Dual-mode Sorption Research Articles

Gas Transport in Glassy Polymers: Prediction of Diffusional Time Lag.

The transport of gases in glassy polymeric membranes has been analyzed by means of a fundamental approach based on the nonequilibrium thermodynamic model for glassy polymers (NET-GP) that considers the penetrant chemical potential gradient as the actual driving force of the diffusional process. The diffusivity of a penetrant is thus described as the product of a purely kinetic quantity, the penetrant mobility, and a thermodynamic factor, accounting for the chemical potential dependence on its concentration in the polymer. The NET-GP approach, and the nonequilibrium lattice fluid (NELF) model in particular, describes the thermodynamic behavior of penetrant/polymer mixtures in the glassy state, at each pressure or composition. Moreover, the mobility is considered to follow a simple exponential dependence on penetrant concentration, as typically observed experimentally, using only two adjustable parameters, the infinite dilution penetrant mobility L10 and the plasticization factor β, both determined from the analysis of the dependence of steady state permeability on upstream pressure. The available literature data of diffusional time lag as a function of penetrant upstream pressure has been reviewed and compared with model predictions, obtained after the values of the two model parameters (L10 and β), have been conveniently determined from steady state permeability data. The model is shown to be able to describe very accurately the experimental time lag behaviors for all penetrant/polymer pairs inspected, including those presenting an increasing permeability with increasing upstream pressure. The model is thus more appropriate than the one based on Dual Mode Sorption, which usually provides an unsatisfactory description of time lag and required an ad hoc modification.

Mixed Matrix Membranes of Boron Icosahedron and Polymers of Intrinsic Microporosity (PIM-1) for Gas Separation.

This work reports on the preparation and gas transport performance of mixed matrix membranes (MMMs) based on the polymer of intrinsic microporosity (PIM-1) and potassium dodecahydrododecaborate (K2B12H12) as inorganic particles (IPs). The effect of IP loading on the gas separation performance of these MMMs was investigated by varying the IP content (2.5, 5, 10 and 20 wt %) in a PIM-1 polymer matrix. The derived MMMs were characterized by scanning electron microscopy (SEM), thermogravimetric analysis (TGA), single gas permeation tests and sorption measurement. The PIM1/K2B12H12 MMMs show good dispersion of the IPs (from 2.5 to 10 wt %) in the polymer matrix. The gas permeability of PIM1/K2B12H12 MMMs increases as the loading of IPs increases (up to 10 wt %) without sacrificing permselectivity. The sorption isotherm in PIM-1 and PIM1/K2B12H12 MMMs demonstrate typical dual-mode sorption behaviors for the gases CO2 and CH4.

Dual Mode Sorption and Diffusion of NaCl in Foodstuffs under Cooking Conditions

Dual Mode Sorption and Diffusion of NaCl in Foodstuffs under Cooking Conditions

Evaluating Nonlinear Sorption of Four Substituted Phenols to Agriculture Soils Using Expanded Polyparameter Linear Free Energy Relationship

Nonlinear sorption of substituted phenols (degradation products of several pesticides) onto soils was often observed. This sorption nonlinearity at low solute concentration ranges could result in higher soil organic carbon-water distribution coefficient (K oc) values than those predicted by their hydrophobicity (K ow). In this study, nonlinear sorption characteristic of four substituted phenols (2,6-dimethylphenol, 2-chlorophenol, 2-nitrophenol, and 2,4-dichlorophenol) onto two agricultural soils was investigated. The sorption nonlinearity gradually approached apparent saturation at low solute activity ranges (e.g., a i < 0.01). At high a i ranges, linear sorption was observed. Thus, partition and adsorption of solutes were successfully evaluated by a dual-mode sorption model. The concentrations of substituted phenols in the environment are pretty low (e.g., usually lower than 1 mg/L). According to our results, nonlinear adsorption is dominant in such low concentration ranges in the environment. To predict varied log K oc values resulted from nonlinear adsorption, especially for low a i range, an expanded polyparameter linear free energy relationship (pp-LFER) is established: log K oc = [(1.829 ± 0.488) + (3.481 ± 0.462) log a i)]E+ [(− 4.307 ± 0.466) log a i]S+ [(− 0.876 ± 0.138) log a i]A+ [(− 0.086 ± 0.529) + (1.209 ± 0.218) log a i]B+ (6.280 ± 0.649)V – (6.814 ± 0.917) (E, the excess molar refraction; S, the dipolarity/polarizability parameter; A, the solute H-bond acidity; B, the solute H-bond basicity; and V, the molar volume). This model can provide a better prediction (within 0.3 log unit) than previous models. This study provides essential parameters for predicting and understanding the environmental behavior of substituted phenols in agricultural soils.

A new characterization method of membranes with nonlinear sorption isotherm systems based on continuous upstream and downstream time-lag measurements

A theoretical foundation for a new characterization method for dual-mode sorption membranes is presented. The method relies on simultaneous monitoring of pressure decay and pressure rise and continuous evaluation of the respective instantaneous upstream and downstream time lags as well as their ratio. Assuming the simplest gas transport model in dual-mode sorption membranes, i.e. complete immobilization and instantaneous equilibrium between Henry's and Langmuir's sites, the expression for the upstream time lag and the ratio of the upstream and downstream time lags for dual-mode sorption membranes has been derived using Frisch's asymptotic solution. The analytically derived ratio between the upstream and downstream time lags is used as an extra equation that reduces the number of permeation experiments required to fully recover the kinetic and sorption parameters of the membrane. The proposed characterization method, developed for the ideal boundary conditions, is extended to the real boundary conditions, in which gas concentrations at the upstream and downstream interfaces of the membrane vary with time, by recognizing a set of unique relationships between the maximum downstream time lag and the time at which the ratio of the upstream and downstream time lags is equal to the unique value that would exist under the ideal boundary conditions. Finite difference numerical modeling was utilized to examine the proposed method under the real boundary conditions using some case studies from the literature. The accuracy of the recovered kinetic and sorption parameters is promising and allows the elimination of challenging sorption measurement involving dense membranes.

CO2/CH4 Sorption Behavior of Glassy Polymeric Membranes Based on Dual Mode Sorption Model

Among the various transport models for gas separation via membrane, the best description is done by solution-diffusion model. The main parameters of this model are the sorption and diffusion of the penetrant through the membrane. Studies conducted by various researchers in the field of gas separation indicate that the thermodynamic interaction (sorption effects) in glassy polymers has the major role in the diffusivity, permeability, and selectivity of the membrane, especially in the multi-component gas mixture. In glassy polymers, dual-mode sorption model is frequently used to describe the equilibrium sorption behavior of a polymer-gas system and, based on this sorption model; the permeation behavior is described by the partial immobilization model. In this study, the difference between the sorption mechanism of CO2 and CH4 in glassy polymeric membranes was analyzed by separating the sorption mode and introducing P50/50 parameter (the pressure at which the contribution of both Henry and Langmuir sorption is equal), which was done using the available experimental data in the scientific references for the sorption values of CO2 and CH4. The contribution of the sorbed molecules in Henry and Langmuir sites was investigated and its changes with pressure were evaluated for CO2 and CH4 permeation. This study was also attempted to provide a correct definition of F factor, used in references, in the form of the multiplication of mobile concentration ratios (CHmobile/CH) by the diffusivity coefficients in two modes (DH/DD).

Impact of Heavy Hydrocarbons on Natural Gas Sweetening Using Perfluorinated Polymeric Membranes

Membranes that are resistant to heavy hydrocarbons are of particular interest in natural gas sweetening, the removal of acidic gases such as CO2 from methane. The perfluorinated polymers Teflon AF1600 and Hyflon AD60 are studied here for this purpose because of their unusual solvent resistant properties. The sorption of hexane and toluene vapor in both perfluorinated polymers was observed to follow standard dual-mode sorption behavior at activities less than 0.8, while multilayer absorption was observed for unity activity. However, the corresponding Henry’s law constants for both hydrocarbons were reduced compared to other classes of glassy polymeric membranes. This was attributed to the solvent resistant properties of the perfluorinated polymers. Under mixed gas conditions, it was observed that the CO2 permeability for both perfluorinated polymers was reduced compared to the single gas permeability, attributed to competitive sorption from CH4. However, CH4 permeability increased slightly for both membranes under mixed gas conditions. Adding hydrocarbon vapors to the mixed gas feed resulted in only small changes to the CO2 and CH4 permeability through both perfluorinated polymers, indicating both perfluorinated polymers have potential as hydrocarbon-resistant membranes in natural gas sweetening.

The retention profile of phosphate ions in aqueous media onto ion pairing immobilized polyurethane foam: Kinetics, sorption and chromatographic separation

A strategy for enriching, separating and determining the inorganic phosphate ions in water based on the use of polyurethane foam (PUFs) physically immobilized with some ion pairing or basic dyes has been reported. The retention of inorganic phosphate from the test aqueous solution containing molybdate ions onto untreated- and ion pairing reagent e.g. crystal violet (CV) or tetrabutylammoniumtetrafluoroborate (Bu4N+·BF4−) impregnated PUFs solid phase extractor was followed the order: unloaded<CV<Bu4N+·BF4− treated PUFs sorbent at pH3.5–4.5. The kinetics of analyte sorption onto Bu4N+·BF4− treated PUFs at optimized pH (pH3.4–4.5) was fast and followed first order rate equation with an overall rate constant of 0.062min−1. Film and intraparticle transport are the main factors controlling the rate of phosphate sorption onto the sorbent from solutions at pH=3.5–4.5. Phosphate uptake followed Langmuir and Freundlich isotherm models. Thus, a dual-mode sorption involving absorption related to “weak base anion exchanger” and an added component for “surface adsorption” is the most probable retention mechanism for phosphate uptake by Bu4N+·BF4− loaded PUFs sorbent. Chromatographic separation of phosphate by Bu4N+·BF4− treated PUFs packed column was carried out. The number (N) and the height equivalent to the theoretical plates (HETP), critical and breakthrough capacities were found equal 81±4, 0.13±0.02mm, 2.5±0.2 and 27.25±2.06mgg−1, respectively. These results suggested the use of the proposed Bu4N+·BF4− treated PUFs in packed column for separation of trace levels of phosphate in water. The retained phosphate were recovered quantitatively (102.6±2.4–106.2±3.2%) with NaOH (0.5molL−1) and analyzed. The proposed column was applied for separation and recovery of phosphate ions in waste- and seawater samples. The results were in good agreement with the spiked and expected phosphate concentrations at 95% confidence.

The effect of the downstream pressure accumulation on the time-lag accuracy for membranes with non-linear isotherms

The time-lag method based on downstream measurements is one of the most popular methods for gas membrane characterization. This method is commonly used to characterize all types of membranes, either glassy or rubbery, and regardless of the type of isotherm for a gas molecule/membrane system. The time-lag method has originally been derived for a linear isotherm system (Henry's relation) under the assumption of vacuum at the downstream side of the membrane. This zero-pressure assumption is obviously violated as a finite downstream volume is required to record a pressure change necessary to estimate the time lag. The objective of this investigation is therefore to critically assess the degree of validity of the downstream time-lag method under more realistic boundary conditions and for systems characterized with nonlinear isotherms such as dual-mode sorption. To achieve this objective, the mass diffusion equation was solved numerically by finite differences and numerical results were compared with known analytical solutions. This study provides a deeper understanding on the behavior of adsorption and the transport of molecules inside a membrane for the cases of linear isotherm and dual-mode sorption with complete immobilization and instantaneous equilibrium. Results clearly show that the sorption parameters have a major impact on the time required to achieve steady state as well as on the transient behavior before reaching steady state under ideal boundary conditions. Under the realistic boundary conditions, steady state can never be achieved and can only be approached due to gas accumulation in the downstream compartment. Conditions under which higher accuracy is obtained in the determination of the diffusion coefficient are discussed.

Novel humidity sensitive materials derived from naphthalene-based poly (arylene ether ketone) containing sulfobutyl pendant groups

A series of novel side-chain-type sulfonated polymer aromatic electrolytes (PAEK-SO3Na-x) with different sulfonation degrees (Ds) were synthesized through a polycondensation of 1,5-bis(4-fluorobenzoyl)-2,6-dimethoxynaphthalene and hydroquinone, followed by a demethylation and sulfobutylation reaction. The structures and Ds of the polymers were confirmed by their 1H NMR spectra and the hydrophilic nature of the membranes was characterized by water uptake and contact angle test. A dual-mode sorption model was applied to analyze the water vapor sorption of the polymers. Humidity sensors were prepared from PAEK-SO3Na-x. Then the sensitivity and responsibility of the sensors were characterized in detail. The variation range of the impedance of the sensors was from 107Ω to 102Ω between 11% RH and 97% RH, which was wider than many other polymeric humidity sensors reported and indicated a good sensitivity. The response time of PAEK-SO3Na-80 was 90s and 100s for desorption and adsorption, respectively. It is for the first time to apply side-chain-type sulfonated polymer electrolyte to humidity sensors and this new kind of sensitivity material showed a good application prospect.

Gas transport properties of LiA type zeolite-filled poly(trimethylsilylpropyne) membranes

The influence of the inorganic filler LiA zeolite on the sorption and transport properties of mixed-matrix membranes based on poly(trimethylsilylpropyne) has been investigated. Isotherms of O2 and N2 sorption for the membranes containing up to 30 wt % (16.2 vol %) zeolite are described well in terms of the dual-mode sorption model. It has been found that the solubility of the gases increases with an increase in the filler fraction and nitrogen sorption is preferable in the pressure range of up to 3–5 atm. Gas permeability of the mixed-matrix membranes also increases with increasing filler loading (approximately by 20%) in comparison with the polymer matrix. Membranes selectivity is almost unaffected by zeolite addition. It has been shown that the formation of the mixed-matrix membranes is accompanied by an increase in volume (approximately by 13% at 30 wt % of zeolite loading). The main reason behind the enhancement of O2 and N2 permeability through the mixed-matrix poly(trimethylsilylpropyne)/LiA membranes is associated with an additional mass transfer through nanovoids localized around the filler particles.

Facilitated transport of gases in polymer hybrid materials containing ionic liquid additives

Gas permeation characteristics (permeability (P), diffusion (D), and solubility (S) coefficients for CH4, C2H4, C2H6, C3H8, CO2, O2, N2, and Ar and gas separation selectivities) for hybrid materials based on ionic-liquid (IL) saturated MF-4SC membranes in H+ and Ag+ forms and in presence of AgCl nanoparticles have been studied. It has been shown that, in the IL-impregnated MF-4SC membranes in H+ and Ag+ forms, the permeation of CO2 and C2H4 occurs by the facilitated transport mechanism. At the same time, in the presence of AgCl nanoparticles, the facilitated transport mechanism is implemented only for CO2. Diffusion coefficients in the polymer matrix, effective diffusion coefficients between “fixed” carriers, sorption Henry constants in the polymer matrix, and effective interaction constants of CO2 and C2H4 with the membrane material have been determined in terms of the dual-mode sorption model formalism for CO2 and C2H4. It has been shown that the most effective material for the separation of CO2 and C2H4 is an IL-impregnated MF-4SC membrane in a hydrogen form. Using model approaches, it has been shown that the main cause of the facilitated transport of ethylene in these membranes is the interaction between the double bond of the ethylene and the IL cation.

Effect of Fabrication and Operation Conditions on CO2 Separation Performance of PEO–PA Block Copolymer Membranes

Poly(ethylene oxide)- (PEO-) based block copolymer membranes have great potential for use in CO2 separation because of their excellent selectivity and moderate permeability. Whereas numerous studies have focused on the permeation performance of such membranes, the influence of the microphase-separated structures on the gas transport is not well understood. This study examined the phase structure of PEO–polyamide (PA) (commercial name, Pebax) block copolymer membranes by scanning probe microscopy (SPM) imaging and thermal analysis. The membranes with the irregular and more disordered phase-separated structure, such as Pebax-1074 membranes, that had longer PA chains and were made using a faster sol-to-gel transition process resulted in higher CO2 permeability than the membranes with the more ordered phase structure. The CO2 solubility coefficient profile as a function of pressure in the Pebax membranes with dual-mode sorption characteristics indicated the involvement of a glassy hard phase in CO2 transport,...

Competitive permeation of gas and water vapour in high free volume polymeric membranes

ABSTRACTHighly permeable glassy polymeric membranes based on poly (1‐trimethylsilyl‐1‐propyne) (PTMSP) and a polymer of intrinsic porosity (PIM‐1) were investigated for water sorption, water permeability and the separation of CO2 from N2 under humid mixed gas conditions. The water sorption isotherms for both materials followed behavior indicative of multilayer adsorption within the microvoids, with PIM‐1 registering a significant water uptake at very high water activities. Analysis of the sorption isotherms using a modified dual sorption model which accounts for such multilayer effects gave Langmuir affinity constants more consistent with lighter gases than the use of the standard dual mode approach. The water permeability through PTMSP and PIM‐1 was comparable over the water activities studied, and could be successfully modeled through a dual mode sorption model with a concentration dependent diffusivity. The water permeability through both membranes as a function of temperature was also measured, and found to be at a minimum at 80 °C for PTMSP and 70 °C for PIM‐1. This temperature dependence is a function of reducing water solubility in both membranes with increasing temperature countered by increasing water diffusivity. The CO2 ‐ N2 mixed gas permeabilities through PTMSP and PIM‐1 were also measured and modeled through dual mode sorption theory. Introducing water vapour further reduced both the CO2 and N2 permeabilities. The plasticization potential of water in PTMSP was determined and indicated water swelled the membrane increasing CO2 and N2 diffusivity, while for PIM‐1 a negative potential implied that water filling of the microvoids hampered CO2 and N2 diffusion through the membrane. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 719–728

Corrigendum: Analysis of gas sorption in glassy polymers with the GAB model: An alternative to the dual mode sorption model

Published in J. Polym. Sci. Part B; Polym. Phys. 52, 1490–1495 (2014) Page 1493, top: Table 1: The units for Ap were incorrectly given in kPa. The correct units should be 103 atm. Page 1495, bottom: Table A1: The units for Ap were incorrectly given in kPa. The correct units should be 103 atm. We apologize for any inconvenience this error may have caused.

Comparison of transport properties of rubbery and glassy polymers and the relevance to the upper bound relationship

A database of permeability, diffusivity and solubility for He, H2, O2, N2, CO2 and CH4 was compiled from the literature for rubbery polymers. These data were compared with results for a similar study conducted for glassy polymers published in 2013. Based on this comparison, glassy polymers have higher solubility coefficients than rubbery polymers due to the excess volume (or free volume) stemming from the non-equilibrium nature of polymers below the glass transition temperature, Tg, which is well described by the dual-mode sorption model. When compared at equal permeability, rubbery polymers have higher diffusion coefficients and lower solubility coefficients. Polymers having permselectivity values at or near the upper bound are virtually all glassy polymers. This phenomenon is widely ascribed to the better diffusivity selectivity of glassy versus rubbery polymers. Comparison of the upper-bound plot of log Pi/Pj versus log Pi (where Pi represents the permeability of the more permeable gas) shows that glassy polymers dominate the upper bounds for all 15 gas pairs possible. However, when log Pi/Pj is plotted versus log Di, many of the gas pairs have overlapping data for glassy and rubbery polymers. Thus, glassy polymers dominate the upper bound due, in part, to their higher solubility coefficients.

Gas Permeability of Hexaphenylbenzene Based Polymers of Intrinsic Microporosity

The synthesis and characterization of a series of novel hexaphenylbenzene (HPB) based polymers of intrinsic microporosity (PIM-HPBs) containing methyl, bromine, and nitrile substituents are reported. The successful formation of thin films from these polymers allowed the evaluation of the influence of the substituents on intrinsic microporosity and gas permeability. Analysis by the time-lag method also yielded information about gas diffusion coefficients and, indirectly, the gas solubility. The gas permeability varies as a function of the polarity of the substituents and shows a significant increase after treatment of the samples with methanol, especially for films cast from THF as the solvent. This enhancement, which is mostly due to an increase in the diffusion coefficient, is only partially lost upon aging of the membranes for 5 months. Measurements at different feed pressures confirm the typical dual mode sorption behavior, with increasing diffusivity and decreasing permeability and solubility as a function of the feed pressure.

Water sorption in poly(ethylene furanoate) compared to poly(ethylene terephthalate). Part 1: Equilibrium sorption

Equilibrium water sorption properties of amorphous poly(ethylene furanoate) (PEF) and amorphous poly(ethylene terephthalate) (PET) were studied at 35 °C over the entire water activity range. PEF exhibits a largely increased equilibrium water sorption capacity of ∼1.8× averaged over the entire concentration range compared to PET, resulting from substitution of the non-polar phenyl ring in PET with the polar furan ring in PEF. Both polyesters exhibit dual-mode sorption up to ∼0.6 activity, after which the onset of swelling produces a noticeable upturn in concentration vs. activity for both polyesters. Excellent agreement was observed between three independent sorption measurement techniques, thereby providing a consistency check for the reported data. Sorption measurements performed at 15, 25, 35, and 45 °C allowed estimation of the effective enthalpy of water sorption for both polyesters, which were similar to the enthalpy of condensation for pure water. The current work demonstrates the importance of structure on sorption and transport properties in these two important polyesters.

Pure- and mixed-gas permeation of CO2 and CH4 in thermally rearranged polymers based on 3,3′-dihydroxy-4,4′-diamino-biphenyl (HAB) and 2,2′-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA)

Permeability coefficients for pure CO2, pure CH4, and CO2/CH4 mixtures containing 50% CO2 are reported for a polyimide synthesized from 3,3′-dihydroxy-4,4′-diamino-biphenyl (HAB) and 2,2′-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA) and for three thermally-rearranged (TR) derivatives thereof. Permeability measurements were made at 35°C for fugacities ranging from 4 to 25atm. The permeability of CO2 and CH4 increased as the degree of TR conversion increased. For example, CO2 permeability at 10atm increased by a factor of 30 between the unconverted polyimide and its TR analog converted at 450°C. In pure-gas experiments, CO2 was observed to plasticize the unconverted polyimide, but it did not appear to plasticize the TR polymers. In mixed-gas experiments, dual-mode competitive sorption caused a depression in CH4 permeability, with very little change in CO2 permeability. In addition, plasticization by CO2 was evident in the CH4 mixed-gas permeability trends, but its impact was small in contrast with dual-mode competitive effects. Consequently, CO2/CH4 mixed-gas permeability selectivity was higher than the ideal selectivity, calculated as the ratio of pure gas permeability coefficients. The dual-mode sorption and permeation model was fit to the experimental data. Dual-mode model parameters and model predictions are reported, along with their confidence intervals. By comparing the dual-mode model predictions with the experimental mixed-gas data, the degree of CO2-induced plasticization was observed to decrease as the degree of TR conversion increased and was completely absent (within experimental uncertainty) for the TR polymer converted at 450°C.

Equilibrium Adsorption Studies of CO2, CH4, and N2 on Amine Functionalized Polystyrene

Adsorption of CO2, CH4, and N2 has been investigated using amine functionalized polymeric resins having diethanolamine, imidazole, dimethylamine, and N-methyl piperazine covalently attached to the styrene-divinyl benzene copolymer (PS) matrix. The equilibrium adsorption of CO2, CH4, and N2 was examined on these functionalized polymers at pressures from atmospheric to 40 atm for CO2 and N2 while up to 10 atm for CH4 at 303 K. PS-Imidazole showed the highest adsorption capacity for CO2 as compared to other functionalized polymers. No significant uptake of CH4 and N2 was observed at low pressures by any of the functionalized polymers. The adsorption isotherms were analyzed using dual mode sorption model and Ideal Adsorbed Solution Theory (IAST).