Diffusion potential is induced by coupled transport of ions through liquid membrane: nonlinear response to pH change in a reverse permeation system

In a cation exchange liquid membrane-aqueous alkali metal chloride system, diffusional flux of alkali metal ion driven by proton was observed. A supported liquid membrane formed on a Teflon filter by impregnating it with stearic acid-doped 1-octanol was used. The internal aqueous phase contained KCl and HCl, and the external aqueous phase also contained KCl. The initial concentrations of K⁺ ions of both phases were 1×10⁻¹ mol dm⁻³ for all the measurements. The concentration of HCl in the internal solution was kept at 1×10⁻² mol dm⁻³. The pH of the external solution was changed successively with HCl, appropriate buffer solution, or KOH. The pH dependence of membrane potential showed hysteresis loop in the range from neutral to alkaline pH, where reverse ion permeation was observed after the flux had been measured in the system with the external solution of an alkaline pH (pH 13). In the acidic range below neutral pH, the hysteresis of the membrane potential as well as reverse ion permeation was not observed. To elucidate the correlation between the appearance of hysteresis loop and the reverse ion permeation driven by proton across the membrane, the time course of the membrane potential in response to pH change was investigated. In the pH range where reverse permeation phenomena appeared, the time dependence of the membrane potential in nonsteady-state showed biphasic behavior. From the time course curve of the membrane potential, the total membrane potential was divided into the Donnan potential and the diffusion potential. From these findings, it was demonstrated that the diffusion potential was generated within the membrane only in the alkaline range where reverse ion permeation occurred. Analyzing the diffusional flux, the diffusion coefficient of potassium ion in the membrane was obtained taking the Donnan potential into account to be much greater than that in the membrane solvent. As a result of comparison of the diffusional fluxes measured by atomic absorption spectrometry and solution conductometry, the flux of the potassium ion was found to be significantly greater than that of the hydrogen ion in the opposite direction, especially at extremely high pH region. This implies the flows of hydroxide ions and neutralization reaction within the membrane facilitate the reverse ion permeation process of potassium ions.     

Mechanistic interpretation of solute permeation through a fully aromatic polyamide reverse osmosis membrane

The objective of this study was to assess the contribution of various mass transfer steps (concentration polarization, partitioning, and diffusive and convective transport) toward overall permeation of major solute sodium chloride (NaCl), and trace component nitrobenzene across the fully aromatic polyamide FT-30^® membrane. Experiments were performed with a closed-loop flat-leaf reverse osmosis apparatus. Feed solutions tested contained 2000, 4000 or 6000 mg/l NaCl, and 10 mg/l nitrobenzene at pH 6 and 25°C. Solute rejection ranged from 95 to 99.2% for NaCl and from 20 to 60% for nitrobenzene. The overall permeation of both NaCl and nitrobenzene appeared to be primarily by partitioning at water/membrane interfaces and diffusion across the membrane phase. Convection accounting for less than 25 and 0.4% of the overall NaCl and nitrobenzene permeation, respectively, appeared to be the result of a small leakage of feed solution through membrane imperfections estimated at 0.14% of the overall product water flux. Solute permeation was affected by concentration polarization taking place primarily within a fouling film of corrosion products. Concentration polarization levels corresponded to solute concentrations next to the feed water/membrane interface ranging from 5 to 70% (NaCl), and from 8 to 140% (nitrobenzene) higher than bulk feed concentrations.     

Membrane potentials and electrolyte permeation in a cation-exchange membrane

A method has been developed which permits one to determine the electrolyte permeation velocities and the membrane system permeability from measurements of the membrane potential as a function of time. The method has been applied to a cation-exchange membrane separating two aqueous KCl solutions, at the same pressure and temperature, but of different concentrations. The experiments have been carried out in two concentration ranges and at different solution stirring rates. The obtained results showed that the membrane system permeability depends notably on the stirring rate, this dependence being greater for the highest concentration range. The intrinsic permeability of the membrane was determined from considerations about the concentration polarization effect, its value being greater in the lower concentration range.     

New differential permeation rate method for determination of membrane transport parameters of gases

A new method for determining the membrane transport parameters (diffusivity, permeability and solubility) of gases through nonporous polymeric membranes is described. The method employs a continuous permeation chamber containing a flat membrane. The most important feature of this method is that, instead of a step concentration change, a rectangular pulse or impulse is sent to the upstream side of the membrane. Consequently, no steady state is approached but a signal peak of typical form can be recorded. The permeability and the diffusivity can be estimated from the height and half-width of the peak, respectively. The method was applied to measure the permeability of hydrocarbons through a polyethylene membrane, the permeation rate being measured by a flame ionization detector. The method and the derived relations are valid for other detectors and gas—membrane combinations as well. The advantages of this novel method are that all the membrane transport parameters can be directly evaluated from data of the response peak, whilst approaching the steady state is not necessary and thus the measuring time can be shortened. Finally, the known and new differential permeation rate methods are compared by generalization of the relationship between the input and output (response) functions.     

Characterization and gas permeation of polycarbonateliquid crystal composite membrane

Polymer/liquid crystal composite membranes were cast from a 1,2-dichloroethane solution of polycarbonate (PC) and N-(4-ethoxybenzylidene-4'-n- butylaniline) (EBBA). The mixing state of the polymer/liquid crystal composite membrane was investigated on the basis of differential scanning calorimetry, x-ray, density, sorption isotherm and sorption—desorption studies and also by electron microscopic observations. EBBA molecules in the composite membrane exist in an almost molecularly dispersed state up to an EBBA fraction of 30 wt%, and in the case of EBBA fractions above 30 wt% form a crystal domain as the mutual continuous phase among the network of polycarbonate fibrils. The composite membrane containing EBBA of 60 wt% can be handled as a homogeneous medium when considering gas permeation.The diffusive permeability coefficient to water reveals a distinct jump in the vicinity of the crystal—liquid crystal phase transition temperature of EBBA. The permeability coefficients, P, to hydrocarbon gases increases 100-200 times over several degrees in the phase transition temperature range. P for hydrocarbon gases decreases with increasing number of carbon atoms below the phase transition temperature, but increases with increasing number of carbon atoms above it. These results suggest that the permeation process is predominantly controlled by diffusion mechanism below the transition temperature of EBBA, while the solubility factor significantly affects gas permeation above it.     

Mechanism of nitromethane liquid membrane oscillator containing sodium oleate

The oscillatory behavior of a liquid membrane oscillator with anionic surfactant was investigated in order to understand the oscillation mechanism at the molecular level. As a theoretical framework, an approach based on chemical kinetics laws has been used. The chosen system involved nitromethane with 2,2(')-bipyridine as liquid membrane. The aqueous donor phase contained sodium oleate and butanol, while sodium chloride was added to the aqueous acceptor phase. It was established that the oscillations take place exclusively at the aqueous acceptor phase/membrane interface. Therefore, liquid membrane oscillators with anionic surfactants behave the same way as oscillators with cationic surfactants as to the location of oscillations. An oscillation mechanism involving three stages is proposed and confirmed by numerical simulations. The oscillations of electrical potential differences between the two aqueous phases are produced by sudden adsorption and desorption of anionic surfactant in solvated form and butanol at the acceptor/membrane interface. The whole process is controlled by the slow diffusion of these species across the liquid membrane. The chaotic character of the oscillations was demonstrated by Lyapunov exponents obtained from the strange attractor of the system.     

Dielectric analysis of nanofiltration membrane in electrolyte solutions: influences of electrolyte concentration and species on membrane permeation

Dielectric properties of a nanofiltration membrane immersed in dilute aqueous electrolyte solutions were measured, and frequency dependence of capacitance and conductance of the systems was analyzed, based on the interfacial polarization theory, giving values of permittivity and conductivity of the membrane and the solutions. Permittivity, epsilon m, of the membrane slightly decreased whereas conductivity, km, of the membrane increased with increasing electrolyte concentration, as a result of entrance of ions into the membrane. The ratio of membrane/solution conductivity, km/kw, also depended on the electrolyte concentration, showing that distribution of ions in the membrane and in solutions follow Donnan equilibrium, due to the presence of negative fixed charges in the membrane. New expressions were derived from Donnan equilibrium principle to explain this phenomenon, and negative fixed charge concentration ce of the membrane was obtained; thus the Donnan potential, DeltaPhi Don, of the membrane in solutions at various concentrations could be calculated. The new expressions could be expected to be usable to analyze ion permeation property through membrane.     

Preparation,structure characteristics and separation properties of thin-film composite polyamide-urethane seawater reverse osmosis membrane

A novel thin-film composite (TFC) seawater reverse osmosis membrane was developed by the interfacial polymerization of 5-chloroformyloxyisophthaloyl chloride (CFIC) and metaphenylenediamine (MPD) on the polysulphone supporting membrane. The performance of the TFC membrane was optimized by studying the preparation parameters, which included the reaction time, pH of the aqueous-MPD solution, monomer CFIC concentration, additive isopropyl alcohol content in aqueous solution, curing temperature and time. The reverse osmosis performance of the resulting membrane was evaluated through permeation experiment with synthetic seawater, and the structure of the novel membrane was characterized by using SEM, AFM and XPS. Furthermore, the separation properties of the TFC membrane were tested by examining the reverse osmosis performances of various conditions, the boron rejection performance and the long-term stability. The results show that the desired TFC seawater reverse osmosis membrane has a typical salt rejection of 99.4% and a flux of about 35 L/m² h for a feed aqueous solution containing 3.5 wt.% NaCl at 5.5 MPa, and an attractive boron rejection of more than 92% at natural pH of 7–8; that the novel seawater reverse osmosis membrane appears to comprise a thicker, smoother and less cross-linking film structure. Additionally, the TFC membrane exhibits good long-term stability.     

Induction time in metal permeation processes through supported liquid membranes

The origin of the induction time observed in the permeation process of cadmium species through trilaurylammonium chloride in triethylbenzene supported liquid membranes is discussed. A model for the non-steady state transference process, where aqueous film diffusion coupled to an interfacial chemical reaction are the main rate determining processes, was developed. By comparison with experimental transference data, the rate constant of the interfacial reaction between cadmium chloride aqueous complexes and the membrane carrier, trilaurylammonium chloride, was evaluated. The time evolution of the concentration profiles through the aqueous diffusion film is also described.     

The permeation liquid membrane as a sensor for free nickel in aqueous samples

There are currently a limited number of techniques to study nickel speciation in aqueous samples. This work reports on the use of the permeation liquid membrane (PLM) for that objective. In this paper, the composition of the organic phase was studied to maximize the Ni flux (thus the sensitivity of the device) over a wide Ni2+ concentration range (50 nM to 100 microM) in order to verify its ability to determine free Ni2+ in the presence of Ni complexes. A mixture containing 1,10-didecyl-1,10-diaza-18-crown-6 ether (22DD) and di(2-ethylhexyl)phosphoric acid (D2EHPA) in toluene/phenylhexane was selected as the optimized organic phase for the PLM. The PLM was shown to be a reliable tool to measure free nickel concentrations down to 10(-7) M. The effect of pH on Ni transport was also studied. Fluxes below pH 6 were reduced significantly, i.e. an order of magnitude smaller than fluxes above pH 7.8. Finally, as part of a broader study examining the ability of trace metals to induce antibiotic resistance in bacteria, we used the PLM to quantify the formation, at pH = 7.2, of a weak complex between Ni and Imipenem (a member of the carbapenem class of beta-lactam antibiotics) or its hydrolysis product(s).     

Coupling between transport phenomena and conformational transitions: Isothermal water transport and “asymmetric osmotic pressure” across a “membrane phase” promoted by a helix-coil transition

A membrane system is described consisting of an aqueous concentrated solution of poly-L-lysine bounded by two rigid conventional membranes permeable to water and to small ions but impervious to the polymer. When a large and steady pH difference is maintained between two external isoosmotic solutions α and β, a vertical volume flow may be observed from the acid solution to the basic solution across the membrane. The flow may give rise to a pressure difference between the basic and the acid solution corresponding to a non-equilibrium steady state of the system. These effects, first reported by Liquori et al. [1] are extensively analyzed from a thermodynamic point of view.It is shown that the pH gradient across the membrane maintains a conformational gradient of poly-L-lysine which is known to undergo a pH regulated helix-coil transition. The latter gradient in turn determines a gradient of chemical potential of water within the membrane phase acting as the main driving force of the volume flow.Using the flux equations of irreversible thermodynamics in connection with a model of helix-coil transition proposed by the author [3,4] compact equations are derived for the isobaric volume flow across the membrane phase and the steady state osmotic pressure in agreement with the experimental results.Possible implications of this study in connection with active transport phenomena across biological membranes are discussed.     

Thermocontrolling ion permeation through binary component membrane composed of crown ether liquid crystal/PVC

In view of the nature of orderness in structure and the mesomorphism in property of liquid crystal, the function of which is further exploited by integrating it with the feature of crown ether. The monoarmed crown ether liquid crystals are successfully applied to the imitation of biomembrane transport. Binary component membrane composed of crown ether liquid crystal and PVC was first developed. Such a novel model of biomimetic membrane is capable of imitating ingeniously the thermocontrolling transport of biomembrane, thus the essential function of liquid crystal in membane transport is more fully exploited. It was suggested, consequently, that the molecules of the crown ether liquid crystal could assemble themselves to form ionic channels, as they exist in mesophase.Of still more significance is that the thermocontrolling transport of ions through the membrane is found to be operative selectively and the permeation of ion is under the direct influence of the thermal turmoil of the crown ether liquid cr     

Extraction of molybdenum by a supported liquid membrane method

This is a report on the extraction of molybdenum(VI) ions using a supported liquid membrane, prepared by dissolving in kerosene, the extractant Alamine 336 (a long-chain tertiary amine) employed as mobile carrier. A flat hydrophobic microporous membrane was utilised as solid support. Appropriate conditions for Mo(VI) extraction through the liquid membrane were obtained from the results of liquid-liquid extraction and stripping partition experiments. The influence of feed solution acidity, the carrier extractant concentration in the organic liquid film and the content of strip agent on the metal flux through membrane were investigated. It was established that maximal extraction of metal is achieved at a pH 2.0 if sulphuric acid is used in the feed solution and at a pH value over 11.0 if Na₂CO₃ is used as strip agent. Moreover, the molybdenum extraction through membrane is enhanced when a 0.02 mol l⁻¹ content of the amine carrier in the organic phase is used. The present paper deals with an equilibrium investigation of the extraction of Mo(VI) by Alamine 336 and its permeation conditions through the liquid membrane, and examines a possible mechanism of extraction.     

Hydraulic permeation of NaCl solution through sulfonated polysulfone-polyvinyl alcohol polysulfone composite reverse osmosis membrane

The hydraulic permeation of NaCl solution through sulfonated polysulfone-polyvinyl alcohol/polysulfone composite reverse osmosis membrane was systematically investigated. It was found that the transport of water in a sulfonated polysulfone-polyvinyl alcohol/polysulfone composite reverse osmosis membrane follows the modified solution diffusion equation developed by Yang and Chu. Moreover, the equation between salt rejection and applied pressure was proposed for describing the hydraulic permeation of salt solution through a sulfonated polysulfone-polyvinyl alcohol/polysulfone composite membrane. The salt rejection was found to fit the greater part of separation transport mechanism. The experimental data and the salt rejection equation appear to be in excellent agreement.     

Study on a novel polyester composite nanofiltration membrane by interfacial polymerization of triethanolamine (TEOA) and trimesoyl chloride (TMC): I. Preparation, characterization and nanofiltration properties test of membrane

A novel thin-film composite (TFC) membrane for nanofiltration (NF) was developed by the interfacial polymerization of triethanolamine (TEOA) and trimesoyl chloride (TMC) on the polysulfone (PSf) supporting membrane. The active surface of the membrane was characterized by using FT-IR, XPS and SEM. The performance of TFC membrane was optimized by studying the preparation parameters, such as the reaction time of polymerization, pH of aqueous phase and the concentration of reactive monomers. It is found that the membrane performance is related to the changes of the monomer content in the aqueous phase rather than in the organic phase. Furthermore, the nanofiltration properties of the TFC membrane were tested by examining the separating performance of various salts at 0.6 MPa operating pressure. The rejection to different salt solutions decreased as per the order of Na₂SO₄ (82.2%), MgSO₄ (76.5%), NaCl (42.2%) and MgCl₂ (23%). Also, streaming potential tests indicated that isoelectric point of the TFC membrane is between pH 4 and 5. Moreover, the investigation of the flux for NaCl solution at different pH showed that the polyester NF composite membrane is also particularly suitable for treating acidic feeds: the flux increased from 8.4 to 11.5 L/m² h when pH of the feed decreased from 9 to 3. Additionally, the TFC membrane exhibits good long-term stability.     

Study of the Selectivity of SnO2 Supported Membranes Prepared by a Sol-Gel Route

In this work the sol-gel process was used to prepare SnO2 supported membranes with an average pore size of 2.5 nm. The effects of salt concentration (NaCl or CaCl2) and of the pH of the aqueous solutions used on the flux and selectivity through the SnO2 membrane were analyzed by permeation experiments and the results interpreted taking account of the zeta potential values determined from the electrophoretic mobility of the SnO2 powder aqueous dispersion. The results show that the ion flux (Na+, Ca2+ and Cl–) throughout the membrane is determined by the electrostatic repulsion among these species and the surface charge at the tin oxide-solution interface.     

Modeling of diffusive transport of benzoic acid through a liquid membrane

A model of diffusive transport of benzoic acid through a liquid membrane (LM) separating two aqueous solutions, based on diffusion layers and the assumption of a steady state, has been developed and tested using experimental results. It has been found that a model with the apparent partition coefficient dependent on the concentration is able to describe the time dependence of acid concentration in LM with and without a maximum on that dependence. The quality of the model fit with the single apparent diffusion coefficient of benzoic acid is the same as the one which takes into account the diffusion of benzoic acid in different forms (undissociated and dissociated form in aqueous phase, monomer and dimer in organic phase); however, in the second case, the model becomes overparameterized. Assuming that the partition and diffusion coefficients are constant, the diffusion layer model corresponds to the model of reversible consecutive reactions. Analytical solution for such case is given. Apart from the partition equilibrium, also kinetics of partitioning was considered. It was shown that in some basic situations both cases yield identical results.     

Development of a molecular recognition ion gating membrane and estimation of its pore size control

We have fabricated a molecular recognition ion gating membrane. This synthetic membrane spontaneously opens and closes its pores in response to specific solvated ions. In addition to this switching function, we found that this membrane could control its pore size in response to a known concentration of a specific ion. The membrane was prepared by plasma graft copolymerization, which filled the pores of porous polyethylene film with a copolymer of NIPAM (N-isopropylacrylamide) and BCAm (benzo[18]crown-6-acrylamide). NIPAM is well-known to have an LCST (lower critical solution temperature), at which its volume changes dramatically in water. The crown receptor of the BCAm traps a specific ion, and causes a shift in the LCST. Therefore, selectively responding to either K(+) or Ba(2+), the grafted copolymer swelled and shrank in the pores at a constant temperature between two LCSTs. The solution flux in the absence of Ba(2+) decreased by about 2 orders of magnitude over a solution flux containing Ba(2+). The pore size was estimated by the filtration of aqueous dextran solutions with various solute sizes. This revealed that the membrane changed its pore size between 5 and 27 nm in response to the Ba(2+) concentration changes. No such change was observed for Ca(2+) solutions. Furthermore, this pore size change occurred uniformly in all pores, as a clear cut-off value for a solute size that could pass through pores was always present. This membrane may be useful not only as a molecular recognition ion gate, but also as a device for spontaneously controlling the permeation flux and solute size.     

Application of permeation liquid membrane and scanned stripping chronopotentiometry to metal speciation analysis of colloidal complexes

The potential of permeation liquid membrane (PLM) to obtain dynamic metal speciation information for colloidal complexes is evaluated by measurements of lead(II) and copper(II) complexation by carboxyl modified latex nanospheres of different radii (15, 35, 40 and 65 nm). The results are compared with those obtained by a well characterized technique: stripping chronopotentiometry at scanned deposition potential (SSCP). Under the PLM conditions employed, and for large particles or macromolecular ligands, membrane diffusion is the rate-limiting step. That is, the flux is proportional to the free metal ion concentration with only a small contribution from labile complexes. In the absence of ligand aggregation in the PLM channels, good agreement was obtained between the stability constants determined by PLM and SSCP for both metals.     

Cyclic voltammetry of highly hydrophilic ions at a supported liquid membrane

Cyclic voltammetry has been used to study the coupling of ion transfer reactions at a liquid membrane. The liquids are either supported by a porous hydrophobic membrane (polyvinylidene difluoride, PVDF) when the organic solvent is non-volatile (o-nitrophenyloctylether) or are merely a free standing organic solvent layer such as 1,2-dichloroethane comprised between two hydrophilic dialysis membranes supporting the adjacent aqueous phases. The passage of current across the liquid membrane is associated with two ion transfer reactions across the two polarised liquid liquid interfaces in series. It is shown that it is possible to study the transfer of highly hydrophilic ions at one interface by limiting the mass transfer of the other ion transfer reaction at the other interface. Indeed, for systems comprising an ion M in one aqueous phase and a reference ion R partitioned between the membrane and the other aqueous phase, the observed and simulated cyclic voltammograms have a half-wave potential determined by the Gibbs energy of transfer of M transferring at one interface and by the limiting mass transfer of R at the other interface. This new methodology opens a way to measure the Gibbs energy of transfer of highly hydrophilic or hydrophobic ions, which usually limits the potential window at single liquid liquid interfaces (ITIES).