Membrane Diffusion Coefficient Research Articles

Measurement of Water Transport Properties Through Membrane-Electrode Assemblies: I. Membranes

A method using a hydrogen pumping cell to measure the electro-osmotic drag (EOD) coefficient of Gore-Select membranes is described. This technique was verified with the measurement of Nafion membranes, yielding results comparable with those reported in the literature and hence supporting its validity for various types of membranes. The EOD coefficient of Gore-Select membranes was measured to be 1.07 over the wide relative humidity range between and 95%. An approach was also proposed to characterize the water concentration at the interface between the membrane and the cathode catalyst layer using high-frequency resistance measurement in an operating fuel cell. Based on the information of water concentrations on both sides of the membrane and the water diffusive flux across the membrane, the water diffusion coefficient of Gore-Select membranes was experimentally determined. It was found that the water diffusion coefficient of Gore-Select membranes is roughly half that of Nafion membranes based on the best fit of our available data in this work.

Measurement of concentration-dependent gas diffusion coefficients in membranes from a psuedo-steady state permeation run

Transient permeation measurements were conducted for carbon dioxide through a PDMS membrane and for carbon dioxide and methane through a ZSM-5 zeolite membrane. After pressurization the feed tank was shut off from the supply and pressure and flux measurements were conducted while the feed-side pressure gradually declined over the course of each run. A series of steady state runs were also conducted for the same range of feed-side pressures. In all runs the transient fluxes were close to the steady state fluxes at corresponding feed-side pressures indicating that the transient evolved at a pseudo-steady state. A dimensionless parameter depending on the feed tank volume, membrane geometry, and adsorption properties was defined to characterize the deviation from steady state. Concentration-dependent diffusion coefficients were estimated from (i) steady state fluxes from a sequence of runs, (ii) transient fluxes from a single run, and (iii) transient pressure measurements from a single run. The fluxes and the diffusion coefficients obtained from a single transient run, and from a sequence of steady state runs are compared and the observed differences are discussed.

Heterogeneous membranes based on a composite of a hypercrosslinked microparticle adsorbent and polyimide binder

This paper deals with the preparation and characterization of heterogeneous membranes based on microparticle hypercrosslinked polymeric adsorbents with a polyimide binder. The polyimide based membrane extension is hindered by their low permeability. We enhanced the permeability of polyimide membranes by changed chemical structure and by adding of the new type fillers. Hypercrosslinked polystyrene microparticles of diameter 1–5 μm were prepared by SnCl 4-catalyzed Friedel–Crafts reaction of polystyrene with chloromethyl methyl ether in 1,2-dichlorethane solution. The precursor polyamic acid (PAA) was synthesized by the reaction of equimolar amounts of 4,4′-oxy(bis(phthalic anhydride)) (ODPA) and bis(4,4′-oxydianiline) (ODA) or 4,4′-[(1,4-phenylene)dipropane-2,2-diyl]dianiline (BIS P) in N-methylpyrrolidone (content 10 wt.%). A PAA solution in N-methylpyrrolidone with the adsorbent was spread onto a glass substrate and kept at 60–240 °C for 12 h. Heterogeneous membranes were characterized by thermal, mechanical and separation measurements. The permeability for membrane ODPA–BIS P filled with 10 wt.% of hypercrosslinked adsorbent was 3.5 × 10 −11 cm 3(STP) cm cm −2 s −1 cmHg −1 for nitrogen and 4 × 10 −9 cm 3(STP) cm cm −2 s −1 cmHg −1 for hydrogen. The permeability of homogeneous polyimide membranes is up to one order of magnitude lower. The diffusion coefficient of heterogeneous membranes increased in the order CH 4 < N 2 < O 2 < CO 2 < H 2. The selectivity of hydrogen–nitrogen separation with the amount of adsorbent decreased from 164 to 69. The prepared membranes are intended for separation of gases and low organic molecules even at enhanced temperature. The present paper aims at giving information on the influence of hypercrosslinked adsorbents and polyimide binding materials on the gas separation properties of membranes.

Modeling of enhanced penetrant diffusion in nanoparticle-polymer composite membranes

We have utilized 2D material point method (MPM) to study the influence of nanoparticles on the diffusivity of penentrants in model polymer membranes comprised of impenetrable spherical nanoparticles dispersed in a matrix with uniform penetrant solubility and diffusivity. Diffusion in the nanoparticle-polymer composite membrane was enhanced by the presence of a thin “skin” of matrix material next to the surface of the nanoparticles with a penetrant diffusion coefficient 100 times that of the bulk matrix. The influence of the skin thickness, nanoparticle area fraction and the manner in which the nanoparticles were distributed in the membrane on penetrant diffusion was studied. For a given skin thickness the penetrant diffusion in the composite membrane was found to increase exponentially with increasing area fraction of nanoparticles both above and below the percolation threshold. Membranes with clustered nanoparticles were found to have higher penetrants diffusivity than the membranes with fully dispersed particles due to the formation of anisotropic nanoparticle clusters. We found the total area fraction of particles + enhanced matrix (skin) to be a valid scaling variable for the effective diffusion coefficient of the nanoparticle-polymer composite membranes for the entire range of skin thickness and nanoparticle loadings investigated. Implications of the observed dependencies of effective penetrant diffusivity in the composite membrane on nanoparticle loading, skin thickness and nanoparticle morphology are discussed.

Diffusion behavior of benzene/cyclohexane molecules in poly(vinyl alcohol)-graphite hybrid membranes by molecular dynamics simulation

Understanding the effects of incorporated inorganic particles in organic–inorganic hybrid membranes at molecular level is crucial to design hybrid membranes with tailored structure and desired properties. In this study, molecular dynamics simulation has been employed to investigate the effects of graphite particle on the diffusion behavior of benzene and cyclohexane in poly(vinyl alcohol)-graphite hybrid membranes. Firstly, the non-bonding interaction energies between PVA polymer and graphite particle with different amount of hydroxyl and carboxyl groups were calculated by DREIDING force field. Next, polymer chain mobility was analyzed by mean-square displacement (MSD) and glass transition temperature ( T g ) , and found that incorporation of graphite particle into PVA increased the polymer chain mobility. Free volume characteristics of membranes were investigated by Connolly surface, and found that free volumes in PVA-graphite hybrid membranes were larger than those in PVA control membranes. This simulation result showed the same tendency with the PALS measurement. Finally, the diffusion coefficients of benzene and cyclohexane in the membranes were calculated by Einstein relation, and found that the diffusion coefficients in hybrid membranes were larger than those in the control membrane. Furthermore, it was found that the diffusion coefficients of benzene and cyclohexane in the membranes exhibited exponential relation with the inverse fractional free volumes and agreed well with the free volume theory.

Microdialysis Sampling Membrane Performance during in Vitro Macromolecule Collection

Microdialysis sampling is well-established for sampling small molecules. Recently, there has been an increased interest toward collecting macromolecules using microdialysis sampling. In this work, fluorescein isothiocyanate-labeled dextrans (FITC-dextrans) with molecular weight between 10 and 70 kDa were chosen as representative molecules to study analyte mass transport properties during microdialysis sampling using different lengths (2 and 10 mm) of 100-kDa MWCO polyethersulfone membranes. Experiments were performed in both well-stirred and quiescent phosphate-buffered saline solutions as well as in a 0.3% agar solution. Different fundamental parameters affecting microdialysis sampling of macromolecules, including effective membrane diffusion coefficients, were evaluated. The applicability of the most-often-cited Bungay et al. mass transfer model was compared to experimental data for the FITC-dextrans. For the larger macromolecules, the membrane provides a significant mass transport resistance most likely caused by hindered diffusion. These experimental aspects that are critical to microdialysis sampling of macromolecules are presented.

Microscopic calculations of local lipid membrane permittivities and diffusion coefficients for application to electroporation analyses

Interaction of electric fields with biological systems has begun to receive considerable attention for applications that include field-assisted drug delivery, medical interventions, and genetic engineering. External fields induce the strongest effects at membranes with electroporation being a common feature. Membrane transport in this context of poration is often based on continuum approaches utilizing macroscopic parameters such as the permittivity, diffusion coefficients, and mobilities. In such modeling, field dependences, local inhomogeneities, and microscopic details are usually ignored. Here, a molecular dynamics (MD) scheme is used for a more rigorous and physically realistic evaluation of such parameters for potential application to electroporative transport model development. A suitable membrane structure containing a nanopore derived from MD analysis is used as the initial geometric configuration. Both static and frequency dependent diffusion coefficients have been evaluated. Permittivities are also calculated and shown to be dramatically non-uniform in the vicinity of membranes under high external fields. A positive feedback mechanism leading to enhanced membrane fields is discussed.

Laser Ablation Inductively Coupled Plasma Mass Spectrometry Assisted Insight into Ion-Selective Membranes

Laser ablation inductively coupled plasma mass spectrometry was used to evaluate ion depth profiles across ion-selective membranes. Advantageously, this approach does not require incorporation of additional components (e.g., chromoionophore) in the membrane composition, as compared to that used in typical potentiometric applications. Moreover, comparison of the distribution of ions in differently pretreated membranes is possible. Concentration profiles of primary and interfering agent (Na+) ions were recorded, for example, of Pb2+-selective poly(vinyl chloride)-based membranes. It was found that the contents and the distribution of Pb(2+) and Na+ ions across the membrane is strongly dependent on the composition of the solutions to which both sides of the membrane are exposed during preconditioning and on the plasticizer included in the membrane formulation. Typical plasticizers, bis(2-ethylhexyl sebacate) (DOS) and the more polar 2-nitrophenyl octyl ether (o-NPOE), were used. It was found that faster ion transport occurs for o-NPOE, and the membrane saturation with Pb2+ ions was achieved within less than 20 h for a 400-microm-thick membrane. In the case of the less polar plasticizer DOS, due to slower rate of ion transport, even after 20 h, the Pb2+ concentration gradients were still visible within the membrane. On the basis of concentration profiles, primary ion diffusion coefficients in both membranes were calculated, and the value obtained for o-NPOE containing membrane was found to be approximately 2 times higher than for its DOS-plasticized counterpart.

Influence of the Non-Perfect Step Input Concentration at the Feed Side of the Membrane Surface on the Diffusion Coefficient Evaluation

Summary: Vapor diffusion coefficients in polymeric membranes were evaluated from dynamic permeation experiments. A membrane separated the diffusion cell into two parts – upstream and downstream. At the start of the experiment the concentration change in the upstream part (feed side) was made by substituting the input stream of pure nitrogen by the stream of permeant vapors. The solution of the Fick's second law with the step input concentration function is used for the evaluation of diffusion coefficients. The realization of the step input function can be difficult and its imperfection can negatively influence the evaluation process. This contribution deals with the description of the experimentally obtained input function and the study of its influence on evaluated values of diffusion coefficients. The mathematical model, which includes the non-perfect step input concentration function and the transport through a polymer was developed. The results of this study enable the estimation of diffusion coefficient evaluation errors as dependence on the experimental arrangements and on the membrane transport properties. Diffusion apparatus for measurement of the steady-state permeation process.

Spines and neurite branches function as geometric attractors that enhance protein kinase C action

Ca2+ and diacylglycerol-regulated protein kinase Cs (PKCs; conventional PKC isoforms, such as PKCγ) are multifunctional signaling molecules that undergo reversible plasma membrane translocation as part of their mechanism of activation. In this article, we investigate PKCγ translocation in hippocampal neurons and show that electrical or glutamate stimulation leads to a striking enrichment of PKCγ in synaptic spines and dendritic branches. Translocation into spines and branches was delayed when compared with the soma plasma membrane, and PKCγ remained in these structures for a prolonged period after the response in the soma ceased. We have developed a quantitative model for the translocation process by measuring the rate at which PKCγ crossed the neck of spines, as well as cytosolic and membrane diffusion coefficients of PKCγ. Our study suggests that neurons make use of a high surface-to-volume ratio of spines and branches to create a geometric attraction process for PKC that imposes a delayed enhancement of PKC action at synapses and in peripheral processes.

Optimum ionic conductivity and diffusion coefficient of ion-exchange membranes at high methanol feed concentrations in a direct methanol fuel cell

In direct methanol fuel cells (DMFCs), the optimum characteristics of ion-exchange membranes are investigated at high concentrations of methanol feed up to 7 M by modifying the diffusion coefficient and the ionic conductivity of the polyelectrolyte material. A Nafion membrane is modified by the incorporation of layered double hydroxide (LDH) nanoplatelets with different Mg 2+:Al 3+ ratios. When the feed concentration of methanol is lower than 3 M, the DMFC is controlled by the ionic conductivity of the polyelectrolyte membrane because methanol cross-over is not relatively significant. When the feed concentration is high, however, the diffusion coefficient of methanol is the key factor that determines the performance of the fuel cell. This is due to a high concentration gradient of methanol across the polyelectrolyte membrane. The open-circuit voltage is increased by the decreased diffusion coefficient in LDH/Nafion nanocomposite membranes at methanol feed concentrations up to 7 M; apparently because methanol cross-over is suppressed by the incorporation of LDH. The maximum power density of the DMFC is determined by the two competing transport processes of ion conduction and methanol diffusion, especially at a relatively high methanol concentration, that can provide optimum operating conditions in the membrane.

LIX ®-loaded polymer inclusion membrane for copper(II) transport : 2. Optimization of the efficiency factors (permeability, selectivity, and stability) for LIX ® 84-I

Polymer inclusion membrane transport of copper(II) in chloride and sulfate medium is studied using CTA (support), tris(2- n-butoxyethyl)phosphate (plasticizer), LIX ® 84-I (carrier), and ethanol (additive). An optimization in the aqueous phases (pH, type of anions and buffer concentration) and membrane compositions (extractant and plasticizer concentrations) was performed and the efficiency factors (permeability, selectivity, and stability) of the system evaluated. It was observed that besides the improvement in metal ion flux and reduction in metal accumulation in the membrane by the incorporation of the additive in the casting solution, an increase in membrane stability is also attained. A transport model for the system in sulfate medium is proposed. Values for the aqueous and organic resistances ( Δ a = 10 4.7 and Δ o = 10 8.0 s m −1, respectively), true organic diffusion coefficient and membrane diffusion coefficient of the Cu(II)–carrier complex in the membrane phase ( D b,o = 10 −13 and D o = 10 −12.2 m 2 s −1, respectively) were calculated assuming that a carrier–diffusion mechanism is mainly responsible for ion transport, as suggested for the transport profiles, and the value of the conditional extraction constant ( log K ′ ext = − 5.6 ) determined in an independent form by solid–liquid extraction experiments. A highly selective separation of Cu(II) from Zn(II) and Fe(III) was achieved as well.

Modelling of copper ion transport through supported liquid membrane containing LIX 984

The transport of copper(II) from an aqueous solution containing zinc, cadmium, nickel and cobalt through supported liquid membrane using LIX 984 dissolved in different diluents as a mobile carrier was studied. A transport rate model has been derived taking into account diffusion through an aqueous feed boundary layer, diffusion of carrier–copper complex through supported liquid membrane and diffusion through an aqueous strip boundary layer as simultaneous controlling factors. For different types of supported liquid membrane (kerosene/Teflon; n-heptane/Teflon; n-octane/Teflon; and kerosene/Durapore), the mass transfer coefficients, k f (m s −1), were calculated from the described transport rate equation to lie between 2.4 to 2.5·10 −5 , whilst the membrane diffusion coefficients of the carrier–copper complex, D R 2Cu, m (m 2 s −1), were 2.6·10 −10, 5.7·10 −10, 1.1·10 −9, and 4·10 −10, respectively. The results showed that Cd 2+, Zn 2+, Co 2+, and Ni 2+ ions were not transported in the pH range 1–5 studied.

Methanol and proton transport control by using layered double hydroxide nanoplatelets for direct methanol fuel cell

Layered double hydroxides (LDH) were incorporated into polyelectrolyte membranes in order to investigate the electrochemical reaction processes affected by transport rates of methanol and protons in direct methanol fuel cell (DMFC) applications. Depending on different ion exchange capacities and its compositions of LDH, the polyelectrolyte membranes gave different diffusion coefficients of methanol and proton conductivities, which were successfully correlated with the maximum power density of a single-cell DMFC. Decreasing the diffusion coefficient of methanol, the open circuit voltage was increased and the overall performance of DMFC was improved by incorporating LDH nanoparticles. An optimal property condition was observed when the proton conductivity and the diffusion coefficient were balanced in a desirable manner providing a 24% increment of the maximum power density compared with the pristine Nafion membrane at 5 M of methanol feed concentration. The developed nanocomposite technique identified the effects of the diffusion coefficient and proton conductivity of polyelectrolyte membranes in the electrochemical reactions of DMFC.

Permeation of water as a tool for characterizing the effect of solvent, film thickness and water solubility in cellulose acetate membranes

Cellulose acetate membranes have been used in many applications; of particular interest are reverse osmosis systems, and as a neutral matrix for incorporation of different polymers (e.g., conducting polymers), inorganic ions (e.g., lanthanides) and organic (e.g., pharmaceutical) compounds. The properties of the new polymers derived from cellulose acetate or blends depend on those of cellulose acetate. This work presents an attempt to find links between thermodynamic and kinetic properties of cellulose acetate membranes in equilibrium with water. Water diffusion coefficients in cellulose acetate membranes are reported, measured with a simple water permeation technique. The comparison of these values with the percentage of water uptake and polymer thickness leads to interesting conclusions related with different polymer properties.

Appropriate Material Properties for Advective–Diffusive Solute Flux in Membrane Soil

A generalized advective–diffusive equation for describing electrolytic solute transport in fine-grained soil (clay) membranes is presented. Clarification is made between the current theory and the widely cited coupled-flow theory by Yeung and Mitchell. Results of recently published experimental data are evaluated to demonstrate that the effective diffusion coefficient associated with Fick’s first law is not an appropriate independent material property for solute transport considerations in membrane soil. Three independent and experimentally attainable material properties are identified, specifically: osmotic efficiency, membrane diffusion coefficient, and hydraulic conductivity. Simple physical experiments by which these properties may be measured are described.

Removal of Ag(l), Cu(II) and Zn(ll) ions with a supported liquid membrane containing cryptands as carriers

The interface behaviour in the facilitated co-transport of Ag(I), Cu(II) and Zn(II) ions through supported liquid membranes (SLMs) made of a flat-sheet polypropylene membrane support containing cryptands (2.2.2 or 2.2.1) as carriers was studied. The liquid-liquid extraction tests showed a maximum distribution coefficient when the carrier concentration was greaterthan 10 −4M. In transport experiments the transmembrane flux increased with increasing carrier concentration reaching a limiting value at greater than 10 −3M concentration. The calculation ofthe diffusion coefficients in membranes showed ahigherdiffusivityof2.2.2-metal complexes with respect to 2.2.1-metal complexes for silver ions. A sequence of diffusivity D(Ag+)>D(Cu 2+)>D(Zn 2+) was obtained, but carrier 2.2.1 showed a higher selectivity through copper ions. A sequence of diffusivity D(Cu 2+)>D(Zn 2+)>D(Ag +) was obtained. The diffusivity was significantly higher when using Celgard 2500 support compared to Celgard 2400 or 2402. Variable metal ion concentrations in the feed phase fluxes almost zero, at less than 10 −2 M concentration, were obtained. In the transient state of the transport through the SLM, different molar flow rates at the feed-membrane and membrane-strip interfaces were observed. The selectivity of the interfaces containing 2.2.2 in the separation binary mixtures of ions showed the following separation factors: SF AgZn = 2.50, SF AgCu = 1.64, SF cuZn = 1.42.

Estimation of membrane diffusion coefficients and equilibration times for low-density polyethylene passive diffusion samplers.

Passive diffusion (PD) samplers offer several potential technical and cost-related advantages, particularly for measuring dissolved gases and volatile organic compounds (VOCs) in groundwater at contaminated sites. Sampler equilibration is a diffusion-type process; therefore, equilibration time is dependent on sampler dimensions, membrane thickness, and the temperature-dependent membrane diffusion coefficient (Dm) for the analyte of interest. Diffusion coefficients for low-density polyethylene membranes were measured for He, Ne, H2, O2, and N2 in laboratory experiments and ranged from 1.1 to 1.9 x 10(-7) cm2 sec(-1) (21 degrees C). Additionally, Dm values for several commonly occurring VOCs were estimated from empirical experimental data previously presented by others (Vroblesky, D. A.; Campbell, T. R. Adv. Environ. Res. 2001, 5(1), 1.), and estimated values ranged from 1.7 to 4.4 x 10(-7) cm2 sec(-1) (21 degrees C). On the basis of these Dm ranges, PD sampler equilibration time is predicted for various sampler dimensions, including dimensions consistent with simple constructed samplers used in this study and commercially available samplers. Additionally, a numerical model is presented that can be used to evaluate PD sampler concentration "lag time" for conditions in which in situ concentrations are temporally variable. The model adequately predicted lag time for laboratory experiments and is used to show that data obtained from appropriately designed PD samplers represent near-instantaneous measurement of in situ concentrations for most field conditions.

A theoretical approach to the measurement of radon diffusion and adsorption coefficients in radonproof membranes

Theoretical expressions for the determination of radon diffusion and adsorption coefficients in radonproof membranes using the standard method for testing the radon insulation effectiveness of a membrane tightly placed between two compartments are derived from the steady-state solution of the differential equation for radon diffusion. These expressions are applied to an experimental set-up designed specifically to carry out such a determination under controlled environmental conditions. Minimum detectable radon diffusion and adsorption coefficients of the method are shown to be typically well below those measured in most of the materials commonly used as radon barriers. Besides the radon diffusion and adsorption coefficients a new parameter called radon flux transmission factor and a more suitable definition of the radon permeability coefficient are also proposed to characterize the applicability of any membrane for radon insulation purposes.

Scaling of Structural and Rheological Response of L3 Sponge Phases in the “Sweetened” Cetylpyridinium/Hexanol/Dextrose/Brine System

We report a study of the shear response of sponge phases in cetylpyridinium chloride (CPCl)/hexanol/brine/dextrose systems by parallel measurements of rheology and structure by small angle neutron scattering (SANS). Our measurements show that this dextrose added to the extensively studied CPCl/hexanol/brine system is taken up exclusively by the brine solvent, resulting in an equivalent CPCl/hexanol membrane structure and phase behavior for this modified system. Adding dextrose to the brine in these systems to volume fractions up to 0.4 allows us to increase the solvent viscosity by more than an order of magnitude. This lowers the cooperative membrane diffusion coefficient in this system as measured by dynamic light scattering by the same factor, resulting in a corresponding slowing of the Helfrich fluctuation dominated membrane dynamics. Our results show clear and consistent evidence of shear-induced sponge to lamellar phase transformations in these systems. Further, both the rheological and microstructural responses of these systems follow universal master curves when plotted against a rescaled applied shear γ̇ηs/φ3, where φ is the membrane volume fraction and ηs is the viscosity of the brine/dextrose solvent. This well-defined shear response is characterized by three distinct regimes. At low shear rates the sponge phases exhibit Newtonian flow behavior and no structural change is observed. For intermediate shear rates, the systems shear thin and SANS measurements show that the sponge phases are progressively transformed into lamellar phases with the CPCl/hexanol membrane normals aligned parallel to the velocity gradient. This continuous process and the absence of a stress plateau in the rheological measurements both rule out the existence of a biphasic state in this region and thus of a first-order transition between sponge and lamellar phases as is observed in equilibrium phase diagrams. At higher shear rates, the systems are apparently again Newtonian, but the induced lamellar phase appears to collapse and a new (and as yet unidentified) large-scale structure forms. We have found these systems to be extremely sensitive to evaporation, and reproducibility of the scattering measurements required the use of a Couette shear cell with a specially designed vapor barrier.