Molecular separation with organic solvent nanofiltration: a critical review

Mechanism and performance relevance of nanomorphogenesis in polyamide films revealed by quantitative 3D imaging and machine learning

Biological morphogenesis has inspired many efficient strategies to diversify material structure and functionality using a fixed set of components. However, implementation of morphogenesis concepts to design soft nanomaterials is underexplored. Here, we study nanomorphogenesis in the form of the three-dimensional (3D) crumpling of polyamide membranes used for commercial molecular separation, through an unprecedented integration of electron tomography, reaction-diffusion theory, machine learning (ML), and liquid-phase atomic force microscopy. 3D tomograms show that the spatial arrangement of crumples scales with monomer concentrations in a form quantitatively consistent with a Turing instability. Membrane microenvironments quantified from the nanomorphologies of crumples are combined with the Spiegler-Kedem model to accurately predict methanol permeance. ML classifies vastly heterogeneous crumples into just four morphology groups, exhibiting distinct mechanical properties. Our work forges quantitative links between synthesis and performance in polymer thin films, which can be applicable to diverse soft nanomaterials.

Large-Area 2D Covalent Organic Framework Membranes with Tunable Single-Digit Nanopores for Predictable Mass Transport

The potential of covalent organic frameworks (COFs) for molecular separations remains unrealized because of challenges transforming nanoscale COF materials into large-area functional COF membranes. Herein, we report the synthesis of large-area (64 cm²), ultrathin (24 nm), β-ketoenamine-linked 2D COFs using a facile interfacial polymerization technique. Angstrom-level control over single-digit nanopore size (1.4-2.0 nm) is achieved by direct integration of variable-length monomers. We apply these techniques to fabricate a series of large-area 2D COF membranes with variable thicknesses, pore sizes, and supporting materials. Tunable 2D COF properties enable control over COF membrane mass transport, resulting in high solvent fluxes and sharp molecular weight cutoffs. For organic solvent nanofiltration, the 2D COF membranes demonstrate an order-of-magnitude greater permeance than the state-of-the-art commercial polymeric membrane. We apply continuum models to quantify the dominance of pore passage resistance to mass transport over pore entrance resistance. A strong linear correlation between single-digit nanopore tortuosity and 2D COF thickness enables solvent fluxes to be predicted directly from solvent viscosity and COF membrane properties. Solvent-nanopore interactions characterized by the membrane critical interfacial tension also appear to influence mass transport. The pore flow transport model is validated by predicting the flux of a 52 nm thick COF membrane.

Revolutionizing Membrane Design Using Machine Learning-Bayesian Optimization

Polymeric membrane design is a multidimensional process involving selection of membrane materials and optimization of fabrication conditions from an infinite candidate space. It is impossible to explore the entire space by trial-and-error experimentation. Here, we present a membrane design strategy utilizing machine learning-based Bayesian optimization to precisely identify the optimal combinations of unexplored monomers and their fabrication conditions from an infinite space. We developed ML models to accurately predict water permeability and salt rejection from membrane monomer types (represented by the Morgan fingerprint) and fabrication conditions. We applied Bayesian optimization on the built ML model to inversely identify sets of monomer/fabrication condition combinations with the potential to break the upper bound for water/salt selectivity and permeability. We fabricated eight membranes under the identified combinations and found that they exceeded the present upper bound. Our findings demonstrate that ML-based Bayesian optimization represents a paradigm shift for next-generation separation membrane design.

Adaptive Synthesis of Functional Amphiphilic Dendrons as a Novel Approach to Artificial Supramolecular Objects

Supramolecular structures, such as micelles, liposomes, polymerosomes or dendrimerosomes, are widely studied and used as drug delivery systems. The behavior of amphiphilic building blocks strongly depends on their spatial distribution and shape of polar and nonpolar component. This report is focused on the development of new versatile synthetic protocols for amphiphilic carbosilane dendrons (amp-CS-DDNs) capable of self-assembly to regular micelles and other supramolecular objects. The presented strategy enables the fine modification of amphiphilic structure in several ways and also enables the facile connection of a desired functionality. DLS experiments demonstrated correlations between structural parameters of amp-CS-DDNs and the size of formed nanoparticles. For detailed information about the organization and spatial distribution of amp-CS-DDNs assemblies, computer simulation models were studied by using molecular dynamics in explicit water.

Use of Boundary-Driven Nonequilibrium Molecular Dynamics for Determining Transport Diffusivities of Multicomponent Mixtures in Nanoporous Materials

The boundary-driven molecular modeling strategy to evaluate mass transport coefficients of fluids in nanoconfined media is revisited and expanded to multicomponent mixtures. The method requires setting up a simulation with bulk fluid reservoirs upstream and downstream of a porous media. A fluid flow is induced by applying an external force at the periodic boundary between the upstream and downstream reservoirs. The relationship between the resulting flow and the density gradient of the adsorbed fluid at the entrance/exit of the porous media provides for a direct path for the calculation of the transport diffusivities. It is shown how the transport diffusivities found this way relate to the collective, Onsager, and self-diffusion coefficients, typically used in other contexts to describe fluid transport in porous media. Examples are provided by calculating the diffusion coefficients of a Lennard-Jones (LJ) fluid and mixtures of differently sized LJ particles in slit pores, a realistic model of methane in carbon-based slit pores, and binary mixtures of methane with hypothetical counterparts having different attractions to the solid. The method is seen to be robust and particularly suited for the study of study of transport of dense fluids and liquids in nanoconfined media.

General synthesis of ultrafine metal oxide/reduced graphene oxide nanocomposites for ultrahigh-flux nanofiltration membrane

Graphene-based membranes have great potential to revolutionize nanofiltration technology, but achieving high solute rejections at high water flux remains extremely challenging. Herein, a family of ultrafine metal oxide/reduced graphene oxide (rGO) nanocomposites are synthesized through a heterogenous nucleation and diffusion-controlled growth process for dye nanofiltration. The synthesis is based on the utilization of oxygen functional groups on GO surface as preferential active sites for heterogeneous nucleation, leading to the formation of sub-3 nm size, monodispersing as well as high-density loading of metal oxide nanoparticles. The anchored ultrafine nanoparticles could inhibit the wrinkling of the rGO nanosheet, forming highly stable colloidal solutions for the solution processing fabrication of nanofiltration membranes. By functioning as pillars, the nanoparticles remarkably increase both vertical interlayer spacing and lateral tortuous paths of the rGO membranes, offering a water permeability of 225 L m-2 h-1 bar-1 and selectivity up to 98% in the size-exclusion separation of methyl blue.

Polybenzimidazole Membrane Crosslinked with Epoxy-Containing Inorganic Networks for Organic Solvent Nanofiltration and Aqueous Nanofiltration under Extreme Basic Conditions

In this study, a novel polybenzimidazole (PBI)-based organic solvent nanofiltration (OSN) membrane possessing excellent stability under high pH condition was developed. To improve the chemical stability, the pristine PBI membrane was crosslinked with a silane precursor containing an epoxy end group. In detail, hydrolysis and condensation reaction of methoxysilane in the 3-glycidyloxypropyl trimethoxysilane (GPTMS) yields organic–inorganic networks within the PBI membrane structure. At the same time, the epoxy end groups on the organosiloxane network (Si–O–Si) reacted with amine groups of PBI to complete the crosslinking. The resulting crosslinked PBI membrane exhibited a good stability upon exposure to organic solvents and was not decomposed even in basic solution (pH 13). Our membrane showed an ethanol permeance of 27.74 LMHbar⁻¹ together with a high eosin Y rejection of >90% under 10 bar operation pressure at room temperature. Furthermore, our PBI membrane was found to be operational even under an extremely basic condition, although the effective pore size was slightly enlarged due to the pore swelling effect. The results suggest that our membrane is a promising candidate for OSN application under basic conditions.

Controlled Transition Metal Nucleated Growth of Carbon Nanotubes by Molten Electrolysis of CO2

The electrolysis of CO2 in molten carbonate has been introduced as an alternative mechanism to synthesize carbon nanomaterials inexpensively at high yield. Until recently, CO2 was thought to be unreactive, making its removal a challenge. CO2 is the main cause of anthropogenic global warming and its utilization and transformation into a stable, valuable material provides an incentivized pathway to mitigate climate change. This study focuses on controlled electrochemical conditions in molten lithium carbonate to split CO2 absorbed from the atmosphere into carbon nanotubes (CNTs), and into various macroscopic assemblies of CNTs, which may be useful for nano-filtration. Different CNT morphologies were prepared electrochemically by variation of the anode and cathode composition and architecture, variation of the electrolyte composition pre-electrolysis processing, and variation of the current application and current density. Individual CNT morphologies’ structures and the CNT molten carbonate growth mechanisms are explored using SEM (scanning electron microscopy), TEM (transmission electron micrsocopy), HAADF (high angle annular dark field), EDX (energy dispersive xray), X-ray diffraction), and Raman methods. The principle commercial technology for CNT production had been chemical vapor deposition, which is an order of magnitude more expensive, generally requires metallo-organics, rather than CO2 as reactants, and can be highly energy and CO2 emission intensive (carries a high carbon positive, rather than negative, footprint).

Carbosilane Glycodendrimers for Anticancer Drug Delivery: Synthetic Route, Characterization, and Biological Effect of Glycodendrimer-Doxorubicin Complexes

The complexity of drug delivery mechanisms calls for the development of new transport system designs. Here, we report a robust synthetic procedure toward stable glycodendrimer (glyco-DDM) series bearing glucose, galactose, and oligo(ethylene glycol)-modified galactose peripheral units. In vitro cytotoxicity assays showed exceptional biocompatibility of the glyco-DDMs. To demonstrate applicability in drug delivery, the anticancer agent doxorubicin (DOX) was encapsulated in the glyco-DDM structure. The anticancer activity of the resulting glyco-DDM/DOX complexes was evaluated on the noncancerous (BJ) and cancerous (MCF-7 and A2780) cell lines, revealing their promising generation- and concentration-dependent effect. The glyco-DDM/DOX complexes show gradual and pH-dependent DOX release profiles. Fluorescence spectra elucidated the encapsulation process. Confocal fluorescence microscopy demonstrated preferential cancer cell internalization of the glyco-DDM/DOX complexes. The conclusions were supported by computer modeling. Overall, our results are consistent with the assumption that novel glyco-DDMs and their drug complexes are very promising in drug delivery and related applications.

Design of an efficient, tunable and scalable freestanding flexible membrane for filter application

To address the global challenge of water pollution, membrane-based technologies are being used as a dignified separation technology. However, designing low-cost, reusable, freestanding and flexible membranes for wastewater treatment with tunable pore size, good mechanical strength, and high separation efficiency is still a major challenge. Herein, we report the development of a scalable, reusable, freestanding, flexible and functionalized multiwalled carbon nanotube (FMWCNT) membrane filter with tunable pore size for wastewater treatment, which has attractive attributes such as high separation efficiency (>99% for organic dyes and ∼80% for salts), permeance (∼225 L h^-1 m^-2 bar^-1), tensile strength (∼6 MPa), and reusability of both the membrane as well as contaminants separately. This FMWCNTs membrane filter has been developed by a simple vacuum-assisted filtration technique followed by the synthesis of MWCNTs using a cost-effective spray pyrolysis assisted chemical vapor deposition (CVD) technique and chemical functionalization. This study deals with understanding the rejection, retrieval, and reusability of both the membranes as well as waterborne contaminants separately. The developed membrane filter has potential utility in many applications such as wastewater treatment, food industry, and life sciences due to its robust mechanical and separation performance characteristics.

From tandem to catalysis – organic solvent nanofiltration for catalyst separation in the homogeneously W-catalyzed oxidative cleavage of renewable methyl 9,10-dihydroxystearate

Phosphotungstic acid is applied as a homogeneous catalyst for oxidative cleavage of methyl 9,10-dihydroxystearate, allowing for retention of the catalyst via organic solvent nanofiltration.

Hydroxypropyl-β-cyclodextrin-incorporated Pebax composite membrane for improved permselectivity in organic solvent nanofiltration

The OSN membrane was prepared via crosslinking HP-β-CD in Pebax layer with TDI. The as-prepared membrane exhibited high methanol permeability and good dye rejections, as well as longer running stability.

La(iii)–curcumin-functionalized gold nanocomposite as a red light-activatable mitochondria-targeting PDT agent

Functionalization of La(iii)–curcumin to gold nanoparticles resulted in remarkable red-shifted UV-visible absorption and exhibited remarkable differential photodynamic ability towards cancer cells upon red-light activation.

Conformationally tunable calix[4]pyrrole-based nanofilms for efficient molecular separation

Preparation of nanofilms which are able to reject water-soluble low molecular weight organic compounds in nanofiltration remains to be a challenge. Herein, we report a new kind of self-standing, defect-free, robust, centimeter-sized and thickness controllable calix[4]pyrrole (C[4]P)-based nanofilms with excellent molecular sieving performance in nanofiltration. The nanofilms were prepared via confined dynamic condensation of the tetra-benzoyl-hydrazine derivative of calix[4]pyrrole (CPTBH) with 1,3,5-benzenetricarboxaldehyde (BTC) at the air/dimethyl sulfoxide (DMSO) interface. Nanofiltration tests under 2 bar pressure with porous polyethylene terephthalate (PET) as the support and a CsF treated CPTBH-BTC nanofilm (∼100 nm) as the selective layer depicted a water permeance of 15 L m^-2h^-1 bar^-1 and a methanol permeance of 45 L m^-2h^-1 bar^-1. High rejection rates (>95%) were found in aqueous solution for most of the tested dyes and pharmaceuticals. Remarkably, the composite membrane also demonstrated good separation performance in aqueous phase to some amino acids and organic dyes with molecular weights around 200 g/mol. High-performance nanofiltration in methanol was also realized. In this case, the molecular weight cutoff value is ∼ 800 g/mol. These findings showed that introduction of macrocyclic hosts is an effective way to develop nanofilms with high solvent permeance but low molecular weight cutoff value.

Multirole Regulations of Interfacial Polymerization Using Poly(acrylic acid) for Nanofiltration Membrane Development

Effective control of monomer diffusion and reaction rate is the key to achieving a controlled interfacial polymerization (IP) and a high-performance nanofiltration (NF) membrane. Herein, an integration of multirole regulations was synchronously realized using poly(acrylic acid) (PAA) as an active additive in a piperazine (PIP) aqueous phase. Thanks to synergistic interactions, including hydrogen bonding, electrostatic interaction, and covalent bonding between PAA and PIP molecules, together with the increased viscosity of the solution, PIP diffusion was rationally controlled. Moreover, interfacial polycondensation was also restrained via the modestly reduced pH of the aqueous solution. These contribute to the formation of a thinner, looser, more hydrophilic, and higher negatively charged PAA-decorated polyamide selective layer with a unique nanostrand-nodule morphology. The harvested NF-PAA/PIP membrane showed an ∼70% rise in water permeability (up to 23.5 L·m-2·h-1·bar-1) while retaining high Na2SO4 and dye rejections. Furthermore, the optimized NF-PAA/PIP membrane presented a superior fouling resistance capability for typical pollutants, as well as long-term stability during successive filtration. Thus, this work offers a straightforward and impactful approach to regulating IP and promoting NF membrane properties.