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.     

Surface modification of phenolphthalein poly(ether sulfone) ultrafiltration membranes by blending with acrylonitrile-based copolymer containing ionic groups for imparting surface electrical properties

Asymmetric ultrafiltration membranes were fabricated from the blends of phenolphthalein polyethersulfone (PES-C) and acrylonitrile copolymers containing charged groups, poly(acrylonitrile-co-acrylamido methylpropane sulfonic acid) (PAN-co-AMPS). From the surface analysis by XPS and ATR-FTIR, it was found that the charged groups tend to accumulate onto the membrane surface. This result indicated that membrane surface modification for imparting surface electrical properties could be carried out by blending charged polymer. Furthermore, with the help of a relatively novel method to measure membrane conduction, the true zeta potentials calculated on the basis of the streaming potential measurements were used to reflect the charge state of membrane surface. In addition, it was noteworthy that, from the profiles of zeta potential versus pH curves and the magnitude of zeta potentials, the determination of zeta potential was dependent not only on the electrical properties of membrane surface but also on its hydrophilicity. At last, based on a relatively elaborate study on the electrostatic interaction between the membrane surface and protein, it was found that these charged membranes could meet different demands of membrane applications, such as resisting protein fouling or protein separation, through adjusting solution pH value.     

Effect of pH on hydrophilicity and charge and their effect on the filtration efficiency of NF membranes at different pH

In this work the effect of pH on membrane structure, its permeability and retention was studied. In addition, we studied whether the possible changes in the membrane properties due to the pH change are reversible. This is important for understanding the performance of nanofiltration membranes at different conditions and for the selection of cleaning processes. Moreover, the results facilitate the choice of membrane for specific applications.

Several commercial NF membranes were studied at different pH values. Their retention and flux were explained by the charge and the hydrophilic characteristics of the membranes. The filtrations were made with uncharged sugar and salt solutions.

The lower the membrane contact angle (i.e., a more hydrophilic membrane) the higher was the change in apparent zeta potential when pH was increased from 4 to 7. As a result, the retention of ions with more hydrophilic membranes changed more than hydrophobic ones when the pH was increased in the feed solution. However, some membranes retained ions well at high pH although their apparent zeta potential or hydrophilicity was relatively low. These membranes had charge inside the pores and it was not detected by streaming potential measurement along the surface or by measuring the contact angle of the surface. Thus, the apparent zeta potential of the exterior membrane surface did not sufficiently describe the ionic transport through the membrane. In addition, some membranes became significantly more open at high pH (i.e., flux increased). This was explained by the chemical nature of the polymer chains in the membrane skin layer, i.e., dissociating groups in the polymer made the surface more hydrophilic and looser when charges of the polymer chains started to repel each other at elevated pH. Generally, the retention of uncharged glucose decreased more at high pH than the salt retention. The changes in permeabilities and retentions were found to be mostly reversible in the pH range studied (very slowly in some cases, however).     

Functionalization of organic membranes by polyelectrolyte multilayer assemblies: application to the removal of copper ions from aqueous solutions

The functionalization of an organic polyethersulfone membrane (PES) was performed by alternating deposition of poly(allylamine hydrochloride) (PAH) and poly(sodium 4-styrene sulfonate) (PSS), leading to the formation of a polyelectrolyte multilayer film (PEM). The resulting assembly was characterized by tangential streaming potential measurements to determine the charge of the modified membranes as a function of the polyelectrolyte solution concentration and as a function of the immersion time of the membrane in the polyelectrolyte solutions. Then, the modified membranes were used to perform the ultrafiltration of aqueous solutions containing copper(II) ions. Different operating conditions were tested including: polyelectrolyte concentration, polyelectrolyte nature, thickness of the PEM film or pH of the Cu(2+) solutions. These filtration experiments demonstrated that it was possible to obtain a satisfactory retention of the copper ions (88%), thus proving that this type of assembly can be useful for the removal of copper ions from contaminated aqueous solutions.     

PREPARATION AND CHARACTERIZATION OF POLYSULFONE AND POLYETHERSULFONE MEMBRANES FOR CALCIUM (Ca2+) AND MAGNESIUM (Mg2+) SEPARATION BY COMPLEXATION ULTRAFILTRATION

ABSTRACT

Asymmetric ultrafiltration membranes were synthesized from locally available polysulfone and polyethersulfone polymers using aprotic solvents and organic additives by the phase inversion method. The membranes were characterized in terms of pure water permeability, separation behavior with respect to polyethylene glycols of various molecular weights and electrolytes. The suitability of using polyethyleneimine (PEI) for selective removal of calcium and magnesium salts by an ultrafiltration process was studied in terms of optimum polymer loading at reasonable permeate flux, irreversible adsorptive fouling of the macromolecular ligand on the polymer as functions of solution pH and ionic strength, and metal ion separation as a function of concentration and pressure. Direct electron microscopic observation of fresh, as well as fouled, membranes are presented.     

Fibrinogen adsorption on mica studied by AFM and in situ streaming potential measurements

Adsorption of fibrinogen from aqueous solutions on mica was studied using AFM and in situ streaming potential measurements. In the first stage, bulk physicochemical properties of fibrinogen and the mica substrate were characterized for various ionic strength and pH. The zeta potential and number of uncompensated (electrokinetic) charges on the protein surfaces were determined from microelectrophoretic measurements. Analogously, using streaming potential measurements, the electrokinetic charge density of mica was determined for pH range 3-10 and the NaCl background electrolyte concentration of 10(-3) and 10(-2) M. Next, the kinetics of fibrinogen adsorption at pH 3.5 and 7.4 in the diffusion cell was studied using a direct AFM determination of the number of molecules per unit area of the mica substrate. Then, streaming potential measurements were performed to determine the apparent zeta potential of fibrinogen-covered mica for different pH and ionic strength in terms of its surface concentration. A quantitative interpretation of these streaming potential measurements was achieved in terms of the theoretical model postulating a side-on adsorption of fibrinogen molecules as discrete particles. On the basis of these results, the maximum coverage of fibrinogen Θ close to 0.29 was predicted, in accordance with previous theoretical predictions. It was also suggested that anomalous adsorption for pH 7.4, where fibrinogen and the mica substrate were both negatively charged, can be explained in terms of a heterogeneous charge distribution on fibrinogen molecules. It was estimated that the positive charge was 12 e (for NaCl concentration of 10(-2) M and pH 7.4) compared with the net charge of fibrinogen at this pH, equal to -21 e. Results obtained in this work proved that the coverage of fibrinogen can be quantitatively determined using the streaming potential method, especially for Θ < 0.2, where other experimental methods become less accurate.     

Tangential flow streaming potential measurements: Hydrodynamic cell characterization and zeta potentials of carboxylated polysulfone membranes

Computational fluid dynamics calculations were carried out to ensure that a self-made tangential flow mode streaming potential measurement cell meets the hydrodynamic stipulations of laminar, steady and established electrolyte flow necessary for reproducible electrokinetic measurements. The calculations show that the cell design meets all of these conditions.Six carboxylated polysulfones with a range of different degrees of substitution (DS) from 0.26 to 1.74 carboxyl groups per polymer repeat unit were synthesized in a two-stage process of lithiation and carboxylation. Ultrafiltration membranes were made from both the unmodified polysulfone and these hydrophilic materials. The zeta potentials of these membrane surfaces were determined in 0.001 M KCl solution as a function of pH. The curves show the theoretically expected profiles for non-ionic and weakly acidic materials. The growing influence of the COOH dissociation on the surface charge formation is indicated by the flattening of the curves at low pH values. The magnitude of the negative zeta potentials plateau values ranged from −52 to −20 mV. While unmodified PSU has a plateau value of −52 mV this value decreases continuously with increasing DS to −20 mV for the PSU-COOH 1.74 material. It is suggested that this arises from a shift of the electrokinetic shear plane into the bulk electrolyte solution due to an extended swelling layer reflecting the enhanced hydrophilicity of these membrane surfaces.     

Electrosurface properties of poly(ethylene terephthalate) films irradiated by heavy ions and track membranes based on these films

The streaming potential method realized in a slit-like setup using 10^–4–10^–2 mol/L KCl solutions has been employed to study the electrosurface characteristics of poly(ethylene terephthalate) films, both initial and irradiated by heavy ions, as well as track membranes with pore sizes of 50 and 210 nm made from these films. Their ζ potentials and surface charges have been calculated. The data obtained suggest that irradiation of the polymer films by heavy ions reduce the ζ potential and surface charge. However, as a result of film etching during the preparation of the track membranes, the ζ potential and surface charge increase and exceed the corresponding values for the initial film.     

Determining the zeta-potential of ceramic microfiltration membranes using the electroviscous effect

The possibility of measuring the zeta-potentials of porous membranes using the electroviscous effect was investigated. The zeta-potential of Membralox^® ceramic microfiltration membranes was determined both with the newly developed electroviscous technique and by streaming potential measurements. It was found that the electroviscous technique provided a simple means of obtaining accurate values of zeta-potential, especially for higher zeta-potentials. The streaming potential measurements were found to be more suitable for the determination of the iso-electric point, i.e. the pH at which the zeta-potential is zero.The iso-electric points of new α-alumina, zirconia, and titania membranes were found to be 8.5, 8.0, and 6.3, respectively. Upon using the membranes and cleaning them with a detergent, the iso-electric point of the α-alumina membrane decreased to 6.5, and that of the zirconia membrane decreased to 5.2, while the iso-electric point of the titania membrane stayed virtually constant. Cleaning these membranes with a strong acid or base could not reverse the observed decreases in iso-electric point.     

Streaming potential in open and packed fused-silica capillaries

The surface properties of novel stationary phases in packed and open tubular columns for capillary electrochromatography (CEC) were examined by measuring the streaming potential in a home made apparatus. The surfaces investigated include materials such as porous styrenic sorbents and octadecyl-silica as well as fused-silica tubing, in both raw and surface modified forms. Functionalization of the surface was carried out, for instance, by reductive amination or organosilane grafting on to capillary inner wall. The dependence of the streaming potential on pH was examined with aqueous solutions in the pH range from 2.5 to 9.0. Electrokinetic properties of 50 microm I.D. fused-silica capillaries have been determined by both streaming potential and electrosmotic flow measurements. Both methods gave similar pH profiles of the zeta-potential and the isoelectric points. This confirms the viability of our approach to evaluate the specific charged groups of the packing which is one of the important factors influencing electrosmotic flow (EOF) velocity and protein adsorption during a chromatographic run. In addition to bare silica capillaries, styrenic monolithic columns with different surface functionalities, which have been extensively used in our laboratory for CEC separation of peptides and proteins, were employed for comparison of two methods. Plots of zeta potential as a function of percent ACN show a complex behavior, indicating that zeta potential cannot be predicted simply from binary mixture solvent properties. It is demonstrated that the evaluation of the zeta potential by the streaming potential method is nondestructive, relatively fast, without untoward effects introduced by Joule heating and yet another means for the characterization of the surfaces under conditions employed in CEC.     

用流动电位法表征纳滤膜的结构及性能

利用测量流动电位的方法考察了纳滤膜的表面电学性能对纳滤膜的截留性能的影响.首先,采用不同功能层材料制备了复合纳滤(NF)膜,考察功能层的交联时间、单体结构等对表面电性能的影响,研究纳滤膜对不同无机盐的选择截留性能与表面电性能的关系.通过流动电位法测定纳滤膜的表面电学参数,如流动电位(ΔE)、zeta电位(ζ)和表面电荷密度(σd).实验表明,这些电学参数的变化与功能层交联时间和纳滤膜截留率的变化一致,在交联时间为45 s时,3种电学参数的绝对值均最大,而纳滤膜对无机盐的截留率也最大.复合纳滤膜zeta电位的绝对值(|ζ|)按照Na2SO4>MgSO4>MgCl2变化,同截留率的变化相同.带侧基单体交联后得到的纳滤膜的表面电性能参数的绝对值小于不带侧基单体的.因此,流动电位法可用于研究复合纳滤膜的截留机理和功能层结构.     

Characterization of chitosan nanofiber fabric by electrospray deposition: electrokinetic and adsorption behavior

Cationic biopolymer nanofiber fabrics were prepared from a chitosan/poly(ethylene oxide) blend solution by electrospray deposition. Their electrokinetic properties and DNA adsorption behavior were analyzed as a function of pH. The zeta potential was determined from streaming potential/streaming current measurements. The adsorption of DNA onto the fabrics was investigated by spectrophotoscopy. The adsorption behavior of DNA correlated well with the electrokinetic properties of the fabrics. This revealed that the electrokinetic approach was a useful option for characterization of novel nanofiber assemblies made by the electrostatic spray process. In addition, these results provided fundamental information about chitosan nanofiber fabrics for both biomedical and analytical applications.     

Streaming potential across cation-exchange membranes in methanol-water electrolyte solutions

Streaming potential measurements across charged membranes separating two equal solutions have been carried out. Two cation-exchange membranes with different cross-linked and swelling properties (Ionics and Nafion membranes) and methanol-water electrolyte solutions of KCl have been used in the experiments. The obtained results show that the streaming potential is higher for the Ionics membrane and that the values depend on the methanol content of the solutions. A different behavior is found in the dependence of the streaming potential on the methanol percentage for each membrane. The study of the relaxation times in the decay of electrokinetic steady states of streaming potential has been carried out from the time dependence of the streaming potential when the pressure difference through the membrane is suppressed. The results show the existence of two different parts or partial relaxations, mechanical and electric. A different behavior of the mechanical relaxation time with the methanol percentage has been found for the two membranes, but any significant difference between their electric relaxation times. These differences have been explained in terms of the different degree of swelling of the membranes used.     

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.     

Problems and results of zeta-potential measurements on fibers

A survey is given on both theoretical background and methodical details of zeta-potential measurements on fibers. Electro-osmosis and streaming potential/streaming current measurements can be used in order to obtain correct zeta-potentials. Both measuring principles yield the same values for zeta-potential if the errors due to resistance measurements are avoided. This agreement as well as the independence of zeta-potential of applied voltage (in the case of electro-osmosis) and hydrostatic pressure (in the case of streaming potential/streaming current) point out that the Stern-potential at the boundary immobile/diffuse layer can be determined. Electrophoresis and measurement of other electrokinetic phenomena give values related to but not identical with the zeta-potential. Applications of electrokinetic measurements for investigating fiber problems in production, processing and finishing are reviewed. Parameters determining the zeta-potential of fibers are discussed.Dedicated to Prof. Dr. E. R. Schwarzl with congratulations for his 60th birthday.     

Surface characterization of hemodialysis membranes based on electrokinetic measurements

Hemodialysis membranes were characterized by means of streaming potential measurements. By variation of the concentrations of different ionic species in the measuring solutions surface potential determining processes can be distinguished: The investigated materials (cellulose derivates) yield surface charges mainly from preferred adsorption of ions. A thermodynamic model of the electrochemical double layer according to STERN (1) was applied to quantify that processes; the resulting set of parameters provide a conjunction between chemical surface properties and the observed interfacial charging processes. Streaming potential measurements can be used for the in situ characterization of the adsorption of biologically relevant molecules like proteins and polysaccharides onto membrane materials. The results given here show the alteration of interfacial properties of different cellulosic membranes through adsorption of human serum albumin and fibrinogen in single, sequential and competitive adsorption.     

Comparison of redox initiated graft polymerisation and sulfonation for hydrophilisation of polyethersulfone nanofiltration membranes

Three polyethersulfone nanofiltration membranes, including two commercial membranes (NFPES10 and N30F) and one lab-made membrane (N71), were hydrophilised in view of an increased water flux and a reduction of membrane fouling.Non-modified as well as modified membranes were characterized using contact angle measurements, water fluxes and retention of sugars.At first, redox initiated graft polymerization was applied, and the influence of reaction time, initiator concentration and monomer concentration were investigated. It was observed that grafting always resulted in a significant reduction of the contact angle. The higher the reaction time, the lower was the contact angle, but the initiator and monomer concentration had no effect when above a threshold value. Grafting results in a decrease of the water flux, while the retention of raffinose increased.A comparison was made with the addition of sulfonated polyethersulfone to the polymer solution. By blending polyethersulfone and sulfonated polyethersulfon, a sufficiently high viscosity could be achieved. The influence of the blending ratio was investigated. The larger the fraction of sulfonated polyethersulfon, the more hydrophilic the membrane became. The addition of sulfonated polyethersulfon, however, has also an influence on the water flux and on the retention of sugars. An important conclusion is also that the membranes containing sulfonated polyethersulfone are significantly more hydrophilic than the ones that have been modified through the grafting technique.     

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.     

Electrochemistry of capillary systems with narrow pores. I. Overview

In capillary systems with narrow pores the Helmholtz electrochemical double layer located at the pore wall extends over the entire cross section of the pores. It loses its character as the “charge on the wall”. It will be shown that not only the electrokinetic phenomena but also the electrical conductivity and the dialysis potential of membranes with narrow pores can be understood from the same point of view, namely: the electrolyte solution in the pores of a membrane with narrow pores is considered to be an approximately homogeneous solution in contact with immobilised charges located at the pore wall. In this case the electrochemical equations contain the fixed ion concentration as a parameter instead of the ζ potential. This makes it possible to describe quantitatively to a good approximation data on the electroosmosis, the electrical conductivity, the streaming potential and the dialysis potential taken from the literature, as well as results of our own measurements, by using a single membrane constant.     

Electrochemistry of capillary systems with narrow pores. I. Overview

In capillary systems with narrow pores the Helmholtz electrochemical double layer located at the pore wall extends over the entire cross section of the pores. It loses its character as the “charge on the wall”. It will be shown that not only the electrokinetic phenomena but also the electrical conductivity and the dialysis potential of membranes with narrow pores can be understood from the same point of view, namely: the electrolyte solution in the pores of a membrane with narrow pores is considered to be an approximately homogeneous solution in contact with immobilised charges located at the pore wall. In this case the electrochemical equations contain the fixed ion concentration as a parameter instead of the ζ potential. This makes it possible to describe quantitatively to a good approximation data on the electroosmosis, the electrical conductivity, the streaming potential and the dialysis potential taken from the literature, as well as results of our own measurements, by using a single membrane constant.