Zeta potential of ion-conductive membranes by streaming current measurements

Surface charge properties have a significant influence on membrane retention and fouling performance. As a key parameter describing the surface charge of membranes used in aqueous applications, zeta potential measurements on membranes of various types have attracted great attention. During the zeta potential characterization of a series of ion-conductive sulfonated poly(sulfone) membranes, it was found that the measured streaming current varied with the thickness of the sample, which is not predicted by the classical Smoluchowski equation. Moreover, for higher conductivity membranes with an increased concentration of sulfonate groups, the zeta potential tended toward zero. It was determined that the influence of membrane bulk conductance on the measured streaming current must be taken into account in order to correctly interpret the streaming current data for ion-conductive polymers and understand the relationship between membrane chemical composition and zeta potential. Extrapolating the measured streaming current to a membrane thickness of zero has proven to be a feasible method of eliminating the error associated with measuring the zeta potential on ion conductive polymer membranes. A linear resistance model is proposed to account for the observed streaming currents where the electrolyte channel is in parallel with the ion-conductive membranes.     

Characterization of polyelectrolyte multilayers by the streaming potential method

Polyelectrolyte multilayer adsorption on mica was studied by the streaming potential method in the parallel-plate channel setup. The technique was calibrated by performing model measurements of streaming potential by using monodisperse latex particles. Two types of polyelectrolytes were used in our studies: poly(allylamine) hydrochloride (PAH), of a cationic type, and poly(sodium 4-styrenesulfonate) (PSS) of an anionic type, both having molecular weight of 70,000. The bulk characteristics of polymers were determined by measuring the specific density, diffusion coefficient for various ionic strengths, and zeta potential. These measurements as well as molecular dynamic simulations of chain shape and configurations suggested that the molecules assume an extended, wormlike shape in the bulk. Accordingly, the diffusion coefficient was interpreted in terms of a simple hydrodynamic model pertinent to flexible rods. These data allowed a proper interpretation of polyelectrolyte multilayer adsorption from NaCl solutions of various concentrations or from 10(-3) M Tris buffer. After completing a bilayer, periodic variations in the apparent zeta potential between positive and negative values were observed for multilayers terminated by PAH and PSS, respectively. These limiting zeta potential values correlated quite well with the zeta potential of the polymers in the bulk. The stability of polyelectrolyte films against prolonged washing (reaching 26 h) also was determined using the streaming potential method. It was demonstrated that the PSS layer was considerably more resistant to washing, compared to the PAH layer. It was concluded that the experimental data were consistent with the model postulating particle-like adsorption of polyelectrolytes with little chain interpenetration. It also was concluded that due to high sensitivity, the electrokinetic method applied can be effectively used for quantitative studies of polyelectrolyte adsorption, desorption, and reconformation.     

Concentration dependence of electroosmosis and streaming potential across charged membranes

Electroosmosis and streaming potential measurements were carried out across charged membranes made of collodion and sulfonated polystyrene. Experiments were confined to the range where linear flux/force relationships hold. Saxén's relationship holds between electroosmosis and streaming potential; for porous charged membranes these exhibit an approximate inverse proportionality to ionic mobility at the limit of low electrolyte concentration. Both tend towards zero at the limit of high electrolyte concentration.     

Tangential streaming potential as a tool in modeling of ion transport through nanoporous membranes

Tangential streaming potential (TSP) measurements have been carried out so as to assess the electrokinetic properties of the active layer of organic nanofiltration (NF) membranes. Due to the porous structure of NF membranes, cares must be taken to convert the experimental data into zeta potential. Indeed, an assumption that is implicitly made in Smoluchowski's theory (or in related approaches accounting for the surface conduction phenomenon) is that both streaming and conduction currents involved in the streaming potential process flow through an identical path. Such an assumption does not hold with porous membranes since the conduction current is expected to flow wherever the electric conductivity differs from zero. Consequently, a non-negligible share of the conduction current is likely to flow through the membrane body filled with the electrolyte solution. This phenomenon has been taken into account by carrying out a series of TSP measurements at various channel heights. Experiments have been conducted with various electrolyte solutions. The inferred zeta potentials have been further converted into membrane volume charge densities which have been used to predict the membrane performances in terms of rejection rates. The conventional NF theory, i.e. based on a steric/Donnan exclusion mechanism, has been found to be unable to describe the experimental rejection rates. Using the volume charge density of the membrane as an adjustable parameter, it has been shown that the conventional theory even predicts the opposite sign for the membrane charge. On the other hand, the experimental rejection rates have been well described by including dielectric effects in the exclusion mechanism. In this case, a noticeable lowering of the effective dielectric constant of the electrolyte solution inside pores has been predicted (with respect to the bulk value).     

The streaming potential method for the characterization of ultrafiltration organic membranes and the control of cleaning treatments

Streaming potential measurement of ultrafiltration (UF) membranes have been realised a new design. This new design is more convenient to determine the streaming potential on a function of the pressure for all kinds of modules (planar, hollow fiber.h.). The effects of pH, ionic strength and size of pores have been studied. Isoelectric points of different materials (polyethersulfone, celloulse acetate, cellulose triacetate and polysulfone membranes) have been experimentally determined from ν variations with pH at a given ionic are, respectively, 3.1, 4.2, 3.4 and 0.5. The study of the charge organic membranes studied has been shown that adsorbing ions are those of water itself. Then the surface charge of the membrane is a dependent on the pH and at the isoelectric point, the charge density and the streaming potential vanished. The polyethersulfone membrane surface has been modified with TX100 adsorption and the modification observed with our design compared to contact angle and permeabilities measurements. The orientation angle of the surfactant at the membrane surface is obtained: θ=5°, and shows that a flat adsorption occurs. The impact of membranes cleaning procedures have been studied in term of permeability completed by streaming potential measurements. It appeared clearly that streaming potential is a useful tool for the control of cleaning procedures.     

Determination of the surface potential for adsorption layers of polyelectrolytes by the streaming potential method

A physicochemical model is proposed to describe electrokinetic phenomena in capillaries and pores the surface of which is coated with a charged porous adsorption layer. The use of this model makes it possible to explain experimental data on the surface potentials of polyelectrolyte adsorption layers upon their deformation resulting from solution flow. The commonly used Smoluchowski equation is shown to lead to large errors in the determination of the potential and charge of the surface of an adsorption layer.     

A note on the streaming potential method for measuring surface potentials

Due to the fact that the classical analysis of the streaming potential method for measuring surface potential is somewhat crude, the problem was analyzed by applying the fundamental principles of electrochemistry. It was found that the classical Helmholz-Smoluchowski equation is valid provided that the duct is long and narrow, in which case the potential in the bulk is uniform over the duct's cross-sectional area and varies linearly over its length.In order to follow how the surface potential of a filter varies with time, investigators have measured the streaming potential across the filter's length as well as across segments of the filter. The problem is what do such measurements mean and do the classical equations apply. It was found that the classical result is expressible in differential form from which it followed that the streaming potential over any section of the filter gives a measure of the average surface potential of that section. This is an important result because it shows that the surface potential profile of a filter can be determined from measurements of the streaming potential profile across the filter.     

Characterisation of the electrokinetic properties of plane inorganic membranes using streaming potential measurements

We present and test a device designed to measure the streaming potential of plane inorganic membrane during filtration.Two kinds of microporous membranes (a membrane made of a mixture of alumina-titania and this same type of membrane covered with an additional titania layer) are studied with different pH, ionic strength and electrolyte nature. The modification of the surface acid-basic equilibriums is analysed from the streaming potential measurements. The pores size of the studied membranes is large enough to avoid overlapping of the double layers. Streaming potential measurements are used to determine the zeta potential of the membranes from the Helmholtz-Smoluchowski relationship, corrected for the lowest ionic strengths studied. The shifting of the isoelectric point of the membranes studied with CaCl₂ and Na₂SO₄ solutions shows specific adsorption of calcium and sulfate ions onto the surface. The additional titania layer on the alumina-titania support does not seem to modify the electrokinetic properties of the membrane.The interactions of the alumina-titania membrane with the H⁺ and OH⁻ ions are analysed by studying the variations of pH between permeate and retentate compartments. These variations allow determining the isoelectric point of the membrane with a reasonable precision.     

Zeta potential of particle bilayers on mica: a streaming potential study

The streaming potential of mica covered by bilayers of latex particles was measured using the parallel-plate channel cell. The size of the first latex (A500) bearing amidine charged groups was 503 nm and the second latex (L800) bearing sulfonate groups was 810 nm (at pH 5.5 and an ionic strength of 10(-2)M). The A500 latex exhibited an isoelectric point at pH 10.5, whereas the L800 latex was strongly negative at all pH. Mica sheets were precovered first by the A500 latex particles under diffusion transport conditions. The coverage of this supporting layer was regulated between 0.02 and 0.5 by changing the bulk concentration of latex and the deposition time. Then, the second layer of the L800 latex of regulated coverage up to 0.55 was deposited under the diffusion transport. The coverage of particles and their distributions in both layers were determined by a direct enumeration of particles by optical microscopy under wet conditions and by AFM. It was shown that the structure of the L800 particle layers and the maximum coverage were in accordance with theoretical simulations performed according to the random sequential adsorption (RSA) model. After forming bilayers of desired composition and structure, streaming potential measurements were carried out. The influence of the mica substrate, the supporting layer coverage, and its zeta potential on the apparent zeta potential of bilayers was systematically studied. It was established that for a bilayer coverage exceeding 0.20, the net zeta potential became independent of the substrate and the supporting layer zeta potentials. Then, the asymptotic values of the zeta potential of the bilayer approach 1/√2=0.71 of the bulk zeta potential of the particles forming the external (second) layer. This behavior was interpreted theoretically in terms of the electrokinetic model derived previously for monolayers. It was also concluded that results obtained in this work can be exploited for interpretation of polyelectrolyte film formation in the layer by layer (LbL) processes and protein adsorption pertinent to the antigen/antibody interactions.     

Characterization of poly(ethylene imine) layers on mica by the streaming potential and particle deposition methods

Deposition kinetics of polystyrene latex (averaged particle size of 0.66 microm) on mica covered by poly(ethylene imine) (PEI), a cationic polyelectrolyte having an average molecular mass of 75,000 g mol(-1), was studied using the impinging-jet method. The hydrodynamic radius of PEI, determined by PCS measurements, was 5.3 nm. The electrophoretic mobility of PEI was measured as a function of pH for ionic strengths of 10(-3) and 10 (-2) M, which made it possible one to determine the amount of electrokinetic charge of the molecule and its zeta potential. Formation of the polyelectrolyte layer on mica was followed by measuring the streaming potential in the parallel-plate channel. From these measurements, the dependence of the apparent zeta potential of mica on the surface coverage of PEI was determined. The amount of adsorbed PEI on mica was calculated from the convective diffusion theory. These results were quantitatively interpreted in terms of the theoretical model postulating a particle-like adsorption mechanism for PEI with not too significant shape deformation upon adsorption. On the other hand, the Gouy-Chapman model postulating the adsorption in the form of flat disks was proved inappropriate. After the surface was fully characterized, particle deposition experiments were carried out with the aim of finding the correlation between the polymer coverage and the initial rate of latex particle deposition. In the range of small polyelectrolyte coverage, a monotonic relation between the polymer coverage and the initial deposition rate of particles, as well as the jamming coverage, was found. For Theta(PEI)>0.25, the initial particle deposition rate attained the value predicted from the convective diffusion theory for homogeneous surfaces. These results were interpreted theoretically by postulating that an effective immobilization of colloid particles occurred on local polyelectrolyte assemblages containing between two and three PEI molecules.     

Investigation of dynamic streaming potential by dimensional analysis

The theory of streaming potential at sinusoidal flow of liquid in a porous medium is a convenient and fruitful tool for determination of the interface properties of materials and also for construction of apparatus for zeta potential measurements and electrokinetic transducers. An investigation of the dynamic streaming potential by the method of dimensional analysis is presented. This method provides a wider approach to the problem under consideration. As a result, relationships between streaming potential in a porous medium and mechanical quantities are established. These quantities include pressure gradient in a liquid inside pores and capillaries, acceleration of capillaries, and the solid part of a porous medium, and the viscous friction force the liquid exerts on the solid part. The corresponding formulas for streaming potential are presented. The relationship between the streaming potential and viscous friction force does not depend on the frequency of oscillation and pore size. All these formulas in particular cases are transformed to known formulas for the streaming potential.     

Measurement of the streaming potential and streaming current near a rotating disk to determine its zeta potential

Methodology for determining the zeta potential of a disk-shaped sample by both streaming potential and streaming current measurements is presented. Integration of Laplace's equation within one radius of the disk surface revealed that the streaming potential decreased strongly in the surface normal direction. With this solution, the zeta potential can be calculated from measurements of the streaming potential near the surface of the disk provided the position of the working electrode near the disk surface is known. Determining the zeta potential of a disk-shaped sample by means of streaming current measurements required determination of a current collection efficiency because not all the streaming current from a disk flows through the auxiliary electronic current path. While the working electrode near the disk should be pointlike, several possible variants on counter electrode shape and size were explored. Although the current collection efficiency was only a few percent in each case, the measured current was of 10 nA order. The current collection efficiency depended only on system geometry and was independent of a disk's zeta potential and solution concentration. Streaming current measurements of zeta potential on silicon wafers in potassium chloride solutions up to 10 mM agreed well with published values.     

Streaming potential measurements as a characterization method for nanofiltration membranes

The streaming potentials of two different nanofiltration membranes were studied with several electrolyte solutions to investigate the influence of salt type and concentration on the zeta potential and kinetic surface charge density of the membranes. The zeta potentials decreased with increasing salt concentration, whereas the kinetic surface charge densities increased. The kinetic surface charge densities could be described by Freundlich isotherms, except in one case, indicating that the membranes had a negligible surface charge. The kinetic surface charge density observed was caused by adsorbed anions. Salt retention measurements showed different mechanisms for salt separation for the two investigated membranes. One membrane showed a salt retention that could be explained by a Donnan exclusion type of separation mechanism, whereas for the other membrane the salt rejection seemed to be a combination of size and Donnan excluion. Comparing the results obtained by the streaming potential measurements with those of the retention measurements, it could be concluded that the membrane with the highest kinetic surface charge density showed the Donnan exclusion type of separation, whereas the membrane with the lower surface charge density showed a separation mechanism that was not totally determined by Donnan exclusion, size effects seemed to play a role as well.     

Separation of bonito extract by composite UF membranes of sulfonated polysulfone coated on ceramics

Separation of ingredients in bonito extract was studied by a composite UF membrane of ceramic/sulfonated polysulfone (SPS). The bonito extract mainly contained inosine-5′-monophosphate (IMP), glutamic acid as a tasty ingredient and hypoxanthine, histidine as a putrefaction ingredient. The composite membrane showed a high rejection against negatively charged IMP, but permeated non-charged hypoxanthine. This is because of the negatively charged repulsion between the membrane and the solute. The permeation of amino acid could be controlled using the difference in isoelectric points of amino acids themselves. When the amino acid solution was filtrated by the composite membrane at pH 7, glutamic acid was rejected and no histidine was rejected. The charges of composite membrane were found to have an effect on the separation of ingredients in bonito extract.The composite membrane was stable within a wide pH range from 3 to 9, and had a thermal durability under 353 K.     

Grafting of temperature sensitive PNIPAAm on hydrophilised polysulfone UF membranes

Poly(N-isopropyl acrylamide) (PNIPAAm) was grafted on a polysulfone UF membrane. The changes of permeability as well as retention of PEG (35 kg/mol) and Dextran (500 kg/mol) between 23 and 60 °C were determined for both grafted and unmodified membranes. The results showed that the viscosity corrected water permeability and solute retention were almost constant for the unmodified membranes within the measured temperature range, the permeability of the grafted membranes increased and the retention of Dextran and PEG decreased with temperature. The variation of changes was most obvious in the temperature range 27–37 °C for the modified membranes due to the lower critical solution temperature (LCST) of PNIPAAm. The location and profile of PNIPAAm inside and on the surface of the membrane were analysed by SEM and FTIR. Depth profile calculation for FTIR spectra showed that PNIPAAm was mostly placed inside the membrane (at a depth of 1.06–1.10 μm from the surface) rather than on the surface. The amount of grafted PNIPAAm was low and did not significantly affect the morphology of the membrane. Therefore, a difference in SEM pictures of modified and unmodified membranes could not be seen. The modified membrane exhibited a clearly different thermal response compared to the unmodified one.     

Photoelectron spectroscopy and theoretical studies of UF5(-) and UF6(-)

The UF(5)(-) and UF(6)(-) anions are produced using electrospray ionization and investigated by photoelectron spectroscopy and relativistic quantum chemistry. An extensive vibrational progression is observed in the spectra of UF(5)(-), indicating significant geometry changes between the anion and neutral ground state. Franck-Condon factor simulations of the observed vibrational progression yield an adiabatic electron detachment energy of 3.82 ± 0.05 eV for UF(5)(-). Relativistic quantum calculations using density functional and ab initio theories are performed on UF(5)(-) and UF(6)(-) and their neutrals. The ground states of UF(5)(-) and UF(5) are found to have C(4v) symmetry, but with a large U-F bond length change. The ground state of UF(5)(-) is a triplet state ((3)B(2)) with the two 5f electrons occupying a 5f(z3)-based 8a(1) highest occupied molecular orbital (HOMO) and the 5f(xyz)-based 2b(2) HOMO-1 orbital. The detachment cross section from the 5f(xyz) orbital is observed to be extremely small and the detachment transition from the 2b(2) orbital is more than ten times weaker than that from the 8a(1) orbital at the photon energies available. The UF(6)(-) anion is found to be octahedral, similar to neutral UF(6) with the extra electron occupying the 5f(xyz)-based a(2u) orbital. Surprisingly, no photoelectron spectrum could be observed for UF(6)(-) due to the extremely low detachment cross section from the 5f(xyz)-based HOMO of UF(6)(-).     

Quenching of *UF6 (A state) by alkanes

The removal of *UF₆ (A state) molecules by selected alkanes has been investigated at 25°C. The following rate constants (units of 10¹¹ l/mol·sec) were evaluated: iso-C₄F₁₀, 0.0432 ± 0.0115; n-C₄F₁₀, 0.0764 ± 0.020; C₂F₆, 0.0192 ± 0.0052; CH₄, 0.0612 ± 0.0061; C₂H₆, 3.78 ± 0.60; C₃H₈, 5.08 ± 0.60; n-C₄H₁₀, 5.05 ± 0.78; iso-C₄H₁₀, 4.17 ± 1.15; neo-C₅H₁₂, 6.59 ± 0.93; CF₃? CH₃, 0.0385 ± 0.0056; CF₂H? CF₂H, 0.0729 ± 0.0074; and CF₂H? CFH₂, 0.149 ± 0.015. The perfluoro-alkane quenching of *UF₆ proceeds via a physical mechanism. The other alkane quenching reactions are consistent with a chemical mechanism also contributing in varying degrees which may involve removal of two hydrogens from the alkane.     

Electrochemical studies on poly(vinylidene fluoride-hexafluoropropylene) membranes prepared by phase inversion method

The copolymer membranes, poly(vinylidene fluoride-hexafluoropropylene) (PVdF-HFP) were prepared by phase inversion method using poly(ethylene glycol) (PEG) as an additive with acetone or dimethyl formamide as solvent. The morphology of the membranes has been greatly varied with the composition of the polymer and solvent. The ionic conductivity of the membranes were measured at various temperatures (−30 to 50 °C). The prepared membranes have been employed as separators in LiCoO₂/Li cells and were subjected to charge-discharge studies at 0.2 C - rate. The polymer membrane prepared with a ratio of PVdF-HFP (50):PEG (50) using dimethyl formamide as solvent delivered higher discharge capacity than the membranes prepared with acetone as solvent.     

Determination of the surface potential for hollow-fiber membranes by the streaming-potential method

A procedure has been proposed for measuring the surface potential of hollow-fiber membranes by the streaming-potential method under the conditions of a tangential flow of a solution. The zeta-potential and surface charge of nanofiltration hollow-fiber polyacrylonitrile membranes have been measured. The measurements have been performed for membranes with different porosities, which were obtained by partial drying of initial humid membranes. The porosity has been determined from the electrical conductivity of a membrane. An equation has been proposed for calculating the charge transfer by a solution flow in a porous layer. It has been shown that the use of the proposed equation makes it possible to obtain more correct values of the membrane surface potential.     

Calculation of the streaming potential near a rotating disk

A corrected theory of the streaming potential in the vicinity of a disk-shaped sample rotating in an electrolytic solution is presented. When streaming-potential measurements on a variety of materials were reduced to a zeta potential according to a prior theory, the results exceeded expected values by a factor of approximately 2, even though other aspects of the same experiments seemed to confirm the theory. Investigation of the source of the discrepancy revealed a flaw in the prior theory. The crucial understanding is that the surface current produced by the rotation of the disk emerges from the diffuse layer and enters the bulk solution at the periphery of the disk. The new treatment accounts entirely for the discrepancy between literature data and results based on the prior theory.