Organogel-Derived Covalent–Noncovalent Hybrid Polymers as Alkali

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Organogel-derived Covalent-Noncovalent Hybrid Polymers as Alkali Metal Ion Scavengers for Partial Deionization of Water Annamalai Prathap, Cijil Raju, and Kana M. Sureshan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03150 • Publication Date (Web): 12 Apr 2018 Downloaded from http://pubs.acs.org on April 13, 2018

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Organogel-derived Covalent-Noncovalent Hybrid Polymers as Alkali Metal Ion Scavengers for Partial Deionization of Water Annamalai Prathap, Cijil Raju and Kana M. Sureshan* School of Chemistry, Indian Institute of Science Education and Research Thiruvananthapuram, Kerala-695 551, India KEYWORDS. crown ether• organogel • de-ionization • water purification • polymer •. ABSTRACT: We show that crown ethers (CEs) 1-5 congeal both polar and non-polar solvents via their self-assembly through weak non-covalent interactions (NCI) such as CH...O and CH...π interactions. Di-isopropylidene-mannitol (6) is a known gelator that self-assembles through stronger OH...O H-bonding. These two gelators together also congeal non-polar solvents via their individual self-assembly. The gelator 6 self-assembles swiftly to fibers, which act as templates and attract CE to their surface through H-bonding and thereby facilitate their self-assembly through weak NCI. Polymerization of styrene gels made from CE and 6 followed by the washing off of the sacrificial gelator 6 yields robust porous polystyrene-crown ether hybrid matrices (PCH), having pore-exposed CEs. These PCHs were not only efficient in sequestering alkali metal ions from aqueous solutions but also can be recycled. This novel use of organogels for making solid sorbents for metal ion scavenging might be of great interest.

Introduction Organogels1-6 formed by low molecular weight organogelators (LMOG) have attracted much interests due to their potential applications in biomedical field,7 pharmaceuticals,8 organic electronics,9 photovoltaics,10-12 material syntheses,13-17 security and forensic applications,18-19 environmental protection,20-26 soft-optics,27 semi-conducting polymer,28 etc. The underlying principle of gelation is the self-assembly of gelators to fibrillar structures, through various non-covalent interactions (NCI), and their entanglement to 3D-fibrous network which immobilize the solvent through capillary forces.29 Though H-bonding is the most common non-covalent interaction responsible for gelation, other weaker interactions such as π-π stacking and van der Waal’s interactions are also known to involve in gelation.30-31 Apart from the requirement of NCI, an important feature of an LMOG is its amphiphilicity. Exploration to identify novel scaffolds for gelators and their exploitation in advanced applications are contemporary research areas of high priority. We herein report a novel class of gelators that use CH...O hydrogen bonding for selfassembly and their use in developing a solid hybrid polymer matrix for partial de-ionization of water. Crown ethers (CE), in view of their remarkable ability to bind metal ions, are interesting compounds.32-33 Simple crown ethers, being highly hydrophilic and water-soluble, are practically not useful for de-ionization of aqueous solutions. Though some of the derivatives of crown ethers are water-insoluble, their use in de-ionization necessitates impractical liquid(water)-liquid(organic solvent) biphasic extraction.34-35 Also, such monomeric crown ethers get

dispersed in aqueous medium, making them unsuitable for direct use as solids for de-ionization of aqueous solutions. Hence there have been attempts to polymerize CE-derived monomers.36-37 However, the efficiencies of such insoluble polymeric materials are poor due to the inaccessibility of metal-binding sites in the core of the matrix.

Figure 1. Proposal for the preparation of porous metal scavenging hybrid polymer.

We envisioned that two different gelators which use different supramolecular glues for gelation viz. a CE-derived gelator (CEG) and a sacrificial gelator (SG) together would congeal styrene through the formation of 3D fibrous network formed by entanglement of two different kinds of non-covalent polymers formed by their self-assembly (Figure 1). Polymerization of this co-gel to polystyrene

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(PS) followed by selective removal (by washing) of the SG fibres from the matrix would lead to a porous covalentnoncovalent hybrid polymer; the non-covalent CE polymer immobilized in the covalent PS. The porous nature of the matrix would allow metal ions present in water to access interior sites. In order to test this hypothesis, we first set out to identify a CEG. Dibenzo-3n-crown-n ethers having two benzene rings are sufficiently amphiphilic and have structural features for CH...O hydrogen bonding, π-π stacking, van der Waals interaction for them to undergo self-assembly. We have thus investigated the gelation ability of various dibenzo-3n-crown-n ethers 1-5 (Figure 2). To our satisfaction, these crown ethers were efficient gelators for various polar and non-polar organic solvents including styrene (Table S1). Though CE-derived gelators were reported and exploited for various applications,38-48 surprisingly, the gelation abilities of simple dibenzo-3n-crown-n ethers were not reported so far.

Figure 2. a) Structures of dibenzo-3n-crown-n ethers (1-5) and the sacrificial gelator 6. b) Arrangement of molecules in the crystal and gel of dibenzo-18-crown-6 ether (2). The blue dotted lines represent C-H...O and C-H…π interactions involved in the organization of molecules in the crystal. SEM images of xerogels (prepared from benzene gels) of CEs c) 1, d) 2, e) 3, f) 4 and g) 5. h) AFM image (height mode) of CE 2 recorded by drop-casting its solution in cyclohexane (1 mg/10 mL).

Scanning Electron Microscopy (SEM) of the xerogels obtained from benzene-gel of these crown ethers revealed the fibrillar morphology of their microstructures (Figure 2c-g). Similarly, Atomic Force Microscopy also confirmed the fibrillar morphology (Figure 2h). Single crystal XRD analysis of the crystals of dibenzo-18-crown-6 ether (CE 2) showed CH...O and C-H...π H-bonded 1D growth of mole-

cules (Figure 2b).50 We found that the PXRD spectra of the crystals and xerogels of CE 2 were identical. This suggests that the crystal packing arrangement is conserved in the gels too (Figure S1). Thus weak CH...O H-bonding and CH...π interactions are the major forces involved in the selfassembly and thus gelation of CE 2. Concentrationdependent 1H NMR spectroscopy of CE 2 showed slight downfield shift of CH2 protons, supporting that CH...O Hbonding is operative in gels (Figure S2). Similarly, analyses of the reported crystal structures of other crown ethers (1, 3-5) revealed that they also use weak CH...O H-bonding and C-H...π interactions for their self-assembly49-52 and comparison of PXRD patterns of their xerogels with the simulated powder patterns from their crystal structures revealed that they also have conserved molecular arrangement in crystals and gels (Figure S3). We have chosen diisopropylidene mannitol (6) as the SG as it uses OH...O hydrogen bond for its self-assembly to form fibrillar structures (Figure S4) and can be selectively removed by washing. The gelator 6 formed gel in styrene at a CGC of 1.6. Having established that CEs and SG 6 individually congeal styrene, we have investigated the ability of the mixture of these two gelators (by taking CE 2 as a representative of CEs) to congeal styrene. Mixture of SG 6 and CE 2 also formed gels at all ratios of total molarity of 124 µM or more in styrene. The Tgel values of mixtures of any ratio were found to be higher than that of the individual compounds suggestive of the higher thermal stability of the co-gel (Figure S5). The rheological analysis of the individual gels and co-gel revealed that both the gelators supplement each other in forming the co-gel and their gelation efficiencies are additive in nature (Figure 3a). SEM imaging of the xerogel made from the co-gel revealed its fibrillar morphology (Figure S6). In order to understand the molecular level interaction between gelators in their co-gel state, we have done 1H NMR titration by serially and incrementally adding CE 2 to a solution of SG 6 in benzene-d6, a gelling solvent. With the addition of CE 2, OH signals and CH signals of SG 6 showed gradual downfield-shift suggesting the OH...O and CH...O hydrogen bonding between CE 2 and SG 6 (Figure 3b). Apart from the down-field shift, these signals became more and more resolved upon addition of CE 2, further supporting their molecular level interaction in the gel. A comparison of the PXRD profiles of xerogels made from gels of individual gelators and mixture of both the gelators (SG and CE) suggested that both the gelators underwent selfassembly rather than co-assembly to form fibers of their own individual traits (Figure 3c). It is thus clear that the gelators 2 and 6 interact with each other without affecting their individual self-assemblies. Fibers formed by self-assembly of one of the two gelators would attract the other gelator on its surface through hydrogen bonding and such adsorbed second gelator undergo self-assembly on the surface of the other reinforcing the fibers. This model not only explains the higher strength of the co-gels but also the interaction between the gelators (NMR) and their self-assembly (PXRD). It is logical to assume that the gelator 6 undergoes faster self-assembly in styrene, through relatively stronger O-H...O hydrogen bonds, to form fibrillar structures. These fibrils act as tem-

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ACS Applied Materials & Interfaces plates and they attract the CE molecules to their surface through hydrogen bonding. Due to the templating effect, the adsorbed CE molecules interact with each other through weaker CH...O, CH...π and π -π interactions. Also the surface-exposed CE assembly would interact with the solvent (styrene) molecules through weak interactions. Even after polymerization, these interactions would anchor the CE assembly to the polystyrene matrix. The template fibers formed by SG 6 can be removed by washing with methanol, which would not only break the OH...O hydrogen bondings but also dissolves the SG selectively, to leave porous PS containing CE in the channels (Supporting Video S1).

Figure 3. (a) Comparison of storage moduli (rheology) of benzene gels of 6 and 2 having 124 mM concentration with equimolar co-gel. (b) Concentration-dependant1H NMR titration of a solution of 6 in benzene-d6 with 2. The downfield shifts of OH signals of 6 indicate their molecular level interaction. (c) Comparison of PXRD patterns of xerogels (prepared from benzene gels) of 2, 6 and their 1:1 (molar ratio) mixture.

Figure 4. SEM images of a) PCH2 (inset is the photograph of the PCH2) b) PCH2 at higher magnification showing the macropores.

As the gelation efficiency of CE 2 is better (CGC 3 wt%) than the other crown ether gelators, we set out to prepare polystyrene matrix from CE 2. Thus styrene gel formed by CE 2 (1 eq) and SG 6 (3 eq) was polymerized by using catalytic amount of AIBN as the initiator (see SI for more information). The polymerized gel was then washed with methanol to remove SG 6 and the porous polystyrenecrown ether hybrid matrix (PCH2) thus obtained was dried under vacuum.1H NMR (Figure S7) analysis of the PCH2 revealed that it contains 38 wt% polystyrene (PS) and 62% of CE 2 and is completely free of SG 6. The CE was intact in the matrix even after several rounds of sonication in water. This suggests that CE is strongly anchored to the polymer matrix and hence would not leach into water during extraction. SEM analysis of PCH2 revealed its porous nature (Figure 4). PXRD analysis of PCH2 revealed that the CE 2 molecules are ordered (self-assembled) in the amorphous PS matrix (Figure S8). By following similar procedure and using CE 1, we have made PCH1. It contains 50.3% of CE 1 and the rest being PS (Figure S9 & Table S2). Matrices from other crown ethers were not made as their gelation efficiencies to congeal styrene were poor. For the control experiment, polystyrene-crown ether matrix wasmade by the polymerization of the styrene gel of CE 2 alone (without the SG 6). This rigid matrix contained 68% of CE 2 (Figure S10) and was non-porous as evidenced from SEM imaging studies (Figure S11). In order to check the efficiencies of PCHs in metal ion extraction, they were made into small pieces and 100 mg were added into 1 mL of aqueous metal picrate solutions of known concentrations and kept undisturbed. After 12 hours, the PCH was removed by filtration and concentration of metal ions (picrate) in the filtrate was estimated by UV-Vis spectroscopy (Figure 5a & SI). From the initial concentration, and the concentration of the filtrate, the percentage of metal ion absorbed by the matrix was calculated. The absorption efficiencies of both PCH1 and PCH2 were found to be good towards alkali metal picrates. While the former extracted 43%, 55% and 76% of Li+, Na+ and K+ ions respectively, the latter matrix extracted 75%, 59% and 88% respectively (Figure S12-S14, For more details see SI). The extractions were done in triplicate and the average of three measurements was taken as the % of extraction (Table S3). Though the non-porous control matrix contains 68%, by weight, of CE 2, it could not absorb metal ions. Thus, it is clear that the use of SG (and its removal) is essential for making the porous matrix and the porosity is very important for metal-ion extraction.

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It is generally accepted that, in solution, 12-crown-4, 15crown-5 and 18-crown-6 ethers selectively bind Li+, Na+ and K+ ions respectively. We noted that the selectivity observed for metal ion extraction in solution by various crown ethers are not observed in our case (Figure S15). In solution, the dissolved crown ether is very flexible and can form planar complex with metal ion in the centre so that all the oxygen atoms are in coordination (this leads to efficient binding). But in our case, we have used selfassembled form (solid form) of crown ether which is not planar (as evident from the crystal structure and PXRD). In solid state the crown ether cannot adjust its conformation to adopt an optimal planar structure. Here the metal ion may be partly coordinated to some of the oxygens of the crown ether or the metal ion may even be sitting in between two or more crown ether molecules which are proximally placed by the self-assembly. The binding selectivity and efficiency in solution and solid states can thus be different.

Figure 5. a) UV-Vis spectra showing the absorption of potassium picrate stock solution and the solutions after extraction by PCH1 and PCH2. b) UV-Vis spectra showing five serial extraction cycles of potassium picrate using PCH2. c) PCH2 as prepared. d) PCH2 after extracting potassium picrate. Potassium picrate solution e) before and f) after 5 cycles of extraction; both solutions were diluted to the same extent.

IR and PXRD analyses of PCH2 before and after extractions revealed the structural integrity of the matrix (Figure S16 & S17). In order to check the recyclability of PCHs, small pieces of PCH2 were sonicated with potassium picrate solution of known concentration. The sorbent was filtered out from the picrate solution and was washed with deionized water for 10-15 times until there was no characteristic picrate color in the washings. Then it was washed with methanol and dried under vacuum to remove the water/methanol. The dried polymer matrix was then used for the next cycle of metal ion absorption from the filtrate obtained above, containing the unabsorbed picrate from first extraction. This process of washing (de-ionization) the matrix and reuse was continued for five cycles (Figure 5b and SI). From the concentration of metal ions in the filtrate calculated from the UV spectroscopic analysis, the percentage of metal ion extracted in each cycle was estimated. Almost 93% of the potassium ions could be extracted in five cycles. This experiment not only shows the recyclability of the matrix without compromising its efficiency but also suggests the possibility of almost complete deionization of metal ion solution through serial extractions. It is known that picrate as anion can positively influence the extraction ability of aromatic group containing crown ethers.54 In order to get realistic data, we have estimated

the metal binding efficiency of PCHs using ICP-AES (Inductively Coupled Plasma Atomic Emission Spectrometry) analysis. Thus PCHs of known weight were soaked in aqueous solutions of alkali metal salts (lithium carbonate, sodium chloride and potassium chloride) of known concentrations and filtered after 12 h (Supporting Information). The metal ion content in the filtrates were analyzed by ICP-AES and the % of metal ion absorbed were calculated and found to be 62, 54 and 72% for Li+, Na+ and K+ ions respectively for the PCH2 treated solution and 35, 52 and 67% of Li+, Na+ and K+ ions respectively for PCH1 treated solution.

Conclusion In conclusion, dibenzo-crown ethers are found to congeal various non-polar solvents through weak noncovalent interactions viz. CH…O H-bonding and CH…π Hbonding. Mixtures of CEs and diisopropylidene-mannitol, a sacrificial gelator (SG) which uses stronger O-H…O hydrogen bonding for its self-assembly, also congeal non-polar solvents through their individual self-assemblies. When both SG and CE are dissolved in a non-polar solvent, the SG, by virtue of its stronger H-bonding, undergoes faster self-assembly to form fibrils and such self-assembled SG fibrils could acts as template by attracting CE through Hbonding and hence facilitate their self-assembly. A styrene co-gel made from CE and SG, was polymerized and the SG template was selectively washed off to get robust porous polystyrene containing crown ethers in its channels. This sorbent was found to be efficient in sequestering alkali metal ions from their aqueous solutions and the sorbent can be re-used for multiple rounds of extraction. This first demonstration of the use of organogels for hybridizing non-covalent polymers of crown ethers into a covalent polymer matrix for making robust solid sorbents for metal ion extraction might find practical use in de-ionizations. This strategy of making porous polymeric metalsequestering matrix using organogelators has the potential of further extension to make sorbents for specific metal ions.

ASSOCIATED CONTENT Experimental procedures for the preparation of matrix and alkali metal ions extractions are given in the SI. Characterization of matrices by 1H NMR spectroscopy, SEM and PXRD are given in SI. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * [email protected]

ACKNOWLEDGMENT KMS acknowledges Department of Science and Technology, India for SwarnaJayanti Fellowship. Mr. Rajesh Ghosh and Mr. Chiranjeevi are acknowledged for their help in BET analysis and ICP-AES analysis respectively.

REFERENCES

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(49) Buchanan, G. W.; Mathias, S.; Lear, Y.; Bensimon, C. Dibenzo-15-crown-5 ether and its Sodium Thiocyanate Complex. X-Ray Crystallographic and NMR Studies in the Solid Phase and in Solution. Can. J. Chem. 1991, 69, 404-414. (50) de Lima, G. M.; Wardell, J. L.; Harrison, W. T. A. Dibenzo18-Crown-6. ActaCryst. 2008, E64, o2001. (51) Wiedemann, D.; Kohl, J. Invariom-Model Refinement and Hirshfeld Surface Analysis of Well-Ordered Solvent-Free Dibenzo21-Crown-7. ActaCryst. 2017, C73, 654-659. (52) Hanson, I. R.; Hughes, D. L.; Truter, M. R.Crystal and Molecular Structure of 6,7,9,10,12,13,20,21,23,24,26,27dodecahydrodibenzo[b,n][1,4,7,10,13,16,19,22]octaoxacyclotetracosin (dibenzo-24crown-8). J. Chem. Soc. Perkin Trans. 2, 1976, 0, 972-976. (53) Bush, M. A.; Truter, M. R. Crystal Structures of Complexes Between Alkali-Metal Salts and Cyclic Polyethers. Part IV. The Crystal Structures of Dibenzo-30-Crown-10 (2,3:17,18-dibenzo1,4,7,10,13,16,19,22,25,28-decaoxacyclotriaconta-2,17-diene) and of its Complex with Potassium Iodide. J. Chem. Soc. Perkin Trans. 2, 1972, 0, 345-350. (54) Talanova, G. G.; Elkarim, N. S. A.; Talanov, V. S.; Hanes, Jr., R. E.; Hwang, H. –S.; Bartsch, R. A.; Rogers, R. D.The “Picrate Effect” on Extraction Selectivities of Aromatic Group-Containing Crown Ethers for Alkali Metal Cations. J. Am. Chem. Soc. 1999, 121, 11281-11290.

Though simple crown ethers bind metal ions selectively and efficiently, they cannot be used for deionization of water due to their water-solubility. Using organogels, we developed porous hybrid matrices containing non-covalent polymer of crown ethers hybridized in covalent polymer, polystyrene. These matrices are superior absorbents for metal ions and have the potentials to be used for deionization of water.

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