Surface Charge-Induced Efficient Recovery of Ionic Liquids from

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Surface Charge-Induced Efficient Recovery of Ionic Liquids from Aqueous Phase Li Li, Li Chang, Xiqi Zhang, Hongliang Liu, and Lei Jiang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b09488 • Publication Date (Web): 09 Aug 2017 Downloaded from http://pubs.acs.org on August 10, 2017

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ACS Applied Materials & Interfaces

Surface Charge-Induced Efficient Recovery of Ionic Liquids from Aqueous Phase Li Li,†, ‡ Li Chang,§ Xiqi Zhang,† Hongliang Liu,*,† and Lei Jiang† †

CAS Key Laboratory of Bio-Inspired Materials and Interfacial Science, CAS Center for

Excellence in Nanoscience, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China. ‡

College of Materials Science & Engineering, Beijing Institute of Petrochemical Technology,

Beijing 102617, P. R. China. §

State Key Laboratory of Applied Organic Chemistry, Key Laboratory of Nonferrous Metal

Chemistry and Resources Utilization of Gansu Province and Department of Chemistry, Lanzhou University, Lanzhou 730000, P. R. China. E-mail: [email protected] KEYWORDS: ionic liquid, recovery, surface charge, wettability, polymer brush

1 Environment ACS Paragon Plus


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ABSTRACT: Ionic liquids (ILs), which consist of pure cations and anions, are widely used in diverse applications and regarded as one of the best choices of “green solvents”. However, the lack of effective green methods for recovering ILs and the safety issues caused by entering environment severely hinder the application of ILs in green chemistry. Here, we show that rationally tuning surface charge of a poly(tert-butyl acrylate) (PtBA)-coated porous mesh can selectively let ILs pass through, and thus provides an efficient and convenient strategy to recover ILs. Surface charge of the porous mesh can be precisely controlled by regulating hydrolysis degree of the chemically grafted PtBA coating. PtBA-coated porous mesh with proper surface charge can be tuned to be IL-philic for a specific IL but hydrophobic, and thus can be applied to recover various kinds of ILs from aqueous phase. This study offers a new platform for development of functional membranes for efficient recovery of ILs.

INTRODUCTION Room-temperature ionic liquids (ILs) are promising candidates to substitute traditional volatile organic solvents in various fields of chemical research, such as synthetic chemistry,1-7 electrochemistry,8-12 and material chemistry.13 For example, hydrophobic ILs like 1-alkyl-3methyl-imidazolium hexafluorophosphate (CnMImPF6) have been successfully used to extract diverse organic compounds and heavy metal ions from aqueous phase.14,15 ILs have been regarded as “green solvents” by virtue of their unique physicochemical properties,11,12,16-19 such as negligible vapor pressure and excellent thermal stability.3,16,20 However, these special properties are a double-edged sword. Extremely low vapor pressure makes it impossible to recover ILs using traditional methods like vacuum distillation. Good thermal stability makes the degradation of ILs quite difficult, and more and more researches have demonstrated that ILs


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have the risk of toxicity.21-23 Thus, once ILs enter the environment, it would bring about serious environmental issues. Decanting is a commonly used simple way to recover hydrophobic ILs from aqueous phase. However, decanting suffers from low recovery efficiency, discontinuous and uncontrollable recovery process. To address these issues, our group has recently developed a membrane-based strategy for efficient recovery of ILs from aqueous phase, by tailoring surface energy of the porous membrane to be hydrophobic but superILphilic.24 However, due to the large number of ILs, it is a big challenge to design a specific functional membrane to meet the criteria for efficient recovery of different ILs from the aqueous phase, especially for those ILs with surface tensions close to water. The structural feature of ILs composed of pure cations and anions gives us a new inspiration that we can precisely control surface charge to construct functional membranes for efficient separation of ILs and immiscible water. Surface charges interacting with ions in ILs, will influence IL wettability through strong electrostatic interaction, which is quite different from wettability by molecular solvents, such as water25-28 and organic solvents29,30 through weak polarization. Therefore, precisely controlling surface charge would be an effective strategy to fabricate functional membrane with distinct wettability by ILs and water for efficient recovery of ILs from aqueous phase. Herein, we achieve precise regulation of surface charge by acidic hydrolysis of chemically grafted poly(tert-butyl acrylate) (PtBA) brushes for efficient recovery of different hydrophobic ILs

from

water

phase.

Taking

1-butyl-3-methyl-imidazolium

hexafluorophosphate

(C4MImPF6)/water system as an example, before hydrolysis, the PtBA coated rough surface, dominated by low-energy terminal methyl (–CH3) of tert-butyl group,31 will repel both water and C4MImPF6 molecules, exhibits hydrophobic and IL-phobic property (Figure 1a). By partial hydrolysis of PtBA with about 63 mol% tert-butyl groups converting to carboxylate ions,32-38 a



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Figure 1. Schematic illustration of surface charge-induced tunable IL wettability on a polymer brushgrafted surface and successful recovery of C4MImPF6 from aqueous phase. (a) A neutral PtBA-coated rough surface exhibits both hydrophobic and IL-phobic. (b) When PtBA is partially hydrolyzed to P(tBAco-AA), the surface becomes IL-philic owing to strong Coulombic attraction generated between the cations of ILs and negative charges on the surface. However, negative charges on the P(tBA-co-AA)coated surface can only polarize water molecules through weak polarization, which is not strong enough to make water CA decrease obviously and the surface is still hydrophobic. (c) Demonstration device for surface charge-induced recovery of C4MImPF6 from aqueous phase. PtBA brush grafted-stainless steel mesh is hydrophobic and IL-phobic, and the mixture of H2O and C4MImPF6 cannot pass through (left). After partially hydrolysis, the mesh becomes IL-philic but hydrophobic, and C4MImPF6 can selectively permeate through the mesh. For better view, C4MImPF6 was dyed by red to distinguish from water clearly.


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negatively charged poly(tert-butyl acrylate-co-acrylic acid) P(tBA-co-AA) coated surface is obtained (Figure 1b). In this case, strong Coulombic attraction generated between the imidazolium cations and negative carboxylate ions facilitates the spreading of C4MImPF6 droplet, achieving an IL-philic surface (right in Figure 1b). However, partially hydrolyzed PtBA can only polarize water molecules through weak polarization,22,39 which is not strong enough to let water droplet spread, still keeping hydrophobic (left in Figure 1b). Based on this design principle, we have assembled a simple device to demonstrate surface charge-induced efficient recovery of C4MimPF6 from aqueous phase (Figure 1c). For a PtBA-grafted stainless steel mesh (Figure S1), it exhibits both hydrophobic and IL-phobic properties owing to PtBA-dominated low surface energy, and the mixture is blocked (left in Figure 1c). After partially hydrolyzing PtBA to P(tBA-co-AA), the stainless steel mesh becomes IL-philic but hydrophobic, and thus C4MimPF6 can selectively pass through the membrane with purity higher than 98% (right in Figure 1c). Considering the solubility of water in C4MimPF6 is about 2%, the recovery efficiency is quite high. Therefore, we have realized efficient recovery of ILs from water phase by tailoring surface charge of the functional membrane.

RESULTS AND DISCUSSION We prepared originally neutral surfaces with PtBA brushes by Surface Initiated Atom-Transfer Radical Polymerization (SIATRP) (Figure S2). The existence of PtBA coating on the surface was indicated by four peak components with binding energies at about 284.6 eV, 285.3 eV, 286.4 eV and 288.7 eV, attributing to CxHy, C-C=O, C-O-C=O, and C-O-C=O species, respectively, of the PtBA units, labelled as A, B, C and D (left in Figure 2a).40 After 90 minhydrolysis under acidic condition, about 63 mol% PtBA converts to PAA, confirmed by a new



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peak at about 288.3 eV ascribing to O=C-O– (right in Figure 2a, mark E). The decreased peak intensity at about 286.4 eV ascribing to C-O-C=O (mark C) and the increased peak intensity at about 288.3 eV ascribing to O=C-O– (mark E) (Figure S3) also proves the hydrolysis process.

Figure 2. Precisely controlling surface charge by regulating hydrolysis degree of the chemically grafted PtBA coating, and the influence of surface charge on water CA and C4MImPF6 CA. (a) XPS results of carbon moieties labeling of PtBA coating before and after 90 min-hydrolysis indicate the appearance of negative charge (–COO–, mark E) on the surface. (b) Typical two-


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dimensional (2D) projections of KFM maps for P(tBA-co-AA) brush-grafted silicon wafer at different hydrolysis time (up) and average surface potential value summarized from corresponding image statistically (bottom). Each KFM image has a scan area of 500 nm by 500 nm. (c) Surface potential and molar content of –COO– increases with increasing hydrolysis time (d) On a PtBA-coated microstructured silicon wafer, C4MImPF6 CA decreases, whereas water CA keeps almost constant as the absolute surface potential increases. We then employed Kelvin force microscopy (KFM) to monitor variation of surface charge during hydrolysis of PtBA. Figure 2b shows typical KFM images for PtBA brush grafted flat silicon substrate before and after hydrolysis, with all data recoded using the same platinumdeposited tip. It can be seen that surface potential becomes more negative on the silicon wafer as hydrolysis time increases. The potential values vary from about 0 V before hydrolysis to about – 7 V after hydrolysis for 120 min. Figure 2c presents the relationship of surface potential and molar content of –COO– with hydrolysis time. The molar content of –COO– is calculated from the degree of hydrolysis by the relative decrease of C-O-C=O peak area divided by the total area of the C1s. After 30 min-hydrolysis, the molar content of –COO– increases to ~35%, and further increases to ~73 % after hydrolysis of 120 min. Accordingly, surface potential is about -1.3 V at 30 min and about -7 V at 120 min. As mentioned before, the successful recovery of hydrophobic ILs from aqueous phase is based on surface charge-induced distinct wettability by ILs and water. So the influence of surface potential on IL and water wettability was investigated in detail (Figure 2d, Figure S4). The underlying surface possesses featured microstructures with 5 µm width and 5 µm spacing of silicon microposts, respectively (Figure S4). As shown in Figure 2d, CA of H2O and C4MImPF6 is ~143.5° and ~119.8°, with surface potential about 0 V, due to the low surface energy of –CH3



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terminated surface. As the surface potential increases, CA of C4MImPF6 decreases gradually, and when surface potential is more than –5 V, it sharply falls to less than ~29.7°. However, water CA is still ~140.8° when surface potential is around -7 V, nearly keeps constant within 150 minhydrolysis (Figure 2d, Figure S4), which is consistent with the results in the literature.35 Therefore, we can achieve simultaneously IL-philic but hydrophobic by precisely controlling surface charge. For the neutral PtBA-coated surface, it is IL-phobic owing to the methyl groups-determined low surface energy,31 and IL CA follows equation 1 (left in Figure 3a):

cosθ = r ⋅

γ SV − γ SL γ LV

(1)

where γ LV , γ SV , and γSL are surface tensions of liquid/gas, solid/gas, and solid/liquid involved in this system. r is surface roughness, defined as the ratio of actual area of a rough surface to the geometric projected area. On the negatively charged P(tBA-co-AA) brush-grafted surface (middle in Figure 3a), Coulombic attraction between IL cations and –COO– groups on the surface generates wetting tension ( Wel ), which is horizontal component of electrostatic force acting on the liquid-gas interface per unit depth due to electrical effect.41-43 The wetting tension would enforce the liquid droplet spread (see Figure S5 and Supporting Information section 6), leading to decreased IL CA. The apparent IL CA (θ) can be expressed as:41-43

cosθ = r ⋅

γ SV − γ SL+Wel γ LV

(2)


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Upon further increasing the negative charges on the surface, larger wetting tension is generated by stronger Coulombic interaction, and the surface becomes more IL-philic with smaller IL CA (right in Figure 3a).

Figure 3. Evaluation of the interaction between C4MImPF6 and PtBA with different hydrolysis degree by using PtBA-modified QCM sensors. (a) With increasing surface charge, Coulombic attraction between the surface and the IL becomes stronger, generating larger wetting tension and decreased C4MImPF6 CA. (b) Typical curves of frequency changes following injection of C4MImPF6 into the QCM chamber with PtBA-modified QCM sensor before and after hydrolysis. (c) Areal mass of C4MImPF6 absorbed on asmodified QCM sensor increases from about 0.2 µg cm–2 before hydrolysis to 2.3 µg cm-2 after hydrolysis



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for 120 min, which indicates strong Coulombic interaction between negative charges of P(tBA-co-AA) chains and C4MIm cations.

We further used quartz crystal microbalance (QCM)44-46 to investigate molecular interaction between PtBA and C4MImPF6 before and after hydrolysis (Figure 3b,c). QCM sensors coated with SiO2 were modified with PtBA brushes by SIATRP. Figure 3b and Figure 3c depict the typical sensorgrams and the statistical binding results for C4MImPF6 conjugated to the PtBAmodified QCM sensors respectively. After injection of C4MImPF6, the frequency changes for PtBA-modified sensor are much more obvious with increasing hydrolysis time (Figure 3b). Accordingly, the areal mass for C4MImPF6 absorbed on PtBA-modified QCM sensors calculated by Sauerbrey equation increases from about 0.27 µg cm–2 before hydrolysis to about 1.3 µg cm–2 after 60 min-hydrolysis, and further rises to ~2.5 µg cm–2 at 120 min hydrolysis time (Figure 3c), which indicates that Coulombic interaction between C4MIm cations and –COO– becomes stronger with increasing amount of –COO– after hydrolysis (Figure 3a). The initial thickness of the PtBA layer is about 5.6 nm according to QCM and ellipsometry measurements (see Figure S6 and Supporting Information section 3), and the area per PtBA chain occupied is ~3.44 nm2/chain (see Supporting Information section 3), which is big enough to allow C4MImPF6 (inset of Figure 3c)47,48 to be inserted between the P(tBA-co-AA) chains. Moreover, during the hydrolysis process, the mean roughness of polymer brush-coated silicon surface is only slightly changed (Figure S7), and thus the contribution of surface roughness to wettability changes can be neglected, and Coulombic interaction between negative charges and C4MIm cations is the main reason for the decreased C4MImPF6 CAs with the increase of hydrolysis time. Importantly, the surface charge-induced tunable wettability can be extended to recover various ILs from the aqueous phase by precisely controlling hydrolysis degree of the PtBA coating.


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Water CAs and various ILs CAs on PtBA-modified stainless steel mesh at different hydrolysis time were summarized in Figure 4a. The transition time from IL-phobic to IL-philic is varied for

Figure 4. Our designed PtBA-coated porous mesh can be applied to recover different ILs from aqueous phase by precisely controlling surface charge. (a) Water CA and IL CA on the PtBA coated mesh at different hydrolysis time. (b) According to the distinct wettability by ILs and water on the PtBA coated mesh, a map including a separable region and a non-separable region for recovery of ILs from water. (c) Demonstration of recovery of three kinds of ILs (C8MImPF6, C6MImPF6, C4MImPF6) from water by PtBA coated stainless steel mesh after certain hydrolysis time. To enhance contrast, the IL phase was dyed by blue color for C8MImPF6, yellow for C6MImPF6, and red for C4MImPF6, respectively.



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different ILs is related to their surface tensions (Table S1). For ILs with higher surface tensions, including C4MImPF6, C5MImPF6, C6MImPF6, C4MImClO4, surface wettability transforms from IL-phobic to IL-philic at certain hydrolysis time (Figure S8, Figure S9). For ILs with lower surface tensions, such as C7MImPF6, C8MImPF6, C4MImNTf2, the surface is IL-philic even for neutral PtBA brush-grafted surface without hydrolysis. According to the distinct wettability by different ILs and water on the porous stainless steel mesh, we draw a map including a separable region and a non-separable region for ILs/water mixtures (Figure 4b). We select three ILs (C4MImPF6, C6MImPF6, C8MImPF6) in different regions to prove the effectiveness of the map (Figure 4c). Before hydrolysis, the PtBA brush-modified stainless steel mesh with surface potential about 0 V is hydrophobic but IL-philic by C8MImPF6 (surface tension of about 33.95 mN m-1). So the mesh can effectively recover C8MImPF6 from the aqueous phase (left in Figure 4c and Figure S10a). After 30 min-hydrolysis, when the mesh has surface potential of about –1.3 V, C6MImPF6, with higher surface tension (38.25 mN m–1) can be recovered from water (middle in Figure 4c and Figure S10b). C4MImPF6, with higher surface tension (47.5 mN m–1) can be recovered from water after further hydrolysis for 90 min when surface potential of the mesh is around –6.4 V (right in Figure 4c and Figure S10c). The purity of all the studied ILs recovered from the aqueous phase is higher than 98%. It should be noted that the influence of the hydrolysis degree of PtBA brushes on recovery efficiency should also be considered. For the same ionic liquid, as long as the hydrolysis of the PtBA brushes can make the porous mesh be ILphilic but hydrophobic, the hydrolysis of the polymer brushes has little influence on the recovery efficiency (Figure S11a). However, the recovery efficiencies of the porous mesh with the same hydrolysis degree of PtBA brushes show slight difference for different ionic liquids (Figure S11b), which may attribute to different water contents on saturation in different ionic


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liquids.49 Anyhow, we can achieve on-demand recovery of different ILs on one porous mesh by simply controlling surface charge. CONCLUSION In summary, we have demonstrated surface charge-induced successful and effective recovery of different ILs from water. Such successful recovery relies on tunable IL wettability by precise control of surface charge on a P(tBA-co-AA) brush-grafted mesh. XPS and KFM characterizations prove the generation and increasing negative charges on the surface with the increase of hydrolysis time. QCM measurements further confirm stronger interactions between IL cations and more negative charges on the surface. This study provides a new opportunity to rationally design functional interfacial materials for purification and recycle of ILs. EXPERIMENTAL SECTION Synthesis of PtBA Brush-based Surface by Surface-Initiated Atom-Transfer Radical Polymerization (SIATRP): In the experiment, monomer tBA (15 mL, 102 mmol) was mixed with solvent DMSO (15 mL) at first, and then PMDETA (0.12 mL, 0.55 mmol) was added into the mixture via syringe. The initiator coated silicon wafer samples (1 cm × 1.5 cm each; preprepared by a typical method50,51) were immersed into the polymerization solution, ensuring that the solution was adequate to submerge each substrate completely, and EBI (15 µL) was added in as well. Thereafter CuBr2 (4.1 mg, 0.018 mmol) was dissolved in the mixed solution, which was degassed by passing a continuous stream of nitrogen gas through the solution at 21 oC for 30 min. Then, CuBr (26.5 mg, 0.18 mmol) was added under nitrogen. After polymerization time of 2 h at 21 oC , the samples were removed and washed with acetone, CH2Cl2 and methanol successively, and finally dried with a flow of nitrogen.



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Fabrication of Surfaces with Negatively Charges: Surfaces with negative charges were obtained by hydrolysis of PtBA brush to P(tBA-co-AA) brush on silicon wafer and stainless steel mesh under acidic condition. The modified substrates were immersed in CH2Cl2 solutions with 1.3 M trifluoroacetic acid (1/10, v/v) in closed beakers for different time at room temperature. After reaction, the samples were thoroughly rinsed three times using deionized water and dried under a nitrogen flow. Contact Angle Measurements: CAs of ILs were measured on an OCA20 machine (DataPhysics, Germany) at ambient temperature. An IL droplet (about 2 L) was dropped carefully onto the modified substrate. CA was adopted as the average value of three to five measurements at different positions of one sample. The sample was washed with deionized water several times before it was exposed to a new aqueous droplet. X-ray photoelectron spectroscopy (XPS): XPS characterization was performed on a Thermo Scientific ESCALab 250Xi X-ray photoelectron spectrometer using 200 W monochromatic Al Kα radiation as the excitation source at a base pressure of about 3×10–10 mbar. The binding energies of XPS data were referenced to C1s line at 284.8 eV from the adventitious carbon. Kelvin force microscopy (KFM): KFM was conducted on LEXT OLS4500 (Olympus, Japan) in ambient environment. KFM tip is a platinum-deposited cantilever, with a platinum layer deposited on the basic silicon probe with the titanium interfacial layer, and the force constant is 1.4–3.3 N/m. Before surface potential measurements, KFM standard sample (stainless steel sample) was used as reference for the calibration of potential. Quartz Crystal Microbalance (QCM) measurements: QCM measurements were performed at 25 oC using Q-Sense E1 system (Sweden). For the binding assays between P(tBA-co-AA) and


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C4MImPF6, the QCM sensor was washed by ethanol, and C4MImPF6 was mixed with ethanol (20 wt%) by stirring. Then C4MImPF6 was injected into the channel at a flow rate of 100 µL min−1. The frequency change was measurable within ± 5 Hz. All the frequency curves were recorded by QSense software and processed by QTools. ASSOCIATED CONTENT Supporting Information. Materials, instruments and characterizations; SEM micrograph of stainless steel mesh; preparation and GPC, XPS, AFM results of PtBA brush-grafted smooth silicon surface; a general principal for IL contact angle on a neutral and a negatively charged surface; changes of wettability by water with the increase of hydrolysis time; wettability changes by ILs with different cations or anions controlled by surface charge; recovery of three selected ILs from water; surface tensions of water and different ILs. (PDF) AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Research Fund for Fundamental Key Projects (2013CB933000 and 2012CB933800), National Natural Science Foundation (21404109, 21421061, 21434009, and 21504098), the Key Research Program of the Chinese Academy of



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Sciences (KJZD-EW-M03), the 111 project (B14009) and Youth Innovation Promotion Association, CAS (2016026). ABBREVIATIONS ILs, ionic liquids; CA, contact angle; PtBA, poly(tert-butyl acrylate); P(tBA-co-AA), poly(tertbutyl acrylate-co-acid acrylate); SIATRP, Surface initiated atom-transfer radical polymerization. REFERENCES (1) Chakraborti, A. K.; Roy, S. R. On Catalysis by Ionic Liquids. J. Am. Chem. Soc. 2009, 131, 6902–6903. (2) Wang L.; Wang H.; Liu F.; Zheng A.; Zhang J.; Sun Q.; Lewis J. P.; Zhu L.; Meng X.; Xiao F. S. Selective Catalytic Production of 5‐Hydroxymethylfurfural from Glucose by Adjusting Catalyst Wettability. Chemsuschem 2014, 7, 402–406. (3) Wasserscheid P.; Keim W. Ionic Liquids—New “Solutions” for Transition Metal Catalysis. Angew. Chem. Int. Edit. 2000, 39, 3772–3789. (4) Dupont J.; de Souza R. F.; Suarez P. A. Z. Ionic Liquid (Molten Salt) Phase Organometallic Catalysis. Chem. Rev. 2002, 102, 3667–3692. (5) Dewilde S.; Dehaen W.; Binnemans K. Ionic Liquids as Solvents for PPTA Oligomers. Green Chem. 2016, 18,1639–1652. (6) Cooper E. R.; Andrews C. D.; Wheatley P. S.; Webb P. B.; Wormald P.; Morris R. E. Ionic Liquids and Eutectic Mixtures as Solvent and Template in Synthesis of Zeolite Analogues. Nature 2004, 430, 1012–1016. (7) Zhou Y.; Antonietti M. Synthesis of Very Small TiO2 Nanocrystals in A Room-Temperature Ionic Liquid and Their Self-Assembly toward Mesoporous Spherical Aggregates. J. Am. Chem. Soc. 2003, 125, 14960–14961.


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Surface Charge-Induced Efficient Recovery of Ionic Liquids from

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