Colloidochemical properties of ultra-and microfiltration membranes in electrolyte solutions

Dependences of the structural, electrokinetic, and adsorption characteristics on solution pH and background electrolyte (NaCl) concentration are extensively studied for Sartorius and Vladisart cellulose acetate microfiltration membranes with pore sizes of 0.45 and 0.2 μm and a Vladisart ultrafiltration membrane with the rejection of 20 kD. It is revealed that effective hydrodynamic pore radii and maximum pore radii of the microfiltration membranes are 1.5-to 2-and 2.5-to 4-fold, respectively, larger than those presented in the catalog, which is related to the membrane calibration relative to the sizes of rejected particles. For the ultrafiltration membrane, it is shown that, when the pressure increased from 0.5 to 8.0 atm, filtration factor of a liquid and streaming potential substantially decrease owing to the contraction of the polymer network. Measurements of membrane conductivity by the difference and contact methods suggest that a structural anisotropy is virtually absent in the microfiltration membranes and that the ultrafiltration membrane has a nonuniform structure. Negative electrokinetic potentials, whose absolute values increase with the pH and dilution of a background electrolyte solution, are observed for all studied membranes. Isoelectric points of the ultrafiltration and microfiltration membranes are observed at pH ≤ 3 and 2.1 ± 0.2, respectively.     

Electrosurface and structural properties of poly(ethylene terephthalate) track membranes

The electrosurface characteristics are studied for poly(ethylene terephthalate) (PET) track membranes (TMs) with pore radii of 6.5?C60 nm, which are used for ultra- and microfiltration. The data obtained enable one to indirectly assess the structure of tracks and variations in the pore space structure of TMs with an increase in the pore radii. Higher porosity values obtained for TMs from the data on their electrical resistance in comparison with those derived from the filtration data lead one to state that the PET pore surface has a loose structure. The thermal treatment of TMs makes the porosity values determined by the methods of electrical resistance and filtration closer to one another. The regularities of variations in the isoelectric point, ?? potential, and surface charge suggest that the properties and structure of PET pore surface depend on the pore radius. The data obtained may be used to predict the separating power of TMs.     

Pore size distribution of ceramic UF membranes by liquid–liquid displacement porosimetry

Commercial ceramic tubular membranes made by Tami^® have been characterized by several techniques. Their pore size distributions (PSD) have been obtained by liquid–liquid displacement porosimetry (LLDP).

Computerized image analysis (CIA) of SEM pictures has been used to get information on the width of the active layer of the studied membranes. These values of thickness have helped to evaluate the porosity of the membranes and to get representative radii from measurements of the permeability to several gases and liquids. A fully automated porosimeter designed by us has been used in the determination of pore size distributions. Results show a good accuracy and reproducibility of LLDP measurements.

Binary and ternary liquid mixtures have been used to wet and penetrate into the membrane pores when performing LLDP leading to quite similar results when an effective surface tension is assigned for the ternary mixture. This procedure can be used to calibrate the technique to be extended to thick ultrafiltration and even to nanofiltration membranes.     

Electrokinetic and surface chemical characterizations of an irradiated microfiltration polysulfone membrane: comparison of two irradiation doses

The effect of ionizing radiation on the surface and electrokinetic characteristic parameters for a porous membrane of pore size 0.2 mum is determined and correlated with the irradiation dose (10 and 80 J/kg). Changes in NaCl permeability and membrane system electrical resistance determined from diffusion and impedance spectroscopy measurements are consistent with the increase of membrane pore radii/porosity, in agreement with SEM micrographs and reported results. Low irradiation dose seems to clean the membrane surface of impurities, according to XPS results, but the increase of irradiation doses could affect surface roughness. Due to the relatively high pore radius, ion transport numbers are practically independent of radiation and dose, but irradiation slightly modifies the membrane solution interface by increasing its weakly electronegative character, which could be of interest in the ultrafiltration of proteins or macromolecules.     

Formation of SnO2 Supported Porous Membranes

In this work, the effect of the substrate microstructure on the formation of SnO₂ membranes and of the sintering conditions on their porosity have been analysed. Samples have been prepared by colloidal suspensions cast on alumina or kaolin substrates. Supported membranes have been characterized by Hg porosimetry, MEV, XRD and N₂ adsorption-desorption isotherms. The results show that the narrower pore size distribution of alumina substrate allowed to prepare membranes more homogeneous and free of cracks than that supported on kaolin. The crystallite and pore sizes of the membranes could be controlled by adjusting the temperature of sintering, allowing materials with adequate microstructure with application for ultrafiltration process.     

Some comments on the applicability of gas permeation methods to characterize porous membranes based on improved experimental accuracy and data handling

The measurement of the gas permeability coefficient as a function of the mean pressure across a membrane can be used to determine a mean pore radius of the membrane. This method has been applied by several authors to characterize microporous and asymmetric ultrafiltration or hyperfiltration membranes. This paper shows how the data acquisition and handling are improved. Experiments are performed on microporous Millipore membranes with a nominal pore radius of 50 nm and on ultrafiltration merebranes of poly(2,6-dimethyl-1,4-phenyleneoxide) with an expectedly sharp pore-size distribution around 2 nm. For the Millipore membrane an unexpected dependence of the calculated pore radius on the type of gas used in the experiment has been found. Measurements on the ultrafiltration membranes indicate that the viscous flow contribution to the permeability coefficient cannot be determined with sufficient accuracy. It is concluded that application of the gas permeation method has some limitations which were not previously recognized.     

The electrostatic and steric-hindrance model for the transport of charged solutes through nanofiltration membranes

Nanofiltration (NF) membranes possess the intermediate molecular weight cut-off between reverse osmosis membranes and ultrafiltration membranes, and also have rejection to inorganic salts. So one can assume that NF membranes have charged pore structure. We have developed the electrostatic and steric-hindrance (ES) model from the steric-hindrance pore (SHP) model and the Teorell-Meyer-Sievers (TMS) model (Wang et al., J. Chem. Eng. Japan, 28 (1995) 372) to predict the transport performance of charged solutes through NF membranes based on their charged pore structure. In this article, by doing the permeation experiments of aqueous solutions of neutral solutes and sodium chloride, the structural parameters (the pore radius and the ratio of membrane porosity to membrane thickness) and the charge density of NF membranes (Desal-S, NF-40, NTR7450 and G-20) were estimated on the basis of SHP model and the TMS model, respectively. Then, we selected an aqueous solution of different tracer charged solutes (sodium benzenesulfonate, sodium naphthalenesulfonate and sodium tetraphenyl-borate) and a supporting salt (sodium chloride) to verify the ES model. The prediction based on the ES model was in good agreement with the experimental results.     

Polyurethane and sulfonated polysulfone blend ultrafiltration membranes. I. Preparation and characterization studies

Polyurethane (ether type) and sulfonated polysulfone (sodium salt form) in the presence of polyethylene glycol 600 were blended in various compositions using N,N'-dimethylformamide as solvent and used for preparing ultrafiltration membranes by the phase inversion technique. Polymer blend composition, additive concentration, and casting and gelation conditions were optimized. Blend membranes were subjected to ultrafiltration characterizations such as compaction, pure water flux, water content, and membrane resistance. The membranes were also subjected to the determination of pore statistics and molecular weight cutoff determination studies using dextran of different molecular weights. Surface morphology of the membranes was analyzed using scanning electron microscopy at different magnifications. The effects of polymer composition and additive concentration on the above parameters were analyzed and the results are compared and discussed with those of pure sulfonated polysulfone membranes. The derived pore size, porosity, and number of pores have a remarkable interrelationship and also have a definite role and relationship with the molecular weight cutoff, morphology, and flux performance of the membranes.     

Nanopore Size Growth and Ultrafiltration Performance of SnO2 Ceramic Membranes Prepared by Sol-Gel Route

Supported ceramic membranes have been produced by the sol-casting procedure from aqueous colloidal suspensions prepared by the sol-gel route. Coatings on a tubular alumina support have been successfully performed leading to crack free layers. Samples have been sintered at 400, 500 and 600°C, and the effect of heating treatment on the nanostructure and on the ultrafiltration properties are analyzed. The characterization has been done by high resolution scanning electron microscopy, nitrogen adsorption-desorption isotherms, water permeation and cut-off determination using polyethylene glycol standard solutions. The micrographs have revealed that grains and pore size increase with the temperature, whereas their shape remains invariant. This results is in agreements with N₂ adsorption-desorption analyses, which have revealed that the mean pore size diameter increases from 4 to 10 nm as the sintering temperature increases from 400 to 600°C, while the total porosity remains constant. Furthermore, the tortuosity, calculated from water permeability, is essentially invariant with the sintering temperatures. The membranes cut-off, determined with a retention rate equal to 95%, are 3500, 6500 and 9000 g·mol^–1 for 400, 500 and 600°C, respectively, showing that the permeation properties of SnO₂ ultrafiltration membranes can easily be controlled by sintering condition.     

聚醚砜/纤维素晶体共混膜材料及其超滤性能

聚醚砜与纤维素晶体等共混成铸膜液,采用浸没沉淀相转化法制备聚醚砜/纤维素晶体共混膜材料.通过超滤装置检测复合膜的水通量、截留率、平均孔径、孔隙率、抗污染性等超滤性能,从而讨论了纤维素晶体含量对共混膜超滤性能的影响.采用抗张测试机、热重分析仪(TGA)、原子力显微镜(AFM)对共混膜的力学性能、热稳定性能、形貌结构进行表征.结果表明,随着纤维素晶体的含量的增加,共混膜的纯水通量先升高后有所降低,截留率均保持在91%～95%,抗张强度、断裂伸长率先增大后有所下降,抗污染性较纯聚醚砜膜显著提高.当纤维素晶体质量分数为1%时,纯水通量达到最大为813.3L·m-2·h-1,孔隙率为88.8%,平均孔径达为70.9nm,抗张强度为7.25MPa,断裂伸长率为11.6%,平均污染度FR值为22.0%,衰减系数m值为35.8%.共混膜具有由纤维素晶体、聚醚砜热降解分别引起的两个失重阶段.共混膜为典型非对称膜结构,表皮层较为致密,多孔支撑层孔径较大.     

Characterization of ultrafiltration membranes. : Part IV. Influence of the deformation of macromolecular solutes on the transport through ultrafiltration membranes.

A theory was developed to point out the important parameters involved in the deformation of a flexible polymer in ultrafiltration through membranes. It appears that the deformation, and thus the easier transport of the polymer solute, occurs when the permeate flux reaches (or exceeds) a critical value which depends on the solution characteristics (solvent viscosity, concentration, temperature) as well as the membrane surface characteristics (porosity and pore radius on the surface). The experimental study was carried out with two flexible polymers: polyethylene glycol (PEG) and dextran. In ultrafiltration under constant pressure through a IRIS 3042 membrane, the increase of the concentration of PEG (of molecular weights 15,000, 20,000, 35,000) beyond a certain value caused a steady drop of rejection from a constant value. On the other hand, the increase of the applied pressure in the ultrafiltration of PEG 35,000 and dextran 70,000 afforded a sharp drop of rejections to a zero value as the fluxes increased steadily. Conversely, the changes in the concentration or the applied pressure did not affect the rejection when membranes of low permeability (Nuclepore 150 A, IRIS 3069) or of low pore size (PTGC) were used. These behaviours are consistent with the established theory.     

Flux reduction of ultrafiltration membranes with different cut-off due to adsorption of a low-molecular-weight hydrophobic solute-correlation between flux decline and pore size

The flux of ultrafiltration membranes may be severely reduced when treating low-molecular-weight hydrophobic solutes even though the cut-off of the membrane is orders of magnitudes greater than the size of the solute molecules. In this investigation, the flux reduction was correlated to the membrane pore size using octanoic acid as a model substance. As a comparison, the pore size was also determined by measuring the retention of a dextran solution and by using the liquid–liquid displacement porometry method (LLDP). The membranes used were four asymmetric polysulphone and polyethersulphone membranes with nominal molecular weight cut-off (NMWCO) between 6 and 50 kDa. It is shown that the use of a low-molecular-weight hydrophobic solute may provide a rapid and simple method of characterising hydrophobic ultrafiltration membranes, both regarding their sensitivity to flux reduction due to adsorption, and their pore-size distribution.     

Correlation Between Structure and Transport Properties of Polymeric Membranes for Immunoisolation

Laboratory-made asymmetric polyurethane membranes designed for immunoisolation were investigated. Two types of EK and ES membranes were prepared in different spinning conditions. The membrane structure was characterised by the skin pore radius measurements using differential scanning calorimetry (DSC). Diffusive transport properties of membranes were determined by in vitro method for albumin and creatinine. The scanning electron microscopy (SEM) was applied to study the morphology of membranes. It has been found that the DSC technique is a useful tool for the evaluation of pore radii in the skin layer for PU membranes. Calculated pore radii were in the range from 1.95 to 2.47 nm for the EK and ES types. A correlation between the skin pore radii and the transport properties was not found in this case of investigated membranes. However, the transport properties data can serve for the estimation of selectivity of membranes. Thus, the selectivity of membranes for solutes of various molecular size was estimated from the D _m/D _w ratio of diffusion coefficients for albumin and creatinine. The SEM micrographs reveal the finger-like internal structure of capillary membranes, as well as various skin thickness. This revised version was published online in July 2006 with corrections to the Cover Date.     

Physico-chemical characterization of nanofiltration membranes.

This study presents a methodology for an in-depth characterization of six representative commercial nanofiltration membranes. Laboratory-made polyethersulfone membranes are included for reference. Besides the physical characterization [molecular weight cut-off (MWCO), surface charge, roughness and hydrophobicity], the membranes are also studied for their chemical composition [attenuated total reflectance Fourier spectroscopy (ATR-FTIR) and X-ray photoelectron spectroscopy (XPS)] and porosity [positron annihilation spectroscopy (PAS)]. The chemical characterization indicates that all membranes are composed of at least two different layers. The presence of an additional third layer is proved and studied for membranes with a polyamide top layer. PAS experiments, in combination with FIB (focused ion beam) images, show that these membranes also have a thinner and a less porous skin layer (upper part of the top layer). In the skin layer, two different pore sizes are observed for all commercial membranes: a pore size of 1.25-1.55 angstroms as well as a pore size of 3.20-3.95 angstroms (both depending on the membrane type). Thus, the pore size distribution in nanofiltration membranes is bimodal, in contrast to the generally accepted log-normal distribution. Although the pore sizes are rather similar for all commercial membranes, their pore volume fraction and hence their porosity differ significantly.     

Dimensional and concentration dependences of diffusion of aqueous alkaline-earth metal nitrate solutions in porous-glass membranes

Diffusion coefficients characterizing penetration of aqueous solutions of magnesium, calcium, strontium, and barium nitrates across porous-glass membranes with predominant pore radii of 4.5–70 nm were determined. A decrease in the pore radii of the membranes is accompanied in all cases by an exponential fall of the diffusion mobility. A reversal of the diffusion mobility series was observed for membranes with pore radii of 4.5 and 70 nm on the background of the general tendency toward a concentration-related rise in the diffusion coefficients.     

Change in molecular weight retention curves of ultrafiltration membranes with respect to lignosulfonates under gel formation conditions

The paper considers ultrafiltration of lignosulfonates (LS) under predominantly the gel formation conditions. An effort is to determine the molecular weight retention (MWR) curves of a series of ultrafiltration membranes differing in their pore size under in turn different operating pressures (1–32 bar). The initial separative properties (both retentivity and volume flux) of all membranes are shown to change because of gel formation occurring actually instantly as a cake layer placed mostly onto the membrane surface. The transmembrane pressure-drop sets up primarily these properties but the initial hydrodynamic permeability coefficient of a membrane (i.e. its mean pore size) is also of concern. As a result, an increase in that pressure results in a shift of the molecular weight retention curves of all membranes under study towards lower molecular weights: the more, the higher their mean pore size. Further, these curves become more abrupt in their form, and such a change depends on the mean pore size of a membrane as well.     

Effects of dip coating parameters on the morphology and transport properties of cellulose acetate–ceramic composite membranes

This work presents the fabrication of cellulose acetate (CA)–ceramic composite membranes using dip coating technique. Ceramic supports used in this work were prepared from kaolin with an average pore size of 560 nm and total porosity of 33%. The dip coating parameters studied experimentally were the concentration of CA solution (varying from 2 wt% to 8 wt%) in acetone and dipping time (varying from 30 s to 150 s). The fabricated composite membranes were characterized using scanning electron microscope, gas permeation, pure water flux and ultrafiltration (UF) experiments using bovine serum albumin (BSA). It was observed that the membrane prepared with 2 wt% and 4 wt% CA were suitable for microfiltration applications and those with 6 wt% and 8 wt% were for ultrafiltration applications. Theoretical investigation was conducted to know the macroporous and mesoporous structure of the prepared membranes using Knudsen and viscous permeability analysis of air. A resistance in series model was applied to identify different resistances responsible for the flux decline. Phenomenological models were proposed to illustrate the dependency of hydraulic resistance of membrane on the structural parameters such as average pore size, effective porosity as well as dip coating parameters like dipping time and concentration of CA. It was found that, the growth rate of CA film on the ceramic support followed exponential growth law with respect to dipping time. The total hydraulic resistance of the membrane was evaluated to be inversely proportional to the ratio of pore sizes of top layer and ceramic support. The resistance due to the CA film was found to be depended to the order of 1.73 with respect to concentration of CA. An increase in the concentration of CA was found to be more effective than dipping time to reduce the membrane pore size.     

Preparation and properties of polysulfone/TiO2 composite ultrafiltration membranes

The influence of inorganic filler TiO₂ nanoparticles on the morphology and properties of polysulfone (PS) ultrafiltration membranes was investigated. PS/TiO₂ composite membranes were prepared by a phase‐inversion method. TiO₂ nanoparticles modified by sodium dodecyl sulfate were uniformly dispersed in an 18 wt % PS casting solution. The addition of TiO₂ resulted in an increase in the pore density and porosity of the membrane skin layer. The pore size distribution changed from the log‐normal distribution to the bimodal distribution because of the presence of TiO₂ nanoparticles, and some large pores were observed when the concentration of the filler was over 3 wt %. The skin layer was gradually thickened; meanwhile, the morphology sublayer changed from macrovoids to spongelike pores, in comparison with PS membranes without the filler. The addition of TiO₂ also induced increases in the hydrophilicity, mechanical strength, and thermal stability. The ultrafiltration experiments showed when the concentration of TiO₂ was less than 2 wt %, the permeability and rejection of the membrane was enhanced and then decreased drastically with a higher filler concentration (>3%). © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 879–887, 2006     

Comparative characterization of the diffusion mobility of aqueous calcium salt solutions in porous-glass membranes

Diffusion coefficients of calcium acetate, nitrate, and chloride, which characterize the transport of their aqueous solutions across porous-glass membranes with predominant pore radii of 4.5–70 nm, were determined. A decrease in the membrane pore radii is accompanied in all cases by an exponential fall of the diffusion mobility. The noticeable difference between the anion mobilities, which favors salt separation, grows with increasing radii of transport channels. In a membrane with a pore radius of 4.5 nm, the salt diffusion mobility series is changed.     

Cellulose acetate and polyethersulfone blend ultrafiltration membranes. Part I: Preparation and characterizations

The overall objective of this investigation is to achieve high‐performance membranes with respect to flux and rejection characteristics, with an interplay of blending polymers having desired qualities. Thus, cellulose diacetate and polyethersulfone as candidate materials, in the presence of polyethylene glycol 600 as a pore forming agent, were blended in 100/0, 95/5, 90/10, 85/15, 80,20 and 75/25% compositions using N,N′‐dimethylformamide as solvent and membranes were prepared by the phase inversion technique. Polymer blend composition, additive concentration, and casting and gelation conditions were standardized for the preparation of asymmetric membranes with various pore statistics and morphology. These blend membranes were characterized for compaction in ultrafiltration experiments at 414 kPa pressure in order to attain steady state flux and is reached within 4–5 hr. The pure water flux was measured at 345 kPa pressure and is determined largely by the composition of polyethersulfone and additive concentration. The flux was found to reach the highest values of 66.5 and 275 1/(cm² hr) at 0 and 10 wt% additive concentrations respectively, at 25% SPS content of the blend. Membrane hydraulic resistance derived by measuring water flux at various transmembrane pressure and by using an algorithm was found to be inversely proportional to pure water flux. Water content is estimated by simple drying and weighing procedures and found proportional to pure water flux for all the membranes. The molecular weight cut‐offs (MWCOs) of different membranes were determined with proteins of different molecular weights and found to vary from 20–69 kDa (globular proteins) depending on the PEG and SPS content in the casting dope. Skin surface porosity of the membranes were analyzed by scanning the frozen membrane samples using scanning electron microscopy (SEM) at different magnifications. The surface porosity is in direct correlation to the MWCO derived from solute retention experiments. Copyright © 2004 John Wiley & Sons, Ltd.