State of the Art Membrane Science and Technology in the Iberian Peninsula 2021-2022.

This Special Issue of the journal Membranes arises from the need to highlight the developments in the field of membrane research and membrane processes that have been emerging in recent years by researchers and research groups based in the Iberian Peninsula [...].

Mixed Conductive, Injectable, and Fluorescent Supramolecular Eutectogel Composites.

Eutectogels are an emerging family of soft ionic materials alternative to ionic liquid gels and organogels, offering fresh perspectives for designing functional dynamic platforms in water-free environments. Herein, the first example of mixed ionic and electronic conducting supramolecular eutectogel composites is reported. A fluorescent glutamic acid-derived low-molecular-weight gelator (LMWG) was found to self-assemble into nanofibrillar networks in deep eutectic solvents (DES)/poly(3,4-ethylenedioxythiophene) (PEDOT): chondroitin sulfate dispersions. These dynamic materials displayed excellent injectability and self-healing properties, high ionic conductivity (up to 10-2  S cm-1 ), good biocompatibility, and fluorescence imaging ability. This set of features turns the mixed conducting supramolecular eutectogels into promising adaptive materials for bioimaging and electrostimulation applications.

Polyphenol Iongel Patches with Antimicrobial, Antioxidant and Anti-Inflammatory Properties.

There is an actual need for developing materials for wound healing applications with anti-inflammatory, antioxidant, or antibacterial properties in order to improve the healing performance. In this work, we report the preparation and characterization of soft and bioactive iongel materials for patches, based on polymeric poly(vinyl alcohol) (PVA) and four ionic liquids containing the cholinium cation and different phenolic acid anions, namely cholinium salicylate ([Ch][Sal]), cholinium gallate ([Ch][Ga]), cholinium vanillate ([Ch][Van]), and cholinium caffeate ([Ch][Caff]). Within the iongels, the phenolic motif in the ionic liquids plays a dual role, acting as a PVA crosslinker and a bioactive compound. The obtained iongels are flexible, elastic, ionic conducting, and thermoreversible materials. Moreover, the iongels demonstrated high biocompatibility, non-hemolytic activity, and non-agglutination in mice blood, which are key-sought material specifications in wound healing applications. All the iongels have shown antibacterial properties, being PVA-[Ch][Sal], the one with higher inhibition halo for Escherichia Coli. The iongels also revealed high values of antioxidant activity due to the presence of the polyphenol, with the PVA-[Ch][Van] iongel having the highest activity. Finally, the iongels show a decrease in NO production in LPS-stimulated macrophages, with the PVA-[Ch][Sal] iongel displaying the best anti-inflammatory activity (>63% at 200 µg/mL).

Natural Deep Eutectic Solvents Based on Choline Chloride and Phenolic Compounds as Efficient Bioadhesives and Corrosion Protectors.

Natural deep eutectics solvents (NADES), owing to their high solvation capacity and nontoxicity, are actively being sought for many technological applications. Herein, we report a series of novel NADES based on choline chloride and plant-derived polyphenols. Most of the obtained phenolic NADES have a wide liquid range and high thermal stability above 150 °C. Among them, small-sized polyphenols, like pyrogallol, vanillyl alcohol, or gentisic acid, lead to low-viscosity liquids with ionic conductivities in the order of 10-3 S cm-1 at room temperature. Interestingly, polyphenols possess valuable properties as therapeutic agents, antioxidants, adhesives, or redox-active compounds, among others. Thus, we evaluated the potential of these novel NADES for two applications: bioadhesives and corrosion protection. The mixture of choline chloride-vanillyl alcohol (2:3 mol ratio) and gelatin resulted in a highly adhesive viscoelastic liquid (adhesive stress ≈ 135 kPa), affording shear thinning behavior. Furthermore, choline chloride-tannic acid (20:1) showed an extraordinary ability to coordinate iron ions, reaching excellent corrosion inhibitive efficiencies in mild steel protection.

Gelatin and Tannic Acid Based Iongels for Muscle Activity Recording and Stimulation Electrodes.

Iongels are soft ionic conducting materials, usually composed of polymer networks swollen with ionic liquids (ILs), which are being investigated for applications ranging from energy to bioelectronics. The employment of iongels in bioelectronic devices such as bioelectrodes or body sensors has been limited by the lack of biocompatibility of the ILs and/or polymer matrices. In this work, we present iongels prepared from solely biocompatible materials: (i) a biobased polymer network containing tannic acid as a cross-linker in a gelatin matrix and (ii) three different biocompatible cholinium carboxylate ionic liquids. The resulting iongels are flexible and elastic with Young's modulus between 11.3 and 28.9 kPa. The morphology of the iongels is based on a dual polymer network system formed by both chemical bonding due to the reaction of the gelatin's amines with the polyphenol units and physical interactions between the tannic acid and the gelatin. These biocompatible iongels presented high ionic conductivity values, from 0.003 and up to 0.015 S·cm-1 at room temperature. Furthermore, they showed excellent performance as a conducting gel in electrodes for electromyography and electrocardiogram recording as well as muscle stimulation.

Overview of Membrane Science and Technology in Portugal.

Membrane research in Portugal is aligned with global concerns and expectations for sustainable social development, thus progressively focusing on the use of natural resources and renewable energy. This review begins by addressing the pioneer work on membrane science and technology in Portugal by the research groups of Instituto Superior Técnico-Universidade de Lisboa (IST), NOVA School of Science and Technology-Universidade Nova de Lisboa (FCT NOVA) and Faculdade de Engenharia-Universidade do Porto (FEUP) aiming to provide an historical perspective on the topic. Then, an overview of the trends and challenges in membrane processes and materials, mostly in the last five years, involving Portuguese researchers, is presented as a contribution to a more sustainable water-energy-material-food nexus.

Ionic liquid/poly(ionic liquid) membranes as non-flowing, conductive materials for electrochemical gas sensing.

Ionic liquids (ILs) are highly promising, tuneable materials that have the potential to replace volatile electrolytes in amperometric gas sensors in a 'membrane-free' sensor design. However, the drawback of removing the membrane is that the liquid ILs can readily leak or flow from the sensor device when moved/agitated in different orientations. A strategy to overcome the flowing nature of ILs is to mix them with polymers to stabilise them on the surface in the form of membranes. In this research, the room temperature ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C2mim][NTf2]), has been mixed with the poly(ionic liquid) (poly(IL), poly(diallyldimethylammonium bis(trifluoromethylsulfonyl)imide), poly[DADMA][NTf2]) to form stable membranes on miniaturised, planar electrode devices. Different mixing ratios of the IL/poly(IL) have been explored to find the optimum membrane that gives both high robustness (non-flowing material) and adequate conductivity for measuring redox currents, with the IL/poly(IL) 60/40 wt% proving to give the best responses. After assessing the blank potential windows on both platinum and gold electrodes, followed by the kinetics of the cobaltocenium/cobaltocene redox couple, the voltammetry of oxygen, sulfur dioxide and ammonia gases have been studied. Not only were the membranes highly robust and non-flowing, but the analytical responses towards the gases were excellent and highly reproducible. The presence of the poly(IL) negatively affected the sensitivity, however the electron transfer kinetics and the limit of detection were actually improved for O2 and SO2, combined with the poly(IL) experiencing less reference potential shifting. These promising results show that membranes containing conductive poly(IL)s mixed with ionic liquids could be used as new 'designer' gas sensor materials in robust membrane free amperometric gas sensor devices.

Poly(ethylene glycol) Diacrylate Iongel Membranes Reinforced with Nanoclays for CO2 Separation.

Despite the fact that iongels are very attractive materials for gas separation membranes, they often show mechanical stability issues mainly due to the high ionic liquid (IL) content (≥60 wt%) needed to achieve high gas separation performances. This work investigates a strategy to improve the mechanical properties of iongel membranes, which consists in the incorporation of montmorillonite (MMT) nanoclay, from 0.2 to 7.5 wt%, into a cross-linked poly(ethylene glycol) diacrylate (PEGDA) network containing 60 wt% of the IL 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C2mim][TFSI]). The iongels were prepared by a simple one-pot method using ultraviolet (UV) initiated polymerization of poly(ethylene glycol) diacrylate (PEGDA) and characterized by several techniques to assess their physico-chemical properties. The thermal stability of the iongels was influenced by the addition of higher MMT contents (>5 wt%). It was possible to improve both puncture strength and elongation at break with MMT contents up to 1 wt%. Furthermore, the highest ideal gas selectivities were achieved for iongels containing 0.5 wt% MMT, while the highest CO2 permeability was observed at 7.5 wt% MMT content, due to an increase in diffusivity. Remarkably, this strategy allowed for the preparation and gas permeation of self-standing iongel containing 80 wt% IL, which had not been possible up until now.

Emerging iongel materials towards applications in energy and bioelectronics.

In the past two decades, ionic liquids (ILs) have blossomed as versatile task-specific materials with a unique combination of properties, which can be beneficial for a plethora of different applications. The additional need of incorporating ILs into solid devices led to the development of a new class of ionic soft-solid materials, named here iongels. Nowadays, iongels cover a wide range of materials mostly composed of an IL component immobilized within different matrices such as polymers, inorganic networks, biopolymers or inorganic nanoparticles. This review aims at presenting an integrated perspective on the recent progress and advances in this emerging type of material. We provide an analysis of the main families of iongels and highlight the emerging types of these ionic soft materials offering additional properties, such as thermoresponsiveness, self-healing, mixed ionic/electronic properties, and (photo)luminescence, among others. Next, recent trends in additive manufacturing (3D printing) of iongels are presented. Finally, their new applications in the areas of energy, gas separation and (bio)electronics are detailed and discussed in terms of performance, underpinning it to the structural features and processing of iongel materials.

Conducting Polymer-Ionic Liquid Electrode Arrays for High-Density Surface Electromyography.

Surface electromyography (EMG) is used as a medical diagnostic and to control prosthetic limbs. Electrode arrays that provide large-area, high density recordings have the potential to yield significant improvements in both fronts, but the need remains largely unfulfilled. Here, digital fabrication techniques are used to make scalable electrode arrays that capture EMG signals with mm spatial resolution. Using electrodes made of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) composites with the biocompatible ionic liquid (IL) cholinium lactate, the arrays enable high quality spatiotemporal recordings from the forearm of volunteers. These recordings allow to identify the motions of the index, little, and middle fingers, and to directly visualize the propagation of polarization/depolarization waves in the underlying muscles. This work paves the way for scalable fabrication of cutaneous electrophysiology arrays for personalized medicine and highly articulate prostheses.

Emerging Ionic Soft Materials Based on Deep Eutectic Solvents.

In the last 5 years, the use of deep eutectic solvents (DESs) have been opening new perspectives toward the creation of novel ionic soft materials as alternatives to expensive ionic liquids. This Mini-Review Article highlights the progress and advances in soft ionic materials or gels, mostly composed by a DES immobilized within difference matrices, such as linear polymers, polymer networks, biopolymers, supramolecular compounds, or sol-gel derived silica networks. By taking advantage of the DESs characteristics and properties in the solid state, this building system delivers a variety of tailor-made materials showing different functionalities (ionic conductivity, self-healing, stretchability and pH-responsiveness) and offers a way to circumvent drawbacks related to shaping and risk of leakage in many technological applications. In this context, we provide a judicious analysis of these emerging ionic soft materials, their properties, and applications open in energy, (bio)electronics, drug delivery, analytical chemistry, and wastewater treatment. Perspectives and opportunities for future research directions on this blossoming field are also discussed.

Elastic and Thermoreversible Iongels by Supramolecular PVA/Phenol Interactions.

Iongels have attracted much attention over the years as ion-conducting soft materials for applications in several technologies including stimuli-responsive drug release and flexible (bio)electronics. Nowadays, iongels with additional functionalities such as electronic conductivity, self-healing, thermo-responsiveness, or biocompatibility are actively being searched for high demanding applications. In this work, a simple and rapid synthetic pathway to prepare elastic and thermoreversible iongels is presented. These iongels are prepared by supramolecular crosslinking between polyphenols biomolecules with a hydroxyl-rich biocompatible polymer such as poly(vinyl alcohol) (PVA) in the presence of ionic liquids. Using this strategy, a variety of iongels are obtained by combining different plant-derived polyphenol compounds (PhC) such as gallic acid, pyrogallol, and tannic acid with imidazolium-based ionic liquids, namely 1-ethyl-3-methylimidazolium dicyanamide and 1-ethyl-3-methylimidazolium bromide. A suite of characterization tools is used to study the structural, morphological, mechanical, rheological, and thermal properties of the supramolecular iongels. These iongels can withstand large deformations (40% under compression) with full recovery, revealing reversible transitions from solid to liquid state between 87 and 125 °C. Finally, the polyphenol-based thermoreversible iongels show appropriated properties for their potential application as printable electrolytes for bioelectronics.

Influence of Anion Structure on Thermal, Mechanical and CO2 Solubility Properties of UV-Cross-Linked Poly(ethylene glycol) Diacrylate Iongels.

Iongel-based CO2 separation membranes were prepared by fast (< 1 min) UV-initiated polymerization of poly(ethylene glycol) diacrylate (PEGDA) in the presence of different ionic liquids (ILs) with the [C2mim]+ cation and anions such as [TFSI]-, [FSI]-, [C(CN)3]- and [B(CN)4]-. The four ILs were completely miscible with the non-ionic PEGDA network. Transparent and free-standing iongels containing between 60 and 90 %wt of IL were obtained and characterized by diverse techniques (FTIR, TGA, DSC, DMTA, SEM, CO2 solubility and pure gas permeability). The thermal and mechanical stability of the iongels, as well as CO2 solubility, were found to be strictly dependent on the IL content and the anion's nature. The TGA results indicated that the iongels mostly follow the thermal profile of the respective neat ILs. The DMTA analysis revealed that the iongels based on fluorinated anions have higher storage modulus than those of cyano-functionalized anions. Conversely, the PEGDA-C(CN)3 iongels presented the highest CO2 solubility values ranging from 72 to 80 mmol/g. Single CO2 permeabilities of 583 ± 29 Barrer and ideal CO2/N2 selectivities of 66 ± 3 were obtained with the PEGDA-70 C(CN)3 iongel membrane. This work demonstrates that the combination of PEGDA with high contents of the best performing ILs is a promising and simple strategy, opening up new possibilities in the design of high-performance iongel membranes for CO2 separation.

Towards Biohydrogen Separation Using Poly(Ionic Liquid)/Ionic Liquid Composite Membranes.

Considering the high potential of hydrogen (H2) as a clean energy carrier, the implementation of high performance and cost-effective biohydrogen (bioH2) purification techniques is of vital importance, particularly in fuel cell applications. As membrane technology is a potentially energy-saving solution to obtain high-quality biohydrogen, the most promising poly(ionic liquid) (PIL)⁻ionic liquid (IL) composite membranes that had previously been studied by our group for CO2/N2 separation, containing pyrrolidinium-based PILs with fluorinated or cyano-functionalized anions, were chosen as the starting point to explore the potential of PIL⁻IL membranes for CO2/H2 separation. The CO2 and H2 permeation properties at the typical conditions of biohydrogen production (T = 308 K and 100 kPa of feed pressure) were measured and discussed. PIL⁻IL composites prepared with the [C(CN)3]- anion showed higher CO2/H2 selectivity than those containing the [NTf2]- anion. All the membranes revealed CO2/H2 separation performances above the upper bound for this specific separation, highlighting the composite incorporating 60 wt% of [C2mim][C(CN)3] IL.

Ionic liquids with anions based on fluorosulfonyl derivatives: from asymmetrical substitutions to a consistent force field model.

Herein, seven anions including four imide-based, namely bis[(trifluoromethyl)sulfonyl]imide (TFSI), bis(fluorosulfonyl)imide (FSI), bis[(pentafluoroethyl)sulfonyl]imide (BETI), 2,2,2-trifluoromethylsulfonyl-N-cyanoamide (TFSAM) and 2,2,2-trifluoro-N-(trifluoromethylsulfonyl) acetamide (TSAC), and two sulfonate anions, trifluoromethanesulfonate (triflate, TF) and nonafluorobutanesulfonate (NF), are considered and compared. The volumetric mass density and dynamic viscosity of five ionic liquids containing these anions combined with the commonly used 1-ethyl-3-methylimidazolium cation (C2C1im), [C2C1im][FSI], [C2C1im][BETI], [C2C1im][TFSAM], [C2C1im][TSAC] and [C2C1im][NF] are measured in the temperature range of 293.15 ≤ T/K ≤ 353.15 and at atmospheric pressure. The results show that [C2mim][FSI] and [C2mim][TFSAM] exhibit the lowest densities and viscosities among all the studied ionic liquids. The experimental volumetric data is used to validate a more consistent re-parameterization of the CL&P force field for use in MD simulations of ionic liquids containing the ubiquitous bis[(trifluoromethyl)sulfonyl]imide and trifluoromethanesulfonate anions and to extend the application of the model to other molten salts with similar ions.

Exploring the effect of fluorinated anions on the CO2/N2 separation of supported ionic liquid membranes.

The CO2 and N2 permeation properties of ionic liquids (ILs) based on the 1-ethyl-3-methylimidazolium cation ([C2mim]+) and different fluorinated anions, namely 2,2,2-trifluoromethylsulfonyl-N-cyanoamide ([TFSAM]-), bis(fluorosulfonyl) imide ([FSI]-), nonafluorobutanesulfonate ([C4F9SO3]-), tris(pentafluoroethyl)trifluorophosphate ([FAP]-), and bis(pentafluoroethylsulfonyl)imide ([BETI]-) anions, were measured using supported ionic liquid membranes (SILMs). The results show that pure ILs containing [TFSAM]- and [FSI]- anions present the highest CO2 permeabilities, 753 and 843 Barrer, as well as the greatest CO2/N2 permselectivities of 43.9 and 46.1, respectively, with CO2/N2 separation performances on top of or above the Robeson 2008 upper bound. The re-design of the [TFSAM]- anion by structural unfolding was investigated through the use of IL mixtures. The gas transport and CO2/N2 separation properties through a pure [C2mim][TFSAM] SILM are compared to those of two different binary IL mixtures containing fluorinated and cyano-functionalized groups in the anions. Although the use of IL mixtures is a promising strategy to tailor gas permeation through SILMs, the pure [C2mim][TFSAM] SILM displays higher CO2 permeability, diffusivity and solubility than the selected IL mixtures. Nevertheless, both the prepared mixtures present CO2 separation performances that are on top of or above the Robeson plot.

Expanding the Applicability of Poly(Ionic Liquids) in Solid Phase Microextraction: Pyrrolidinium Coatings.

Crosslinked pyrrolidinium-based poly(ionic liquids) (Pyrr-PILs) were synthesized through a fast, simple, and solventless photopolymerization scheme, and tested as solid phase microextraction (SPME) sorbents. A series of Pyrr-PILs bearing three different alkyl side chain lengths with two, eight, and fourteen carbons was prepared, characterized, and homogeneously coated on a steel wire by using a very simple procedure. The resulting coatings showed a high thermal stability, with decomposition temperatures above 350 °C, excellent film stability, and lifetime of over 100 injections. The performance of these PIL-based SPME fibers was evaluated using a mixture of eleven organic compounds with different molar volumes and chemical functionalities (alcohols, ketones, and monoterpenes). The Pyrr-PIL fibers were obtained as dense film coatings, with 67 µm thickness, with an overall sorption increase of 90% and 55% as compared to commercial fibers of Polyacrylate (85 µm) (PA85) and Polydimethylsiloxane (7 µm) (PDMS7) coatings, respectively. A urine sample doped with the sample mixture was used to study the matrix effect and establish relative recoveries, which ranged from 60.2% to 104.1%.

Ionic liquid-based materials: a platform to design engineered CO2 separation membranes.

During the past decade, significant advances in ionic liquid-based materials for the development of CO2 separation membranes have been accomplished. This review presents a perspective on different strategies that use ionic liquid-based materials as a unique tuneable platform to design task-specific advanced materials for CO2 separation membranes. Based on compilation and analysis of the data hitherto reported, we provide a judicious assessment of the CO2 separation efficiency of different membranes, and highlight breakthroughs and key challenges in this field. In particular, configurations such as supported ionic liquid membranes, polymer/ionic liquid composite membranes, gelled ionic liquid membranes and poly(ionic liquid)-based membranes are detailed, discussed and evaluated in terms of their efficiency, which is attributed to their chemical and structural features. Finally, an integrated perspective on technology, economy and sustainability is provided.

Poly(ionic liquid)s as phase splitting promoters in aqueous biphasic systems.

Aqueous biphasic systems (ABSs) provide a sustainable and efficient alternative to conventional liquid-liquid extraction techniques with volatile organic solvents, and can be used for the extraction, recovery, and purification of diverse solutes. In this work, and for the first time, ABSs composed of poly(ionic liquid)s (PILs) and inorganic salts were measured at 25 °C and atmospheric pressure. New PILs having pyrrolidinium polycations combined with different counter-anions, namely acetate [Ac](-), trifluoroacetate [TFAc](-), hexanoate [Hex](-), adipate [Adi](-), and citrate [Cit](-) were synthesized, by a simple and environmentally-friendly procedure, and characterized. The effect of the PIL features, namely molecular weight and anionic character, and other experimental variables, such as temperature, on the phase splitting ability was researched. The aptitude of the studied ABS to be implemented as separation technologies was also evaluated through the use of a model biomolecule, tryptophan.

Playing with ionic liquid mixtures to design engineered CO2 separation membranes.

Ionic liquids have been explored as attractive alternative media for CO2 separation not only due to their low volatility but also due to their highly tuneable nature. Aiming at designing highly efficient liquid phases for flue gas separation and natural gas purification, this work focuses on the use of binary ionic liquid mixtures containing sulfate and/or cyano-functionalized anions. Several mixtures were prepared and their gas transport properties through supported ionic liquid membranes (SILMs) were investigated. The thermophysical properties of these mixtures, namely viscosity and density (data presented and discussed in ESI), were also measured so that trends between transport properties and thermophysical properties could be evaluated. The results obtained indicate that depending on the anions mixed, membranes with fine-tuned gas permeabilities, diffusivities and solubilities can be obtained. Additionally, SILMs prepared with these ionic liquid mixtures are on the upper bound of the CO2/N2 separation, or even may surpass it, indicating their potential for separating CO2 in low-pressure post-combustion processes. Overall, the use of ionic liquid mixtures combining the most selective anions with the least viscous anions is a highly promising strategy to design advanced engineered liquid phases for CO2 separation membranes.