Jovan Kamcev

Maximizing the right stuff: The trade-off between membrane permeability and selectivity

Filtering through to whats important Membranes are widely used for gas and liquid separations. Historical analysis of a range of gas pair separations indicated that there was an upper bound on the trade-off between membrane permeability, which limits flow rates, and the selectivity, which limits the quality of the separation process. Park et al. review the advances that have been made in attempts to break past this upper bound. Some inspiration has come from biological membranes. The authors also highlight cases where the challenges lie in areas other than improved separation performance. Science, this issue p. eaab0530 BACKGROUND Synthetic membranes are used for desalination, dialysis, sterile filtration, food processing, dehydration of air and other industrial, medical, and environmental applications due to low energy requirements, compact design, and mechanical simplicity. New applications are emerging from the water-energy nexus, shale gas extraction, and environmental needs such as carbon capture. All membranes exhibit a trade-off between permeability—i.e., how fast molecules pass through a membrane material—and selectivity—i.e., to what extent the desired molecules are separated from the rest. However, biological membranes such as aquaporins and ion channels are both highly permeable and highly selective. Separation based on size difference is common, but there are other ways to either block one component or enhance transport of another through a membrane. Based on increasing molecular understanding of both biological and synthetic membranes, key design criteria for new membranes have emerged: (i) properly sized free-volume elements (or pores), (ii) narrow free-volume element (or pore size) distribution, (iii) a thin active layer, and (iv) highly tuned interactions between permeants of interest and the membrane. Here, we discuss the permeability/selectivity trade-off, highlight similarities and differences between synthetic and biological membranes, describe challenges for existing membranes, and identify fruitful areas of future research. ADVANCES Many organic, inorganic, and hybrid materials have emerged as potential membranes. In addition to polymers, used for most membranes today, materials such as carbon molecular sieves, ceramics, zeolites, various nanomaterials (e.g., graphene, graphene oxide, and metal organic frameworks), and their mixtures with polymers have been explored. Simultaneously, global challenges such as climate change and rapid population growth stimulate the search for efficient water purification and energy-generation technologies, many of which are membrane-based. Additional driving forces include wastewater reuse from shale gas extraction and improvement of chemical and petrochemical separation processes by increasing the use of light hydrocarbons for chemicals manufacturing. OUTLOOK Opportunities for advancing membranes include (i) more mechanically, chemically, and thermally robust materials; (ii) judiciously higher permeability and selectivity for applications where such improvements matter; and (iii) more emphasis on fundamental structure/property/processing relations. There is a pressing need for membranes with improved selectivity, rather than membranes with improved permeability, especially for water purification. Modeling at all length scales is needed to develop a coherent molecular understanding of membrane properties, provide insight for future materials design, and clarify the fundamental basis for trade-off behavior. Basic molecular-level understanding of thermodynamic and diffusion properties of water and ions in charged membranes for desalination and energy applications such as fuel cells is largely incomplete. Fundamental understanding of membrane structure optimization to control transport of minor species (e.g., trace-organic contaminants in desalination membranes, neutral compounds in charged membranes, and heavy hydrocarbons in membranes for natural gas separation) is needed. Laboratory evaluation of membranes is often conducted with highly idealized mixtures, so separation performance in real applications with complex mixtures is poorly understood. Lack of systematic understanding of methodologies to scale promising membranes from the few square centimeters needed for laboratory studies to the thousands of square meters needed for large applications stymies membrane deployment. Nevertheless, opportunities for membranes in both existing and emerging applications, together with an expanding set of membrane materials, hold great promise for membranes to effectively address separations needs. From intrinsic permeability/selectivity trade-off to practical performance in membranes. Polymer membranes for liquid and gas separation applications obey a permeability/selectivity trade-off—highly permeable membranes have low selectivity and vice versa—largely due to broad distributions of free-volume elements (or pores in porous membranes) and nonspecific interactions between small solutes and polymers. We highlight materials approaches to overcome this trade-off, including the development of inorganic, isoporous, mixed matrix, and aquaporin membranes. Further, materials must be processed into thin, typically supported membranes, fashioned into high surface/volume ratio modules, and used in optimized processes. Thus, factors that govern the practical feasibility of membranes such as mechanical strength, module design, and operating conditions are also discussed. Increasing demands for energy-efficient separations in applications ranging from water purification to petroleum refining, chemicals production, and carbon capture have stimulated a vigorous search for novel, high-performance separation membranes. Synthetic membranes suffer a ubiquitous, pernicious trade-off: highly permeable membranes lack selectivity and vice versa. However, materials with both high permeability and high selectivity are beginning to emerge. For example, design features from biological membranes have been applied to break the permeability-selectivity trade-off. We review the basis for the permeability-selectivity trade-off, state-of-the-art approaches to membrane materials design to overcome the trade-off, and factors other than permeability and selectivity that govern membrane performance and, in turn, influence membrane design.

Chemically enhancing block copolymers for block-selective synthesis of self-assembled metal oxide nanostructures.

We report chemical modification of self-assembled block copolymer thin films by ultraviolet light that enhances the block-selective affinity of organometallic precursors otherwise lacking preference for either copolymer block. Sequential precursor loading and reaction facilitate formation of zinc oxide, titanium dioxide, and aluminum oxide nanostructures within the polystyrene domains of both lamellar- and cylindrical-phase modified polystyrene-block-poly(methyl methacrylate) thin film templates. Near-edge X-ray absorption fine structure measurements and Fourier transform infrared spectroscopy show that photo-oxidation by ultraviolet light creates Lewis basic groups within polystyrene, resulting in an increased Lewis base-acid interaction with the organometallic precursors. The approach provides a method for generating both aluminum oxide patterns and their corresponding inverses using the same block copolymer template.

Nanoscale transport enables active self-assembly of millimeter-scale wires.

Active self-assembly processes exploit an energy source to accelerate the movement of building blocks and intermediate structures and modify their interactions. A model system is the assembly of biotinylated microtubules partially coated with streptavidin into linear bundles as they glide on a surface coated with kinesin motor proteins. By tuning the assembly conditions, microtubule bundles with near millimeter length are created, demonstrating that active self-assembly is beneficial if components are too large for diffusive self-assembly but too small for robotic assembly.

Partitioning of mobile ions between ion exchange polymers and aqueous salt solutions: importance of counter-ion condensation

Equilibrium partitioning of ions between a membrane and a contiguous external solution strongly influences transport properties of polymeric membranes used for water purification and energy generation applications. This study presents a theoretical framework to quantitatively predict ion sorption from aqueous electrolytes (e.g., NaCl, MgCl2) into charged (i.e., ion exchange) polymers. The model was compared with experimental NaCl, MgCl2, and CaCl2 sorption data in commercial cation and anion exchange membranes. Ion sorption in charged polymers was modeled using a thermodynamic approach based on Donnan theory coupled with Mannings counter-ion condensation theory to describe non-ideal behavior of ions in the membrane. Ion activity coefficients in solution were calculated using the Pitzer model. The resulting model, with no adjustable parameters, provides remarkably good agreement with experimental values of membrane mobile salt concentration. The generality of the model was further demonstrated using literature data for ion sorption of various electrolytes in charged polymers, including HCl sorption in Nafion.

Charged Polymer Membranes for Environmental/Energy Applications

Ion exchange membranes are used in various membrane-based processes (e.g., electrodialysis, fuel cells). Charged solute transport is largely governed by the charged groups on the polymer backbone. In this review, fundamental relationships describing salt permeability and ionic conductivity, as well as water permeability, in charged polymers are developed within the framework of the Nernst-Planck and solution-diffusion models. The influence of fixed charge groups and polymer structure on water sorption and diffusion is discussed. Current understanding of ion partitioning in charged polymers, focusing on the use of thermodynamic models (i.e., Donnan theory) to describe such phenomena, is summarized. Ion diffusivity data from the literature are interpreted using a model developed by Mackie and Meares to assess relative and absolute effects of the polymer and fixed charge groups on ion diffusivity. Furthermore, membrane requirements for several important technologies are listed. Knowledge gaps and opportunities for fundamental research are also discussed.

Microtubule nanospool formation by active self-assembly is not initiated by thermal activation

Biotinylated microtubules partially coated with streptavidin and gliding on a surface coated with kinesin motors can cross-link with each other and assemble into nanospools with a diameter of a few micrometres. The size distribution of these nanospools is determined, and it is shown with simulations of microtubule gliding that these spools are too small to be formed by thermally activated turns in the gliding direction (a Brownian ratchet mechanism). Instead, spool formation is primarily the result of two processes: pinning of gliding microtubules to inactive motors and simultaneous cross-linking of multiple microtubules.

Predicting Salt Permeability Coefficients in Highly Swollen, Highly Charged Ion Exchange Membranes

This study presents a framework for predicting salt permeability coefficients in ion exchange membranes in contact with an aqueous salt solution. The model, based on the solution-diffusion mechanism, was tested using experimental salt permeability data for a series of commercial ion exchange membranes. Equilibrium salt partition coefficients were calculated using a thermodynamic framework (i.e., Donnan theory), incorporating Mannings counterion condensation theory to calculate ion activity coefficients in the membrane phase and the Pitzer model to calculate ion activity coefficients in the solution phase. The model predicted NaCl partition coefficients in a cation exchange membrane and two anion exchange membranes, as well as MgCl2 partition coefficients in a cation exchange membrane, remarkably well at higher external salt concentrations (>0.1 M) and reasonably well at lower external salt concentrations (<0.1 M) with no adjustable parameters. Membrane ion diffusion coefficients were calculated using a combination of the Mackie and Meares model, which assumes ion diffusion in water-swollen polymers is affected by a tortuosity factor, and a model developed by Manning to account for electrostatic effects. Agreement between experimental and predicted salt diffusion coefficients was good with no adjustable parameters. Calculated salt partition and diffusion coefficients were combined within the framework of the solution-diffusion model to predict salt permeability coefficients. Agreement between model and experimental data was remarkably good. Additionally, a simplified version of the model was used to elucidate connections between membrane structure (e.g., fixed charge group concentration) and salt transport properties.

Effect of fixed charge group concentration on equilibrium ion sorption in ion exchange membranes

Despite their increasing importance in many energy and water purification applications, few systematic studies of ion sorption in ion exchange membranes exist where fixed charge group concentration and water content are varied independently. Such studies are critical for developing fundamental structure/property relations important for rationally tailoring such materials. Here, cation and anion exchange membranes having different fixed charge group concentrations but similar water content were synthesized to investigate the influence of fixed charge group concentration on equilibrium ion sorption in such materials. Co-ion sorption decreased with increasing membrane fixed charge group concentration, as expected, presumably due to enhanced Donnan exclusion. However, the extent to which co-ion sorption was suppressed was different for the cation and anion exchange membranes, despite similar changes in membrane fixed charge group concentration. A thermodynamic model, based on Donnan theory and Mannings counter-ion condensation theory, was used to interpret the data. The model predicted equilibrium co-ion concentrations in the anion exchange membranes with no adjustable parameters. However, good agreement between the model and experimental data for the cation exchange membranes was only obtained by treating the Manning parameter as an adjustable constant, presumably due to phase separation during polymerization, which produced inhomogeneous membranes.