Search

Polymer inclusion membranes (PIMs) have been prepared using cellulose triacetate (CTA) as polymer and derivatives of the commercial ionic liquid (IL) Aliquat 336, trioctyl methylammonium chloride (AlqCl), as extractants. The different ILs were prepared by exchanging chloride anion from AlqCl for other more lipophilic anions, obtaining IL derivatives AlqNO 3 and AlqSCN. PIMs containing these extractants at two different concentrations (30% and 60%) were prepared and characterized using X-ray photoelectron spectroscopy (XPS), infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), contact angle and impedance spectroscopy to obtain information on the material properties of both the surface and bulk membranes. The IL counter-anion affects the membrane's electrical parameters (dielectric constant and conductivity) as well as its hydrophobic character, which also depends on the IL concentration in the PIM formulation. Passive transport across the different PIMs using HCl and NaCl aqueous solutions was also considered, and the results obtained seem to be directly related to the hydrophilic/hydrophobic character of the studied membranes The financial support of the Spanish government through research project CTM2013-48967-C2-2-P is acknowledged. R. Vera acknowledges a grant from the Spanish Ministerio de Economía y Competitividad (ref. BES-2014-068314)

The effect of the base-polymer and carrier concentration on the physical–chemical characteristics of polymer inclusion membranes (PIMs) is investigated. Two typical polymers used to manufacture PIMs have been tested, i.e. poly(vinyl chloride) (PVC) and cellulose triacetate (CTA), and different amounts of the ionic liquid (IL) Aliquat 336, used as extractant, were the PIMs constituents. The resulting PIMs have been characterized using different techniques to provide information on both the surface and bulk material properties. XPS results do not practically show differences in the surfaces of CTA and PVC based membranes with similar Aliquat 336 content, and the total surface coverage for Aliquat 336 concentration higher than 40% (w/w) was obtained, which was also corroborated with the results of contact angle measurements. However, membrane elastic response seems to be strongly dependent on both base-polymer and Aliquat 336 concentration, which affect Young modulus and elongation at break. The IL concentration also increases dielectric constant and the conductivity of the PIMs from both polymers according to impedance spectroscopy results, providing a rather conductive character to both kind of samples for Aliquat 336 content higher than 40% (w/w) (average conductivity around 10 −3 (Ω m) −1 ). CTA and PVC-based PIMs with Aliquat 336 content around 45% have been used in order to compare the influence of the polymer on the transport of As(V).

Two highly hydrophilic regenerated cellulose planar sheets (RC-6 and RC-MD) with different morphological, structural and transport parameters were used as supports of lipid (lecithin-triestearine) nanoparticles (RC-6/LNPs and RC-MD/LNPs samples), which were embedded by immersion in a water dispersion of LNPs. Salt and ions transport (salt diffusion, ion transport numbers and effective fixed charge) were determined from measurements carried out with the samples in contact with NaCl solutions at different concentration and, consequently, osmotic pressures. Results do not show differences in the small fixed charge of the membranes as a result of nanoparticles inclusion (−0.012 M and −0.008 M for RC-6 and RC-MD, respectively) and stability of the LPNs-modified membranes for osmotic pressure up to 5 bars was established. TEM images shows the presence of LNPs in the RC-supports and Raman spectroscopy analysis was used for physic-chemical characterization of the RC-supports and RC/LNPs-modified membranes but also to determine the stability of modified samples under flow conditions. Different transport behaviour depending on the RC-support was obtained, and results seem to indicate the maintenance of nanoparticles into the dense RC-support structure till a concentration gradient of 0.1 M.

Ultrasound and high-pressure homogenized lipid (lecithin–tristearin)nanoparticles (NPs) were prepared and embedded into a regenerated cellulose (RC-MD) commercial membrane; the RC-support was also modified with the deposition of a lecithin–tristearine layer (RC-MD/LL sample). The lipid nanoparticles, RC-support and modified membranes were topographically and chemically characterized by Atomic Force Microscopy (AFM) and Raman spectroscopy. Electrical parameters (conductivity and capacitance) of both the original and modified membranes were determined by the impedance spectroscopy (IS) technique using a parallel resistance–capacitance equivalent circuit as a model. The results allow the lipid particle inclusion to be distinguished from the lipid layer deposition. Moreover, the paper shows the possibility of lipid nanoparticle inclusion into a solid support maintaining particle integrity.

Water effect in different physical–chemical and elastic parameters determined for a dense cellulose regenerated (RC) membrane is presented. Membrane samples characterization was carried out by obtaining XPS spectra, X-ray diffraction patterns and elasticity curves, and the significant differences determined when dry and wet samples are compared are basically associated to the water plasticization effect in this high hydrophilic material. Transport of different aqueous electrolyte solutions (NaCl, KCl, LiCl, MgCl 2 and CaCl 2 ) was study by measuring membrane potentials and diffusion, which allows the determination of effective membrane fixed charge concentration, ion transport numbers and permselectivity as well as diffusional permeability and diffusion coefficient of the different salts across the RC membrane. Differences associated to the kind of electrolyte (1:1 or 2:1 electrolytes) were obtained as a result of membrane–solute electrical interactions. Membrane dense structure significantly reduce both salt and water diffusional transport, being this latter determined by using tritiated water as permeate.

Nafion membranes were modified by incorporation of ionic liquid (IL) cations, at controllable degrees. These modified membranes were then characterised with the use of different techniques. X-ray photoelectron spectroscopy (XPS) demonstrated that the IL cations are incorporated in the membrane material and allowed to evaluate the distribution of the cation in the membrane surface layer. Electrochemical impedance spectroscopy (EIS) was used to monitor membrane modification by incorporation of IL cations, and to study the electrical properties of the modified membranes, with different degrees of cation incorporation. These results were adjusted to mathematical models of equivalent electrical circuits to extract data of membrane ionic conductivity. Finally, thermogravimetry studies allowed assessing the stability of the modified membranes with increasing temperature in terms of their water content, and these results were used to establish a possible relation between the membrane stability at high temperatures with their water content and degree of structuring. It was concluded that it is possible to obtain membranes with tailored properties depending on the type of IL cation and its degree of incorporation.

Asymmetric sulfonated-polysulfone/polyamide membranes were modified by incorporating different amounts of lignosulfonate (LS) into the network (5, 10, 20 and 40 wt.%) in order to increase membrane conductivity due to the presence of sulfonic acid groups in the LS structure. LS-modified membranes were geometrically characterized by scanning electron microscopy (SEM) micrographs, which show two different substructures: a dense top layer (around 2.7 μm thickness) and a thick porous support (thickness between 95 and 102 μm, and porosity ranging between 37 and 42%). Chemical top layer characterization was carried out by XPS analysis, which shows the complete covering of the PS/PA membrane by LS and it is correlated with the increase of LS in the casting solution. Electrical characterization was carried out by impedance spectroscopy with the membranes in contact with HCl solution at different concentrations by using equivalent circuits as models. These results allow the separate determination of the electrical contribution associated to the top layer and the porous support; modified membranes and top layers conductivities were calculated from electrical resistance and geometrical values previously determined from SEM analysis. According to these results, 10 wt.% of LS seems to be the more appropriated figure to increase both membrane and top layer conductivity (σm ∼ 0.06 (Ω m)−1 for 1 M HCl solution).

Supported liquid membranes (SLMs), prepared by immobilising the room temperature ionic liquids (RTILs) [CnMIM]PF6 (n = 4 and 8) and [C10MIM]BF4 in a polyvinylidene (PVDF) supporting membrane, were prepared and characterised by electrochemical impedance spectroscopy (IS). This non-invasive technique allows the determination of the electrical properties of a given sample, such as the electrical resistance and capacitance under working conditions, i.e., in contact with saline solutions. Bearing in mind that the water content of the ionic liquids can drastically affect their physicochemical properties, impedance measurements of the SLMs, placed between two aqueous solutions, were carried out at regular time intervals, in order to assess the impact of the presence of water inside the RTILs on the electrical properties of supported ionic liquid membranes. The electrical resistance of the SLMs and its variation during long-term operation was also used as a physical parameter to identify the presence/loss of ionic liquid from the pores of the support. Additionally, the comparison of the IS results obtained for the SLMs with those obtained for the supporting membrane was carried out, in order to gather information about electrical changes associated with the presence of ionic liquid in the pores. © 2005 Elsevier B.V. All rights reserved.

Chemical, structural and electrochemical characterizations of two cellophane membranes depending on both cellulose content (20 and 25 kg/m 2 ) and thermal treatment (60 ◦ C during 4 h), have being carried out. Different transport parameters were determined by measuring water and salt fluxes, membrane potential and impedance spectroscopy using NaCl solutions at different concentrations. Chemical degradation of the membrane surfaces due to membrane heating was obtained from XPS spectra. Electrochemical results shown that treated and untreated samples behave as weak cation-exchangers, but small differences in their electrical parameters (membrane fixed charge concentration, cation transport number and electrical resistance) due to both the thermal treatment and the amount of cellulose were obtained. Results also indicate a more open structure for the membrane with higher content of cellulose. The influence of concentration/polarization layers on salt permeation was also studied by measuring salt diffusion at different stirring rates. Moreover, variation of water permeability with temperature for untreated samples seems indicate changes in the membrane structure around 40 ◦ C, which was confirmed by differential scanning calorimetry (DSC) thermograms. © 2003 Elsevier Science B.V. All rights reserved.

Because of the diversity of streaming potential and zeta potential values reported in the literature, even for similar membranes and external conditions (hydrodynamics and solution chemistry), the influence of measuring cells and procedures in electrokinetic parameters has been studied. For this purpose, two different experimental systems (named EKA and NIS) with similar hydrodynamic parameters, but different electrodes and electrical devices, are used for the electrokinetic characterization of a commercial polysulfone membrane. Detailed descriptions of measurement procedures and the influence of external conditions (flow direction, electrode polarization, etc.) are made since they could contribute to the variability of experimental results. On the other hand, an electrokinetic characterization of the polysulfone membrane in contact with NaCl solutions at different concentrations and pH was carried out by using the EKA system. An heterogeneous specific adsorption of anions on the membrane surface (Freundlich isotherm) was assumed in order to determine parameters such as the membrane surface charge density, the Gibb’s free energy and the number of adsorption sites per unit of membrane area. However, at low salt concentration (∼10 −3 M) the membrane proper charge is mainly due to dissociation of some kind of acid groups on the membrane surface (isoelectric point: 3.1 ± 0.1), and the site dissociation model has been used to obtain the membrane acid equilibrium constant and the surface density of acid sites. © 2001 Elsevier Science B.V. All rights reserved.

Electrokinetic, structural and surface chemical characterizations of both active and porous sublayers of a composite (polyamide/polysulfone) reverse osmosis membrane were carried out by different techniques: (i) electrochemical impedance spectroscopy (EIS) allows both electrical and geometrical characterizations using equivalent circuits as models; (ii) surface chemical composition of the active layer was determined by X-ray photoelectron spectroscopy (XPS); (iii) salt diffusion, tangential streaming potential and membrane potential measurements allow the determination of different transport and electrokinetic parameters such as salt and ionic permeabilities, zeta potential and surface charge density. Electrical and electrokinetic parameters were determined with the membrane in contact with NaCl solutions at different concentrations, although the influence of pH and the use of the membrane on zeta potential values was also studied. Changes in the electrokinetic parameters as a result of the membrane use were obtained and, according to XPS results, they can be attributed to the hydratation of the polyamide active layer. For comparison, electrochemical measurements with a polysulfone ultrafiltration membrane similar to the porous sublayer of the composite RO membrane were also carried out. Good agreement was obtained for the electrical resistance and ionic permeabilities determined for both the ultrafiltration membrane and the porous sublayer of the RO membrane, which confirms the two-layer model assumed for composite membranes.

In this work, three samples of sulfonated polysulfones of different sulfonation degree have been characterized, and the electrical properties of dense membranes based on them have been evaluated. The polymers were characterized spectroscopically, and by their inherent viscosity, and water absorption. Electrical characterization of membranes was carried out on dense membranes in contact with NaCl solutions, by impedance spectroscopy (IS) measurements using equivalent circuits as models. The results showed how the sulfonation clearly affected the membrane electrical characteristics, strongly reducing the membrane resistance, and also changing the type of circuit associated to the membrane, which could be related to the increase of electrolyte taken by the membrane when the sulfonation degree increased. Concentration dependence of the electrical parameters was also studied.

Pressure induced electrical potential across a composite polyamide/polysulfone reverse osmosis membrane and a microporous polysulfone membrane similar to the porous layer of the composite one (Δ E and Δ φ st PS , respectively) were measured with NaCl and MgCl 2 solutions at different feed concentrations. Because of the high salt rejection and the two-layered structure assumed for reverse osmosis membranes, Δ E values do not represent the streaming potential in the active layer since other electrical potentials (diffusion potential in the active layer, streaming potential in the porous sub-layer and boundary potential) are also included. These different terms were evaluated and the streaming potential for the active and porous layers of the composite membrane, Δ φ st a and Δ φ st p , were estimated under the assumption of similar streaming potential coefficient g =Δ φ st /Δ P for the porous layer of the composite membrane and the microporous polysulfone membrane ( g PS = g p ). Differences between Δ E m i values obtained under filtration (Δ E m F , different feed and product concentrations) and dialysis conditions (Δ E m D , equal feed and product concentrations), for similar volumen fluxes show the influence of the porous sub-layer. A comparison between Δ E m F /Δ P and active layer streaming potential coefficient, g a , is also made. Results show that the diffusion potential in the active layer is 10–15% of the total electrical potential difference across the composite membrane, while the influence of the porous layer is practically lower than 3%. The electrical potential in filtration measurements was calculated from dialysis results, once diffusion and streaming potential in the porous layer are known, and good agreement between experimental and calculated values was obtained. Salt concentration dependences for streaming potential coefficient in porous and active layers of the reverse osmosis membrane were studied, while for the microporous polysulfone membrane the zeta potential ( ζ ) was determined by the Hemholtz–Smoluchowski expression.

The transport of NaCl and CaCl 2 solutions across isolated pepper cuticular membranes was studied by means of conductivity, membrane potential and diffusion experiments. Some characteristic membrane parameters such as the electrical resistance, ionic and salt permeabilities were obtained as a function of the electrolyte concentrations. Cuticle morphological asymmetry accounts for differences in membrane potential values under external reverse gradients. The influence of temperature on the membrane structure was also considered, but only small changes in the electrokinetic parameters were obtained. From the NaCl diffusion experiments two activation energies were determined (54.8 kJ/mol for temperature ranging between 15 and 35°C, and 20.6 kJ/mol for the interval of temperature between 40 and 60°C), which could be associated to thermal transitions in the molecular structure of the cuticle for the interval 30–40°C.

Retention curves with a composite membrane (HR 95) have been measured for different solutions of Na 2 SO 4 and MgSO 4 . Salt permeabilities in the skin and the porous layer of the composite membrane have been calculated. The results show that the salt permeability in the skin layer is 15% of that corresponding to the porous layer. Electrical resistances for the composite membrane and another membrane similar to the supported membrane (without skin layer) have been measured using both direct and alternating current. From the a.c. values, assuming that no concentration polarization exists at the skin-porous layer interface, the electrical resistance for the skin layer at different concentrations was estimated. The membrane resistance values were also determined by impedance spectroscopy measurements, and the results agree with those previously found by a.c. measurements.

Retention curves, membrane potential, diffusion and electrical resistance for two membranes: a composite membrane (HR 95) and the support membrane (without the skin layer), have been measured for different solutions of NaCl and MgCl2. Salt permeabilities in the skin and porous layers of the composite membrane were calculated and these latter values were compared with those obtained with the porous polysulfone membrane by diffusion experiments. From the experiments it was concluded that 92% of the total flux-resistance was due to the selective skin layer, whereas only 78% of the total diffusion resistance under dialysis conditions was due to the skin layer. A comparison of the experimental and calculated values for the electrical resistance of the polysulfone membrane has also been made. In both cases, good agreements were found. Cation transport numbers in the skin and porous layers of the composite membrane have also been obtained.

This work evaluates the use of room temperature ionic liquids (RTILs) in supported liquid membranes (SLMs). The physicochemical properties of RTILs, namely their reduced solubility in water and high interfacial tension, make their use very attractive in order to obtain stable supported liquid membranes. Preliminary results showed that, under relatively mild stirring conditions, there was no evidence of ionic liquid displacement from the membranes’ pores. In this work, further studies were conducted in order to assess the SLMs’ operational stability under dynamic conditions. Dynamic stability studies were performed to evaluate the stability of a SLM with [C8MIM][PF6] when submitted to more aggressive and well-defined hydrodynamic conditions. Additionally, X-ray photoelectron spectroscopy (XPS) was used to characterise the SLMs obtained by immobilizing the ionic liquids [CnMIM][PF6 ]( n = 4, 8) and [C10MIM][BF4] in a polyvinylidene fluoride (PVDF) supporting membrane. This technique allows for the determination of the surface chemical composition of a sample and was used for the characterization of both the supported liquid membranes and the ionic liquids. The SLMs were analysed immediately after preparation and after being immersed for 1 week in de-ionised water. This way, it was possible not only to assess the presence of the RTILs in the membrane after preparation, but also to identify whether there were any chemical modifications on the SLMs’ surface after contact with an aqueous solution, thus obtaining valuable information about their integrity and stability. © 2005 Elsevier B.V. All rights reserved.

Electrokinetic phenomena in porous membranes were studied in the framework of the linear thermodynamics of irreversible processes. Phenomenological coefficients of two commercial porous membranes were determined. The variation of these coefficients with the different concentrations of NaCl used was also studied. Onsager's reciprocal relationship between cross-coefficients holds within the limits of experimental error. Membrane potentials were measured at concentrations ranging from 10 −3 M to 75 X 10 −3 M . The apparent transport number of the cation, , was calculated from the diffusion potential expression. An electrometric method was used to obtain the true cation transport number, , and the water transport number, , for different solution concentrations, ranging from 15 x 10 −3 M to 75 x 10 −3 M . The true transport number of the cation, , was also determined, taking into account water transport across the membrane. A good agreement was found between and values.