Permeability of hydrophilic and hydrophobic Shirasu-porous-glass (SPG) membranes to pure liquids and its microstructure

Morphological properties of hydrophilic and hydrophobic Shirasu-porous-glass (SPG) membranes were investigated over a wide range of mean pore sizes (0.252–20.3 μm) by liquid permeability measurements, scanning electron microscopy and Hg porosimetry. Hydrophobic modification of membrane surface was made by surface coating with silicone resin. The results are discussed using the non-uniform capillary bundle model of membrane permeability. The mean pore tortuosity of 1.28 was kept constant over the whole range of mean pore sizes investigated. The SEM images confirmed that the geometry of pore network was similar for all SPG membranes, irrespective of their mean pore size. The span of pore size distribution ranged from 0.28 to 0.68 and the number of pores per unit cross-sectional membrane area from 10⁹ to 10¹³ m⁻². The membrane resistance was unchanged after surface treatment with silicone resin, which means that the pores were not plugged by the resin, even in the submicron range of mean pore sizes.     

浸没凝胶相转化制备聚合物膜的孔结构及其形成机理

全面地综述了浸没凝胶相转化法制备的聚合物微孔膜的表面和膜中存在的各种孔的结构和形态,从制膜体系的热力学性质和成膜动力学角度解释了各种孔结构形态的形成和生长机理,即膜表面与膜中孔的结构形态由此时制膜体系发生的相分离类型决定,由此可推断出不同的膜层可能有不同的成膜机理。因此,只要掌握了各种膜孔结构形成的机理,通过改变膜的制备条件,控制体系的热力学性质与成膜时动力学扩散是可以实现相转化膜结构的控制。     

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.     

Determination of the pore radius of regenerated cellulose membranes by a dyeing technique

Membrane structure strongly affects the transport of solutes through dialysis membranes. This suggests that knowledge of membrane structure and its effects on permeability is required in order to improve the membranes. Solute transport in membrane pores is limited by steric hindrance at the pore entrances, frictional resistance of the pore walls, and the tortuosity of the pores. Differences in dyeing properties are found among the various tubular dialysis membranes made of regenerated cellulose (RC) that are commercially available. The objective of the present study is to determine intramembrane diffusivity for dyes, and from this the pore radius of RC membranes based on pore model calculations. Values of the pore radius of RC membranes obtained from intramembrane diffusivity data are in disagreement with our previously reported values obtained from solute and pure water permeability data. This indicates that RC membranes are of asymmetrical structure and slightly tight near the outside surfaces.     

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.     

Intra-layer flow in fouling layer on membranes

The structure of fouling layer determines the pressure drop across the fouling layer. Three-dimensional distributions of nucleic acids, proteins, α-d-glucopyranose polysaccharides, β-d-glucopyranose polysaccharides and lipids in the biofouling layer that is formed on a mixed cellulose ester membrane were determined using a six-fold staining protocol combined with confocal laser scanning microscopy (CLSM). Based on the three-dimensional volumetric grid model of the fouling layer structure observed from the series of CLSM images, the intra-layer flow field during filtration was simulated using commercial software. The effective permeability of the fouling layer was estimated to be 2.65 × 10⁻¹² m², which determines the upper estimate on the permeability of the fouling layer. The pores were categorized according to their diameters, using the maximum convex perimeter approach, and then the effects of the blocking pores on the permeability of the fouling layer were investigated. Blocking the large pores that accounted for 15% of the porosity reduced the mean permeability by 58%.     

CsxH3-xPW12O40的孔结构

Ｎ２静态吸附容量法的测定结果表明，磷钨酸铯盐（ＣｓｘＨ３－ｘＰＷ１２Ｏ４０）的孔窝和孔分布与ｘ值的大小相关。ｘ〈１．５的ＣｓｘＨ３－ｘＰＷ１２Ｏ４０孔容相近，孔分布近似；当ｘ〉１．５时，ＣｓｘＨ３－ｘＰＷ１２Ｏ４０的孔主要是孔径小于１０ｎｍ的中孔和微孔，平均孔径及孔容随ｘ的增加而增大。ＳＥＭ和ＴＥＭ的观测结果表明，ＣｓｘＨ３－ｘＰＷ１２Ｏ４０的孔是微细粒子堆积留下的空隙孔，可能不存在晶内孔。     

Formation of cellulose acetate membranes using a supercritical fluid assisted process

Microporous cellulose acetate membranes have been prepared from polymer–acetone solutions using a supercritical fluid phase inversion process in which CO₂ acts as the non-solvent. Series of experiments were performed at various polymer concentrations, temperatures and pressures. The structure of the resulting membranes was analysed using scanning electron microscopy. We operated with polymer concentrations ranging between 5 and 40% (w/w) in acetone obtaining different pore dimensions and membrane structures. Increasing the percentage of polymer in the solution, the structure of the membranes changed from beads-like structure to cellular structure. Polymer concentration also influenced the mean diameter of the pores that ranged from 2 to 50 μm for polymer concentrations from 40 to 5% (w/w). We also tested membrane formation pressures between 100 and 200 bar and at temperature between 45 and 65 °C. Pressure influences the change in membrane structure from cellular to beads-like, whereas temperature has a minor influence on pore size: both the effects can be partially related to CO₂ density. Cellulose acetate membrane formation mechanisms have also been discussed.     

Elaboration and characterisation of a plasticized cellulose triacetate membrane containing trioctylphosphine oxyde (TOPO): Application to the transport of uranium and molybdenum ions

A cellulose triacetate (CTA) membrane containing trioctylphosphine oxyde (TOPO) as carrier and 2-nitrophenyloctyl ether (NPOE) as a plasticizer was prepared. The membrane CTA + NPOE + TOPO was characterised using chemical techniques as well as Fourier Transform InfraRed (FTIR) spectroscopy, X-ray diffraction and Scanning Electron Microscopy (SEM). The CTA membrane is characterised by well-defined pores; these pores are completely filled with the NPOE and carrier. Surfaces of membranes with TOPO are smooth. The systems constituted by the mixture of CTA + NPOE, CTA + NPOE + TOPO do not give any diffraction. This can be due to the absence of crystallization within the membrane. On the other hand, this result should be attributable to the amorphous state of the structure, which permits us to eliminate the mechanism of transfer of the ions by electron jump. A comparative study of transport across a polymer inclusion membrane (PIM) and a supported liquid membrane (SLM) containing the same carrier in chloroform has shown that uranium or molybdenum transport efficiency was increased using PIM instead of SLM. PIM showed higher stability than SLM, the flux of transport remain constant in the former case after 2 weeks.     

Effect of trace amounts of water on organic solvent transport through gamma-alumina membranes with varying pore sizes

The transport behavior of toluene and n-hexane in gamma-alumina membranes with different pore diameters was studied. It was shown that the permeability of water-lean hexane and toluene is in agreement with Darcy's law down to membrane pore diameters of 3.5 nm. The presence of molar water fractions of 5-8 x 10(-4) in these solvents led to a permeability decrease of the gamma-alumina layer by a factor of 2-4 depending on pore size. In general, a lower permeability was found for hexane than for toluene. Moreover, in the presence of water a minimum applied pressure of 0.5-1.5 bar was required to induce net liquid flow through the membrane. These phenomena were interpreted in terms of capillary condensation of water in membrane pores with a size below a certain critical diameter. This is thought to lead to substantial blocking of these pores for transport, so that the effective tortuosity of the membrane for transport of hydrophobic solvents increases.     

Structures of cellulose acetate membranes for reverse osmosis

Conclusions Studies under the scanning electron microscope have shown that the cellulose acetate membranes used for reverse osmosis are high-molecular-weight condensation structures of the cellular type resulting from the dropwise separation of a new liquid phase under diffusional enrichment of the polymer solution by water, the solvent. The pore diameter, and the total pore volume, both diminish on approaching the membrane surface; the diffuse character of the active layer traces back to the concentration distribution resulting from vaporization of acetone, the volatile component, from the acetone- formamide cellulose acetate solution.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 1, pp. 111–115, January, 1977.     

Creation of regenerated cellulose microspheres with diameter ranging from micron to millimeter for chromatography applications

Regenerated cellulose microspheres (RCM) with different diameters were prepared from cellulose solution using 7 wt% NaOH/12 wt% urea aqueous solvent pre-cooled to −12 °C by the sol–gel transition method via a “green” process. By varying the hydrophile–lipophile balance, the amount of the surfactants, the proportion of the water to the oil phase and the stirring speed, the mean diameter of the cellulose microsphere with nano-scale pore size could be controlled easily from 5 μm to 1 mm. The structure and physicochemical properties of the microspheres were characterized by FT-IR spectroscopy, scanning electron microscopy, X-ray diffraction, mercury intrusion-porosimetry and particle size analyzer. The RCM microspheres exhibited spherical shape with the cellulose II structure. A preparative size-exclusion chromatography (SEC) column packed with the cellulose microspheres was used for the fractionation of a polyethylene oxide (PEO) in water, which indicated high efficiency for the fractionations and a large daily throughput of 4 g. Moreover, they had good adsorption capacity to dye particles through physical interaction. The cellulose microspheres would have potential applications in the fields of purification, separation and fractionation of polymers as chromatography packing and adsorbent both at laboratory and industrial scale.     

Supported liquid membranes modification with sulphonated poly(ether ether ketone): Permeability,selectivity and stability

The development of a new type of composite membrane consisting of a microfiltration support membrane, an immobilised liquid membrane phase and a hydrophilic, charged polymer layer and its function as a supported liquid membrane (SLM) for copper selective transport are described. The ion-exchange layers function as stabilisation layers to improve the membrane lifetime and consist of sulphonated poly(ether ether ketone) (SPEEK). This polymer shows a high permeability for copper ions due to the presence of fixed negative charges and to its swelling capacity in an aqueous phase.A method was developed to prepare composite membranes composed of the support membranes Celgard with one stabilisation layer on either the feed or strip side of the membrane or on both sides. Good adhesion of homogeneous, negatively charged, hydrophilic SPEEK layers to the hydrophobic macroporous support membranes could only be established when the support membranes were first hydrophilised with a concentrated sulphuric acid solution containing 5 wt% free SO₃.The lifetime of the SLMs is significantly improved when one stabilisation layer is applied at the strip side or two layers at both sides of the SLM. A second advantage of this composite SLM is the increase in copper flux caused by a decrease in thickness of liquid membrane phase. However, when SPEEK penetrates entirely through some pores of the support membrane, ions diffuse non-specifically through the SPEEK matrix resulting in an undesired selectivity loss. This phenomenon occurs only when thin Celgard membranes are used as support membranes.     

The influence of membrane ion-permselectivity on electrokinetic concentration enrichment in membrane-based preconcentration units

The performance of nanoporous hydrogel microplugs with varying surface charge density is described in concentrating charged analytes electrokinetically in a microfluidic device. A neutral hydrogel plug with a mean pore size smaller than the size of charged analytes acts as a simple size-exclusion membrane. The presence of fixed charges on the backbone of a nanoporous hydrogel creates ion-permselectivity which results in charge-selective transport through the hydrogel. This leads to the development of concentration polarization (CP) in the adjoining bulk electrolyte solutions under the influence of an applied electrical field. CP strongly affects the distribution of the local electrical field strength, in particular, in the vicinity of the hydrogel plug which can significantly reduce the concentration enrichment factors compared to the neutral hydrogel. A theoretical model and simulations are presented, together with experimental data, to explain the interplay of hydrogel or membrane cation-selectivity, electrical field-induced CP, and the distribution of the local electrical field strength with respect to concentration enrichment of negatively charged analytes at the cathodic membrane-solution interface.     

Impact of membrane immobilization on particle formation and trichloroethylene dechlorination for bimetallic Fe/Ni nanoparticles in cellulose acetate membranes

The use of membrane immobilization to carry out the batch dechlorination of trichloroethylene (TCE) using bimetallic Fe/Ni (4:1, Fe to Ni) nanoparticles in cellulose acetate membranes is examined using modeling of transport phenomenon based on experimental results. Membranes are synthesized using both gelation and solvent evaporation techniques for phase inversion. The reduction of metal ions within cellulose acetate phase-inversion membranes was accomplished using sodium borohydride reduction to obtain up to 2 wt % total metals. Characterization of the mixed-matrix structure reveals a bimodal particle distribution ranging between 18 and 80 nm within the membrane cross section. The distribution is the result of changes in the morphology of the cellulose acetate support. The diffusivity and linear partitioning coefficient for the chlorinated organic were measured and are 2.0 x 10(-8) cm2.s-1 and 3.5 x 10(-2) L.g-1, respectively. An unsteady-state model for diffusion through a membrane with reaction was developed to predict experimental results with an error of only 7.2%. The error can be attributed to the lack of the model to account for loss of reactivity through pH effects, alloy effects (bimetallic ratio), and oxidation of nanoparticles. Simulations were run to vary the major transport variables, partitioning and diffusivity, and determine their impact on reaction kinetics. Of the two, diffusivity was less significant because it really only influences the time required for maximum TCE partitioning to the membrane to be achieved and has no effect on the limiting capacity of the membrane for TCE. Therefore, selection of an appropriate support material is crucial for development of highly reactive mixed-matrix membrane systems.     

单分散聚苯乙烯微球的制备及表征

应用膜乳化-液中干燥法成功制备出粒径为2～20μm的单分散聚苯乙烯(PS)微球.PS微球的粒径主要由膜孔径决定,其值约为膜孔径的2倍;PS溶液的浓度对其也有一定的影响.膜乳化过程中的压力对微球粒径的分散性有很大的影响,在一定压力范围内,粒径呈单分散.在分散相中加入致孔剂,制备出表面多孔的PS微球.采用复乳-液中干燥法制备出中空PS微球.     

Modification of crystallinity and pore size distribution in coagulated cellulose films

In this study the effects of altering the coagulation medium during regeneration of cellulose dissolved in the ionic liquid 1-ethyl-3-methylimidazolium acetate, were investigated using solid-state NMR spectroscopy and NMR cryoporometry. In addition, the influence of drying procedure on the structure of regenerated cellulose was studied. Complete conversion of the starting material into regenerated cellulose was seen regardless of the choice of coagulation medium. Coagulation in water predominantly formed cellulose II, whereas coagulation in alcohols mainly generated non-crystalline structures. Subsequent drying of the regenerated cellulose films, induced hornification effects in the form of irreversible aggregation. This was indicated by solid-state NMR as an increase in signal intensity originating from crystalline structures accompanied by a decrease of signal intensity originating from cellulose surfaces. This phenomenon was observed for all used coagulants in this study, but to various degrees with regard to the polarity of the coagulant. From NMR cryoporometry, it was concluded that drying induced hornification generates an increase of nano-sized pores. A bimodal pore size distribution with pore radius maxima of a few nanometers was observed, and this pattern increased as a function of drying. Additionally, cyclic drying and rewetting generated a narrow monomodal pore size pattern. This study implies that the porosity and crystallinity of regenerated cellulose can be manipulated by the choice of drying condition.     

Effect of Bacteria‐Virus Interaction on Mechanism of Virus Removal Using Microfiltration Membranes

In this study, 0.22 μm hydrophobic GVHP membrane was challenged with host bacteria, Escherichia coli (ED 8656) and bacteriophage (λ wild type). For investigation of filtration mechanism, we employed blocking laws, which are based on the profiles of t/v versus time (t) and t/v versus volume (v). For prominent demonstrating the linearity, the profiles were reconstructed. When feed contained bacteriophage or a mixture of bacteriophages and host bacteria, the profiles indicated that at the beginning of the filtration, standard blocking (pore plugging) was the dominant mechanism. Over time the bacteriophages filled the pores and formed a cake layer on the membrane surface. The presence of bacteria facilitated the cake layer build up. Nevertheless the filtration mechanism started with pore blockage and ended with cake formation. The comparison of the present study with another research project carried out by the authors indicates that the removal mechanisms do not rely upon the interactions between microorganisms. However the details of cake layer specifications depend on the nature of the microorganisms and operating conditions.     

Membrane potential in charged porous membranes

For charged porous membranes, the separation efficiency to charged particles and ions is affected by the electrical properties of the membrane surface. Such properties are most commonly quantified in terms of zeta-potential. In this paper, it is shown that the zeta-potential can be calculated numerically from the membrane potential. The membrane potential expression for charged capillary membranes in contact with electrolyte solutions at different concentrations is established by applying the theory of non-equilibrium thermodynamic to the membrane process and considering the space-charge model. This model uses the Nernst–Planck and Navier–Stokes equations for transport through pores, and the non-linear Poisson–Boltzmann equation, which is numerically solved, for the electrostatic condition of the fluid inside pores. The integral expressions of the phenomenological coefficients coupling the differential flow (solute relative to solvent) and the electrical current with the osmotic pressure and the electrical potential gradients are established and calculated numerically. The mobilities of anions and cations are individually specified. The variations of the membrane potential (or the apparent transport number of ions in the membrane pores) are studied as a function of different parameters: zeta-potential, pore radius, mean concentration in the membrane, ratio of external concentrations and type of ions.