Enhanced dehumidification performance of PVA membranes by tuning the water state through incorporating organophosphorus acid

A novel thin film composite membrane with superior propylene dehumidification performance was prepared by coating a high hydrophilic organophosphorus acid ethylene diamine tetra(methylene phosphonic acid) (EDTMPA) doped poly(vinyl alcohol) (PVA) on polysulfone (PS) hollow fiber membranes. Experimental studies and molecular dynamics simulations were combined to probe the existing states and the transport mechanism of water in the membranes. Water vapor sorption experiments revealed that the enhanced dehumidification performance was governed by the diffusion process. Water states and water distribution were investigated by molecular dynamics simulation. At low EDTMPA content (<10 wt.%), states of the water were not obviously changed and the increase of water diffusion coefficient was mainly attributed to enlarged free volume of the membrane. At high EDTMPA content (10–30 wt.%), the increase in the water diffusion coefficient mainly arose from the variations in the water states. Strong interaction between PVA and EDTMPA reduced the amount of water that bounded to the PVA and increased the proportion of free water. The diffusion coefficients of water increased with increasing proportion of free water, since the mobility of free water was higher than that of bound water. The permeance of water reached 997.7 GPU for the PVA–EDTMPA/PS membrane with a 20 wt.% EDTMPA content when the proportion of free water was the highest, and the separation factor increased to infinity.     

Nonstationary diffusion of polystyrenes through a cellophane membrane

Diffusion of five polystyrene fractions at various concentrations in toluene through cellophane membranes has been observed. The results have been used to calculate friction coefficients between solvent and solute, and between solute and membrane. The calculation requires measurement of the diffusion coefficient and the reflection coefficient of the solute, of the permeability for the solvent, of the pore volume of the membrane, and of the partition coefficient of the solute between membrane and solvent. By comparing the friction coefficient between solvent and solute in the membrane with this coefficient in free solution, the tortuosity factor and the pore diameter of the membrane can be estimated. The dependence of the friction coefficients on molecular weight M₂ of the solute is determined. For large values of M₂, the friction between solute and solvent is the determining factor. The friction coefficient between solute and solvent increases more strongly with M₂ in the membrane than in free solution owing to an entrance effect for the permeating solute at the interface.     

Prediction of skin permeability of drugs. I. Comparison with artificial membrane

In order to measure the contribution of lipid and pore (aqueous) pathways to the total skin permeation of drugs, and to establish a predictive method for the steady state permeation rate of drugs, the relationship between permeability through excised hairless rat skin and some physicochemical properties of several drugs were compared with those through polydimethylsiloxane (silicone) and poly(2-hydroxyethyl methacrylate) (pHEMA) membranes, as typical solution-diffusion and porous membranes, respectively. A linear relationship was found between the permeability coefficients of drugs for the silicone membrane and their octanol/water partition coefficients. For the pHEMA membrane, the permeability coefficients were almost constant independent of the partition coefficient. On the other hand, the skin permeation properties could be classified into two types: one involves the case of lipophilic drugs, where the permeability coefficient is correlated to the partition coefficient, similar to the silicone membrane; and the other involves hydrophilic drugs, where the permeability coefficients were almost constant, similar to pHEMA membrane. From the above results, the stratum corneum, the main barrier in skin, could be described as a membrane having two parallel permeation pathways: lipid and pore pathways. An equation for predicting the steady state permeation rate of drugs was derived based on this skin permeation model.     

Solubilization of Hydrophilic Compounds in Copolymer Aggregates

The solubilization of five hydrophilic water-soluble aroma compounds in self-aggregating triblock amphiphilic copolymers of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO), with similar percentages of PEO and different molecular weights, was studied. The five hydrophilic compounds (diacetyl, 2-methylpyrazine, pyrrole, furfural, guaiacol) were carefully selected to represent hydrophilic molecules with a similar molecular weight and molecular volume, but with different abilities to interact with the micellar core of PPO moieties and with the PEO palisade side chains. It was found that the solubilized solute mole fraction increased and the aggregate-water partition coefficients of the solutes decreased with increasing free solute concentration in the aqueous phase. The partition coefficients were smaller than those obtained for hydrophobic compounds and equilibrium was reached at lower solubilization values. Guaiacol was the least hydrophilic molecule and had the highest partition coefficient. Diacetyl was the most water-soluble compound and exhibited the smallest partition coefficient. The data reveal that the higher molecular weight polymers solubilized more solute than the low-molecular-weight polymers. Moreover it is supposed that at low solute concentrations, guaiacol (containing a hydroxyl electron acceptor group) penetrates the core of the micelle and displaces water while at more elevated concentrations it seems to be solubilized in the micelle corona. Diacetyl, the most hydrophilic solute investigated (consisting of electron donor groups), prefers mainly the corona since its affinity for the polymeric core is very weak. The solubilization occurs in the palisade layer and the partition coefficient is independent of the free solute concentration. Selective site (palisade vs core) solubilization of hydrophilic compounds in polymeric micelles can be a powerful tool to protect sensitive materials from reactants present in the continuous water phase and to conduct surface-sensitive organic reactions. Furthermore, selective release properties of reactants and products can be designed. Copyright 2000 Academic Press.     

Permeation of aromatic compounds in aqueous solutions through thin,dense cellulose acetate membranes

The partition and diffusion coefficients of aqueous solutions of aromatic compounds through a thin, dense cellulose acetate membrane were measured at 20°C. The water content and the thickness of the prepared membranes varied from 0.121 to 0.610 by volume fraction and from 17 to 88 μm, respectively. The aromatic solutes used were phenol, aniline, hydroquinone and p-chlorophenol. The solute concentration ranged between 9.0 x 10^-5 and 1.0 x 10^-3 mol/l. The partition coefficients had the following order: p-chlorophenol, phenol, aniline, hydroquinone; they were experimentally correlated with the water content of the swollen membranes.The dependence of the diffusion coefficients on the water content of the membrane was examined using as basis a pore model and a free volume model, respectively. The diffusion coefficients were adequately correlated with the water content of the membrane according to the relation given by the free volume model.     

Factors influencing reverse osmosis rejection of inorganic solutes from aqueous solution

Reverse osmosis (RO) rejection is strongly influenced by the distribution of solute between the membrane and solvent phases. For this reason, we examined the partition coefficients of inorganic compounds between water and cellulose acetate (CA) membranes. Cation and anion partition coefficients were determined by independent analyses. Effects of fixed (negative) membrane charges on CA are clearly apparent at low solute concentrations. The mean cation/anion partition coefficients decrease with the product of the cation and anion valence, and increase with increasing ionic size. Un-ionized inorganic compounds, HgC1₂ and HAuC1₄, are strongly sorbed by CA membranes. All of these observations are consistent with electrostatic theory.Experimental membrane/water partition coefficients are influenced by temperature, pH, and ion-pairing. CA membranes exhibit swelling and shrinkage when exposed to certain aqueous solutions. Swelling and shrinkage influence solute partition and diffusion coefficients, the water content of the membranes, and their RO rejection.The present results provide a comprehensive experimental basis for understanding the mechanism of RO rejection by CA membranes. Moreover, these results can be used to predict RO behavior under a wide variety of experimental conditions. The potential use of reverse osmosis in a variety of wastewater applications is considered in some detail.     

The use of moment theory to interpret diffusion and sieving measurements in terms of the pore size distribution in heteroporous membranes

Moment theory has been applied to model porous membranes to show that one can place reasonable bounds on the cumulative pore size distribution, the hindered diffusivity or the reflection coefficient of large solutes in a heteroporous membrane by measuring the diffusive permeability to a small solute, the hydraulic permeability and one or two additional transport characteristics. These additional measurements involve either the flux of a small solute at Pe1, the hindered diffusivity of a large solute or the reflection coefficient of a large solute at Peå1. Membrane heteroporosity is incroporated in the predicted bounds without requiring one to make any a priori assumptions about the nature of the pore size distribution. In this paper, the results from calculations performed with different model membranes containing log-normal pore size distributions are reported. A comparison of the results obtained with three different membranes shows that one can distinguish between membranes with the same average pore size but different pore size distributions by measuring either the hindered diffusion coefficient or the reflection coefficient of two different sized solutes. A comparison of the bounds on D and the bounds on σ predicted from different types of transport measurements shows that, under certain conditions, one can place tighter bounds on one transport characteristic by measuring a different one.     

Pore structure of gel chitosan membranes. I. Solute diffusion measurements

The capillary pore model of water-swollen gels was used to interpret solute diffusion through gel chitosan membranes. Diffusive permeability coefficients of 12 solutes ranging in molecular radius from 2.5 Å (methanol) to 14 Å (polyethylene glycol 4000) were measured for an untreated chitosan membrane, for four chitosan membranes crosslinked with glutaraldehyde of concentrations between 0.01 and 1% and coated with a protein and also, for comparison, for a commercial Cuprophan membrane. Through the capillary pore-model correlation of the above coefficients with the membrane water content, the following structural factors of the examined membranes were calculated: pore radius, surface porosity and tortuosity factor. Knowledge of these factors is required if the desired membranes are to be designed for a given application (e.g. dialysis).     

Hydrogel membranes with mesh size asymmetry based on the gradient crosslinking of poly(vinyl alcohol)

Poly(vinyl alcohol) (PVA) hydrogel membranes with mesh size asymmetry were prepared and their transport properties were studied. Homogeneous membranes with water contents of 82%, 76% and 72% were prepared by crosslinking PVA with glutaraldehyde. These membranes were then modified to create asymmetry by establishing a glutaraldehyde concentration gradient across the hydrogel thickness. The reaction time and magnitude of the glutaraldehyde concentration gradient were varied to determine the optimum values of permeability and selectivity. Permeation experiments with creatinine, Fab and IgG were performed in a stirred diffusion cell through homogeneous and asymmetric PVA hydrogels. A modified version of the multiple-membrane technique was used to determine boundary layer resistance in order to determine the intrinsic membrane permeability. As expected, the selectivity of creatinine over IgG increased as the modification time increased. However, the selectivity of Fab over IgG initially increased as the modification time increased, but then decreased at longer times, indicating that the increased crosslinking at the surface effectively blocks both proteins. At a given value of IgG rejection, the asymmetric membranes had higher creatinine and Fab permeabilities than the corresponding homogeneous membranes. This indicates that creating mesh size asymmetry in a hydrogel can result in a high-flux, high-selectivity membrane for cell encapsulation or bioseparations.     

耗散粒子动力学模拟Nafion膜和PVA/Nafion共混膜的介观结构

采用耗散粒子动力学模拟方法研究了水化Nafion膜和水化聚乙烯醇(PVA)/Nafion共混膜的微结构.模拟结果表明水化Nafion膜和水化PVA/Nafion共混膜均能形成相分离的微结构.在水化Nafion膜中,水与磺酸根混合形成管状的水团簇.随着膜内水含量增多,管状水团簇的尺寸逐渐变大并在膜内形成连续的水通道.在水化PVA/Nafion共混膜中,PVA、水、磺酸根混合形成亲水性区域.共混膜中PVA的质量分数和水含量共同影响膜的微结构.当膜中PVA质量分数较低时,PVA主要分布在Nafion的磺酸根基团周围;PVA质量分数升高后,PVA会在膜内单独成一相.当膜中的水含量相对较低时,水分子会溶解于PVA中,此时膜内不存在单独的水团簇;膜中的水含量增多后,膜内会形成接近于球形的水团簇.本文工作可为直接甲醇燃料电池用的PVA改性Nafion膜的开发提供参考.     

Effects of Multisolute Steric Interactions on Membrane Partition Coefficients

A key parameter in membrane and chromatographic separations is the partition coefficient, the equilibrium ratio of the solute concentration in a porous or fibrous material to that in bulk solution. The theoretical effects of solute size on partition coefficients in straight pores or randomly oriented fiber matrices have been investigated previously for very dilute solutions, where solute-solute interactions are negligible, and also for more concentrated solutions consisting of spherical solutes of uniform size. For concentrated solutions it has been found that steric and other repulsive interactions among solutes increase the partition coefficient above the dilute limit. To extend the results for porous or fibrous media to include concentrated mixtures of solutes with different sizes or shapes, we used an excluded volume approach. In this formulation, which describes steric interactions only, partition coefficients were computed by summing all volumes excluded to a solute molecule by virtue of its finite size, the finite size of other solutes, and the presence of fixed obstacles (pore walls or fibers). For a mixture of two spherical solutes, the addition of any second solute at finite concentration increased the partition coefficient of the first solute. That increase was sensitive to the size of the second solute; for a given volume fraction of the second solute, the smaller its radius, the larger the effect. When the total volume fraction of solutes was fixed, an increase in the amount of a second, smaller solute increased the partition coefficient of the first solute, whereas an increase in the amount of a second, larger solute had the opposite effect. Results were obtained also for oblate or prolate spheroidal solutes and for fibrous media containing fibers of different radii. For constant total fiber volume fraction, an increase in the amount of a second, smaller fiber decreased the partition coefficient of a spherical solute, whereas an increase in the amount of a second, larger fiber had the opposite effect. Overall, the theory suggests that the introduction of heterogeneities, whether as mixtures of solute sizes or mixtures of fiber sizes, may cause partition coefficients to differ markedly from those of uniform systems. Copyright 2000 Academic Press.     

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.     

A spray technique for the determination of membrane diffusion and distribution coefficients by the time-lag method: evaluated for electrolyte transport through charged and uncharged membranes

Using solution sprays, a sharp concentration step may be effected at the surface of a membrane in less than 0.1 sec. This technique allows the time-lag method to be used to determine rapidly both solute permeability and diffusion coefficient in a single experiment. From the mathematical analysis of diffusion into a cell with finite collecting volume, limiting conditions are derived which define experimental conditions under which the conventional linear, steady state analysis of time lag will be valid. These limiting conditions were used to determine the dimensions of the experimental diffusion cell. Diffusion, permeability and distribution coefficients were measured for a variety of electrolytes in both charged and uncharged membranes and shown to be in good agreement with conventional methods. A Donnan analysis is given by which the charged nature of an unknown membrane may be assessed precisely and rapidly.     

Pore structure of gel chitosan membranes. III. Pressuredriven mass transport measurements

The capillary pore model of water-swollen gels was used to interpret pressure-driven mass transport properties of gel chitosan membranes. Pure water hydraulic permeability coefficients, L_p, and rejection coefficients, R, of 13 solutes ranging in molecular radius from 2.4 Å (methanol) to 16 Å (polyethylene glycol 6000) were measured for an untreated chitosan membrane, for two chitosan membranes crosslinked with glutaraldehyde of concentrations 0.01 and 0.1% and coated with a protein, and for comparison for a commercial Cuprophan membrane. Pore radii of the membranes were determined from these results by three methods: (1) L_p method that uses water hydraulic permeability coefficient, (2) σ method that uses reflection coefficients, and (3) P/L_p method that uses water diffusive permeability coefficient and water hydraulic permeability coefficient.     

Effects of concentration on the partitioning of macromolecule mixtures in agarose gels

To test the effects of solute concentration on the equilibrium partitioning of single macromolecules and macromolecule mixtures between bulk solutions and gels, the partition coefficient in agarose was measured for BSA and for four narrow fractions of Ficoll with Stokes radii of 30-59 A. Solutions of each test macromolecule were equilibrated with a known volume of gel, final liquid concentrations measured, and partition coefficients (gel concentration divided by bulk concentration) calculated by applying a material balance. The partition coefficient of each macromolecule was measured in 4 and 6% gels under dilute conditions and with BSA present at initial concentrations up to 13.5 g/dl. As expected, the partition coefficients decreased with increasing agarose concentration and with increasing macromolecular size. Moreover, increasing the BSA concentration increased the partition coefficient of BSA itself and that of all four Ficolls. This effect was most pronounced for the largest test solutes. Measurements at two ionic strengths confirmed that electrostatic interactions were negligible under the conditions used. The experimental results were compared with predictions from a previously developed excluded volume theory for the partitioning of mixtures of rigid, spheroidal macromolecules in fibrous media. Agarose was represented as a randomly oriented array of cylindrical fibers, BSA as a prolate spheroid, and Ficoll as a sphere. The quantitative agreement between the model predictions and the data was generally quite good, indicating that steric interactions among solute molecules and between solute molecules and gel fibers could explain the partitioning results. The theory is simple enough computationally to be applied to a variety of processes that are influenced by the equilibrium partitioning of macromolecules.     

Anti-trade-off in dehydration of ethanol by novel PVA/APTEOS hybrid membranes

Novel organic–inorganic hybrid membranes were prepared through sol–gel reaction of poly(vinyl alcohol) (PVA) with γ-aminopropyl-triethoxysilane (APTEOS) for pervaporation (PV) separation of ethanol/water mixtures. The membranes were characterized by FTIR, EDX, WXRD and PALS. The amorphous region of the hybrid membranes increased with increasing APTEOS content, and both the free volume and the hydrophilicity of the hybrid membranes increased when APTEOS content was less than 5 wt%. The swelling degree of the hybrid membranes has been restrained in an aqueous solution owing to the formation of hydrogen and covalent bonds in the membrane matrix. Permeation flux increased remarkably with APTEOS content increasing, and water permselectivity increased at the same time, the trade-off between the permeation flux and water permselectivity of the hybrid membranes was broken. The sorption selectivity increased with increasing temperature, and decreased with increasing water content. In addition, the diffusion selectivity and diffusion coefficient of the permeants through the hybrid membranes were investigated. The hybrid membrane containing 5 wt% APTEOS has highest separation factor of 536.7 at 50 °C and permeation flux of 0.0355 kg m⁻² h⁻¹ in PV separation of 5 wt% water in the feed.     

Separation of aromatic substances from aqueous solutions using a reverse osmosis technique with thin,dense cellulose acetate membranes

Investigations were made of the water flux rate and rejection characteristics of aromatic substances in aqueous solutions using a thin, dense cellulose acetate membrane in reverse osmosis experiments. The aromatic substances used were phenol, aniline, hydroquinone and p-chlorophenol. The permeate became more enriched in aromatic compounds as compared to the feed solution as the water content of the membrane increased. By considering both the effects of pressure on the chemical potential of a component and the contribution of viscous flow to the overall transport of that component in the hydrated membrane, a theoretical relationship was developed to predict the negative solute rejection of the membrane. Based on this proposed theory, the permeability coefficients of water and organic solute were estimated from experimental solute rejection data, including negative values. The permeability coefficients of components were in good agreement with previously established correlations in measurements of partition and diffusion coefficients.     

Pervaporation separation of water + isopropanol mixtures using novel nanocomposite membranes of poly(vinyl alcohol) and polyaniline

Novel nanocomposite polymeric membranes containing nanosized (30–100 nm) polyaniline (PANI) particles dispersed in poly(vinyl alcohol) (PVA) were prepared and used in the pervaporation separation of water–isopropanol feed mixtures ranging from 10 to 50 mass% of water at 30 °C. Of the three nanocomposite membranes prepared, the membrane containing 40:60 surface atomic concentration ratio of PANI:PVA produced the highest selectivity of 564 compared to a value of 77 observed for the plain PVA membrane. Flux of the nanocomposite membranes was lower than those observed for the plain PVA membrane, but selectivity improved considerably. Membranes were characterized by differential scanning calorimetry, dynamic mechanical thermal analyzer, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy and scanning electron microscopy. The highest selectivity with the lowest flux was observed for 10 mass% water containing feed mixture. Flux increased with increasing amount of water in the feed, but selectivity decreased considerably. These results were attributed to the acid-doped PANI particles in the PVA membrane as a result of change in the micromorphology of the nanocomposite membranes. In addition, molar mass between cross-links and fractional free volume of the membranes are responsible for the varying membrane performance. Temperature effect on permeability was investigated for 10 mass% water containing feed with the membrane containing higher concentration of PANI particles, the presence of which could be responsible for varied effect of water permeation through the membrane. Membranes of this study could remove as much as 98% of water from the feed.     

Permeability of membranes containing ionogenic groups

The present paper deals with the transport properties of membranes made of hydrophilic gels containing ionogenic groups. Introduction of ionogenic groups into a gel based on 2-hydroxyethyl methacrylate will affect the permeability of the investigated membranes for sodium chloride by an order or more. Dependences of the permeability on the content of ionogenic groups, three-dimensional network density, and pH were established. The permeability for NaCl was compared for that for bivalent salt (MgSO₄). It is shown, on the basis of independently determined distribution coefficients, that an increase in the permeability of ampholytic membranes in comparison with the neutral ones is primarily due to an increase in the diffusivity of the salt in the membranes with modified structure. It can also be concluded that an approximation of the free volume from the volume of the solvent in the membrane cannot be applied to the poly(2-hydroxyethyl methacrylate) gel.