Membrane design and morphology characterization. The bioinspired nano-ordered liquid membranes are constructed from ion/molecule self-assembly strategy to imitate the structure of cellular membrane. For the structural design of bioinspired nano-ordered liquid membranes, AgNO3 is chosen as the carrier because of its low cost, facile availability and excellent ability of transporting ethylene; PSs, as a subclass of ionic liquids, are chosen due to their same anion with carrier, useful protic acidic properties to stabilize silver carrier and their tunable intermolecular forces, especially amphiphilicity and electrostatic interactions; the natural and renewable polyols such as glycerol have conspicuous advantages of strong hydrogen-bond interactions and low material cost. A variety of PSs such as EAN, DEAN, TEAN, MIMN, CPAN and three typical polyols of EG, G and TEG are screened for the investigation of their structural effects on the self-assembly behaviors and corresponding separation performances of membranes (Figure S1). The physical property of as-selected PSs and their structural characterizations by 1H NMR and FTIR spectra are shown in Table S1 and Figure S2-S3. A series of binary liquid eutectics of PSs and polyols, and ternary eutectics of PSs, polyols and silver salt carrier (also referred to as membrane liquid) are formed with nano-ordered structure due to strong interactions, which is verified by their low melting points (Table S2). The bioinspired liquid membranes are easily fabricated by spin-coating of as-designed nano-ordered membrane liquid onto a home-made PES support (Figure 1a and Figure S4), where nanopores of support (about 100 nm) and the suitable viscosity of membrane liquid confer facile accumulation of membrane liquid on the support surface to form stable selective separation layer. Moreover, the chemical synthesis of membrane liquid is also highlighted by taking EAN (PS) and G (polyol) as an example (Figure 1a1), which comprises two steps of synthesizing EAN by proton transfer reaction and mixing as-prepared EAN, AgNO3 and G. The amphiphilicity, hydrogen bonding and electrostatic attractions endow the resultant membranes with inhomogeneous and nano-ordered liquid structures, where molecular and ionic clusters are found within the complex and ordered hydrogen-bond networks. The amphiphilicity interactions of the PS cations make themselves aggregate together into apolar nano-regions. The hydrogen bonding and electrostatic attractions lead to the formation of polar nano-domains, which are composed of with polyol, silver cation and NO3- anions. Therefore, the self-assembled nano-ordered liquid membrane share the distinct structural similarity with cellular membrane (Figure 1b), where the self-assembly of PSs mimics the lipid bilayer (Figure 1b1), the self-assembly of glycerol embedded within carrier mimics the carrier protein (Figure 1b2), being expected to possess incomparable gas permeability and selectivity like cellular membrane.
The distinctive feature of bioinspired liquid membrane is the precise manipulation of carrier distribution, including the continuity, the aggregation and the corresponding activity. Optimally, the arrangement of the nano-ordered polar and apolar domains can be tailored by the content of PS, polyol or their molar ratios, which results in the formation of regular carrier wires and enormous 3D interconnected ethylene transport nanochannels (Figure 1d). The continuous carrier distribution and high carrier activity contribute to the ultrafast and selective ethylene transport, which afford the combined high ethylene permeability and super-high ethylene/ethane selectivity (Figure 1d1). In contrast, the traditional polymer or liquid membranes generally possesses discrete and disordered carrier distribution with low carrier activity (Figure 1e), suffering from the very sluggish transport of ethylene molecules (Figure 1e1). Moreover, the impertinent combination of Ps and polyol or their molar ratio will lead to the lousily carrier aggregation with poor carrier activity, resulting in unattractive ethylene/ethane selectivity (Figure 1c). The morphology of bioinspired nano-ordered liquid membranes is investigated by SEM (Figure 1f-1k). The home-made PES support has relatively uniform pore sizes of ~100 nm and hierarchical cross-section structure with a relative dense top layer and a finger-like macroporous sublayer (Figure 1f and 1g). After spin-coating for several times, the support is completely covered by membrane liquid and a selective separation layer of nano-ordered liquid with ~5 µm is clearly obtained (Figure 1h and 1i). Figure 1j and 1k show the photos of pristine support and as-designed bioinspired nano-ordered liquid membrane, which further confirm the successful membrane fabrication. The facile membrane manufacturing by spin-coating making them more attractive for large scale industrial applications. Finally, the bioinspired nano-ordered liquid membranes exhibit desirable thermal stability as revealed by TG curves (Figure S5)
Characterizations of non-covalent intermolecular forces. The intermolecular forces driving self-assembly of nano-ordered liquid structure are investigated by the DSC, 1H NMR, ATR-FTIR and FT-Raman spectra (Figure 2), especially for the electrostatic and strong hydrogen-bonding interactions.50 The eutectic property among PSs, polyols and carrier is confirmed by the low melting points, which vary from -68 to -91.5 oC and are significantly manipulated by the combination of PS and polyol and the corresponding molar ratio. The incorporation of AgNO3 within binary eutectics of PS and polyol further increases melting points slightly (Figure 2a). The eutectic property indicates the collapse of crystal structure of PS and the weakened electrostatic interactions between the cation and anion by mixing. Meanwhile, eutectics make membrane remain liquid state and possess similar dynamic fluidity with cellular membranes, which renders the membrane with elasticity and flexibility, contributing to the defect-free membranes. Other physical properties of the membrane liquid, such as density, ion conductivity and the viscosity, are also collected in Table S2. As shown in Figure 2b, the NH3+ and OH chemical shifts in pure [EAN] locate at 7.832 and 4.590 ppm. Upon mixing with G, the chemical shift of NH3+ moves to up-filed gradually with the molar ratios of EAN to G changing from 1:1 to 1:3, which manifests that the electrostatic interactions and double ionic hydrogen bonds between cation and anion in PSs are weakened by the intercalation of PS by G. Concomitantly, the chemical shift of OH in G moves to up-fields by mixing PS, suggesting the hydrogen bond interactions among G molecules are broken and new hydrogen bond interactions between [G] and [EAN] are built. As seen from Figure 2c, the FTIR spectra of PS-polyol binary eutectic exhibits the characteristic bands of [EAN] and [G], and the hydrogen bonds existing as N-H···O and O-H···O have been confirmed by the broad peaks between 3600 and 3200 cm-1. The deformation vibrations of NH3+ and stretching vibrations of NO3- are found to be blue-shifts with the molar ratio of EAN to G decreasing from 1:1 to 1:3, which further indicates the collapse of long-range ordered structure of the PS.
The stretching modes of the OH in PS-polyol eutectic exhibit blue-shifts, implying that the hydrogen bond interactions of G-PS cation and G- PS anion are weaker than that of G-G. The 1H NMR and FTIR spectra of EG and TEG based binary eutectics exhibit similar behaviors with that of G-based binary eutectics (Figure S6-S7). However, the extent of shifts varies with different polyol and molar ratios, which indicates the tunable nano-ordered structures of bioinspired liquid membranes with different polyols. Subsequently, the incorporation of silver salt carrier to form membrane liquid (ternary eutectics), where the intermolecular forces between carrier and PS or polyol are also probed by ATR-FTIR spectra (Figure 3d). Upon introduction of silver salt, the OH stretching vibrations of polyols exhibit different red shifts for G, EG and TEG based membrane liquids, which indicated the diverse coordinative interactions between silver cation and polyols.51, 52 Meantime, the red-shifts of NO3- stretching vibrations are also observed, hinting the possible formation of hydrogen bonds between NO3- of silver salt and polyols. The G based membrane liquid exhibits stronger red shifts than EG and TEG based membrane liquids, which implies the stronger coordinative and hydrogen bond interactions. As elucidated in Figure 2e-2g, the deconvoluted FT-Raman spectra further gain insight of the ionic species in bioinspired nano-ordered liquid membranes with different PS/polyol molar ratios. Note that the NO3- stretching bands of free ions, ion pairs, and ion aggregates are located at 1034, 1040, and 1045 cm-1, respectively. For the PS/polyol molar ratio of 1:2, the ion aggregates are the dominant ionic constituents. For the PS/polyol molar ratio of 1:1, the content of ion aggregates decreases, while ion pairs and free ions increase. When the molar ratio of is 1:3, the free ions and ion pairs predominate. FT-Raman clearly indicates the PS/polyol molar ratios also govern the nano-ordered structure of bioinspired liquid membranes.
Visualization of nano-ordered membrane structure. The visualization of nano-ordered structure of bioinspired liquid membranes and the resultant carrier distributions are conducted by molecular dynamics simulations and small- and wide-angle X-ray scattering (SWAXS) at nanoscale level. The highlighted snapshot of bulk structure of bioinspired liquid membrane (EAN: glycerol = 1:1) in equilibrated simulation box reveals a bicontinuous nano-ordered structure analogous to the morphology of cellular membrane (Figure 3a), where the EAN cations (yellow) are assembled into apolar nanodomains to imitate lipid molecular layer, while the EAN anions (red), silver salt and G (blue) are assembled into polar nanodomains to act as carrier protein. The snapshot of bulk structure of bioinspired liquid membrane are shown in Figure 3b, and the isolated snapshots of cations, anions and glycerol further discern the self-assembled nano-ordered structure clearly (Figure 3c, 3e and 3f).
Cation alkyl chains are solvophobically associated together into apolar domains due to amphiphilic interactions, where carbon atoms orient towards each other with the ammonium groups facing away (figure 3c). The NO3- anions and G molecules are assembled together to form polar nanodomains due to strong electrostatic and hydrogen-bonding interactions, where NO3- anions are aligned upon the corresponding positions of cation charge group, and the G molecules or their clusters locate around charge groups of EAN (Figure 3e and 3f). Nearly all the silver salts confine themselves within polar nanodomains because of strong coordinative and hydrogen bond interactions. The periodic and well-defined nano-order structure of bioinspired liquid membrane are further confimed by radial distribution functions of ion-ion or ion-molecular, gij(r). The cation-cation and anion-anion peaks further suggest their aggregations within apolar and polar nanodomains, respectively. The stronger G-G peak in relative to G-Cation and G-anion indicates the presence of clusters of G molecules due to stronger hydrogen bonding interactions among G molecules (Figure 3j). The radial distribution functions of atom-atom highlight the strong electrostatic interactions between the cation and anion are retained in membrane liquid, where the NH3+ tends to approach the NO3− at closer distances (Figure 3k). The radial distribution functions of silver-anion, silver-G and silver-cations suggest the silver cations are encircled by NO3- anions and solvated by G molecules due to electrostatic and hydrogen-bonding interactions, respectively (Fig. S8). The coordination number quantificationally describe the intensity of hydrogen bonds (Table S3), which follows the order: G-G > G-cation > G-anion. The nano-ordered structure of bioinspired liquid membranes with EAN/G molar ratio of 1:2 is shown in Figure 3d and the isolated snapshots of cations, anions and glycerol are shown in Figure S9, which suggests the continuous polar domains but discrete apolar domains with the increase of G molecules. The polar domains occupy a larger fraction of the stimulated box, which results in the severe compression of apolar domains. The distributions of silver cations within bioinspired liquid membranes with EAN/G molar ratios of 1:1 and 1:2 are shown in Figure 3g and Figure 3h. The bicontinuous and interpenetrating networks of polar and apolar within bioinspired liquid membranes (1:1) lead to the continuous and uniform carrier distribution to create the regular carrier networks (highlighted in Figure 3i). Therefore, enormous 3D interconnected ethylene transport nanochannels are formed and contribute to the ultrafast and selective ethylene transport, which afford the excellent separation performances of bioinspired liquid membranes (1:1). In contrast, the distribution of silver cations within bioinspired liquid membranes (1:2) can be divided into two situations: uniform but discrete distribution and severe aggregation of most silver cations, which lead to poor carrier activity, resulting in poor separation performances of bioinspired liquid membranes (1:2). It can be inferred that the apolar domains will be further diluted by G clusters with the G molecules increasing continuously. The silver cations within bioinspired liquid membranes (1:3) are uniformly but discretely dispersed in the polar domains and solvated by G molecules, which render the membrane with moderate separation performances.
Moreover, the SWAXS further reveals the intermediate-range order of bioinspired liquid membrane. For the bioinspired liquid membrane (Ag/[EAN:G=1:1]), the peak at low q ( < 1 Å-1) can be assigned as a correlation peak arising from segregation of the alkyl chains into apolar domains, which clearly indicates the intermediate range nano-ordered structure (Fig. 3l). Correlation distance between the ammonium head groups can be approximated from the q values of the peaks through the use of Bragg’s Law of d = 2π/q, which is about 10 Å. The other correlation peak located at ∼1.7 Å-1 has been attributed to the distance between alkyl chains. However, no peak is observed for [EAN: G] at low q (< 1 Å-1), which suggests the small disordered cluster structure. This difference further confirms the vital role of carrier for the construction of nano-ordered structure, which is indicative by the compact alkyl chain configuration.
Evaluation of separation performances. The separation performances of bioinspired nano-ordered liquid membranes are evaluated by carrying out equimolar ethylene/ethane mixture separation on our home-made facility (Figure S10), and the operational procedure can be found in Supporting Information. The effects of the PS, Polyol, and their molar ratio on the membrane separation performances are investigated systematically, which gains insight of the membrane structure-performance relationships. As shown in Figure 4a, the PSs greatly affect the gas permeability. The effect of PSs on the ethylene permeability follows the sequence of MIMN > EAN > DEAN > TEAN > CAPN, which suggests ethylene permeability can be improved by skillful selection of PSs. As expected, the order of ethane permeability is in good agreement with the viscosities of membrane liquid due to that its permeation is mainly dependent on Fick diffusion (Figure S11). With the introduction of more hydroxyl groups into PSs, the decrease of ethane permeability is quicker than that of ethylene, which is probably attributed to the compact nano-ordered membrane structure induced by the presence of more hydroxyl groups. Therefore, the gas selectivity sharply increases with the more hydroxyl groups into PSs and reaches up to 265 for [TEAN] derived bioinspired nano-ordered liquid membrane, which is the highest ethylene/ethane selectivity ever reported. As shown in Figure 4b and Figure S12, the ethylene permeability of the membranes based on three different polyols follows the order of EG > G > TEG, while the ethylene/ethane selectivity is as follows: G > EG >TEG, which is arisen from by the different carrier activity within the different polyol derived polar domains. The silver cations surrounded by G molecules are more active due to stronger coordinative and hydrogen bond interactions. The PS/polyol molar ratios significantly determine carrier distribution within bioinspired nano-ordered liquid membranes, resulting in diverse separation performances. As expected by the above molecular dynamics simulation, the ethylene permeability obtains maximum at the molar ratio of 1:1, median at the molar ratio of 1:3 and minimum at molar ratio of 1:2 (Figure 4c). In contrast, the ethane permeability possessed maximum at the molar ratio of 1:2, which is almost 1~2 orders of magnitude higher than values obtained at the molar ratios of 1:1 and 1:3. The unusual ethane permeability at the molar ratio of 1:2 is ascribed to lousily carrier aggregation. Therefore, ethylene/ethane selectivity decreases initially and then increases with the PS/ polyol molar ratio of changing from 1:1 to 1:3, obtaining a maximum at the molar ratio of 1:1, which originates from enormous 3D interconnected ethylene transport nanochannels (Figure 4d). As seen from Figure S13, the ethylene permeability increases gradually with the silver salt content increasing from 0 to 5 mol/L, and the gas selectivity initially increases slightly and then increased sharply, varying from 3.4 to 226 for EAN derived membrane, which is attributed to more continuous and available carrier for ethylene transport. Moreover, an initial parameter optimization is conducted to promote the process efficiency, including transmembrane pressure, operating temperature, the feed ratio of ethylene/ethane and the flow rate of sweep gas (Figure S14), where the decrease of the transmembrane pressure favors separation selectivity but goes against the gas permeability, while the decrease of operating temperature has a completely opposite effect on the gas selectivity and permeability, the permeability decreases but selectivity increases. Moreover, the increase of the flow rate of sweep gas and the feed ratio not only improves the gas selectivity but also enhances the gas permeability. Fortunately, bioinspired nano-ordered liquid membranes present long-term stability during continuous operating, which clearly show a tempting prospect for commercial success. Finally, the excellent separation performances of as-designed membranes are highlighted by in comparison with those of traditional and advanced ethylene/ethane separation membranes (Figure 4f and Figure S15), and the highly competitive permeability and superhigh slectivity make them more promising than previously reported polymeric membranes, carbon molecular sieve membranes (CMS), mixed matrix membranes (MMMs), carrier-facilitated transport membranes (FTMs), IL and deep eutectic solvent (DES) based liquid membranes and metal organic framework membranes (MOF).
In summary, we have developed a series of bioinspired nano-ordered liquid membranes constructed from ion/molecule self-assembly of PS, polyol and ethylene-transport carrier (AgNO3), which mimic the structure of cellular membranes to achieve high-performance ethylene/ethane separation. The eutectics of PS and polyol make membrane remain liquid state and possess similar dynamic fluidity with cellular membranes, contributing to the facile scalable fabrication of defect-free membranes by spin-coating. The intermolecular forces are investigated by various spectroscopic characterizations and the visualization of nano-ordered membrane structure is conducted by molecular dynamics simulations. The amphiphilicity, hydrogen bonding and electrostatic attractions within membranes drive the self-assembly of ion/molecule into nano-ordered liquid structure with interpenetrating and continuous apolar domains and polar domains, which can precisely manipulate of the carrier distribution such as the continuity, aggregation and activity. By optimizing the PS and polyol and their compositions, the regular carrier wires with continuous and uniform carrier distribution are obtained, resulting in the formation of enormous 3D interconnected ethylene transport nanochannels. The plenty of nanochannels effectively reduce the mass transport distances as well as enable continuous and fast transport of ethylene molecules throughout nano-ordered liquid membranes. The elucidation of stucture-performance relationship further confirms the 3D nanochannels and suggests that ethylene permeability and gas selectivity can be greatly improved by rational selection the PSs, polyol, and their molar ratios. Moreover, the bioinspired nano-ordered liquid membranes exhibit good long-term stability. Therefore, the excellent separation performances and good long-term stability may herald this new class of bioinspired membranes as promising alternatives to the existing separation technology, and the pioneering exploration to construct fast transport nanochannels by ion/molecule self-assembly will shed light on the development of channel-based membranes, highly conductive electrolytes and controlled metal-distributed catalysts for energy and environmental applications.