polymers in a commonly used membrane fabrication process. The PHBHV based membranes were made by non-solvent induced phase separation (NIPS) using N-methyl-2-pyrrolidone (NMP) as solvent. The process parameters and dope solution composition were developed in order to give structures suitable for performances characterization. In that sense, the microstructures were characterized in case of non-supported and supported membranes. The addition of hydrophilic additives, ethylene glycol (EG) and polyethylene glycol 300 g mol−1 (PEG300), was linked to the morphology changes. Porous asymmetric membranes were obtained in case of the non-supported membranes or supported membranes with a low amount of additive (1 wt%). Otherwise, symmetric porous membranes, made of interconnected crystal lamellas, were observed. The particularly high crystallinity of this biomaterial involved some different microstructures compared to classic polymers. Both types of structures demonstrated decent rejections of a clay dispersion. Due to the increased pore size, the permeabilities were greatly improved with the additives and values up to 480 L m−2 h−1.bar−1 were achieved.

Experimental Investigation of the Oxidative Coupling of Methane in a Porous Membrane Reactor: Relevance of Back-Permeation.

Novel reactor configurations for the oxidative coupling of methane (OCM), and in particular membrane reactors, contribute toward reaching the yield required to make the process competitive at the industrial scale. Therefore, in this work, the conventional OCM packed bed reactor using a Mn-Na2WO4/SiO2 catalyst was experimentally compared with a membrane reactor, in which a symmetric MgO porous membrane was integrated. The beneficial effects of distributive feeding of oxygen along the membrane, which is the main advantage of the porous membrane reactor, were demonstrated, although no significant differences in terms of performance were observed because of the adverse effects of back-permeation prevailing in the experiments. A sensitivity analysis carried out on the effective diffusion coefficient also indicated the necessity of properly tuning the membrane properties to achieve the expected promising results, highlighting how this tuning could be addressed.

Nonlinear stability of explicit self-similar solutions for the timelike extremal hypersurfaces in $${\mathbb {R}}^{1+3}$$

This paper is devoted to the study of the singularity phenomenon of timelike extremal hypersurfaces in Minkowski spacetime $${\mathbb {R}}^{1+3}$$ . We find that there are two explicit lightlike self-similar solutions to a graph representation of timelike extremal hypersurfaces in Minkowski spacetime $${\mathbb {R}}^{1+3}$$ , the geometry of them are two spheres. The linear mode unstable of those lightlike self-similar solutions for the radially symmetric membranes equation is given. After that, we show those self-similar solutions of the radially symmetric membranes equation are nonlinearly stable inside a strictly proper subset of the backward lightcone. This means that the dynamical behavior of those two spheres is as attractors. Meanwhile, we overcome the double roots case (the theorem of Poincare can’t be used) in solving the difference equation by construction of a Newton’s polygon when we carry out the analysis of spectrum for the linear operator.

Development of Hydrophilic PVDF Membrane Using Vapour Induced Phase Separation Method for Produced Water Treatment.

During the production of oil and gas, a large amount of oily wastewater is generated, which would pollute the environment if discharged without proper treatment. As one of the most promising treatment options, membrane material used for oily wastewater treatment should possess desirable properties of high hydraulic performance combined with high membrane fouling resistance. This project employs the vapor induced phase separation (VIPS) technique to develop a hydrophilic polyvinylidene fluoride (PVDF) membrane with polyethylene glycol (PEG) as an additive for produced water treatment. Results show that thanks to its slow nonsolvent intake, the VIPS method hinders additive leaching during the cast film immersion. The results also reveal that the exposure of the film to the open air before immersion greatly influences the structure of the developed membranes. By extending the exposure time from 0 to 30 min, the membrane morphology change from typical asymmetric with large macrovoids to the macrovoid-free porous symmetric membrane with a granular structure, which corresponds to 35% increment of steady-state permeability to 189 L·m−2h−1bar−1, while maintaining >90% of oil rejection. It was also found that more PEG content resides in the membrane matrix when the exposure time is extended, contributes to the elevation of surface hydrophilicity, which improves the membrane antifouling properties. Overall results demonstrate the potential of VIPS method for the fabrication of hydrophilic PVDF membrane by helping to preserve hydrophilic additive in the membrane matrices.

Pivotal Role of Interdigitation in Interleaflet Interactions: Implications from Molecular Dynamics Simulations.

The asymmetric lipid composition in plasma membranes within the inner leaflet is not typically suitable for domain formation. Thus elucidation of the likelihood of the formation or stability of a raft-like domain in the inner leaflet is necessary. Herein we investigated the phase behavior of asymmetric membranes using coarse-grained molecular dynamics simulations. The lipid leaflet comprising dioleoylphosphatidylcholine (DOPC) and cholesterol (Chol) does not typically show well-developed domains in symmetric bilayer membranes; however, it does separate into liquid ordered (Lo) and liquid disordered (Ld) phases when the opposing leaflet containing sphingomyelin (SM), DOPC, and Chol demonstrates domain formation. We determine that interdigitated acyl chains modulated the partitioning of Chol in the opposing leaflet, resulting in phase separation. Similarly, the acyl chain length of SM within the opposing leaflet affected the phase behavior of the leaflet. Our results reveal the crucial role of interdigitation in determining the phase status in asymmetric membranes.

New high permeable polysulfone/ionic liquid membrane for gas separation

In this study, the effects of 1-Ethyl-3-methylimidazolium tetrafluoroborate ionic liquid on CO2/CH4 separation performance of symmetric polysulfone membranes are investigated. Pure polysulfone membrane and ionic liquid-containing membranes are characterized. Field emission scanning electron microscopy (FE-SEM) is used to analyze surface morphology and thickness of the fabricated membranes. Energy dispersive spectroscopy (EDS) and elemental mapping, Fourier transform infrared (FTIR), thermal gravimetric (TGA), X-ray diffraction (XRD) and Tensile strength analyses are also conducted to characterize the prepared membranes. CO2/CH4 separation performance of the membranes are measured twice at 0.3 MPa and room temperature (25 °C). Permeability measurements confirm that increasing ionic liquid content in polymer-ionic liquid membranes leads to a growth in CO2 permeation and CO2/CH4 selectivity due to high affinity of the ionic liquid to carbon dioxide. CO2 permeation significantly increases from 4.3 Barrer (1 Barrer=10-10 cm3(STP)·cm·cm-2·s-1·cmHg-1, 1cmHg=1.333kPa) for the pure polymer membrane to 601.9 Barrer for the 30 wt% ionic liquid membrane. Also, selectivity of this membrane is improved from 8.2 to 25.8. mixed gas tests are implemented to investigate gases interaction. The results showed, the disruptive effect of CH4 molecules for CO2 permeation lead to selectivity decrement compare to pure gas test. The fabricated membranes with high ionic liquid content in this study are promising materials for industrial CO2/CH4 separation membranes.

Free-standing, thin-film, symmetric membranes: Next-generation membranes for engineered osmosis

The support layer of an asymmetric thin-film composite membrane results in structural resistance (internal concentration polarization) that significantly undermines engineered osmosis. Increasing the porosity and reducing the thickness and tortuosity of the membrane support layer reduces structural resistance; however, internal concentration polarization still impacts membrane performance. A novel, ultrathin, free-standing and symmetric membrane has been synthesized using sulfonated polyether ketone and tested for forward osmosis applications. This membrane is composed of a protonic acid group containing an aromatic polyether resin with sulfonated structural units. Polyether ketone provides high mechanical strength essential for ultrathin free-standing membranes, while sulfonation enhances the membrane hydrophilicity. These sulfonated polyether ketone membranes show promising water flux performances with impressive mechanical strength under the hydraulic operating conditions used for a FO process.

Formation of Asymmetric and Symmetric Hybrid Membranes of Lipids and Triblock Copolymers.

Hybrid membranes formed by co-assembly of AxByAx (hydrophilic-hydrophobic-hydrophilic) triblock copolymers into lipid bilayers are investigated by dissipative particle dynamics. Homogeneous hybrid membranes are developed as lipids and polymers are fully compatible. The polymer conformations can be simply classified into bridge- and loop-structures in the membranes. It is interesting to find that the long-time fraction of loop-conformation () of copolymers in the membrane depends significantly on the hydrophilic block length (x). As x is small, an equilibrium always results irrespective of the initial conformation distribution and its value depends on the hydrophobic block length (y). For large x, tends to be time-invariant because polymers are kinetically trapped in their initial structures. Our findings reveal that only symmetric hybrid membranes are formed for small x, while membranes with stable asymmetric leaflets can be constructed with large x. The effects of block lengths on the polymer conformations, such as transverse and lateral spans ( and ) of bridge- and loop-conformations, are discussed as well.

Peptide Kinetics in Symmetric and Asymmetric Membrane Mimics of Gram-Negative Bacteria

Antimicrobial peptides (AMPs) are well-studied compounds with high potential for application against multi-resistant bacterial strains. The AMPs L18W-PGLa and MG2a from the magainin family, exhibit a tenfold increase in killing efficacy when applied as a cocktail. Using Large Unilamellar Vesicles (LUVs) as simple lipid-only mimics of bacterial cytoplasmic membranes we were recently able to correlate this synergistic effect to the formation of a sponge phase with a co-existing lamellar phase. Here we used time-resolved small angle X-ray scattering and dye-leakage experiments to investigate the kinetics of the single peptides and their equimolar mixture, including a chemically linked L18W-PGLa/MG2a heterodimer (hybrid). For the hybrid peptide as well as the equimolar mixture, the sponge phase was already present after the experimental deadtime (i.e. 1 min after mixing). All peptides induced a transition from LUVs to multilamellar vesicles with a collapsed interbilayer spacing, apart from MG2a which just lead to an adhesion of the LUVs. The positional correlations of lamellae within the multibilayers increased with time for all systems but for the mixture, fastest for the hybrid, slowest for MG2a. For the peptide mixture, the multibilayers showed an increasing disorder after their initial formation indicating additional ongoing processes during the 30 minute time span of the experiment. Further, also the lamellar repeat distance exhibited non-monotonic behavior during the multibilayer formation process, showing first a decrease, followed by a swelling that started several minutes after the addition of the peptides. Finally, we report on the significance of cardiolipin and membrane asymmetry for the interaction kinetics with the AMPs.

Interleaflet Interaction in Phase Separated Asymmetric Lipid Bilayers

Ordered lipid domains (OLD) may serve as signaling platforms provided both membrane leaflets contribute. Interleaflet registration of phase separated regions in symmetric bilayers is driven by (i) the line tension around the thicker ordered domains (1) and (ii) membrane undulations (2, 3). How these forces act to generate membrane spanning domains in compositionally different leaflets is unknown. Here we use both fluorescence microscopy and fluorescence correlation spectroscopy of folded asymmetric planar lipid bilayers (4) to observe the enslavement of disordered lipids into ordered domains by ordered lipids in the opposing leaflet and, vice versa, the partial dissolution of ordered phases by disordered lipids in the opposing leaflet. These results were mirrored by the transition of lipids from the gel phase to the liquid phase and vice versa as seen by molecular dynamics simulations of compositionally symmetric, but - due to temperature differences - phase asymmetric bilayers. Our analysis of elastic lipid deformations, thermal membrane fluctuations and the structure of the fluid/gel boundary shows that the alignment of the original domain in one leaflet with the induced domain in the opposing leaflet is the energetically favored state and that the phase boundaries in the individual leaflets are shifted relative to each other by a few nanometers along the membrane surface. We conclude that similar forces govern domain registration in both symmetric and asymmetric membranes. 1. Galimzyanov, et al. Phys. Rev. Lett. 2015, 115:088101. 2. Horner, Antonenko, Y, Pohl. Biophys. J. 2009, 96:2689. 3. Galimzyanov, et al. Biophys. J. 2017, 112:339. 4. Horner; Akimov; Pohl. Phys. Rev. Lett. 2013, 110:268101.

Feasibility of concentrating textile wastewater using a hybrid forward osmosis-membrane distillation (FO-MD) process: Performance and economic evaluation

The forward osmosis-membrane distillation (FO-MD) hybrid process has shown great promise in achieving zero liquid discharge in the textile industry, recovering valuable dye molecules while producing large amounts of clean water. However, the progress of this technology seems to have stagnated with the direct coupling of commercial asymmetric FO and MD membranes, because water management in the system is found to be rather complicated owing to the processing of the different membranes. Herein, we propose, for the first time, an FO-MD hybrid process using a custom-made self-standing and symmetric membrane and a hydrophobic polytetrafluoroethylene membrane in the FO and MD units, respectively. Three types of operation modes were investigated to systematically study the process performance in the concentration treatment of model textile wastewater; two commercial FO membranes were also tested for comparison. Owing to its low fouling propensity and lack of an internal concentration polarization effect, the water transfer rate of our symmetric FO membrane quickly reaches equilibrium with that in the MD unit, resulting in continuous and stable operation. Consequently, the hybrid process using the symmetric FO membrane was found to consume the least energy, as indicated by its lowest total cost in both lab- and large-scale systems. Overall, our study provides a new strategy for using a symmetric FO membrane in the FO-MD hybrid process and highlights its great potential for use in the treatment of textile wastewater.

Developing symmetrical traditional double-stage membrane distillation system based on air-gap and water-gap configurations

Developing symmetrical traditional double-stage membrane distillation system based on air-gap and water-gap configurations

Influence of operating conditions on the desalination performance of a symmetric pre-conditioned Ti3C2Tx-MXene membrane capacitive deionization system

Introduction of new nanomaterials with conductivity, salt adsorption capacity (SAC) and rate (SAR) exceeding that of carbon electrodes may greatly improve capacitive deionization of water. However, those materials show a different electrochemical behavior, which must be studied and optimized for practical use. Here, effects of operating conditions on desalination performance of pre-conditioned Ti3C2Tx-MXene-based electrodes in a symmetric membrane capacitive deionization (MCDI) system were investigated. Specifically, influences of discharge potential, half-cycle length (HCL), and flow rate were systematically studied. Results showed different degrees of performance dependence on operating conditions. For instance, lower discharge potentials increased SAC and SAR by 152%. However, longer HCL increased SAC by 32% while decreasing SAR by 54%. Finally, faster flow rates decreased both SAC and SAR by 20%. Desalination performances of symmetric pre-conditioned MXene and activated carbon cloth (ACC) electrodes were gravimetrically and volumetrically compared in MCDI system. Pre-conditioned MXene electrodes gravimetrically performed 30% lower than ACC due to their notably higher density. Yet, pre-conditioned MXene electrodes volumetrically outperformed ACC by 162%. Results suggest that although MXenes offer high electrochemical activity and hydrophilicity, making them promising candidates for CDI applications, the strong dependence of desalination performance of MXenes on operating conditions requires in-depth understanding and warrants further research.

Spontaneous Curvature, Differential Stress, and Bending Modulus of Asymmetric Lipid Membranes

Lipid bilayers can exhibit asymmetric states, in which the physical characteristics of one leaflet differ from those of the other. This most visibly manifests in a different lipid composition, but it can also involve opposing lateral stresses in each leaflet that combine to an overall vanishing membrane tension. Here, we use theoretical modeling and coarse-grained simulation to explore the interplay between a compositional asymmetry and a nonvanishing differential stress. Minimizing the total elastic energy leads to a preferred spontaneous curvature that balances torques due to both bending moments and differential stress, with sometimes unexpected consequences. For instance, asymmetric flat bilayers, whose specific areas in each leaflet are matched to those of corresponding tensionless symmetric flat membranes, still exhibit a residual differential stress because the conditions of vanishing area strain and vanishing bending moment differ. We also measure the curvature rigidity of asymmetric bilayers and find that a sufficiently strong differential stress, but not compositional asymmetry alone, can increase the bending modulus. The likely cause is a stiffening of the compressed leaflet, which appears to be related to its gel transition but not identical with it. We finally show that the impact of cholesterol on differential stress depends on the relative strength of elastic and thermodynamic driving forces: if cholesterol solvates equally well in both leaflets, it will redistribute to cancel both leaflet tensions almost completely, but if its partitioning free energy prefers one leaflet over the other, the resulting distribution bias may even create differential stress. Because cells keep most of their lipid bilayers in an asymmetric nonequilibrium steady state, our findings suggest that biomembranes are elastically more complex than previously thought: besides a spontaneous curvature, they might also exhibit significant differential stress, which could strongly affect their curvature energetics.

Sandwich-like symmetric dual-phase composite membrane with an ultra-thin oxygen separation layer and excellent durability

The Zr0.84Y0.16O1.92-La0.8Sr0.2Cr0.5Fe0.5O3-δ (YSZ-LSCrF) dual-phase composite membrane is known to possess appreciable oxygen permeability and excellent stability under stringent operation conditions. In the present study, a sandwich-like symmetric YSZ-LSCrF membrane was prepared using the phase-inversion tape casting/lamination/sintering technique. The as-prepared membrane consisted of a 5 μm thick dense oxygen separation layer sandwiched between two 300 μm thick finger-like porous layers. To promote the surface oxygen exchange, the membrane was impregnated with Sm0.2Ce0.8O2-δ (SDC) nanoparticles. The oxygen permeation flux through the membrane was measured by exposing one side of the membrane to a flowing air stream and the other side to a flowing CO stream. An oxygen permeation rate of 1.95 ml (STP) cm–2 min–1 was obtained at 900 °C. The oxygen permeation rate increased with increasing temperature as expected, and the apparent activation energy for oxygen permeation was calculated to be 29.5 kJ mol–1. The membrane remained intact after the oxygen permeation measurement and thermal cycle stability test. The sandwich-like symmetric YSZ-LSCrF membrane with an ultra-thin oxygen separation layer demonstrated desired oxygen permeability and satisfactory durability, promising for membrane reactor applications. The preparation method developed in the present study can be applied to other ceramic membranes and devices requiring the similar sandwich-like symmetric structure.

Stimuli-Responsive Membranes through Sustainable Aqueous Phase Separation.

Polymeric membranes are used on huge scales for kidney dialysis, wastewater treatment, and drinking water production. However, almost all polymeric membranes are fabricated by a process reliant on the use of unsustainable, expensive, and reprotoxic dipolar aprotic solvents. In this work, we propose an aqueous phase separation approach for preparing porous membrane films. Poly(4-vinylpyridine) (P4VP), a pH-responsive polymer, is first dissolved at low pH where the polymer is charged and subsequently cast as a thin film. Switching to a high pH where the polymer is uncharged and insoluble results in controlled phase separation and solidification of the polymer into porous membrane structures. This approach gives a large degree of control over membrane structure, leading to symmetric porous microfiltration membranes and asymmetric dense nanofiltration membranes. Moreover, the use of a pH-responsive polymer leads directly to a pH-responsive membrane, where the degree of responsive behavior can be tuned by the degree of cross-linking. Such responsive behavior allows effective cleaning of the membrane, without the use of harsh chemicals. This work outlines an approach toward preparing membranes in a more sustainable fashion—an approach that allows control over the membrane structure and one that naturally leads to advanced membranes with responsive properties.

Understanding Interactions of Curcumin with Lipid Bilayers: A Coarse-Grained Molecular Dynamics Study

The interactions of curcumin with various lipid bilayers (POPC, DOPC, oxidized POPC, and oxidized DOPC) and model biomembranes (symmetric bacteria and yeast plasma membranes, as well as asymmetric mammalian plasma membrane) are investigated. A nonlinear thinning effect of curcumin with respect to its concentration is demonstrated in PC membranes and in the yeast. Curcumin induces asymmetry to the symmetric yeast membranes but reduces the degree of asymmetry of the mammalian plasma membranes when the molecule is placed facing the outer leaflets. The molecule is found to diffuse through oxidized PC bilayers, POPC bilayers at a curcumin to lipid ratio C/L = 1/5, yeast membranes at C/L = 1/100, and the mammalian plasma membranes at C/L = 1/5 and when the molecule placed facing the outer leaflets. The results of this work demonstrate that the lipid type, the lipid distribution, and curcumin amount play a critical role in defining the interactions of curcumin with the lipids and their transport behavior through the bilayers.

The Role of Interface Ions in the Control of Water Transport through a Carbon Nanotube.

Controlling the water transport toward a given direction is still challenging, particularly due to thermal fluctuations of water motion at the nanoscale. While most of the previous works focus on the symmetric hydrophobic membrane systems, the role of the membrane in affecting the water transport remains largely unexplored. In this work, by using extensive molecular dynamics simulations, we find an interesting electropumping phenomenon, that is, the flowing counterions on an asymmetric hydrophobic-hydrophilic membrane can significantly drive the single-file water transport through a carbon nanotube, suggesting a nanometer water pump in a highly controllable fashion. The ion-water coupling motion in electric fields on the charged surface provides an indirect driving force for this pumping phenomenon. The water dynamics and thermal dynamics demonstrate a unique behavior with the change in electric fields, surface charge density, and even charge species. Particularly, due to the ion flux bifurcation for the positive and negative surfaces, the water dynamics such as the water flow, flux, and translocation time also exhibit similar asymmetry. Surprisingly, the positive surface charge induces an abnormal three-peak dipole distribution for the confined water and subsequent high flipping frequency. This can be attributed to the competition between the surface charge and interface water orientation on it. Our results indicate a new strategy to pump water through a nanochannel, making use of the counterion flowing on an asymmetric charged membrane, which are promising for future studies.