Deep-Eutectic Solvents as MWCNT Delivery Vehicles in the Synthesis

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Deep-eutectic Solvents as MWCNT Delivery Vehicles in the Synthesis of Functional Poly(HIPE) Nanocomposites for Applications as Selective Sorbents Arturo Carranza, María Guadalupe Pérez-García, Kunlin Song, George M Jeha, Zhenyu Diao, Rongying Jin, Nina Bogdanchikova, J. Félix Armando Soltero, Mauricio Terrones, Qinglin Wu, John Anthony Pojman, and Josue David Mota-Morales ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b09589 • Publication Date (Web): 25 Oct 2016 Downloaded from http://pubs.acs.org on October 31, 2016

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ACS Applied Materials & Interfaces

Deep-eutectic Solvents as MWCNT Delivery Vehicles in the Synthesis of Functional Poly(HIPE) Nanocomposites for Applications as Selective Sorbents Arturo Carranza,a María G. Pérez-García,b Kunlin Song,c George M. Jeha,a Zhenyu Diao,d Rongying Jin,d Nina Bogdanchikova,e Armando F. Soltero,f Mauricio Terrones,g Qinglin Wu,c John A. Pojman,a Josué D. Mota-Morales h* a

Department of Chemistry, Louisiana State University, Baton Rouge, LA 70820, USA.

b

Centro Universitario de Tonalá, Universidad de Guadalajara, Tonalá, Jal. 45425, México.

c

School of Renewable Natural Resources, Louisiana State University Agricultural Center, Baton Rouge, LA 70803, USA. d

Department of Physics & Astronomy , Louisiana State University, Baton Rouge, LA 70820, USA.

e

Centro de Nanociencias y Nanotecnología, Universidad Nacional Autónoma de México, Ensenada, BC 22860, México. f

Departamento de Ingeniería Química, Universidad de Guadalajara, Guadalajara, Jal 44430, México.

g

Department of Physics and Center for 2-Dimensional and Layered Materials, Pennsylvania State University, University Park, PA 16802, USA.

h

CONACYT - Centro de Nanociencias y Nanotecnología, Universidad Nacional Autónoma de México, Ensenada, BC 22860, México. Correspondence to: Josué D. Mota-Morales (E-mail: [email protected]; [email protected]) Keywords: Carbon nanotubes; Deep-eutectic solvent; High internal phase emulsion; Hydrophobic; Nanocomposite; Fuel sorbents.

Abstract We report an alternative green strategy based on deep-eutectic solvents (DES) to deliver multiwalled carbon nanotubes (MWCNTs) for a bottom-up approach that allowing the selective interfacial functionalization of nonaqueous poly(high internal phase emulsions), poly(HIPEs). The formation and polymerization of methacrylic and styrenic HIPEs were possible through the stabilization with nitrogen doped carbon nanotube (CNX) and surfactant mixtures using a urea-choline chloride DES as a delivering phase. Subtle changes in CNX concentration (less than 0.2 wt % to the internal phase) produced important changes in the macroporous monoliths functionalization, which in turn led to increased monolith hydrophobicity and pore openness. These materials displayed great oleophilicity with water contact angles as high as 140o making them apt for biodiesel, diesel and gasoline fuel sorption applications. Overall, styrene divinylbenzne (StDvB) based poly(HIPEs) showed hydrophobicity and fuel adsorption capacities as high as 4.8 (g/g). Pore hierarchy, namely pore openness, regulated sorption capacity and sorption times where greater openness resulted in faster adsorption and increased sorption capacity. Monoliths were subject to 20 sorption-desorption cycles demonstrating recyclability and stable sorption capacity. Finally, CNx/surfactant hybrids made it possible to reduce surfactant requirements for successful HIPE formation and stabilization during polymerization. All poly(HIPEs)

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retained acceptable conversion as a function of CNX loading nearing 90% or better with thermal stability as high as 283oC. Introduction Porous sponge-like materials (e.g. foams and monoliths) provide many ideal characteristics for oil recovery as they possess interconnected pores and are tunable for target adsorbates. There is, however, a growing need to develop new macroporous materials that, while keeping their bulk properties, their surface properties can be tailored for specific applications. Recently, several top-down approaches to produce super hydrophobic foams have been proposed.1,2 While these methods hold some promise, they suffer from requiring several volatile organic solvents thus decreasing their renewable aspect. Carbon-based materials like, nanotubes and graphene sheets, present an opportunity to integrate hydrophobicity, functionality, and bulk property enhancement during sorbent synthesis. Recently Li and coworkers have proposed the synthesis of saturated carbon nanotube sponges,2 while Cho and coworkers have increased hydrophobicity and adsorption efficiency of a sponge through the introduction of a few layer graphene sheets.3 Although these methods allow for void size control, there are finer architectural features like pore openness where a bottom-up approach could be more convenient. In this regard, high internal phase emulsions (HIPEs) and most notably Pickering HIPEs offer versatility and control of finer architectural features when used as templates for porous materials, which have find applications as separation foams4 and metallic ion sorbents.5-7 High internal phase emulsions are highly viscous systems characterized by an internal phase volume exceeding 74% that is dispersed within a continuous phase,8 as polyhedral motifs. 9 Introduction of a polymerizable continuous phase and subsequent extraction of the internal phase affords a porous functional matrix with smaller interconnecting pore voids, commonly known as poly(HIPEs). The introduction of nano or micro objects brings a new facet to poly(HIPE) architectural design while providing an accessible method of functionalization. These emulsions are termed Pickering HIPEs. With the advent of nanotechnology poly(HIPE) matrixes have been adorned with wide range of nanomaterials to fulfill specific target adsorption including aromatic hydrocarbons,10 water and methanol vapor,11 and oils.12-14 Pickering HIPEs can be prepared with relatively low nanoparticle loading using titania,15 silica16 as well as single-walled17 and multi-walled carbon nanotubes.18,19 Most recently, the concept has extended to green synthesis with the inclusion of biomass as stabilizers such as cellulose nanocrystals,20 chitin nanocrystals,21 as well as lignin.22 However, much room has been left to explore the influence of nanomaterials and their method of delivery23 that result in more efficient adsorption at the HIPE interface. Pickering HIPEs are notably resilient against coalescence due to nanoparticles’ tendency to adsorb quasi-irreversibly to the oil-water interface in homogeneous emulsions;24,25 contrary to traditional HIPES where a limiting factor is the high amount of surfactant needed, which has the tendency to lower mechanical properties.26 Pickering HIPEs eliminate the need for surfactant while in some cases providing functionality and additional structural support. Despite all this, Pickering HIPEs result in closed cavity materials independent of continuous phase volume ratio.15,27-30 Surfactant/particle hybrids and composites create a new opportunity to integrate Pickering HIPEs’ stability and functionality to traditional surfactant HIPEs’ pore tunability. Integration of both techniques will also permit surfactant reduction requirements leading to more resilient materials.


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Although typical routes for poly(HIPE) synthesis involve aqueous media as an internal or continuous phases, non-aqueous systems offer new possibilities in the inclusion of water sensitive monomers and polymerization techniques while broadening the range of working temperatures available. 31,32 Within the Green Chemistry framework, recently we have reported the use of urea choline chloride (U:ChCl) deep-eutectic solvent (DES) as internal phase for acrylic and styrenic poly(HIPE) synthesis.33-35 Additionally, Pérez-García and coworkers have developed a polymerizable DES as a continuous phase in a biodegradable poly(HIPE).14 DESs are a new generation of green solvents comprised of a eutectic mixture possible through association of hydrogen bond donors and ammonium or phosphonium salts.36,37 They share many notable qualities with ionic liquids like high thermal and chemical inertness, negligible vapor pressure and the ability to dissolve a wide range of solutes, including the efficient dispersion of carbon nanotubes.38 DES properties can be tailored to their end use by choosing both the nature and the ratio of their components, which added to their low cost, biocompatibility, and biodegradability, have quickly positioned DES as designer solvents, often called advanced ionic liquids. We report an alternative green strategy based on DES to deliver multiwalled carbon nanotubes for a bottom-up approach that allows the selective introduction of interfacial functionality into poly(HIPEs). The formation and polymerization of methacrylic and styrenic HIPEs were possible through the stabilization with nitrogen doped carbon nanotube (CNX) and surfactant mixtures using U:ChCl DES as the delivery phase. CNX/Span 60 hybrids demonstrated improved emulsion stability and lead to a significant reduction in surfactant requirements, while CNX contributed to pore openness modulation. CNX functionalized poly(HIPEs) showed enhanced hydrophobicity. Styrene divinylbenzene (StDvB )CNX poly(HIPEs) had the best fuel adsorption performance demonstrating improved sorption capacity and adsorption times. Materials and methods All reagents were used as received without any further purification. Methyl methacrylate (MMA) 99%, stearyl methacrylate (SMA) technical grade, ethylene glycol dimethacrylate (EGDMA) 98%, styrene (St) 99%, divinyl benzene (DvB) 99%, 2,2’-Azobis(2-methylpropionitrile) (AIBN), urea 99%, choline chloride (ChCl) 98%, Span 60 (sorbitan stearate), glycerol 99.5%, and hexane were purchased from Sigma Aldrich. The nonionic surfactant, Cithrol (Arlacel P135), an ABA terblock copolymer, was generously contributed by Croda Europe Ltd. Carbon nanotubes were synthesized by the CVD method as previously described for CNX39 and COX.40 The continuous phase (20 vol %) was prepared by dissolving 2.0 mol% AIBN (as thermal initiator) with respect to the total concentration of C=C reactive groups in a 2:1 monomer (MMA or SMA) to crosslinker (EGDMA) molar ratio for acrylates or a 10:1 molar ratio for St and DvB and surfactant mixture. The amount of surfactant used was 7 wt % (Cithrol) for acrylates and 4 wt% (Span 60) for StDvB with respect to the total weight of the emulsion. To prepare the internal nonaqueous DES phase (80 vol %), ChCl was recrystallized in ethanol and dried at 90oC in an oven. A 2:1 molar ratio of urea and ChCl were combined and oven heated to 60oC until a clear viscous, homogeneous liquid was obtained. Several internal phases were prepared by adding 0.04, 0.08, 0.12, 0.16 or 0.20 wt%, with respect to the continuous phase of either nitrogen functionalized (CNX) or carbonyl functionalized (COX) MWCNTs. The different internal phases were then subject to three iterations of vortexing at 3200 rpm for 1 minute followed by a water ultrasonicating bath at 60oC for 30 minute. HIPEs were prepared by mixing both


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phases in an 8 mL glass vial and vortexing at 3200 rpm for at least 10 min. until homogenous emulsion was obtained. The emulsion did not flow upon the inversion of the vial (Fig 1a inset). HIPEs were polymerized in an oven at 60oC for 24 hrs. After polymerization was completed, the internal phase and surfactant were removed by Soxhlet extraction with ethanol for 12hrs. The resulting monoliths were oven dried at 60OC until constant weight was reached. Dried monolith conversion was determined gravimetrically. The microstructures of DES-based emulsions were studied using deconvolution microscopy (Leica DM RXA). The morphologies of all poly(HIPE) nanocomposites were investigated by scanning electron microscopy (SEM; JSM 6610 LV) with an accelerating voltage of 10 kV. Samples were platinum coated for 240 seconds in an inert argon atmosphere at 1 x 10-5 mbar (Emmitech 550). The average droplet size, pore, and pore window diameters were calculated in sets of 100 using ImageJ analysis software. Additionally, the degree of pore openness was estimated using the equation proposed by Pulko and Krajnc.41 Poly(HIPEs)’ thermal stability was assessed by thermogravimetric analysis (TGA 2950 thermogravimetric analyzer) in an inert nitrogen atmosphere from 25oC to 500oC with a heating rate of 10oC min-1. TGA was carried out using 5 to 10 mg of sample in standard aluminum pans. Thermal analysis was performed using TA Universal Analysis software. All rheological measurements were performed in a controlled stress rheometer (AR2000ex, TA Instruments, New Castle, DE, USA) using parallel plate geometry consisting of stainless steel upper plate (diameter 25 mm) and an aluminum lower plate (diameter 40 mm) with a 1 mm gap width. Stress-sweep data were collected at a frequency of 0.1 Hz at 25oC. Apparent viscosities were recorded at shear rates ranging from 0.1 to 1000 s-1 for each suspension at a constant temperature of 25oC. A solvent trap cover was used to avoid solvent evaporation during the measurements. The monoliths’ mechanical properties were evaluated according to the ASTM D1621 in an Instron Model 5996 using a 5 kN load cell using a 4 mm min-1 compression rate. Samples were measured in triplicates by compressing to 75% of their initial height and their elastic modulus was determined from the initial linear slope obtained from the stress-strain plot. The stress at yield was recorded to show monoliths compression strength. Thermal conductivity measurements were calculated by the four leads method using physical property measurement system Quantum Design 1684-100B cooled by liquid helium from -38oC to 120oC at 0.5OC min-1. The contact angles (CA) were measured using a VCA 2000 contact angle system (VCA, Billerica, MA) at room temperature. For each measurement a 3 µL drop was allowed to equilibrate on the surface for 1 minute prior to measuring the contact angle. Surface energy measurements (γT) and both polar (γP) and dispersive (γd) components were obtained from measurements using glycerol and nanopure water using the Fowkes equation 1:

+ + + = (1 + ) (1)

For all calculations each liquid was measured six times on different places on the surface to ensure homogeneity of the material. Data for the liquids are water: γt = 72.8 mJ m-2, γd = 21.8 mJ m-2, γP = 51.0 mJ m-2; glycerol: γt = 64.0 mJ m-2, γd = 34.0 mJ m-2, γP = 30.0 mJ m-2.


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The sorption capacity (Q) of the poly(HIPE) nanocomposites was tested for different types of solvents including water, biodiesel, diesel, gasoline and hexane. The monolith’s adsorption was monitored at time intervals until the adsorption equilibrium was reached. Q was calculated using the equation 2: =

( )

(2)

Where M0 and M are the monoliths’ dry weight and adsorption weight respectively, and where Q represents (g/g) adsorption quantity of solvent per sorbent. Solvent recovery was performed using two methods: centrifugation at 4000 rpm for 5 minutes or low pressure distillation. Additionally, monolith recyclability was tested through 20 times cycled adsorption experiments for selected sets of representative monoliths in each group. The material was immersed in excess solvent for 10 minutes when Q was immediately calculated. Solvent was removed from the monolith through centrifugation, the material was reweighed and the regenerated monolith was used in the next adsorption cycle. Results and discussion Normally, nano objects are incorporated directly to the polymer matrix to enhance mechanical or thermal properties, but in special cases like porous polymers made from HIPE, nano objects can be specifically designed to interact at the emulsion interface serving as stabilizers during polymerization as well as structural support post polymerization. Two types of functionalized multiwalled carbon nanotubes (MWCNTs), carbonyl COX and nitrogen CNX, were chosen due to their demonstrated dispersibility in DES.38 After a careful screening, CNX was selected to conduct this study showing better dispersibility and stable HIPE formation. Emulsion morphology was affected significantly by the amount of CNX used (Table 1). Deconvolution microscopy showed closely packed polyhedral morphologies separated by a thin film of continuous phase (Figure 1A, C and E). In the case of monomers containing a short alkyl chain, St and MMA CNX, droplets adopted a loose conformation at low CNX loading when compared to their parent emulsion without CNx. However, upon increased CNX loading StDvB and MMA emulsions adopt a more compact conformation. SMA CNX HIPEs, on the other hand, do not show this transition, but instead go directly into a more compact droplet conformation, which could be indicative of increased emulsion stability as a function of CNX. A good measure to compare stability during polymerization is to analyze size change from HIPE droplet to poly(HIPE) pore, where little to no morphological change indicates good stability. Nevertheless, it has been demonstrated that nanofillers (e.g. silica nanoparticles or graphene oxide)42,43 can interact directly with the monomer matrix during polymerization resulting in HIPEs’ phase separation during polymerization and hierarchization of porosity.44 Scanning electron micrograph measurements reveal a significant yet consistent increase in pore diameter for StDvB HIPEs (between 8.8 and 10.4 µm) as compared to droplet size of their precursing emulsion. It is speculated that this difference is due to better interaction or mixing of CNX with StDVB which leads to phase separation during polymerization. On the other hand, acrylate CNX poly(HIPEs) pore diameters show very little change relative to their precursor emulsion with the exception of higher CNX concentrations of SMA, namely 0.08 to 0.20 wt% CNX, which suggest better mixing of CNX with monomer during emulsion preparation leading to phase separation in this range of CNT concentration. When compared to their parent poly(HIPE), StDvB showed an increased pore window diameter yet consistent throughout all CNX concentrations. MMA showed no change from its parent poly(HIPE) but resulted in closed cell monoliths at higher CNX concentrations (0.12 to 0.20 wt%). SMA showed a steady increase in pore window diameter but resulted in a closed cell monolith at 0.20


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wt% CNX loading. The degree of openness for StDvB CNX poly(HIPEs) doubled when compared to its parent poly(HIPE) and remained consistent throughout all samples. In the case of acrylates the degree of openness was significantly reduced as more CNX was added eventually resulting in closed cell monoliths.

Figure 1. Optical micrographs of HIPEs A) StDVB CNX 0.04 (inset: image of HIPE) C) MMA CNX 0.04 E) SMA CNX 0.04 and scanning electron micrographs of poly(HIPEs) post-DES extraction B) pStDVB CNX 0.04 (inset: image of poly(HIPEs)) D) pMMA CNX 0.04 F) pSMA CNX 0.04.


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Table 1. Structural morphology of HIPEs and Poly(HIPEs). HIPE

Avg. Drop Diameter [µ µm]

StDvB 5.0±1.3 StDvB CNx 0.04 5.9±1.4 StDvB CNx 0.08 7.0±2.6 StDvB CNx 0.12 4.3±1.3 StDvB CNx 0.16 3.1±0.7 StDvB CNx 0.20 2.1±0.3 MMA 7.4±2.7 MMA CNx 0.04 10.4±2.9 MMA CNx 0.08 10.3±2.5 MMA CNx 0.12 11.2±2.3 MMA CNx 0.16 6.8±2.4 MMA CNx 0.20 5.8±2.9 SMA 6.9±2.0 SMA CNx 0.04 6.1±1.9 SMA CNx 0.08 5.2±2.4 SMA CNx 0.12 4.1±1.8 SMA CNx 0.16 5.8±2.0 SMA CNx 0.20 7.9±2.5 * Cf indicates closed cell monolith

Avg. Pore Diameter [µ µm] 5.0±1.3 10.3±2.3 8.8±1.0 10.4±1.0 8.8±2.9 8.6±2.2 6.3±2.4 10.3±1.2 9.9±2.5 11.6±2.9 7.8±1.2 5.1±2.0 6.8±2.0 6.7±2.0 8.3±0.9 9.0±1.1 9.5±0.8 20.1±3.3

Avg. Pore Window [µ µm]

Deg. of Openess [%]

1.2±0.4 3.9±0.5 2.5±0.3 2.9±0.4 2.4±0.3 2.5±0.3 1.8±0.6 1.6±0.9 1.6±0.9 * Cf * Cf * Cf 1.4±0.6 1.3±0.8 1.8±0.9 2.0±0.8 3.7±1.0 * Cf

11.7±2.6 24.4±3.8 23.3±3.9 22.4±2.3 21.5±4.2 24.4±3.1 23.2±5.1 7.0±4.4 7.5±5.0 — — — 16.8±4.0 10.9±3.9 13.8±3.6 14.3±3.0 14.8±3.0 —

Shear-viscosity as a function of shear rate was obtained for UChCl DES for some CNx ratios at 25oC (Figure S2). Samples exhibits a Newtonian behavior at lower shear rates values, at a critical shear rate of around 60 s-1 a shear-thinning behavior was detected. From curves, the zero-shear viscosity (η0) was obtained. Data were analyzed using the Carreau model (Equation S1), from the results infinitumshear viscosity (η∞), η0 and the characteristic time of the system (λ) were obtained. In Figure S3, η0 and η∞ as a function of CNx ratio are depicted; it is evident that both, η0 and η∞ increased slightly with nanotubes concentration. Yuan and Williams have demonstrated two distinct mechanisms arising from surfactant and nanoparticle interactions.45,46 When surfactant and nanoparticles have no specific interaction interfacial adsorption of both nanoparticles and surfactant proceed competitively. On the other hand, when surfactant molecules interact directly with the nanoparticles, the modified surfaces undergo faster wetting at the O/W interface resulting in improved stabilization. Stress-sweep plots of storage modulus versus shear stress for all three monomers at different CNX concentrations show a linear response where the storage modulus is independent of shear stress until a critical shear stress value is reached (Fig S5). Lag phase vs shear stress plots were used to determine the yield point where a pronounced increase in the lag phase angle was taken to be the yield point. Figure 2A shows a summary of poly(HIPE) yield points as a function of CNX. Acrylates had the highest overall yield stress values where a long hydrocarbon tail was a defining stability feature.


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These results suggest competitive interfacial adsorption between CNX and Cithrol surfactant leading to destabilization, as demonstrated by lower yields stresses. Surprisingly, CNX addition, even at its lowest concentration of 0.04 wt %, resulted in a substantial yield stress decrease in both acrylic systems. Additional CNX loading on acrylate HIPEs gradually increases yield stress values, which were always lower than that of the original pure monomer HIPEs (MMA and SMA). On the other hand, StDvB showed a cosurfactancy effect, immediately increasing in yield stress upon initial CNX loading until it reached a maximum value at 0.16 wt% CNX and a decline at 0.20 wt% CNX. These results indicate significant interaction between CNX and Span 60. Altogether, these trends in the emulsions stability correspond well with the average drop radii in the HIPEs. For instance, the behavior of MMA emulsions is consistent with the larger drop size of the HIPE having the lowest CNx concentration (the less stable) and their reduction as CNx concentration increases in comparison with the parent HIPE. In the case of SMA this trend is less pronounced; while in StDVB the cosurfactancy effect give rise, in general, to smaller drops (more stable) as CNx concentration increases, with the exception of the last point (0.2 wt.% of CNx).

Figure 2. poly(HIPE) yield-stress A) as a function of CNX composition for StDvB, MMA, and SMA and B) as a function of surfactant composition at a constant CNX concentration (CNX 0.16) for StDvB.


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In order to evaluate the impact of surfactant concentration on StDvB CNX HIPEs, a series of additional stress-sweep experiments were carried out by choosing an optimal CNX concentration (0.16 wt%) and varying its original Span 60 concentration (Figure 2B). When concentrations of Span 60 are smaller than 2.50 wt% insufficient surfactant molecules can establish an adequate adsorption desorption interfacial equilibrium resulting in much lower stability. Similarly, when quantities above 0.16 wt% CNx are used, excess CNTs are adsorbed onto the interface, limiting the surfactant adsorption desorption equilibrium. Interestingly, relatively smaller amounts of CNX can replace a substantial amount of surfactant used to stabilize StDvB HIPEs as evidenced by the linear regime overlap of both plots on Figure 2B. Stress-strain curves also indicate that increasing CNX or surfactant within these critical values provides deformation resistance (Figure S6). Most importantly, CNx/Span 60 surfactant hybrids can support emulsion formation at lower surfactant concentrations where it would otherwise not be possible using surfactant alone.33 Thermal conductivity measurements were performed for selected CNX composites to test the influence of CNTs on thermal conductivity of the finished bulk material (Figure S7). It was found that thermal conductivity increases proportionally to the amount of CNX loading suggesting a certain degree of percolation and homogeneity throughout the monolith which was also supported by FTIR showing similar signals to previously reported CNX signals47 (Figure S4). Thermal conductivity values are in range with reported values for polymer CNT composites.48-50 The bulk density (ρb) of all poly(HIPEs) are in agreement with previously reported values for similar macroporous materials reported in the literature (Table S3).51 Contact angle measurements were performed using two different solvent systems (glycerol and nanopure water) to determine total surface energy (Figure 3). Water contact angles of native StDvB, MMA, and SMA were observed to be 109o, 78o, and 68o respectively. CNX monolith functionalization yields higher contact angle values ranging from 130 to 132o, 112 to 132o, and 133 to 140o for StDvB, MMA, and SMA respectively depending on the amount of CNX loading. These values are comparable to previously reported graphene functionalized sponges3 (143.5o) and super hydrophobized sponges1,2 (155o). Figure 4 shows a plot of the significant water contact angle change from native monoliths to CNX functionalized monoliths. When comparing the pure parent poly(HIPEs), SMA had the lowest native surface energy (γΤ) (22 mJ m-2) followed closely by StDvB (24 mJ m-2) while MMA had the highest (γΤ) (37 mJ m-2). Adding as little as 0.04 CNX weight percent drastically reduced surface energy (γT) values making all monolith surface hydrophobic where StDvB (10 mJ m-2) had the most hydrophobic surface followed by SMA (13 mJ m-2) and MMA (17 mJ m-2). MMA and SMA had their lowest values upon adding 0.20 wt CNX at 9 mJ m-2 and 8 mJ m-2 respectively.


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Figure 3. A) Total surface energy (γΤ) as a function of CNX B) Nonpolar contribution as a function of CNX for StDvB, MMA, and SMA.


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Figure 4. A) Water contact angle measurements as a function CNX concentration for StDvB, MMA, and SMA. B) pSMA poly(HIPE) contact angle photograph. C) pSMA CNX 0.04 poly(HIPE) contact angle photograph.

Sorption capacity (Q) of selected poly(HIPEs) pStDvB, pStDvB CNX 0.04, pStDvB CNX 0.16, pMMA, pMMA CNX 0.04, pSMA and pSMA CNX 0.04 was tested against water, biodiesel, diesel, gasoline, and hexane by immersing each monolith in excess solvent and determining their equilibrium mass (Table 2). Q is per gram quantity defined as mass of solvent sorbed per gram of dry monolith. Figure 5 shows a typical solvent sorption curve for pstDvB and pStDvB CNX 0.04. Overall, pStDvB had the highest sorption capacities and fastest for all solvents relative to all poly(HIPEs) tested. On average, pStDvB CNX 0.04 reached maximum sorption capacity in 10 minutes with the exception of gasoline and water which required about 35 minutes and 120 minutes respectively. Rapid sorption is attributed to the sum of enhanced hydrophobicity and larger interconnected pores as a result of CNT addition. When comparing the parent monomer to their CNT versions, it was found that the overall sorption capacity was only increased for StDvB monoliths where acrylic monoliths showed a decrease in maximum sorption capacity. While acrylic monomers displayed increase hydrophobicity as a result of CNX functionalization, their sorption capacity is significantly reduced by the addition of CNTs. This phenomenon is explained by the reduction in pore openness in acrylic poly(HIPEs) as a result of CNT addition. Smaller sorbent pores reduce the migration rate into porosity causing a monolayer buildup of adsorbate. On the other hand, larger pore openness allows faster and increase sorption capacity as in the case pStDvB CNX 0.04. This effect is also observable in StDvB poly(HIPEs) where adding more CNX results in a pore openness decrease. Our results are comparable with the lower bound of porous sorbents, 2.5 g/g,14 within the limitations of the selected monomer matrix and degree of cross-linking where other approaches like swellable copolymers, hyper cross-linking,52-54 or carbonization55 could afford materials within the sorbance upper-bound 80 g/g3 to 450 g/g1.


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Table 2. Poly(HIPE) saturation times and maximum sorption capacity. Sample pStDvB pStDvB CNX 0.04 pStDvB CNX 0.16 pMMA pMMA CNX 0.04 pSMA pSMA CNX 0.04

Water tsat [min] 120 120 120 360 120 120 360

Qmax [g/g] 0.13±0.02 0.32±0.01 0.17±0.02 1.16±0.17 0.96±0.04 0.33±0.03 0.86±0.11

Biodiesel tsat Qmax [min] [g/g] 5 2.71±0.01 10 4.16±0.05 55 3.14±0.14 30 2.11±0.16 10 1.45±0.06 60 1.66±0.08 120 0.95±0.09

Diesel tsat [min] 10 10 60 20 30 5 60

Qmax [g/g] 3.56±0.08 3.60±0.15 2.87±0.10 2.25±0.14 2.25±0.08 3.64±0.13 1.58±0.15

Gasoline tsat Qmax [min] [g/g] 60 4.53±0.09 35 4.80±0.04 45 3.72±0.18 60 2.92±0.18 30 2.81±0.16 60 3.17±0.16 60 2.55±0.11

Hexane tsat [min] 15 10 50 60 30 60 60

Figure 5. Typical sorption capacities (Q) vs. time for water, biodiesel, gasoline, and hexane in A) pStDvB poly(HIPEs) and B) pStDvB CNX 0.04 poly(HIPEs). Insets in each plot show the first 60 mins.


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Qmax [g/g] 3.36±0.11 3.68±0.14 3.27±0.07 2.40±0.17 2.09±0.08 2.74±0.15 2.05±0.17

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A determining factor on final sorbate use is the sorption time that is the time it takes for the sorbent to reach sorption equilibrium with the sorbate. Sorption time should happen within a reasonable amount of time for a materials practical use. Most super sorbents can achieve this goal, but are limited by diffusion and in several cases require additional mechanisms (e.g. pumping) to facilitate sorption. The sorbate chemical nature also influenced sorption capacity and sorption time. In this study, maximum sorption occurs almost instantaneously though a combination of oleophilicity and capillary action. For StDvB samples it was demonstrated that higher boiling point hydrocarbons like biodiesel and diesel had the fastest adsorption times. Biodiesel sorption capacity was drastically affected by its molecular nature owing its lower absorbance to esters and carbonyl groups that interact less efficiently with the polymer matrix. On the other hand, it was shown that lower boiling point hydrocarbons have a high affinity for StDvB poly(HIPEs) resulting in relatively high adsorption capacities, but have longer adsorption times. Adsorption times, however, could be increased by modifying the degree of openness via CNTs. In contrast, acrylates had significant increase in adsorption times and showed preferential adsorption towards low boiling point adsorbates like gasoline and hexane. The only exception was pSMA which had an exceptional sorption capacity and time relative to any other poly(HIPE) tested. In general, due to the hydrophobic nature of the poly(HIPEs), water had the lowest sorption capacity and longest equilibrium times where adsorption capacities were controlled by their pore openness and hydrophilicity/hydrophobicity character. Fuel desorption was tested using two different methods: low pressure distillation and centrifugation. Low vacuum distillation provided the best means of recovery for volatile components recovering 99.2% of gasoline after 30 minutes and 66.9% and 73.1% of diesel after 30 and 60 minutes (Figure S9). Centrifugation at 4000 rpm for 5 minutes provided much simpler method of solvent recovery, however, decreasing the amount of fuel that could be recovered to between 60% and 70%. Due to their economic importance, gasoline, diesel, and biodiesel were chosen as targets for adsorption cycled experiments. Adsorption cycles were performed (Figure 6) by immersing monoliths in excess solvent until adsorption equilibrium was reached. Monoliths were then centrifuged at 4000 rpm for 5 minutes to extract the adsorbate. Regenerated poly(HIPEs) were then used for the next cycle and the procedure was repeated for a total of 20 cycles. Sorption capacity was calculated based on monoliths dry weight before the collection process. A general feature of the cycle curves is that around the same amount of residual adsorbate was left on the material after each cycle, and there was little decline in the overall sorption capacity. Gasoline had the best overall sorption capacity followed closely by biodiesel and diesel. Out of all three fuels tested, diesel seemed to have the best overall recoverability between cycles while biodiesel had the poorest results. Nanomaterial delivery into a HIPE via DES prospects is easily envisaged in combination with the use of copolymers as continuous phase and post-functionalization, e.g. hypercross-linking or carbonization,55 to enhance their performance for specific targets and fuel sorbents. In the same line, although urea-choline chloride DES suited well the dispersion requirements for this specific N-doped CNT, a plethora of ammonium or phosponium salts mixed with hydrogen bond donors can be explored to efficiently disperse and deliver other types of carbon-based nanomaterials56,57 for poly(HIPES) nanocomposites preparation. On the other hand, the protocol presented for the incorporation of CNT into the Poly(HIPES) porosity may trigger new applications where the presence of a conductive nanomaterial (either electric or thermal) in an interconnected macroporosity is advantageously. We can forsee poly(HIPES)-containing carbon nanotubes being applied as biomaterials for nerve tissue


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engineering, as sensor where changes in conductivity can be associated with the presence of certain analyte, as support for catalyst in electrochemical reactions or in flow systems.

Figure 6. Adsorption-desorption cycles of pStDvB CNX 0.04 for different selected fuels biodiesel, diesel, and gasoline.

Conclusion We have demonstrated an alternative green strategy based on DES to deliver MWCNTs for a bottom-up approach that allows to selectively introducing interfacial functionality. Resulting poly(HIPEs) had tunable hierarchical morphologies and hydrophobicity through the introduction of small CNX concentrations (e.g. 0.04-0.20 wt%) where water contact angles values ranged between 130o and 140o. CNX loading impacted the overall emulsion stability having a cosurfactancy effect on StDvB-derived nanocomposites but being detrimental in the case of acrylic monomers. Relatively small amounts of CNX can replace a substantial amount of surfactant where HIPE stabilization would otherwise not be possible using surfactant alone. Surfactant/particle(CNx) hybrids and carbon nanotube-based composites create a new opportunity to integrate Pickering HIPEs’ stability and functionality to traditional surfactant HIPEs’ pore tunability. Integration of both techniques will also permit surfactant reduction requirements leading to more resilient materials without the limitation of closed porosity. CNX poly(HIPEs) were tested for their selective sorption of fuels. The improved oleophilicity and porosity through the CNX modification improved pStDVB HIPEs performance and selectivity. Adsorption capacities ranged between 3.6 and 4.8 times the original monolith mass for top performing poly(HIPEs). Experiments showed consistent performance even after 20 cycles in monolith recyclability and fuel recovery by either low pressure distillation (effective only for volatiles) or centrifugation. CNX poly(HIPEs) provide an alternative route for selective sorbent synthesis for scale up applications due to their tunability, selective adsorption, high sorption capacity, and recyclability, ease of preparation and overall green character.


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Acknowledgements The authors gratefully acknowledge the help of Natalie Suarez. Financial support by CONACYT through projects 261425 and 252774, is greatly appreciated.

Supporting Information Available: chemical structures, DES viscosity as a function of CNX loading, zero and high shear viscosity, CNX nacocomposite FTIR, stress-sweep analysis, stress-strain curves, thermal conductivity, compressive strength, and volatile solvent recovery using low pressure distillation. This material is available free of charge via the Internet at http://pubs.acs.org.

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Deep-Eutectic Solvents as MWCNT Delivery Vehicles in the Synthesis

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