Polyelectrolyte multilayers on PTMSP as asymmetric membranes for gas separations: Langmuir-Blodgett versus self-assembly methods of anchoring

Tuning Electrostatics to Promote Ordered Monolayers of Phosphole-Lipids

Heteroatom-doped π-conjugated systems have been exploited for electronic device applications. Phosphole-containing backbones are particularly intriguing with regard to electron delocalization and ultimately control over the photophysical, redox, and charge transfer characteristics. However, practical application of these materials commonly relies on well-structured, thin film architectures. Self-assembly offers a route to generate ordered 2D structures deposited on solid substrates. The orientation of these deposited films can be manipulated by the introduction of alkyl chains of varying length (to form phosphole-based lipids) and chemical modification of the phosphole-derived headgroup. External controls can also be considered to tune these films' properties, e.g., the electrostatic interactions within the film can be controlled by varying the environment with the addition of simple salt counterions. A series of lipids with phosphole-based π-conjugated headgroups have been designed and exhibit intramolecular conformational changes in response to external conditions. Herein the 2D film structure in Langmuir and Langmuir-Blodgett films is reported in the presence and absence of halide salts. The film morphology obtained from Brewster angle and atomic force microscopy shows the formation of a condensed phase but also 3D aggregates, and grazing incidence X-ray diffraction confirmed the presence of untilted, hexagonally packed chains. The size of the counterion influences its ability to intercalate between the phosphole headgroups, which ultimately provides a means to induce the formation of a well-ordered, single monolayer film without aggregates that can be transferred to the solid substrate.

Increased CO2/N2 selectivity of PTMSP by surface crosslinking

The surface crosslinking of poly[1-(trimethylsilyl)-1-propyne] (PTMSP) membranes by dithiothreitol under thiol-ene click reaction conditions has yielded membranes having CO₂/N₂ selectivities in excess of 30 with CO₂ permeances in excess of 300 GPU (gas permeation units). The simplicity of this surface crosslinking strategy together with these permeation results suggests that PTMSP that is modified in such ways could lead to useful materials for the separation of CO₂/N₂ from flue gas and for certain other gaseous mixtures.

Defect Repair of Polyelectrolyte Bilayers Using SDS: The Action of Micelles Versus Monomers

Defects within single, double, and triple polyelectrolyte bilayers derived from poly(sodium 4-styrenesulfonate) (PSS) and poly(diallyldimethyammonium chloride) (PDDA) have been repaired using aqueous solutions of sodium dodecyl sulfate (SDS), as evidenced by a reduction in their permeability and an increase in their permeation selectivity. In contrast to the use of monomer solutions of SDS, which were moderately effective in repairing only double and triple bilayers, micellar solutions proved highly effective for all three assemblies. Evidence for intact micelles or micellar fragments being deposited on the surface of single bilayers of PSS/PDDA has been obtained from a combination of atomic force microscopy, X-ray photoelectron spectroscopy, ellipsometry, and contact angle measurements. Observed CO₂ permeances of ca. 200 GPU and CO₂/N₂ selectivities of ca. 30 for SDS-repaired, single bilayers of PSS/PDDA suggest that further development of such assemblies could have the practical potential for the separation of CO₂ from N₂ in the flue gas.

Don't Forget Langmuir-Blodgett Films 2020: Interfacial Nanoarchitectonics with Molecules, Materials, and Living Objects

Designing interfacial structures with nanoscale (or molecular) components is one of the important tasks in the nanoarchitectonics concept. In particular, the Langmuir-Blodgett (LB) method can become a promising and powerful strategy in interfacial nanoarchitectonics. From this viewpoint, the status of LB films in 2020 will be discussed in this feature article. After one section on the basics of interfacial nanoarchitectonics with the LB technique, various recent research examples of LB films are introduced according to classifications of (i) growing research, (ii) emerging research, and (iii) future research. In recent LB research, various materials other than traditional lipids and typical amphiphiles can be used as film components of the LB techniques. Two-dimensional materials, supramolecular structures such as metal organic frameworks, and biomaterials such as DNA origami pieces are capable of working as functional components in the LB assemblies. Possible working areas of the LB methods would cover emerging demands, including energy, environmental, and biomedical applications with a wide range of functional materials. In addition, forefront research such as molecular manipulation and cell fate control is conducted in LB-related interfacial science. The LB technique is a traditional and well-develop methodology for molecular films with a ca. 100 year history. However, there is plenty of room at the interfaces, as shown in LB research examples described in this feature article. It is hoped that the continuous development of the science and technology of the LB method make this technique an unforgettable methodology.

Ultrathin permselective membranes: the latent way for efficient gas separation

Membrane gas separation has attracted the attention of chemical engineers for the selective separation of gases. Among the different types of membranes used, ultrathin membranes are recognized to break the trade-off between selectivity and permeance to provide ultimate separation. Such success has been associated with the ultrathin nature of the selective layer as well as their defect-free structure. These membrane features can be obtained from specific membrane preparation procedures used, in which the intrinsic properties of different nanostructured materials (e.g., polymers, zeolites, covalent-organic frameworks, metal-organic frameworks, and graphene and its derivatives) also play a crucial role. It is likely that such a concept of membranes will be explored in the coming years. Therefore, the goal of this review study is to give the latest insights into the use of ultrathin selective barriers, highlighting and describing the primary membrane preparation protocols applied, such as atomic layer deposition, in situ crystal formation, interfacial polymerization, Langmuir-Blodgett technique, facile filtration process, and gutter layer formation, to mention just a few. For this, the most recent approaches are addressed, with particular emphasis on the most relevant results in separating gas molecules. A brief overview of the fundamentals for the application of the techniques is given. Finally, by reviewing the ongoing development works, the concluding remarks and future trends are also provided.

Clicking the Surface of Poly[1-(trimethylsilyl)propyne] (PTMSP) via a Thiol-Ene Reaction: Unexpected CO2/N2 Permeability

The surface modification of poly[1-(trimethylsilyl)propyne] (PTMSP) film via a thiol-ene click reaction with sodium 3-mercapto-1-propanesulfonate has yielded membranes having a CO₂ permeance as high as 530 GPU with a CO₂/N₂ selectivity of 21. This level of performance, together with the simplicity of this surface modification, suggests that such materials could become viable alternatives to some of the most promising membrane materials that are currently being explored for the practical capture of CO₂ from flue gas.

Top-Down Polyelectrolytes for Membrane-Based Post-Combustion CO2 Capture

Polymer-based CO2 selective membranes offer an energy efficient method to separate CO2 from flue gas. `Top-down' polyelectrolytes represent a particularly interesting class of polymer materials based on their vast synthetic flexibility, tuneable interaction with gas molecules, ease of processability into thin films, and commercial availability of precursors. Recent developments in their synthesis and processing are reviewed herein. The four main groups of post-synthetically modified polyelectrolytes discern ionised neutral polymers, cation and anion functionalised polymers, and methacrylate-derived polyelectrolytes. These polyelectrolytes differentiate according to the origin and chemical structure of the precursor polymer. Polyelectrolytes are mostly processed into thin-film composite (TFC) membranes using physical and chemical layer deposition techniques such as solvent-casting, Langmuir-Blodgett, Layer-by-Layer, and chemical grafting. While solvent-casting allows manufacturing commercially competitive TFC membranes, other methods should still mature to become cost-efficient for large-scale application. Many post-synthetically modified polyelectrolytes exhibit outstanding selectivity for CO2 and some overcome the Robeson plot for CO2/N2 separation. However, their CO2 permeance remain low with only grafted and solvent-casted films being able to approach the industrially relevant performance parameters. The development of polyelectrolyte-based membranes for CO2 separation should direct further efforts at promoting the CO2 transport rates while maintaining high selectivities with additional emphasis on environmentally sourced precursor polymers.

Self-Assembled Polymeric Membranes and Nanoassemblies on Surfaces: Preparation, Characterization, and Current Applications

Biomembranes play a crucial role in a multitude of biological processes, where high selectivity and efficiency are key points in the reaction course. The outstanding performance of biological membranes is based on the coupling between the membrane and biomolecules, such as membrane proteins. Polymer-based membranes and assemblies represent a great alternative to lipid ones, as their presence not only dramatically increases the mechanical stability of such systems, but also opens the scope to a broad range of chemical functionalities, which can be fine-tuned to selectively combine with a specific biomolecule. Tethering the membranes or nanoassemblies on a solid support opens the way to a class of functional surfaces finding application as sensors, biocomputing systems, molecular recognition, and filtration membranes. Herein, the design, physical assembly, and biomolecule attachment/insertion on/within solid-supported polymeric membranes and nanoassemblies are presented in detail with relevant examples. Furthermore, the models and applications for these materials are highlighted with the recent advances in each field.

Hyperthin Membranes for Gas Separations via Layer-by-Layer Assembly

Thin film formation via the Layer-by-Layer method is now a well-established and broadly used method in materials science. We have been keenly interested in exploiting this technique in the area of gas separations. Specifically, we have sought to create hyperthin (<100 nm) polyelectrolyte-based membranes that have practical potential for the separation of CO₂ from N₂ (flue gas) and H₂ from CO₂ (syngas). In this personal account, we summarize recent studies that have been aimed at measuring the influence of a variety of factors that can affect the permeability and permeation selectivity of hyperthin polyelectrolyte multilayers (PEMs).

Bioelectrochemical Properties of Enzyme-Containing Multilayer Polyelectrolyte Microcapsules Modified with Multiwalled Carbon Nanotubes

This work investigated changes in the biochemical parameters of multilayer membrane structures, emerging at their modification with multiwalled carbon nanotubes (MWCNTs). The structures were represented by polyelectrolyte microcapsules (PMCs) containing glucose oxidase (GOx). PMCs were made using sodium polystyrene sulfonate (polyanion) and poly(allylamine hydrochloride) (polycation). Three compositions were considered: with MWCNTs incorporated between polyelectrolyte layers; with MWCNTs inserted into the hollow of the microcapsule; and with MWCNTs incorporated simultaneously into the hollow and between polyelectrolyte layers. The impedance spectra showed modifications using MWCNTs to cause a significant decrease in the PMC active resistance from 2560 to 25 kOhm. The cyclic current-voltage curves featured a current rise at modifications of multilayer MWCNT structures. A PMC-based composition was the basis of a receptor element of an amperometric biosensor. The sensitivity of glucose detection by the biosensor was 0.30 and 0.05 μA/mM for PMCs/MWCNTs/GOx and PMCs/GOx compositions, respectively. The biosensor was insensitive to the presence of ethanol or citric acid in the sample. Polyelectrolyte microcapsules based on a multilayer membrane incorporating the enzyme and MWCNTs can be efficient in developing biosensors and microbial fuel cells.

Layer-by-layer assembly of a polymer of intrinsic microporosity: targeting the CO2/N2 separation problem

A polymer of intrinsic microporosity has been successfully incorporated into 6 nm thick polyelectrolyte multilayers and found to exhibit exceptional permeability properties with respect to CO₂ and N₂.

A plug and socket approach for tightening polyelectrolyte multilayers

A plug and socket approach for tightening polyelectrolyte multilayers is introduced based on the use pendant β-cyclodextrin groups.

Ultrathin Composite Polymeric Membranes for CO2 /N2 Separation with Minimum Thickness and High CO2 Permeance

The use of ultrathin films as selective layers in composite membranes offers significant advantages in gas separation for increasing productivity while reducing the membrane size and energy costs. In this contribution, composite membranes have been obtained by the successive deposition of approximately 1 nm thick monolayers of a polymer of intrinsic microporosity (PIM) on top of dense membranes of the ultra-permeable poly[1-(trimethylsilyl)-1-propyne] (PTMSP). The ultrathin PIM films (30 nm in thickness) demonstrate CO₂ permeance up to seven times higher than dense PIM membranes using only 0.04 % of the mass of PIM without a significant decrease in CO₂ /N₂ selectivity.

Preparation of polyelectrolyte-modified membranes for heavy metal ions removal

Polyethersulfone membranes were modified by polyelectrolyte (PE) multilayers, made of poly(allylamine hydrochloride) with poly(styrene sulfonate), to remove Cu²⁺, Zn²⁺ and Ni²⁺ heavy metal cations from aqueous solutions in a wide range of metal concentration (50-1200 ppm). After characterization of the modified membranes, the efficiency of the process was estimated for single heavy metal ions solution leading to high rejection rates (>90% for 50 ppm) and good adsorption capacities (7.0-8.5 mg cm^-2) whatever the metal ion tested. The stability in time of the modified membranes was proved by repeating successive filtrations with the same membrane. The filtration process was also used with mixed solutions composed of Cu²⁺, Zn²⁺ and Ni²⁺ ions. The rejection rates obtained for these ternary systems were very similar to the ones obtained for the single metal solutions, showing that the filtration process is still efficient for mixed solutions and can be applied for the decontamination of complex solutions. The long-term stability of the modified membranes was also demonstrated for mixed solutions. The high efficiency of the filtration process and the good adsorption capacities of the modified membranes are due to the ability of the PEs used to complex all the metallic dications tested in this study.

pKa-Dependent Facilitated Transport of CO2 across Hyperthin Polyelectrolyte Multilayers

Hyperthin (ca. 20-30 nm thick) polyelectrolyte multilayers have been fabricated that are capable of facilitated transport of CO₂. These membranes were fabricated from polycations bearing pendant groups of varying basicity plus poly(sodium 4-styrenesulfonate) as a polycounterion. A strong dependency of such transport on the basicity of the pendant groups (i.e., fixed carrier sites) has been found, where pK_a values in the range of ca. 5-7 appear optimal.

Gas Permeability of Hyperthin Polyelectrolyte Multilayers Having Matched and Mismatched Repeat Units

A series of polyelectrolyte multilayers (PEMs) has been fabricated using polyanions and polycations that have repeat units (i) similar in structure and composition (matched), (ii) partially similar in structure and composition (semimatched), and (iii) very different in structure and composition (mismatched). The primary aim of this investigation was to determine whether the matching of the polyelectrolytes can significantly influence the permeability properties of hyperthin PEMs. While matching, per se, was not found to be a key factor in defining membrane permeability, large differences in permeability were observed (the permeances of N₂ varied by a factor of 20), which were correlated with the concentration of pendant aryl groups present, i.e., the greater the concentration of these groups, the higher the permeability. Analysis by AFM indentation measurements further revealed that high-permeability PEMs tend to be more compliant than low-permeability PEMs. These findings underscore the need for considering a broad range of polyelectrolyte combinations when optimizing a particular functional property of PEMs.

Tightening Polyelectrolyte Multilayers with Oligo Pendant Ions

A series of polycations bearing one, two, and three pendant quaternary ammonium groups per repeat unit has been synthesized and combined with poly(sodium 4-styrenesulfonate) to produce hyperthin polyelectrolyte multilayers. Incremental addition of quaternary ammonium groups per repeat unit leads to decreased permeances for H₂, CO₂, and N₂ and increased H₂/CO₂ and CO₂/N₂ permeation selectivities. These results, together with analysis of the composition from X-ray photoelectron spectra and values of Young's modulus from nanoindentation analysis, show that oligo pendant ions provide a means for increasing volume charge density and tightening polyelectrolyte multilayers.

Consequences of Tacticity on the Growth and Permeability of Hyperthin Polyelectrolyte Multilayers

A series of polyanions and polycations have been synthesized from atactic, syndiotactic, and isotactic forms of poly(methyl methacrylate) and used to construct polyelectrolyte multilayers (PEMs). Polymer tacticity has been found to have a large influence on film growth but only a slight effect on H2/N2 permeation selectivities and no significant influence on CO2/N2 permeation selectivities. The permeances of H2, CO2, and N2 across those PEMs exhibiting the highest H2/N2 selectivities were found to vary by as much as factors of 3, 6, and 5, respectively, depending on the tacticities employed. A simple model that accounts for the strong dependency of film growth on tacticity is presented.

Splaying hyperthin polyelectrolyte multilayers to increase their gas permeability

The concept of splayed, hyperthin polyelectrolyte multilayers (PEMs) is introduced in which a bulky, hydrophilic and charged pendant group is used to increase the gas permeability of a PEM without reducing its permeation selectivity.