Transport processes through track-etch membrane filters in a reagent delivery cell

A reagent delivery cell with a track-etch membrane filter for on-line dilution of concentrated salt solutions is described. The influence of several system parameters such as concentration of the stock solution, temperature. transmembrane pressure and the dependence on the diffusion coefficients of several salt components on the dilution was evaluated. As an application example, the use of the reagent delivery cell for on-line calibration of an atomic absorption spectrometer was studied. Fluxes through the membrane filter of 10 to 50 nL mm(-2) min(-1) with relative standard deviations of 0.8% within a day and 1.9% from day to day were achieved. The permeation experiments with the track-etch membrane filter for the dilution of aqueous solutions of several chlorides and sodium salts confirm a diffusion process. Flux rates can be estimated mathematically using Fick's first law with an agreement between measured and calculated dilution factors within 86 to 113%.     

Bilayer lipid membranes from falling droplets

We describe a system that provides a rapid and simple way of forming suspended lipid bilayers within a microfluidic platform from an aqueous droplet. Bilayer lipid membranes are created in a polymeric device by contacting monolayers formed at a two-phase liquid–liquid interface. Microdroplets, containing membrane proteins, are injected onto an electrode positioned above an aperture machined through a conical cavity that is filled with a lipid–alkane solution. The formation of the BLM depends solely on the device geometry and leads to spontaneous formation of lipid bilayers simply by dispensing droplets of buffer. When an aqueous droplet containing transmembrane proteins or proteoliposomes is injected, straightforward electrophysiology measurements are possible. This method is suitable for incorporation into lab-on-a-chip devices and allows for buffer exchange and electrical measurements.

Transport processes through track-etch membrane filters in a reagent delivery cell

A reagent delivery cell with a track-etch membrane filter for on-line dilution of concentrated salt solutions is described. The influence of several system parameters such as concentration of the stock solution, temperature, transmembrane pressure and the dependence on the diffusion coefficients of several salt components on the dilution was evaluated. As an application example, the use of the reagent delivery cell for on-line calibration of an atomic absorption spectrometer was studied. Fluxes through the membrane filter of 10 to 50 nL mm^–2 min^–1 with relative standard deviations of 0.8% within a day and 1.9% from day to day were achieved. The permeation experiments with the track-etch membrane filter for the dilution of aqueous solutions of several chlorides and sodium salts confirm a diffusion process. Flux rates can be estimated mathematically using Fick’s first law with an agreement between measured and calculated dilution factors within 86 to 113%.     

Breakup promotion of deformed EPDM droplets by the migration of nanoparticles during extrusion

The effect of migration of calcium carbonate (CaCO₃) nanoparticles on the breakup dynamics of Ethylene-Propylene-Diene Monomer (EPDM) droplets in Polypropylene (PP) matrix during melt extrusion was investigated in situ. The breakup process of EPDM droplets was sped up dramatically when the migration of CaCO₃ nano-particles from dispersed phase to matrix was introduced to PP/EPDM melts. It was found that both the total breakup time and the shape stability of slender EPDM droplets decreased with the increase of CaCO₃ concentration. Both the maximum value in equivalent diameter d and aspect ratio AR of EPDM droplets were also reduced by increasing the composition of CaCO₃ nanoparticles. Results were discussed in consideration of interfacial tension and migration of CaCO₃ nanoparticles. Reduction in interfacial tension is mainly responsible for the improved breakup process in the two-step composites with CaCO₃ nanoparticles (<2 wt%). Higher composition of CaCO₃ (≥2 wt%) induced the CaCO₃ aggregates in the EPDM phase. These aggregates acted as stress concentration when the EPDM droplets break up.     

Polysaccharide functionality through extrusion processing

Extrusion processing is a technology applied in the food and pharmaceutical industry for affecting product microstructure, product chemistry or the macroscopic shape of products. Starch based products are often extruded to break down the starch granule to render it digestible and to produce a shaped product. Encapsulation of flavors, nutrients and drugs is another frequent application of extrusion processing. This short review article is concerned with the use of extrusion processes to modify polysaccharide functionality. Extrusion processes are applied to polysaccharides for specific purposes such as physical modification or chemical modification (reactive extrusion), manufacture of confectionary gels and encapsulation of flavors or drugs. Non-starch polysaccharides and confectionary gels have also been extruded. Another application area is in the field of dietary fibers, obtained through extrusion processing of by- or waste-products of the food industry. The focus of this article is on extruding starch and other polysaccharides as an ingredient rather than as part of a final food product obtained by extrusion processing. It concludes with a discussion on extrusion as microstructure generating process and the relevance of this application to taste perception in semi-liquid foods.     

In-line monitoring of droplets deformation and recovering and polymer degradation during extrusion

An extruder can be operated as a torque rheometer by setting an external control of the processing variables and adding an in-line optical detector and an on-off mechanical valve at the extruder die exit. Various operational modes can be used including constant, ramp and sinusoidal changes of the die-head pressure. With the valve closed, a fixed amount of polymer is added and the extruder put into operation, controlling the screw rotation speed via software, having a proportional/integral/derivative controller. Polymer degradation can be followed recording changes in barrel pressure and torque. After processing, the valve is opened and the molten polymer discharged under a controlled die-head pressure, manipulating again the screw rotation speed. The polymer mixture morphology can be scanned during the discharge of the melt flow by the in-line turbidimeter, showing the deformation/recovery of the second phase droplets.     

Preferential glycerol transport by electroosmosis through ionic membranes

This paper describes a process in which glycerol is preferentially sorbed relative to water by an ionic membrane and transported into the product solution by electroosmosis at a significantly higher concentration than that in the feed solution. Direct measurement of the membrane distribution coefficient demonstrated preferential glycerol sorbtion for Neosepta CL-25T and AV-4T membranes. Both DC and AC electric currents significantly increase glycerol transport over that of a concentration difference alone.     

Separation of liquids by pervaporation through polymeric membranes

Fundamental and applied aspects of liquid separation by means of pervaporation through polymeric membranes are considered. The review gives the state of the art as well as prospects of development of this branch of membrane science and technology.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 208–219, February, 1994.     

Influence of environmental stresses on stability of oil-in-water emulsions containing droplets stabilized by beta-lactoglobulin-iota-carrageenan membranes

An oil-in-water emulsion (5 wt% corn oil, 0.5 wt% beta-lactoglobulin (beta-Lg), 0.1 wt% iota-carrageenan, 5 mM phosphate buffer, pH 6.0) containing anionic droplets stabilized by interfacial membranes comprising of beta-lactoglobulin and iota-carrageenan was produced using a two-stage process. A primary emulsion containing anionic beta-Lg coated droplets was prepared by homogenizing oil and emulsifier solution together using a high-pressure valve homogenizer. A secondary emulsion containing beta-Lg-iota-carrageenan coated droplets was formed by mixing the primary emulsion with an aqueous iota-carrageenan solution. The stability of primary and secondary emulsions to sodium chloride (0-500 mM), calcium chloride (0-12 mM), and thermal processing (30-90 degrees C) were analyzed using zeta-potential, particle size and creaming stability measurements. The secondary emulsion had better stability to droplet aggregation than the primary emulsion at NaCl 相似文献   

Transport of alkali-metal picrates through liquid membranes: Coupled action of w/o microemulsion droplets and lipophilic crown-ether carriers

The transport of potassium and sodium picrates is measured by a liquid membrane technique in the presence of two mobile carriers: water-in-oil microemulsion droplets and lipophilic crown-ethers. The fluxes are compared for two cases: (i) the crown-ether (dicyclohexano-18-crown-6 is assumed to be independent of the microemulsion droplet and (ii the crown-ether (dodecano- 18-crown-6) is assumed to be anchored in the droplet. A lower flux is observed when the microemulsion droplet and the crown-ether diffuse as a single entity. These results suggest that larger synergistic effects are obtained when the extractant can diffuse independently of the microemulsion droplet.     

Pertraction through liquid membranes

A survey of liquid membranes and related transport mechanisms is presented. Possibilities and advantages of application of pertraction as a separation and concentration operation are discussed. Pertraction into emulsions is analyzed from the process point of view in more detail. A further development of this operation will be connected with a better understanding of interphase phenomena.     

Deep desalination of water by evaporation through polymeric membranes

Transport properties and structural features of a number of already known and new types of polymeric membranes in desalination of aqueous salt solutions by membrane distillation and pervaporation were studied. The possibility of obtaining distilled water in a single stage by pervaporation from dilute or concentrated aqueous salt solutions was examined for the example of membranes fabricated from celluloses of varied origin (wood, cotton, bacterial).     

Separation of isomeric xylenes by pervaporation through cellulose ester membranes

The interaction between the isomeric xylenes and different cellulose esters was investigated using solubility parameter considerations and through measurements of swelling values. p]Hansen's three-dimensional solubility parameters δ_d, δ_p, δ_h of all the components have been calculated. These values have been used to predict the interaction between polymer and penetrant. A measure for this interaction is given by Δ, which is the distance between polymer and penetrant in the δ_d, δ_r, δ_h space. As expected, the experimental swelling values varied in inverse proportion to the calculated Δ values. p]Pervaporation characteristics of different cellulose ester membranes were determined by measuring product rates and selectivity. The differences in membrane characteristics have been explained qualitatively in terms of the solubility parameter concept.     

Transport of sulfate anions through inorganic membranes modified by ion-exchange material

Resistance properties of systems consisting of H₂SO₄ solutions and composite ceramic membranes modified by amphoteric ion-exchange material—hydrated zirconium dioxide (HZD)—are investigated by impedance spectroscopy. Transport numbers of sulfate ions through these membranes are calculated, and their values reach 0.86–0.92 as a function of the amount of HZD inserted into the membrane. Values of the transport numbers, estimated from impedance measurements, are found to be in good agreement with data obtained in investigations of sulfate transport at constant current under conditions in which the membrane charge is determined only by anions.     

Inert gas permeation through homopolymer membranes

New data are reported for the permeation of inert gases through polyethylene, polytetrafluoroethylene, and silicone and natural rubbers. Additional data are compiled from the literature. The relative solubilities of these gases are practically insensitive to chemical variations in the homopolymer. Hence variations in structure at the glass transition (T_g) and melting (T_m) temperatures that affect diffusion also unambiguously affect permcation. Consequently an equivalence results between permeation at a given temperature for different polymers and permeation at different temperatures for a given polymer. Although the diffusion coefficient changes continuously with temperature, the Arrhenius parameters D_o and E_d apparently change discontinuously at T_g and T_m. Their magnitudes and variations with atomic weight reach maxima at about T_g. These data indicate a dependence of the classical correlation between D_o and E_d on polymer properties. A perturbed diameter for the permeant, specific for each polymer, is proposed for correlating the D_o and E_d data. This correlation makes the changes observed at T_g and T_m more perceptible.     

Ion transport modelling through nanofiltration membranes

The aim of this work is to study the transport mechanism of ions through nanofiltration membranes. A model based on extended Nernst–Planck and film theory equations is reported. This model can be characterized by three transport parameters: the water permeability L_p, the salt transmittance Φ and the effective salt transfer coefficient K_eff. The knowledge of the feed and permeate concentration and of the permeate volumetric flux enable us to calculate these transport parameters. The model is used to estimate cadmium salts rejection by a NANOMAX 50 membrane. Experimental and calculated results are shown to be in good agreement. The model is then successfully extended to experimental data reported in the literature.     

Competitive facilitated transport through liquid membranes

A mathematical model is presented which solves the dimensionless, transient, non-linear partial differential equations governing the competitive facilitated transport of two gases through a liquid membrane. The model incorporates the mass transfer coefficients in the boundary conditions for the free gas concentrations. Several studies were carried out. A comparison of this model with a steady-state “equilibrium core” model was excellent. Through varying the dimensionless parameters, it was found that gas I would have a higher steady-state facilitation factor than gas 2 if k₁ >k₂ and k_-1k_-2. The boundary conditions and mass transfer coefficients were also varied to see their effects on the facilitation factors. The idea of pumping one of the gases against its concentration gradient was shown to be theoretically possible.     

Gas permeation through water-swollen hydrogel membranes

This work deals with water-swollen hydrogel membranes for potential CO₂ separation applications, with an emphasis on elucidating the role of water in the membrane for gas permeation. A series of hydrogel membranes with a wide range of water contents (0.9–10 g water/g polymer) were prepared from poly(vinyl alcohol), chitosan, carboxyl methyl cellulose, alginic acid and poly(vinylamine), and the permeation of CO₂, H₂, He and N₂ through the membranes at different pressures (200–800 kPa) was studied. The gas permeabilities through the dry dense membranes were measured as well to evaluate the resistance of the polymer matrix in the hydrogel membranes. It was shown that the gas permeability in water-swollen membrane is lower than the gas permeability in water, and the selectivity of the water-swollen membranes to a pair of gases is close to the ratios of their permeabilities in water. The permeability of the water-swollen membranes increases with an increase in the swelling degree of the membrane, and the membrane permeability tends to level off when the water content is sufficiently high. A resistance model was proposed to describe gas permeation through the hydrogel membranes, where the immobilized water retained in the polymer matrix was considered to form transport passageways for gas permeation through the membrane. It was shown that the permeability of hydrogel membranes was primarily determined by the water content in the membrane. The model predictions were consistent with the experimental data for various hydrogel membranes with a wide range of water contents (0.4–10 g water/g polymer).     

Transport through zeolite filled polymeric membranes

In this paper the effect of zeolite particles incorporated in rubbery polymers on the pervaporation properties of membranes made from these polymers is discussed. Pervaporation of methanol/toluene mixtures was carried out with membranes prepared from the toluene selective polymer EPDM and the methanol selective polymers Viton and Estane 5707. From the results of the pervaporation experiments it could be concluded that the addition of the hydrophilic zeolite NaX as well as the hydrophobic zeolite silicalite-1 leads to an increase in methanol flux and a decrease in toluene flux through the membranes. Pervaporation experiments with bi-layer membranes consisting of an unfilled polymer layer filled with zeolite particles demonstrated that the effect of addition of particles depends on their position in the membrane. Furthermore, the component flux through the membranes as a function of the volume fraction of zeolite is modelled with existing theories describing the permeability of heterogeneous materials. The results show that the apparent permeability of the dispersed phase is lower than the intrinsic permeability of the dispersed phase when the flux through the particle is restricted by the polymer phase. This phenomenon was confirmed by numerical simulation of the transport in the membrane through a plane parallel to the transport direction. The simulations are carried out for an unfilled membrane, a membrane filled with an impermeable particle, a rubber particle and with a particle which shows Langmuir sorption behaviour. The reason for the discrepancy between the apparent permeability and the intrinsic permeability is that the apparent permeability of the zeolite phase is calculated by dividing the flux with the driving force over the entire membrane which is larger than that over the particle. In case of numerical simulation the concentration in every position in the plane is known and therefore the intrinsic permeability of the filler can be calculated on basis of the actual driving force. This treatment results in a permeability which is correct over several orders of magnitude.     

Quantum model for transport through membranes

In the present paper a new model for the transport through membranes, introduced previously by Hosur, is considered. In this model, the membrane is assimilated to a potential barrier, and the material flow through the membrane is determined from the transmission coefficient of the barrier, assuming the existence of an energy difference among the molecules placed at both sides of the membrane. An equation for the transport, in the case of small energy differences, is obtained, which is particularized to the cases that the energy difference is caused by a temperature gradient, a concentration gradient, and both gradients acting together. In all cases, under certain limitations, formally identical equations to those of the thermodynamics of irreversible processes are obtained.