Graphynes for Water Desalination and Gas Separation

Selective transport of mass through membranes, so-called separation, is fundamental to many industrial applications, e.g., water desalination and gas separation. Graphynes, graphene analogs yet containing intrinsic uniformly distributed pores, are excellent candidates for highly permeable and selective membranes owing to their extreme thinness and high porosity. Graphynes exhibit computationally determined separation performance far beyond experimentally measured values of commercial state-of-the-art polyamide membranes; they also offer advantages over other atomically thin membranes like porous graphene in terms of controllability in pore geometry. Here, recent progress in proof-of-concept computational research into various graphynes for water desalination and gas separation is discussed, and their theoretically predicted outstanding permeability and selectivity are highlighted. Challenges associated with the future development of graphyne-based membranes are further analyzed, concentrating on controlled synthesis of graphyne, maintenance of high structural stability to withstand loading pressures, as well asthe demand for accurate computational characterization of separation performance. Finally, possible directions are discussed to align future efforts in order to push graphynes and other 2D material membranes toward practical separation applications.

Metal-coordinated sub-10 nm membranes for water purification

Ultrathin membranes with potentially high permeability are urgently demanded in water purification. However, their facile, controllable fabrication remains a grand challenge. Herein, we demonstrate a metal-coordinated approach towards defect-free and robust membranes with sub-10 nm thickness. Phytic acid, a natural strong electron donor, is assembled with metal ion-based electron acceptors to fabricate metal-organophosphate membranes (MOPMs) in aqueous solution. Metal ions with higher binding energy or ionization potential such as Fe³⁺ and Zr⁴⁺ can generate defect-free structure while MOPM-Fe³⁺ with superhydrophilicity is preferred. The membrane thickness is minimized to 8 nm by varying the ligand concentration and the pore structure of MOPM-Fe³⁺ is regulated by varying the Fe³⁺ content. The membrane with optimized MOPM-Fe³⁺ composition exhibits prominent water permeance (109.8 L m⁻² h⁻¹ bar⁻¹) with dye rejections above 95% and superior stability. This strong-coordination assembly may enlighten the development of ultrathin high-performance membranes.

Ultrathin membranes have demonstrated great promise for water purification technologies owing to their high permeance. Here the authors fabricate sub-10 nm, defect-free, robust membranes for dye remediation from water through the coordination-driven assembly of metal-organophosphates.

Interfacial Junctions Control Electrolyte Transport through Charge-Patterned Membranes

Distinct transport mechanisms emerge when nanostructured substrates are patterned with multiple chemistries. For example, charge-patterned mosaic membranes possess surfaces functionalized with discrete domains of both positive and negative charge. These oppositely charged domains provide pathways for both the cation and anion from a dissolved salt to permeate through the membrane without violating the macroscopic constraint of electroneutrality. Here, by systematically varying the geometry and size of the charge pattern, we elucidate the molecular interactions that promote the transport of salts under the action of pressure-driven flow. For patterns that consist of equivalent areal coverages of positively charged and negatively charged domains, the effects of the geometric parameters were encapsulated in a single variable, the interfacial packing density, that quantified the fraction of the membrane surface covered by junctions between oppositely charged domains. Experimentally, the transport of symmetric electrolytes (i.e., KCl and MgSO₄) increased with the value of the interfacial packing density, while the interfacial packing density did not significantly affect the transport of asymmetric electrolytes (i.e., K₂SO₄ and MgCl₂). Simulations of the electrical potential near the membrane surface demonstrate that for symmetric electrolytes the structural charge heterogeneity reduces the barrier to ion partitioning, thereby promoting salt transport through the membranes. For asymmetric electrolytes, the charge heterogeneity skews the local availability of ions from the stoichiometric ratio of the salt, thus hindering salt transport. These findings demonstrate the promise of accessing transport mechanisms, which could find utility in a diverse range of chemical separations and sensing applications, through chemical patterning of membranes.

Molecular Insights into the Composition-Structure-Property Relationships of Polyamide Thin Films for Reverse Osmosis Desalination

A molecular-level understanding of the structure-property relationship of polyamide (PA) active layers in thin-film-composite membranes remains unclear. We developed an approach to build and hydrate the PA layer in molecular dynamics simulations and reproduced realistic membrane properties, which enabled us to examine the composition-structure-permeability relationships at the molecular level. We discovered the variation of pore size distributions in the dry PA structures at different monomer compositions, leading to different water cluster distributions and wetting properties of hydrated PA films. Membrane swelling was linearly dependent on the degree of cross-linking (DC), and higher water flux was obtained in the more swelling-prone PA films because of the transition in water transport mechanisms. Continuum-like and jumping transport both occurred in PA films with smaller DC, where visible and more persistent channels existed. In the denser films, water molecules relied only on the on-and-off channels to jump from one cavity to another; however, jumping transport was more pronounced even in the less dense PA films, and all the PA structures exhibited oscillations, which provided evidence for the solution-diffusion model rather than the pore-flow model. The results not only contribute to fundamental understanding but also provide insights into the molecule-level design for next-generation membranes.