Electrokinetic characterization of porous plugs from streaming potential coupled with electrical resistance measurements

The zeta potential of mixed nickel-iron oxide particles is evaluated by a new laboratory instrument. This latter allows the measurement of streaming potential together with the electrical resistance of porous plugs. The conductivity of electrolyte inside plug (pore conductivity) is deduced from electrical resistance measurements and is used together with streaming potential to evaluate the zeta potential by accounting for the surface conduction phenomenon. It is shown that neglecting the surface conduction phenomenon leads to a substantial underestimation of the zeta potential. The coupled measurements of streaming potential and plug electrical resistance yield zeta potential values that are in very good agreement with those obtained by electrophoresis. The densification of the porous plug with increasing pressure increments is put in evidence by the decrease in measured streaming potentials. Electrical resistance measurements make it possible to account for the increase in surface conductivity resulting from the more compacted structure of the plug. By doing so, the calculated zeta potential is found to be virtually independent of the pressure difference involved in streaming potential experiments, whereas the negligence of surface conduction phenomenon leads to a decrease in the apparent zeta potential with increasing pressure level.     

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).     

Caractérisation des propriétés électriques des parois de pores d’une membrane

The characterisation of the surface electrical properties of membrane materials is critical for understanding and predicting the filtration performances of membranes. In this paper, four simple experimental methods – streaming potential, electro-osmosis, pore conductivity and membrane potential –, allowing the characterisation of the charge state of membrane pore walls are presented. The four experimental parameters provide information about the sign of the electrical net charge. Examples illustrating the influence of the pH, electrolyte concentration and nature on the four experimental parameters are given. The zeta potential can be then deduced from these measurements by means of a model.     

Theoretical study of the electrokinetic and electrochemical behaviors of two-layer composite membranes

A theoretical study concerning the effect of structure (porosity, pore radius and layer thickness) and surface characteristics (zeta potential) of two-layer composite membranes on global streaming potential (SP_g), membrane potential (Em_g) and membrane conductivity (λ_g) is presented. To this end, each layer of the composite membrane (composed of a support layer and a filtering layer) was modeled as a bundle of identical capillary tubes with connections between pores of the two layers (the pores in the filtering layer being smaller than those of the support layer). The global parameters SP_g, Em_g and λ_g were calculated by using the theory of thermodynamics of irreversible processes and a space charge model. SP_g, Em_g and λ_g were expressed as a function of the individual parameters of each layer SP⁽ⁱ⁾, Em⁽ⁱ⁾ and λ⁽ⁱ⁾, respectively, the length fraction of the support layer, the porosity and pore radius ratios. It was shown that the electrokinetic (streaming potential and membrane conductivity) and electrochemical (membrane potential) behaviors of such composite membranes vary between that of single layers. For streaming potential, the results indicate that the contribution of the filtering layer to the global streaming potential is very little influenced by zeta potentials of both types of pores. It appears that the individual streaming potential of the filtering layer greatly dominates the global streaming potential. This is due to the fact that the streaming potential of the filtering layer is weighted by the pore radius ratio which is a predominant parameter in determining the global streaming potential. In contrast to the streaming potential, the contribution of the filtering layer to the global membrane potential (Em_g) or membrane conductivity (λ_g) depends more or less on the zeta potentials of both kinds of pores and the corresponding electrokinetic radii as well. As to the membrane potential, the contribution of the filtering layer to Em_g is all the more sensitive to the zeta potentials than the electrokinetic radii are small. The filtering layer greatly dominates the global membrane potential when its pores are narrow (with regard to the Debye length) and strongly charged. For the electrolyte conductivity inside pores, the smaller pores (inside the filtering layer) have an effect all the more dominant on the apparent membrane conductivity than their zeta potential is low and that of larger pores (inside the support layer) is high.     

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.     

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.     

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 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.     

Influence of steric, electric, and dielectric effects on membrane potential

The membrane potential arising through nanofiltration membranes separating two aqueous solutions of the same electrolyte at identical hydrostatic pressures but different concentrations is investigated within the scope of the steric, electric, and dielectric exclusion model. The influence of the ion size and the so-called dielectric exclusion on the membrane potential arising through both neutral and electrically charged membranes is investigated. Dielectric phenomena have no influence on the membrane potential through neutral membranes, unlike ion size effects which increase the membrane potential value. For charged membranes, both steric and dielectric effects increase the membrane potential at a given concentration but the diffusion potential (that is the high-concentration limit of the membrane potential) is affected only by steric effects. It is therefore proposed that membrane potential measurements carried out at high salt concentrations could be used to determine the mean pore size of nanofiltration membranes. In practical cases, the membrane volume charge density and the dielectric constant inside pores depend on the physicochemical properties of both the membrane and the surrounding solutions (pH, concentration, and chemical nature of ions). It is shown that the Donnan and dielectric exclusions affect the membrane potential of charged membranes similarly; namely, a higher salt concentration is needed to screen the membrane fixed charge. The membrane volume charge density and the pore dielectric constant cannot then be determined unambiguously by means of membrane potential experiments, and additional independent measurements are in need. It is suggested to carry out rejection rate measurements (together with membrane potential measurements).     

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.     

Modification of electrochemical properties of pore wall of track-etched mica membranes

The electrochemical properties of the pore wall of track-etched mica membranes are modified (a) by covalent binding of positively and negatively charged groups, and (b) by adsorption of cationic and amonic polyelectrolytes. The electrochemical properties of the pore wall are characterized by measurements of membrane potential, electrical conductivity and streaming potential.By these methods it is possible to change the sign of the surface charge density of the pore wall and to increase its absolute value by a factor of about 30 compared with that of the unmodified pore wall. Changes of electrochemical properties of the pore wall are desirable in studies of negative osmosis and incongruent electrolyte transport in membranes with known pore structure.     

Direct Current Conductometry of Track Membranes

The electrical conductivity measured for a KCl solution in pores of poly(ethylene terephthalate) track membranes has been studied as depending on electrolyte concentration and pore diameter with the use of a direct-current source. The difference between the experimentally determined conductivity and the standard value has been shown to decrease with increasing electrolyte concentration and pore diameter. At the same time, its value is significantly lower than that determined by impedance spectroscopy. This result is related to a decrease in the contribution of a gel layer formed on the pore surface upon coming into the contact with the electrolyte to the electrical resistance of a membrane.     

Theory of pressure-driven transport of neutral solutes and ions in porous ceramic nanofiltration membranes

Hindered transport theory and homogeneous electro-transport theory are used to calculate the limiting, high volume flux, rejection of, respectively, neutral solutes and binary electrolytes by granular porous nanofiltration membranes. For ceramic membranes prepared from metal oxides it is proposed that the membrane structural and charge parameters entering into the theory, namely the effective pore size and membrane charge density, can be estimated from independent measurements: the pore radius from the measured hydraulic radius using a model of sintered granular membranes and the effective membrane charge density from the hydraulic radius and the electrophoretic mobility measurements on the ceramic powder used to prepare the membrane. The electro-transport theory adopted here is valid when the membrane surface charge density is low enough and the pore radius is small enough for there to be strong electrical double layer overlap in the pores. Within this approximation the filtration streaming potential is also derived for binary electrolytes.     

Streaming potential and surface charge density of microporous membranes with pore diameter in the range of thickness

A system formed by two phases bathing a microporous membrane is studied considering its behavior as a dynamic system. So, the natural frequencies for each used membrane is determined and then, applying a flow ramp with a rate sufficiently small, the streaming potential can be obtained from the slope of the pressure values versus electrical potential.The determination of the electric potential inside the pores, φ, requires to solve the Poisson–Böltzmann equation in the case of membranes with pore diameter in the range of thickness, for which the radial components of velocity of the fluid must be considered. Since there is no analytical solution, a numerical method was used to obtain φ. The electrical potential value at a distance equal to hydrodynamic radius from pore axis (zeta potential) is used to evaluate the streaming potential by the Helmholtz–Smoluchowsky relation. These values were compared with the experimental data accomplishing the suitable iterations over the surface charge density until coincidence.The values of the surface charge density for the studied membranes show a concentration dependence described by Langmuir’s model for the greatest pore diameters and Freundlich’s model for the smallest pore diameters.     

Effect of solution chemistry on the surface charge of polymeric reverse osmosis and nanofiltration membranes

A streaming potential analyzer has been used to investigate the effect of solution chemistry on the surface charge of four commercial reverse osmosis and nanofiltration membranes. Zeta potentials of these membranes were analyzed for aqueous solutions of various chemical compositions over a pH range of 2 to 9. In the presence of an indifferent electrolyte (NaCl), the isoelectric points of these membranes range from 3.0 to 5.2. The curves of zeta potential versus solution pH for all membranes display a shape characteristic of amphoteric surfaces with acidic and basic functional groups. Results with salts containing divalent ions (CaCl₂, Na₂SO₄, and MgSO₄) indicate that divalent cations more readily adsorb to the membrane surface than divalent anions, especially in the higher pH range. Three sources of humic acid, Suwannee River humic acid, peat humic acid, and Aldrich humic acid, were used to investigate the effect of dissolved natural organic matter on membrane surface charge. Other solution chemistries involved in this investigation include an anionic surfactant (sodium dodecyl sulfate) and a cationic surfactant (dodecyltrimethylammonium bromide). Results show that humic substances and surfactants readily adsorb to the membrane surface and markedly influence the membrane surface charge.     

Ionic Diffusivity, Electrical Conductivity, Membrane and Thermoelectric Potentials in Colloids and Granular Porous Media: A Unified Model

Ionic diffusivity, electrical conductivity, membrane and thermoelectric potentials in isotropic and homogeneous colloidal suspensions, and granular porous media saturated by a binary symmetric 1:1 electrolyte are four interrelated phenomena. The microstructure and the surface properties of the solid grains-water interface influence directly these properties. The ionic diffusivities (and the electrical conductivity, respectively) in colloids and porous media have contributions from diffusion (and electromigration, respectively) through the bulk solution occupying the pores, together with electromigration occurring at the grains-water interface in the electrical double layer. Surface diffusion in porous materials has no contribution from concentration gradients along the grains-water interface. Instead, surface diffusion is envisioned as a purely electromigration process due to the membrane potential. The tortuosities of the transport of anions and cations are equal to the bulk tortuosity of the pore space only at high ionic strength. As the ionic strength decreases, the dominant paths for transport of the ion corresponding to the counterion of the electrical double layer shift from the pore space to the solid grains-water interface. Because anions and cations do not move independently, the membrane potential created by the charge polarization alters the velocity of the anions and influences the mutual diffusivity coefficient of the salt in the porous material. An electric potential of thermal origin is also produced in nonisothermal conditions. The ionic contributions to the electrical conductivity are based on a differential effective medium approach. These ionic contributions to the electrical conductivity are used to derive the ionic diffusivities and the membrane and thermoelectric potentials. The influence of the temperature and the presence, in the pore space, of a second immiscible and nonwetting phase is also considered in this model. Porosity is shown to affect the membrane potential. Several predictions of the model are checked with success by comparing the model to a set of experimental data previously published. Copyright 1999 Academic Press.     

Colloid chemical characteristics of nanoporous glasses with different compositions in solutions of simple and organic electrolytes. 3. Calculation of electrochemical characteristics of membranes in terms of a homogeneous model

The electrosurface characteristics of nanoporous glass membranes–ion concentrations in pores with taking into account the specificity of counterions, electrokinetically mobile charge, the convective component of pore solution electrical conductivity, electroosmotic mobility of a liquid in the field of streaming potential and ion mobilities in pore space–were calculated within the homogeneous model. The effects of the type of counterion (sodium, potassium, ammonium, tetramethylammonium, and tetraethylammonium ions), solution concentration, glass composition, and pore size on the equilibrium and transport characteristics of membrane systems have been analyzed. A method for the determining of electrolyte activity coefficients in the membranes has been proposed.     

Evaluation of the "DSPM" model on a titania membrane: measurements of charged and uncharged solute retention, electrokinetic charge, pore size, and water permeability

The DSPM (Donnan steric partitioning pore model) was evaluated in the case of a titania membrane with "nanofiltration properties" by measuring the electrokinetic charge, pore size, and water permeability of the membrane, along with charged and uncharged solute retention. The zeta potential values (zeta) were determined from measurements of the electrophoretic mobility (EM) of titania powder forming the filtering layer of the membrane. Zeta potential values were converted into membrane volume charge (X) by assuming two limiting cases: a constant surface charge (sigma(s)(cst)) and a constant surface potential (psi(s)(cst)). The mean pore radius and thickness/porosity ratio of the membrane were determined by permporometry and from water permeability measurements, respectively. Retention measurements were carried out as a function of the permeate volume flux for both neutral solutes (polyethylene glycol PEG of different size) and salts (KCl, MgSO4, K2SO4, and MgCl2) at various pH values. Ionic retentions showed minimum values near the IEP of the membrane. Retention data were analyzed using the DSPM. Very good agreement was found between the pore radius calculated by the model and that determined by permporometry. X values calculated from fitting retention data using the DSPM were also in satisfactorily agreement with X values calculated from EM measurements assuming a constant surface potential for a large pH range. Furthermore, the DSPM leads to X values (X(DSPM)) between those calculated from EM (X(EM)) using the two limiting bounds. In other words, X(DSPM) was higher than X(EM) assuming psi(s)(cst) at pH values far from the isoelectric point (IEP) and lower than X(EM) assuming sigma(s)(cst). These results show that the DSPM is in qualitative agreement with the charge regulation theory (increase of the pore surface potential and decrease of the pore surface charge density with decreasing the pore size). On the other hand, the thickness/porosity ratio of the membrane calculated from solute retention data differed significantly from that determined from water permeability measurements. Moreover, a single value of Deltax/Ak could not be determined from PEG and salt retention data. This means that the Deltax/Ak parameter loses its physical meaning and includes physical phenomena which are not taken into account by the DSPM. Nevertheless, the model satisfactorily predicted the limiting retention, as this is not influenced by the Deltax/Ak parameter.     

Wettability of polypropylene capillary membranes during the membrane distillation process

Studies of membrane wettability in the membrane distillation process were performed with the application of hydrophobic capillary membranes made of polypropylene. Three kinds of Accurel PP membranes (Membrana GmbH, Germany) differing in the diameter of capillaries and pores as well as in the wall thickness were used. It was confirmed that membranes with lower wall thickness and larger pore size provide higher yields of the process. The studies demonstrated that the pores of used membranes located close to the external surface of capillaries are several times larger than those located inside the membrane wall. Based on air permeability measurements it was found that external surface of the membranes with such large pores was completely wetted by water after 50–80 h of membrane distillation. However, the pores located inside the wall with the diameter below 1 μm were not wetted and electrical conductivity of the obtained distillate was maintained at the level of 3–6 μS cm⁻¹.