Pickering Emulsion-Templated Encapsulation of Ionic Liquids for

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Surfaces, Interfaces, and Applications

Pickering Emulsion-Templated Encapsulation of Ionic Liquids for Contaminant Removal Qinmo Luo, Yifei Wang, Zehao Chen, Peiran Wei, Esther Yoo, and Emily B. Pentzer ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b21881 • Publication Date (Web): 11 Feb 2019 Downloaded from http://pubs.acs.org on February 11, 2019

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ACS Applied Materials & Interfaces

Pickering Emulsion-Templated Encapsulation of Ionic Liquids for Contaminant Removal Qinmo Luo, Yifei Wang, Zehao Chen, Peiran Wei, Esther Yoo, and Emily Pentzer* Department of Chemistry, Case Western Reserve University, 10900 Euclid Ave. Cleveland, OH 44106 USA Key words: Ionic liquid, Pickering emulsion, Interfacial polymerization, Capsules, Contaminant removal Abstract Ionic liquids (ILs) have received attention for a diverse range of applications, but their liquid nature can make them difficult to handle and process and their high viscosities can lead to suboptimal performance. As such, encapsulated ILs are attractive for their ease of handling and high surface area, and have potential for improved performance in energy storage, gas uptake, extractions, etc. Herein, we report a facile method to encapsulate a variety of ILs using Pickering emulsions as templates, graphene oxide (GO)-based nanosheets as particle surfactants, and interfacial polymerization for stabilization. The capsules contain up to 80% IL in the core and the capsule shells are composed of polyurea and GO. We illustrate that capsules can be prepared from IL-in-water or IL-in-oil emulsions and explore the impact of monomer and IL identity, thereby accessing different compositions. The spherical, discrete capsules are characterized by optical microscopy, scanning electron microscopy, infrared spectroscopy, Raman spectroscopy, thermogravimetric analysis, and 1H NMR spectroscopy. We illustrate the application of these IL capsules as a column material to remove phenol from oil, demonstrating ≥98% phenol removal after passage of >170 column volumes. This simple method to prepare capsules of IL will find widespread use across diverse applications.

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Introduction Ionic liquids (ILs) are organic salts with melting points lower than 100 °C that have distinct fluidity, conductivity, thermal and electrochemical stability, and negligible vapor pressure.1–3 As such, ILs have garnered much research interest for applications in energy storage,3–5 separations,6 and gas handling,7–9 among others. For example, ILs were widely reported to be used in contaminant removal from water or oil. De Gaetano et al. demonstrated the removal of pesticides from waste water with novel bio-sourced ILs;10 likewise, Platzer et al. prepared thioglycolate-based ILs for metal extraction11 and Prashant et al. reviewed the use of ILs for desulfurization of diesel fuel.12 Imidazolium- or pyrrolidinium-based ILs can transfer phenol and its analogs to an IL phase from other media (i.e., extraction) due to favorable van der Waals forces, electrostatic interactions, hydrogen-bonding, hydrophobic interactions, and π−π stacking.13–17 Unfortunately, extraction of contaminants by simple addition of IL to oil or water solutions is complicated by miscibility of the two phases or emulsion formation; thus, separation and recovery the purified water or oil is limited. Encapsulation with polymer or composite shells is an attractive route to compartmentalizing fluids,18–20 and thus one approach to facilitating the use of ILs is their encapsulation, essentially converting a liquid-liquid separation to a liquid-solid one. Encapsulation has the added benefit of overcoming the inherent viscosity of an IL and increasing its surface area, as well as allowing the IL to be handled as a solid powder. The shell used for encapsulation must be permeable to the contaminant to be removed and its chemical composition determines whether the shell is inert or active in the extraction process. In 2017 Elizarova et al. used a layer-by-layer method to encapsulate [P14,6,6,6][TFSI] in nano-emulsion droplets and used these structures for the removal of heavy metals from aqueous solutions.21 Other routes to encapsulate ILs include impregnation of hollow capsules,22–25 and extruding IL with a solidifying reagent into a solution to induce solid formation at the fluid-fluid interface.26,27 ILs can also be encapsulated by using a simple emulsion as a template. IL-in-water and IL-in-oil emulsions have be prepared using small molecule, polymer, or particle surfactants.28–31 Whereas these systems have been applied in chemical sensing, lubrication, and drug delivery,32–34 they can also be used to prepare capsules of IL by incorporating reactive species in one or both phases. Composite capsules prepared by this approach have been used as micro-reactors,35 as active materials in supercapacitors,5 and for the extraction of contaminants.21,36 For example, Weiss et al. used [Bmim][PF6]-in-water emulsions and interfacial polymerization (i.e., a polyurea shell) to access capsules used for compartmentalized hydrosilylation and Michael addition reactions.35 Luo et al. used a similar approach to encapsulate [Bmim][PF6] in a shell of polyurea and graphene oxide (GO) nanosheets, then used these structures in electrochemical double layer capacitors, providing both the active electrode material (reduced GO) and electrolyte (IL).5 Common approaches to encapsulate ILs with molecular surfactants typically require highly specialized surfactant composition (e.g., block copolymers)37 and ~20 wt.% of surfactant relative to IL. As such, the resulting capsules contain only ~20-30 wt.% IL.21,35,37 To make full use of the benefits of encapsulated ILs as discussed above, a facile and robust synthetic method is required; an ideal system would be amenable to a variety of ILs and shell compositions, and utilize readily accessible surfactant materials at low loadings. Herein, we report the preparation of various IL capsules with a shell of GO and polyurea and use of these hybrid capsules as active column material for the extraction of phenol from oil. This work builds upon recent reports from our group demonstrating GO can serve as a particle surfactant for different IL-containing emulsions at loadings as low as 1 wt% relative to the dispersed phase,38–41 with the nanosheet polarity dictating the continuous and discontinuous phases.28 As prepared GO and alkylated GO (C18-GO) were used as particle surfactants to


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stabilize IL-in-water and IL-in-oil emulsions, respectively, and interfacial polymerization between multifunctional amines and isocyanates stabilized the structures through formation of polyurea shells (Scheme 1). We illustrate the encapsulation of seven different ILs using eleven different polymers. The ILs used have different miscibility with water, melting points above or below room temperature, and are composed of different cations or anions. Most capsules prepared are composed of ~80 wt% IL and had a core-shell structure. We then demonstrate the removal of phenol from hexanes using columns packed with only the IL capsules, achieving >98% removal over 170 column volumes. As these structures improve the performance and facilitate handling of ILs, this facile and scalable approach is likely to find use in various applications.

Scheme 1. Illustration of the work presented herein: GO and C18-GO are used as particle surfactants to stabilize IL-in-water and IL-in-oil emulsions, respectively, and subsequent interfacial polymerization yields capsules of IL with a composite shell. The capsules are then used as a column material for the extraction of phenol from hexanes. The chemical structures of the ILs used in this study are shown on the right. Experimental Section Materials. Graphite flakes, sulfuric acid (H2SO4), hydrogen peroxide (H2O2), toluene, and N,Ndimethylformamide (DMF), 1-ethyl-3-methylimidazolium dimethyl phosphate ([Emim][DMP], 98%), 1-Butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF4], 98%), tolylene-2,4diisocyanate (TDI, 95%), tris(2-aminoethyl)amine (Tren, 96%), and mesitylene (98%), were purchased from Sigma Aldrich. Potassium permanganate (KMnO4), n-octane (98%) and 1octadecylamine (97%) were purchased from Alfa Aesar. 1,6-Diisocyanatohexane (HDI, 99%), 1,6-hexanediamine (HMDA, 99.5%), glycerol (99+%), and diethylenetriamine (DETA, 98+%) were purchased from Acros Organics. Poly(ethyleneimine)(PEI) solution (50% w/v in H2O) was purchased from Fluka Analytics. Ethylenediamine (EDA) was purchased from BASF. Sodium carbonate (Na2CO3) was purchased from Fisher. 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI], 99%), 1-Butyl-1-methylpyrrolidinium hexafluorophosphate ([Bmim][PF6], 99%), 1-Hexyl-3-methylpyrrolidinium hexafluorophosphate ([Hmim][TFSI], 99%), 1-Methyl-1-propylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([Mpp][TFSI], 99%), and 1-Butyl-2,3-dimethylimidazolium hexaflourophosphate ([Bm2im][PF6], 99%) were purchased from IoLiTec.


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Instrumentation and sample preparation. Centrifugation was performed with either an Eppendorf 5804 centrifuge (5000 rpm) or a SORVALL Evolution RC centrifuge (15000 RCF). Xray photoelectron spectroscopy (XPS) was performed using a PHI Versaprobe 5000 scanning X-ray photoelectron spectrometer, sample solid was immobilized on the holder with double sided tape. Scanning electron microscopy was accomplished with Nova NanoLab 200 FEGSEM/FIB with secondary electrons scattering mode; samples were prepared by immobilizing capsules on a conductive carbon double sided tape, and excess particles were blown off using a cleaning duster. Cross sections of capsules were prepared by removing part of the sample with a focus ion beam (FIB). Optical images were taken with an AmScope M150C microscope with AmScope MU500-CK 5.0 MP USB microscope camera. Samples for optical microscopy were prepared by dispersing isolated capsules in dodacane, then drop casting them onto a glass slide. The samples after glass slide pressing were observed directly. Particle size analysis was performed with Image J using optical microscopy images; features out of focus, overlapped or fragmented were manually removed. Laser diffraction analysis was performed with a Malvern Mastersizer 2000, capsule samples were dispersed into 1 L water until an appropriate opacity was observed; particles were assumed to be spherical. Fourier Transform infrared (FTIR) spectroscopy was performed using an Agilent Cary 630 FTIR in ATR mode. Atomic force microscopy (AFM) was performed on a NX-10 Park System in tapping mode; samples were prepared by drop casting solution onto a mica substrate. Ultrasonication was completed with a Branson M3800 bath sonicator. Vortex mixing was accomplished with a Fisher Model 9454FIFSUS vortex mixer. The hand-held emulsifier used was from BioSpec Products, Model 985370 and highest energy level was used. Raman spectra were collected using Xplora (HORIBA Instruments Inc). TGA was performed using a TA Instruments Q500-TGA, heating from 100-600 °C at 10 °C min-1 under nitrogen. 1H NMR spectra were collected on a Bruker Ascend III HD 500 MHz NMR. To perform the measurement, ~20 mg of IL capsules were mixed with acetone-D6 containing mesitylene (0.047 mM) as the internal standard. The wt% IL of capsules was calculated by comparing the 1H NMR integral of IL and mesitylene signals.5 BET measurement was accomplished with Micromeritics Instrument Corporation, TriStar II Serial # 692. Synthesis of GO. Graphene oxide (GO) was synthesized from graphite flakes following a reported method.42 Briefly, graphite flakes (1.0 g) was magnetically stirred in concentrated H2SO4 (134 mL) at room temperature. Then KMnO4 (1.0 g, 0.063 mol) was slowly added to the suspension. The mixture appeared dark green and was stirred at 25 °C for 24 h; the addition of KMnO4 was repeated three more times every 24 h, until a total of 4 g of KMnO4 was added. At the end of reaction, the mixture was pink and had become more viscous. The mixture was transferred to ice–water (0.70 L), followed by the slow addition of an aqueous H2O2 solution (30%), until the pink color changed to bright yellow, indicating quenching of excess KMnO4. Finally, centrifugation led to the isolation of a yellow brown solid, which was washed repeatedly with 2-propanol until the supernatant had a neutral pH, and it was then dried under reduced pressure at room temperature. The dry solid was blended into a powder. To make GO solutions in water, a suspension of GO was vortexed and sonicated at a concentration of 2.0 mg/mL. Preparation of Octadecylamine-Functionalized Graphene Oxide (C18-GO). C18-GO was prepared following a reported method.28 GO (100 mg) was dispersed in DMF (50 mL), and octadecylamine was dissolved (1.0 g) in DMF (50 mL), both at 50 °C until fully dispersed/dissolved. The two solutions were mixed together and stirred for 5 min at 50 °C, after which a brown precipitate was isolated by centrifugation, and then dispersed in toluene (50 mL); octadecylamine (2.0 g) was dissolved in toluene (50 mL) at 50 °C until dissolved and then added to the first suspension and stirred overnight at 50 °C. A dark brown solid was isolated by centrifugation, washed with toluene and octane, and dried under reduced pressure at room


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temperature. The products was dispersed in octane (50 mL) to achieve C18-GO dispersion, with a concentration of 2.0 mg/mL standardized by starting material (GO). Preparation of capsules in IL-in-water emulsions (IL-w capsules). The preparation of the ILin-w emulsions was modified from a reported literature.31 The monomers used in the dispersed phase and continuous phase were marked as monomer M1 and monomer M2, respectively. The identity and amount of monomers are listed in Table S1. Unless otherwise specified in Table 1, In a representative procedure, [Bmim][PF6] was encapsulated by interfacial polymerization in a Pickering emulsion. The aqueous phase was prepared by mixing an aqueous suspension of GO (5 mL, 2 mg/mL), and aqueous Na2CO3 (0.050 mL, 1.0 M). The IL phase was prepared by dissolving monomer M1 (1.30 mol) in [Bmim][PF6] (1.00 mL), then mixed with the aqueous phase in a vial and agitated by three cycles of emulsification (20 s) with a hand-held emulsifier with two breaks (15 s each) in between. Water (0.5 mL) was then added to dilute the formed emulsion. Monomer M2 was dissolved in water (1.33 mol/mL) and the solution (1.25 mL) was added drop wise to the previously formed emulsion and the vial swirled by hand. The mixture was left unagitated for 72 h before addition of water (100 mL), and aqueous ammonium hydroxide (30 wt%, 5 mL), then left unagitated for 5 h to quench unreacted isocyanate functional groups (to prevent intra capsule crosslinking). The solid particles were collected by gravity filtration and washed with water to neutral pH. Capsules were obtained by drying the isolated solid in the vacuum oven, yielding a light brown color powder. The procedure of fabrication IL-w capsules with different ILs was the same as described above, with M1=HDI, and M2=HMDA, and varying IL identities to [Bmim][PF6], [Emim][TFSI], [Hmim][TFSI], [Mpp][TFSI] and [Bm2im][PF6]. Of note, the melting point of [Bm2im][PF6] is 43 °C, therefore the procedure was carried out under 50 °C. Preparation of capsules in IL-in-oil emulsions (IL-o capsules). The monomers in the dispersed phase and continuous phase were marked as monomer M1 and monomer M2 respectively. The identity and amount of addition for monomers are listed in Table S1. Unless otherwise specified, the IL phase was prepared by dissolving monomer M1 (1.30 mmol) in IL (1.00 mL). The IL phase was then mixed with a C18-GO in octane dispersion (5.00 mL, 2.0 mg/mL standardized by parental GO) in a vial and agitated by three cycles of emulsification (20 s) with a hand-held emulsifier with 15 s breaks in between. Octane (0.5 mL) was added to the emulsion for dilution. Then monomer M2 was dissolved in octane (1.33 mol/mL), and the monomer solution (1.25 mL) was added drop wise to the previously formed emulsion solution while swirling the vial by hand. The mixture was left unagitated for 72 h, then a solid isolated by gravity filtration and thoroughly washed with hexane. The capsules were then dispersed in hexane (100 mL) and propylamine (5 mL) was added, then the solution left unagitated for 5 h to quench unreacted isocyanate functionalities. The solid particles were collected by gravity filtration and washed with hexane to neutral pH. Finally, the capsules were dried in the vacuum oven, yielding a light brown color powder. The procedure of fabrication IL-o capsules was the same as described above, with M1=EDA, and M2=HDI, and varying IL identities to [Bmim][PF6], [Emim][TFSI], [Hmim][TFSI], [Mpp][TFSI] [Bm2im][PF6], [Bmim][BF4], and [Emim][DMP]. Of note, the melting point of [Bm2im][PF6] is 43 °C, therefore the procedure was carried out under 50 °C. Phenol removal. 20 mg cotton was packed into the neck of a 5 3/4 ’’ Pasteur pipette with inner diameter of 0.6 cm. Then [Bmim][BF4]-o capsules (0.10 g) were introduced to the pipette and hexanes passed through the column until the powder was compacted, forming a plug of capsules ~1 cm high. Hexanes (~1.5 mL) remained in the column to keep the IL capsules wetted. Then, the phenol standard solution (0.57 mM in hexanes) was added to the column. Pressure was applied manually by a pipette bulb to maintain a flow of 1.5 mL/min. The column eluent was collected in 1.5 mL aliquots, and the phenol concentration for each was determined by UV-Vis absorption spectroscopy, comparing the absorption intensity at 271 nm against a


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standard calibration curve (Figure S8). To generate [Bmim][BF4]-o capsules shell, IL was removed by washing the capsules in acetone (0.02 g of shell material were isolated from 0.1 g of capsules); this IL-free material was used to as the column material, and profile of phenol removal was measured in the same process. The phenol removal profile of [Emim][TFSI]-o capsules was determined with the same method. Of note, though same mass of samples used as [Bmim][BF4]-o capsules, the formed column plug of capsules length was ~0.6 cm. Results and Discussion GO and C18-GO stabilized emulsions. GO and alkylated GO nanosheets were prepared and characterized as previously reported (Figure S1).28,42,43 Briefly, graphite flakes were oxidized using potassium permanganate in concentrated H2SO4, isolated, and characterized by X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and Fourier transform infrared (FTIR) spectroscopy. XPS revealed that the GO nanosheets have a C:O ratio of ~3:2, indicating that they are highly oxidized. AFM revealed that GO nanosheets are microns in diameter and ~1 nm in height, in agreement with single layer nanosheets.44 The FTIR spectrum of GO is also consistent with previous reports, with characteristic absorption peaks at ∼1720 cm−1 and ∼1620 cm−1, indicating the presence of C=O and C=C functionalities, respectively.45 To access C18-GO, as prepared GO was functionalized with octadecylamine;28,43,46 XPS revealed the C:O:N ratio of C18-GO was ∼35:6:1, consistent with the addition of 18 carbon atoms for each nitrogen atom, within the limits of XPS. Compared to GO, the FTIR spectrum of C18-GO has significantly more intense peaks from 2700 to 3000 cm−1, assigned to the C−H bonds from the alkyl chain, supporting successful functionalization. GO nanosheets were dispersed in water and C18-GO nanosheets were dispersed in octane, forming clear or opaque dispersions (both solutions were 2 mg/mL based on concentration of GO). IL capsules fabricated with different monomers. To evaluate the ability to prepare capsules of IL using different monomers for interfacial polymerization, [Bmim][PF6] was used as the model IL, as it is immiscible with both water and oil, and thus capsules could be made using either as the continuous phase. Figure 1A shows the different multifunction amines, alcohols and isocyanates used to generate polyurea upon interfacial polymerization. To prepare the capsules, two methods were used: 1) For IL-in-water systems (IL-w), diisocyante was dissolved in IL and this solution was added to an aqueous suspension of GO, followed by agitation, and then diamine or diol was added to the continuous water phase; or 2) For IL-in-oil systems (IL-o), diamine was dissolved in IL and this solution was added to an octane suspension of C18-GO, followed by agitation and addition of diisocyanate to the continuous oil phase. Upon standing unagitated, the diamine in one phase reacted with the diisocyanate in the second phase, leading to the formation of polymer at the fluid-fluid interface and thus realization of a capsule of IL with a composite shell of polymer/(C18-)GO. Figures 1B and 1C show the optical microscopy images of [Bmim][PF6]-o and [Bmim][PF6]-w capsules successfully formed by this interfacial polymerization approach, with the inset showing the sample after compression which expels the IL from the capsule. The chemical composition of the shell largely impacts the morphology of capsules. [Bmim][PF6]-w capsules synthesized using HMDA x HDI were ~10-25 μm in diameter (Figure 1Bi, see Figure S2A, S2B for analysis of capsule diameter) and those prepared using EDA x HDI were ~30-60 μm in diameter (Figure 1Bii); the latter had residual nanosheet floccus, which may be attributed to suboptimal emulsion formation, as EDA and [Bmim][PF6] are miscible. Figures 1Ci and 1Cii show [Bmim][PF6]-o capsules formed using EDA x HDI and EDA x TDI, respectively, revealing spherical capsules with diameters ranging from ~30-60 µm (see Figure S2A, S2B for analysis of capsule diameter). In contrast, [Bmim][PF6]-o capsules prepared with DETA x HDI have a broader distribution in diameter (Figure 1Ciii, S3), which may be attributed to basicity of the amine or higher volume added (see Table S1). Figure 1Civ shows [Bmim][PF6]-o capsules formed with Tren x TDI,


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revealing less spherical structures and aggregates. We then illustrated that C18-GO and the same monomers could be used to prepare capsules from [Bmim][BF4]-o emulsions. Similar to the [Bmim][PF6] capsules, the EDA x HDI and EDA x TDI systems produced spherical, individual capsules, whereas other monomer combinations led to irregular and aggregated structures (Figure S2C). A number of other monomer combinations were evaluated for the stabilization of [Bmim][PF6]-w, [Bmim][PF6]-o, and [Bmim][BF4]-o capsules, with mixed success (Table S1). Of note, capsules could only be formed with glycerol and diisocyanates in the presence of acid or base catalyst and at elevated temperature (Figure S2D shows optical microscopy images of these capsules).

Figure 1. A) Chemical composition of multifunctional amines and isocyanates used as monomers for interfacial polymerization; B) Optical microscopy images of [Bmim][PF6]-w capsules synthesized with i. HMDA x HDI, ii. EDA x HDI, C) [Bmim][PF6]-o capsules synthesized with combinations as i. EDA x HDI, ii. EDA x TDI, iii. DETA x HDI, iv. Tren x TDI.

All scale bars are 120 μm; insets show optical microscopy images of capsules after compression. Capsules with different ILs. We then illustrated that IL-o and IL-w emulsions can be used to encapsulate ILs bearing different cations and anions: [Emim][TFSI], [Hmim][TFSI], [Mpp][TFSI], [Bm2im][PF6], [Emim][DMP], [Bmim][PF6], and [Bmim][BF4] (the latter two were discussed above). All ILs were able to be encapsulated both in oil-based and water-based emulsions, with the exception of [Bmim][BF4] and [Emim][DMP] which are water-miscible and thus only IL-o systems were used. The interfacial polymerization of HDI x HMDA was used for all IL-w


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capsules, and HDI x EDA for all IL-o capsules, following the procedure outlined above. Of note, [Bm2im][PF6] is a solid at room temperature (mp = 43 °C), and thus these capsules were prepared with emulsions heated to 50 °C. IL-w capsules of [Bmim][PF6], [Emim][TFSI], [Hmim][TFSI], [Mpp][TFSI] and [Bm2im][PF6] were prepared using HDI x HMDA and isolated by simple filtration. Representative SEM and optical microscopy images are shown in Figure 2A-E. SEM images reveal a slightly rough surface of the capsules, with the exception of [Bm2im][PF6]-w capsules which have a rougher, more highly crumbled structure. This difference may be attributed to contraction of the core upon solidification of the IL when cooled to room temperature. Optical microscopy images in Figure 2 show distinct spherical capsules ~10-25 μm in diameter dominate, with little-to-no inter-particle cross-linking. The insets of the optical microscopy images show the capsules after pressing between two glass slides (Figure 2Eii was heated to 50 °C); for all samples, compression led to expulsion of IL. The particle size distribution can be characterized using laser diffraction or Image J analysis of optical microscopy images, with the two techniques giving consistent results for the samples evaluated. The size distribution of all samples can be found in Figure S3; all ILw capsules are 10-35 μm in diameter, except for the [Bmim][BF6]-w capsules which are ~10-25 μm. SEM images of the cross section of these capsules reveal a core-shell structure and shell thickness of 1 to a few microns, except for [Hmim][TFSI]-w capsules, which shown a spongelike core architecture, which may be attributed to diffusion of HMDA into the IL and polymerization occurring inside the droplets (Figure S4). These data support that capsules of different ILs can easily be prepared, templated by an emulsion stabilized by GO-based surfactants. The compositions of the IL-w capsules were characterized by FTIR and Raman spectroscopies, thermogravimetric analysis (TGA), and extraction of the encapsulated IL and quantification by 1H NMR spectroscopy. Taking [Emim][TFSI]-w capsules as an example, the FTIR spectrum of the capsules is consistent with the presence of neat [Emim][TFSI], including distinct features at 1347 cm-1, 1345 cm-1 ,1178 cm-1, and 1131 cm-1 (Figure 2F). The spectrum of the capsules has additional stretching frequencies at 1616 cm-1 and 1579 cm-1, which are attributed to C=O functionalities of the polyurea, while the signal at 3321 cm-1 corresponds to the polyurea N-H stretch. Figure S5 shows the Raman spectra of GO and [Emim][TFSI]-w capsules, both revealing the distinctive D and G bands of the nanosheets (1600 and 1310 cm-1, respectively), and thus supporting the present of GO nanosheets in the capsule shell. The TGA weight loss profiles of [Emim][TFSI]-w capsules and neat [Emim][TFSI] are shown in Figure 2G; both samples lose mass from 360-500 °C, indicative of the decomposition of the IL. [Emim][TFSI]-w capsules lose ~35 wt% from 200-360 °C, suggesting decomposition of the polyurea shell.5 Only the capsules have residual mass above 500 °C (~5 wt%), giving an estimation of IL composition around 60 wt.% (Figure S6). The wt% IL was definitively established by extraction of the IL with an acetone-D6 solution containing an internal standard (mesitylene), and subsequent integration of peaks in the 1H NMR spectra. Using this method, [Emim][TFSI]-w capsules were determined to be 55 ± 2 wt% IL, in line with the TGA data. Other IL-w capsules showed similar characterization data (Figures S5-S7; Table S2).


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Figure 2. IL-w capsules prepared with HDI x HDMA: A) [Bmim][PF6]-w capsules, B) [Emim][TFSI]-w capsules, C) [Hmim][TFSI]-w capsules, D) [Mpp][TFSI]-w capsules, and E) [Bm2im][PF6]-w capsules: (i) SEM images and (ii) optical microscopy images (inset shows after compression all scale bars 120 μm unless otherwise specified); F) FTIR spectra of [Emim][TFSI]-w capsules and neat [Emim][TFSI]; G) TGA weight loss profiles (solid lines) and first derivatives (dotted lines) of [Emim][TFSI]-w capsules and neat [Emim][TFSI]. A series of IL-o capsules were also prepared using C18-GO as surfactant, octane as the continuous phase, HDI x EDA for interfacial polymerization, and [Bmim][PF6], [Emim][TFSI], [Hmim][TFSI], [Mpp][TFSI], [Bm2im][PF6], [Bmim][BF4] or [Emim][DMP] as IL. Capsules were isolated by washing with hexane under gravity filtration and dried under vacuum. Figure 3 shows the SEM and optical microscopy images of the IL-o capsules, similar to those presented in Figure 2. Most capsules ([Bmim][PF6]-o, [Emim][TFSI]-o, [Hmim][TFSI]-o, and [Mpp][TFSI]-o) have spherical shapes and slightly rough surfaces, but [Bm2im][PF6]-o capsules are more highly crumbled, again likely due to contraction of the core upon solidification. Of note, the [Emim][DMP]-o capsules showed charging in the SEM (glow around the capsule), which may be attributed to IL (Figure 3Gi), despite a powdery appearance when handling. Particle size distribution of the capsules was determined by Image J analysis of optical microscopy images, and can be found in Figure S3. For all the samples, the majority of capsules show diameters 1550 μm, except for [Bmim][PF6]-o which were 30-60 μm in diameter. Overall these capsules are


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more uniform in size and less aggregated than the IL-w capsules discussed above, most likely because the IL-o emulsions are stable to basic conditions (i.e., the presence of an amine), but the GO-stabilized emulsions are not.28 The insets in the optical images show the capsules after compression, illustrating expulsion of IL ([Bm2im][PF6]-o capsules were heated to 50 °C). Cross sectional SEM images of the IL-o capsules are shown in Figure S4, again showing a core-shell structure and shell thicknesses of 1 to a few micros; the exception is [Emim][TFSI]-o capsules which had a sponge-like structure throughout, which again, may be attributed to diffusion of HDI into the IL. All IL-o capsules were characterized by FTIR, Raman, TGA, and 1H NMR to verify their chemical compositions, similar to that discussed above for the IL-w capsules. Taking [Emim][TFSI]-o capsules as an example, Figure 3H compares the FTIR spectra of [Emim][TFSI]-o capsules, [Emim][TFSI]-w capsules, and [Emim][TFSI]. The spectrum of [Emim][TFSI]-o capsules coincides with that of [Emim][TFSI]-w, verifying the presence of IL and polyurea shell (of note, the diamine used is different, but the spectroscopic signature of the polyurea is similar). Figure S5 shows the Raman spectra of [Emim][TFSI]-o capsules compared to both C18-GO and GO nanosheets, all presenting the distinctive D and G bands (1600 cm-1 and 1310 cm-1), supporting the presence of nanosheets in the capsule shell. The TGA weight loss profiles of [Emim][TFSI]-o capsules and neat [Emim][TFSI] are shown in Figure 3I; both samples lose mass from 360-500 °C, indicative of IL decomposition. [Emim][TFSI]-o capsules lose ~20 wt% from 200-360 °C, indicative of the decomposition of the polyurea shell. Using extraction and quantification by 1H NMR integration, [Emim][TFSI]-o capsules were determined to be 83 ± 2 wt% IL, in agreement with the estimation from TGA data (~80 wt%). The FTIR, Raman, TGA, and %IL characterization data for all IL-o capsules are consistent with these results (see Figures S5-S7, Tables S2).


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Figure 3. IL-o capsules prepared with HDI x EDA: A) [Bmim][PF6]-o capsules, B) [Emim][TFSI]o capsules, C) [Hmim][TFSI]-o capsules, D) [Mpp][TFSI]-o capsules, E) [Bm2im][PF6]-o capsules, F) [Bmim][BF4]-o capsules, and G) [Emim][DMP]-o capsules: (i) SEM images and (ii) optical microscopy images (inset shows after compression, all scale bars 120 μm unless otherwise specified); H) FTIR spectra of [Emim][TFSI]-o, [Emim][TFSI]-w capsules and neat [Emim][TFSI]; and I) TGA weight loss profiles (solid lines) and first derivatives (dashed lines) of [Emim][TFSI]-o capsules and neat [Emim][TFSI]. Phenol removal Given that imidazolium based ionic liquids can be used to remove phenol from oil and water solutions using liquid-liquid extractions13–16 or IL-based liquid membranes,47,48 we envisioned that our IL capsules could be used in a similar application, provided the shell is permeable to phenol. Moreover, given the increased surface area of the encapsulated IL compared to the bulk, difficulties associated with unwanted emulsion formation (in the case of liquid-liquid extractions), or delayed mass transfer (due to limited interfacial area of liquid membranes) could be overcome. Our IL capsules were packed into a column and a phenol-contaminated solution was passed through. Two types of capsules were evaluated: 1) [Bmim][BF4]-o, as this IL was reported to effectively remove phenol from oil at room temperature;14 2) [Emim][TFSI]-o, to evaluate the impact of IL structure, IL content, and capsule morphology. Two different glass columns were packed with [Bmim][BF4]-o and [Emim][TFSI]-o capsules as the absorbent material and a standard solution of phenol (0.57 mM in hexanes) was passed through (Figure 4A). The column eluent was characterized by UV-Vis absorption spectroscopy, with the concentration of phenol determined using a standard calibration curve (see Figure S8). We note that the capsule shells are not porous, as determined by BET analysis (see Table S3), but rely on permeability of the shell to phenol, to allow for efficient extraction. Figure 4B shows the percentage of phenol removal as a function of passed volume for [Bmim][BF4]-o capsules (blue trace) and the [Bmim][BF4]-o capsule shells after extraction of the IL (red trace); each data point is averaged over 3 experiments. These data illustrate that the encapsulated IL, and not the composite shell, is responsible for a majority of the phenol removal. Greater than 98% of phenol is removed upon passage of up to 12 mL of the phenolcontaminated solution; as the column material contains ~0.07 mL of IL (based on wt% IL and overall mass of sample loaded), removal efficiency is maintained upon passage of >170 “column” volumes. The phenol-removal efficiency slowly drops upon passing of more solution. The eluent is a clear liquid, with the lack of solid and or phase separation supporting the integrity of the capsule shell and sustained encapsulation of the IL. Moreover, after use in the extraction of phenol, the IL capsules had no significant change in wt% of IL or capsule morphology (Table S2 and Figure S10). The extraction profile of the [Bmim][BF4]-o capsule shell suggests it has some ability to extract phenol (possibly due to H-bonding interactions), but removal efficiency rapidly dropped after 1.5 mL of solution was passed through and is 170 column volumes, and [Emim][TFSI]-o capsules had the same efficiency up to 110 column volumes. This easily operated phenol removal method overcomes limitations of liquid-liquid separations, and illustrates a novel and efficient direction to fully utilize the separation abilities of ILs. The method developed here for the encapsulation of ILs is scalable and compatible with a wide variety of ILs, and thus will find application in numerous targeted applications that require high mass transport, such as heavy metal removal,49 bioactive substance extraction,50 gas handling,8 etc.

SUPPORTING INFORMATION:


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The Supporting Information is available free of charge on the ACS Publications website at DOI: Characterization data for GO, C18-GO, IL-o capsules, and IL-w capsules; list of all experiments; phenol calibration curve. (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID Qinmo Luo: 0000-0003-4269-2641 Yifei Wang: 0000-0003-0110-8147 Peiran Wei: 0000-0001-7820-1716 Emily Pentzer: 0000-0003-4269-2641

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors thank NSF CAREER Award no. 1551943 for financial support. They would also like to thank the Swagelock Center for Surface Analysis of Materials (SCSAM) for XPS, the CWRU Physics Department for SEM, Professor Rigoberto Advincula (CWRU Macromolecular Science and Engineering) for AFM instrumentation, Professor Joseph Ortiz (Kent State University, Geology Department) for laser diffraction particle analysis, Professor Burcu Gurkan and Mr. Yun-Yang Lee (CWRU Chemical Engineering) for porosity measurements, and NSF MRI1334048 for NMR instrumentation. References (1)

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Cover Art


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Pickering Emulsion-Templated Encapsulation of Ionic Liquids for

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