Performance of solvent-resistant membranes for non-aqueous systems: solvent permeation results and modeling

Membrane processes like reverse osmosis (RO) and nanofiltration (NF) can be low energy consuming operations as compared to the traditional chemical engineering unit operations and have been widely used for aqueous systems. Since such membrane processes are low energy consuming operations, their use in non-aqueous systems would offer considerable energy savings. Thus, the study is directed towards development and experimental verification of membrane materials and transport models to explain permeation properties of non-aqueous solvent systems. The understanding of polymer–solvent interactions is critical towards the development of suitable materials and also the prediction of the transport mechanisms.Pure solvent permeation studies were conducted to understand the mechanism of solvent transport through polymeric membranes. Different membrane materials (hydrophilic and hydrophobic) as well as different solvents (polar and non-polar) were used for the study. Pure solvent fluxes for hydrophilic membranes used showed that polar solvents (methanol, ethanol, iso-propanol) had a significantly higher flux (8–10 times) than that of the non-polar solvents (pentane, hexane, octane). On the contrary, the non-polar solvent flux was two to four times that of the polar solvents for hydrophobic membranes. For example, hexane flux at ∼13 bar through a hydrophobic silicone based NF membrane was ∼0.6×10⁻⁴ cm³/cm² s. And that through a hydrophilic aromatic polyamide based NF membrane was ∼6×10⁻⁴ cm³/cm² s. A simple model based on a solution-diffusion approach is proposed for predicting the pure solvent permeation through hydrophobic polymeric membranes. The model uses molar volume and viscosity of the solvent as parameters for predicting the pure solvent permeability. The model reasonably predicts the pure solvent permeation (R²=0.89, S.E.∼4%) for hydrophobic membranes. The model has also been experimentally verified using high solution temperatures and also literature experimental data. To extend the predictions to different membranes (hydrophilic and hydrophobic), surface energy and sorption values have been used as a parameter along with the solvent physical properties.     

Effect of membrane pretreatment on performance of solvent resistant nanofiltration membranes in methanol solutions

This paper reports the effect of membrane pretreatment using different organic solvents on the performance of polyamide, polyimide and polydimethylsiloxane (PDMS) membranes in methanol solutions. Membrane pretreatment using acetone, methanol and toluene results in significant changes of membrane flux and rejection for polyamide- and polyimide-based membranes (Desal-DK and STARMEM 228) due to membrane swelling. The Performance of a polydimethylsiloxane (PDMS)-based membrane (MPF-50) in methanol solutions was not significantly affected by membrane pretreatment.     

Solvent flux through dense polymeric nanofiltration membranes

This work examines the flux performance of organic solvents through a polydimethylsiloxane (PDMS) composite membrane. A selection of n-alkanes, i-alkanes and cyclic compounds were studied in deadend permeation experiments at pressures up to 900 kPa to give fluxes for pure solvents and mixtures between 10 and 100 l m⁻² h⁻¹. Results for the chosen alkanes and aromatics, and subsequent modelling using the Hagen–Poiseuille equation, suggest that solvent transport through PDMS can be successfully interpreted via a predominantly hydraulic mechanism. It is suggested that the mechanism has a greater influence at higher pressures and the modus operandi is supported by the non-separation of binary solvent mixtures and a dependency on viscosity and membrane thickness. The effects of swelling that follow solvent–membrane interactions show that the relative magnitudes of the Hildebrand solubility parameter for the active membrane layer and the solvent(s) are a good indicator of permeation level. Solvents constituting a group (e.g. all n-alkanes) induced similar flux behaviours when corrections were made for viscosity and affected comparable swelling properties in the PDMS membrane layer.     

Polymer membranes with selective gas and vapor permeation properties

Since many years synthetic membranes have been used in reverse osmosis or ultrafiltration for the separation of aqueous mixtures. More recently the separation of gases and vapors by selective membrane permeation has gained significant technical and commercial interest. The recovery of hydrogen from petrochemical purge gases and ammonia production processes or the removal of CO₂ from natural gas by selective membrane permeation are today state of the art procedures. The recovery of organic solvents from waste air streams is another very promising application of synthetic membranes. In this paper the main parameters determining the performance of a membrane in gas and vapor separation are described. The requested intrinsic properties of the polymer to be useful as a barrier for a selective gas and vapor transport are discussed. The preparation of appropriate membranes is described. Their performance in practicle applications is illustrated in selected examples.     

基团贡献法研究环型聚苯乙烯稀溶液流体力学参数

首次采用基团贡献法, 根据内聚能、摩尔体积、摩尔吸引常数和临界特性粘数等分别计算并通过实验数据验证了环型和线型聚苯乙烯的溶度参数, 特性粘数、摩尔极限粘数函数、临界分子量和Mark-Houwink参数, 详细地讨论了计算大环高分子的K_(θ,r)值所取用的临界分子量应具有的理论意义。     

Inverse gas chromatographic characterization of triblock copolymer of polystyrene–poly(ethylene oxide)–polystyrene

The thermodynamic properties of triblock copolymer of polystyrene–poly (ethylene oxide)–polystyrene (PS‐b‐PEO‐b‐PS) were investigated by means of inverse gas chromatography (IGC) using 15 different kinds of solvents as the probes. Some thermodynamic parameters, such as specific retention volume, molar heats of sorption, weight fraction activity coefficient, Flory‐Huggins interaction parameter, partial molar heats of mixing and solubility parameter were obtained to judge the interactions between PS‐b‐PEO‐b‐PS polymers and solvents and the solubility of the polymers in these solvents. It was found that increasing PEO content in PS‐b‐PEO‐b‐PS resulted in the increase in the solubility of PS‐b‐PEO‐b‐PS in alkanes and acetates solvents, but the solubility in alcohols had no change, and more PEO content in polymer caused a small decrease in the solubility parameter of PS‐b‐PEO‐b‐PS polymer, © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2015–2022, 2007     

Influence of solvent on the thermal stability and organization of self-assembling fibrillar networks in methyl-4,6-O-(p-nitrobenzylidene)-α-d-glucopyranoside gels

Sugar based low-molecular-mass organogelator (LMOG) methyl-4,6-O-(p-nitrobenzylidene)-α-d-glucopyranoside, is a unique gelator because its small and weakly-interacting molecules can form large supramolecular structures in nonpolar, but also in polar, solvents and cause their gelation. The self-assembling properties of the gelator were studied in selected nonpolar and polar solvents. It was shown that the driving forces for both types of solvent are the intermolecular hydrogen bond interaction. The effect of the nature of the solvent on the thermal stability of the gels and on the three-dimensional network organization was determined. Different solvent parameters, such as dielectric constant, one-component solubility parameter, the polarity parameter and the Kamlet-Taft parameters were considered to quantify solvent effects on the gelation. Some correlation between these parameters and the gel stability, microstructure and the enthalpy of the phase transition were established. The effort to correlate the Kamlet-Taft parameters to the thermal stability and gelation ability is also possible but applies only to the studied gelator.     

超临界水的性质及其在化学反应中的应用

介绍了水的热物理性质、结构性质、离子积、扩散系数和粘度等在超临界区域的特殊性，以及超临界水溶液在介电常数、偏摩尔体积、溶解性和极性等方面的特殊性质，并阐述了超临界水的这种特殊性在化学反应和环境治理中的应用。     

Experimental contribution to the understanding of transport through polydimethylsiloxanenanofiltration membranes: Influence of swelling,compaction and solvent on permeation properties

This study aims to better understand the permeation properties of polydimethylsiloxane (PDMS) membranes. The compressibility and nanofiltration fluxes were measured for swollen PDMS films using several solvents at applied pressures ranging from 5 to 50 bar. The degree of swelling varied according to the solvent and the pressure applied. To show the correlation between the behaviour of the swollen PDMS under pressure and its permeation performance, the thickness reduction of the membrane was mimicked using uniaxial compression tests. The evolution of the nanofiltration flux as a function of the transmembrane pressure proved to be non-linear. Linearization was achieved by taking into account both the swelling and the thickness reduction previously measured, confirming that these phenomena may have occurred during the nanofiltration experiments. Moreover, the solvents' viscosity and affinity for the polymer were confirmed to have a great influence on their ability to permeate the membrane. Finally, employing the most commonly used models, a study of transport through the membrane led to the conclusion that the experimental results were in agreement with the hydraulic theory of transport.     

The correlation between free volume and gas separation properties in high molecular weight poly(methyl methacrylate) membranes

Poly(methyl methacrylate) membranes of different fractional free volume (FFV) were prepared by dry casting from different solvents. Free volume data were determined by means of Bondi method and positron annihilation lifetime spectroscopy (PALS). It was found that both the boiling point and the solubility parameter of casting solvent affect the membrane’s free volume. It was believed that the difference in free volume was arisen from the difference in polymer packing.The gas permeability is higher when membranes are cast from higher molecular weight PMMA. But the plasticizing effect of CO₂ is less serious compared with the low molecular weight one. The high molecular weight PMMA membrane also has an extremely high O₂/N₂ selectivity, indicating its high structure uniformity. These results indicate that membranes made from polymer of higher molecular weight have the advantages of high permeability, gas selectivity and are less sensitive to CO₂ plasticization. The intrinsic gas transport properties such as the permeability, solubility and diffusivity of O₂, N₂, and CO₂ are measured or calculated. The effects of fractional free volume on membrane gas separation properties were investigated. It was found that the fractional free volume had no definite effects on gas solubility, but the gas permeability and diffusivity increased accordingly to the measured free volume.     

Relationship between ionic conductivity of perfluorinated ionomeric membranes and nonaqueous solvent properties

Ionic conductivity and swelling data are measured for Nafion^® perfluorinated ionomeric membranes in nonaqueous solvents and solvent mixtures and correlated with solvent physical properties. The dependence of ionic conductivity on solvent uptake and cation type is examined for Nafion^® 117 membranes with a nominal equivalent weight of 1100 g/eq. The most important factors determining ionic conductivity in membranes swollen with polar nonaqueous solvents are the solvent viscosity, molar volume, donor properties, and the solvent uptake by the membrane. Ionic conductivity is generally limited by dissociation of the cation from the fixed anion site indicating that the ionomer fixed anion site basicity is the critical membrane property. Means for increasing membrane ionic conductivity are discussed.     

Thermodynamic interactions and characterisation of naphthenic oil by inverse gas chromatography

The thermodynamic properties of naphthenic oil, a plasticiser, were investigated by means of inverse gas chromatography (IGC) using 10 different kinds of solvents as probes. Some thermodynamic parameters, such as specific retention volume, weight fraction activity coefficient, Flory–Huggins interaction parameter, partial molar heats of mixing and solubility parameter were obtained to judge the interactions between oil and solvents and the solubility of oil in these solvents. The results indicated that n-heptane, n-hexane, cyclohexane, chloroform, benzene and diethyl ether are good solvents for oil at experimental temperatures. The solubility parameters of oil varied from 13.94 to 13.21?(J?cm^?3)^1/2 at temperature range 323–353?K. The solubility parameter of oil was calculated to be 14.38?(J?cm^?3)^1/2 at room temperature, which is consistent with that obtained using surface tension–solubility parameter relation method.     

Sorption and transport behavior of gasoline components in polyethylene glycol membranes

Desulphurization mechanism of polyethylene glycol (PEG) membranes has been investigated by the study of solubility and diffusion behavior of typical gasoline components through PEG membranes with various crosslinking degrees. The sorption, diffusion and permeation coefficients were calculated by the systematic studies of dynamic sorption curves of gasoline components such as thiophene, n-heptane, cyclohexane, cyclohexene and toluene in PEG membranes. Furthermore, the temperature dependence of diffusion and solubility coefficients and the influence of crosslinking degree on sorption and diffusion behaviors were conducted to elucidate the mass-transfer mechanism. According to the discussions on dynamic sorption curves, transport mode, activation energy and thermodynamic parameters, thiophene species were the preferential permeation components. Crosslinking is an effective modification way to improve the overall performance of PEG membranes applied in gasoline desulphurization. The pervaporation (PV) and gas chromatography (GC) experiments results corresponded to the conclusions. All these investigations will provide helpful suggestions for the newly emerged membrane desulphurization technology and complex organic mixture separation by pervaporation.     

改性PPO的溶解性能和溶度参数的初步估计

研究了聚苯醚PPO、羧化PPO、苯酰化PPO、苯磺酰化PPO和磺化PPO在不同溶剂中的溶解能力,分别讨论了各种取代基以及取代度对聚合物溶解性能的影响,由PPO衍生物的特性粘数估算了它们的溶度参数。     

Solubility Parameter of Gatifloxacin and Its Correlation with Antibacterial Activity

The solubility parameter of gatifloxacin was calculated theoretically by Fedors’ method and also determined experimentally using the standard solubility method. The molar volume of gatifloxacin was determined experimentally by the flotation method. Three solvents, ethyl acetate, propylene glycol and water were used to prepare nine binary mixtures having different solubility parameter values varying from 8.9 to 23.4 H. The δ ₂ solubility parameter of gatifloxacin was found to be 12.4 H. The cumulative transport of gatifloxacin from the binary solvent mixtures (ethyl acetate–propylene glycol and propylene glycol–water) exhibited an inverse parabolic relationship to the solubility parameter of the solvent mixtures. Transport studies revealed that the extent of gatifloxacin rejection from solvent mixtures was dependent on its solubility parameter and system composition. In the selected organisms (bacteria), a minimum zone of inhibition was observed in a solvent mixture having the solubility parameter nearest to the solubility parameter of gatifloxacin. Two models were employed for the evaluation of the antibacterial activity of gatifloxacin in binary mixtures.     

The rate of chloroacetic acid alkaline hydrolysis: Medium effect

Rate constant of chloroacetic acid alkaline hydrolysis in the mixed aqueous-organic solvents depends on the organic co-solvent nucleophilic solvation; however, the effect of the medium polarity on the rate constant is not clear. The rate constant can be reasonably estimated with a multi-parameter equation accounting for the organic co-solvent basicity as well as its density of cohesion energy and molar volume.     

Effect of molecular association on solubility,diffusion, and permeability in polymeric membranes

The filling‐type membrane is composed of grafted polymer and solvent‐resistant substrate; the calculation of solubility, diffusivity and swelling‐suppression effect by the substrate permits the prediction of solvent permeability. As noted in our previous article, the use of this approach, called membrane design, resulted in accurate prediction of the permeability of aromatic compounds. In this study, the influence of hydrogen bonding on solubility and diffusivity is investigated both theoretically and experimentally. The solubility of chloroform and dichloromethane in poly(acrylate)s increases, and their diffusivity decreases, compared with that estimated without considering the hydrogen‐bonding effect. Solubilities predicted by the lattice‐fluid hydrogen‐bonding (LFHB) model show good agreement with the results of vapor sorption. Comparison of diffusion coefficients measured by vapor permeation with those predicted from free volume theory reveals that the decrease of solvent diffusion coefficient is approximately proportional to the fraction of associated molecules. Fluxes of chloroform and dichloromethane were measured by vapor permeation experiments through filling‐type acrylate membranes, and predictions agree well with experiments. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 171–181, 2000     

改性PPO的溶解性能研究

研究了 PPO、羧化 PPO(C-PPO)、苯酰化 PPO(BA-PPO)、苯磺酰化 PPO(BS-PPO)和磺化 PPO(S-PPO)在不同溶剂中的溶解能力,分别讨论了各种取代基以及取代度对溶解性能的影响,测定了各种改性 PPO 在溶剂中的特性粘数,估计了它们的溶度参数。发现,在 PPO 的溶解过程中,色散力起重要作用,S-PPO 样品的溶解过程则是极性力和氢键起主导作用,而 BA-PPO 和 BS-PPO 的溶解过程则是三种力相互作用的结果。对于不同取代度的 C-PPO,随取代度增加,在极性和形成氢键的溶剂中的溶解能力增加。     

1-烷基-3-甲基咪唑硫酸氢盐的物理性状研究

合成了系列酸性离子液体1-烷基-3-甲基咪唑硫酸氰盐（[CnMIm]HSO4,n=0,2,4,6,8）,并用FT-IR和^1H NMR表征其结构．测定了其密度、黏度、溶解性、酸度、电导率和热稳定性等物理性质,考察了不同烷基对物理性状的影响．结果表明,密度随咪唑环上烷基的增长而减小;由于受氢键与范德华力的共同作用,使[C4MIm]HSO4的黏度最小,其数值为16．359×10^-3Pa·s;在水溶液中,[CnMIm]HSO4的极限摩尔电导率均高于KCl,其中以[C4MIm]HSO4的极限摩尔电导率最大,其数值为570．93S·cm^3·mol^-1;在氢键和空间效应的共同影响下,使[C6MIm]HSO4的pKa最小,其数值为0．695;离子液体的溶解度与溶剂的极性有关,易溶于强极性溶剂,难溶于弱极性溶剂;[CnMIm]HSO4在低于240℃具有良好的热稳定性．