Ionic Liquid-Containing Pickering Emulsions Stabilized by Graphene

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Ionic Liquid-Containing Pickering Emulsions Stabilized by Graphene Oxide-Based Surfactants Qinmo Luo,§ Yifei Wang,§ Esther Yoo, Peiran Wei, and Emily Pentzer* Department of Chemistry, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, Ohio 44106, United States

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S Supporting Information *

ABSTRACT: Emulsions stabilized by particles (i.e., Pickering emulsions) are complementary to those stabilized by small molecules or polymers and most commonly consist of oil droplets dispersed in a continuous water phase, with particles assembled at the fluid−fluid interface. New particle surfactants and different fluid−fluid interfaces are critical for developing next-generation systems for a number of advanced applications. Herein we report the preparation of IL-containing emulsions stabilized by graphene oxide (GO)-based nanoparticles using the IL [Bmim][PF6]: GO nanosheets stabilize IL-in-water emulsions, and alkylated GO nanosheets (C18-GO) stabilize IL-in-oil emulsions. The impact of particle concentration, fluid−fluid ratio, and addition of acid or base on emulsion formation and stability is studied, with distinct effects for the water and oil systems observed. We then illustrate the broad applicability of GO-based particle surfactants by preparing emulsions with different ILs and preparing inverted emulsions (water-in-IL and oil-in-IL emulsions). The latter systems were accessed by tuning the polarity of GO nanosheets by functionalization with a perfluorinated alkyl chain such that they were dispersible in IL. This work provides insight into the preparation of different IL-containing emulsions and lays a foundation for the architecture of dissimilar materials into composite systems.

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individual GO nanosheet is ∼1 nm thick and microns in diameter).27−29 GO nanosheets are composed of a 2D framework of carbon atoms decorated with oxygen-based functional groups including hydroxyls, carboxylic acids, and epoxides that can be modified using simple chemical reactions.30,31 Experimental studies have demonstrated that graphene oxide (GO) nanosheets are suitable surfactants for Pickering emulsions, complimented by computational studies that indicate substantially more energy is required to remove a nanosheet from the fluid−fluid interface compared to small molecules or spherical particles.17,32,33 GO is also a multifunctional material, as it has antimicrobial properties and can decrease gas permeability and improve mechanical strength of polymers, as well as impart electrical conductivity.34−37 GO nanosheets and their modified derivatives have been used as surfactants for various Pickering emulsions, with the polarity of the nanosheet and identity of fluids impacting emulsion formation. When oil-in-water Pickering emulsions are stabilized with GO nanosheets, salt concentration, pH, and oil identify all impact emulsion formation and stability, as reported in the works of Huang et al., Gao et al., and Thickett and Zetterlund.27,38,39 Such emulsions are applied as templates to fabricate porous materials.23 Alternatively, Rodier et al.

INTRODUCTION Emulsions are mixtures of two or more immiscible fluids, in which droplets of one (e.g., oil) are dispersed in a continuous phase of the other (e.g., water). Surfactants can be used to lower the interfacial tension between the two fluids and improve the kinetic stability of the systems.1,2 Most commonly, surfactants are amphiphilic small molecules, such as sodium dodecyl sulfate (SDS), which is composed of a charged headgroup and hydrophobic tail; however, solid particles can also serve as surfactants,3,4 yielding what are called Pickering emulsions. Most commonly, spherical (0D) particles microns in diameter are used to stabilize Pickering emulsions, although 1D rod-like particles as well as 2D sheet-like particles can also be used.5−10 The key characteristic of particle surfactants is their ability to assemble at the fluid−fluid interface rather than remain dispersed in either phase.11,12 Pickering emulsions have gained increased attention due to their applications in the food industry, drug delivery, catalysis, etc., and superior stability and biocompatibility compared to emulsions stabilized by small molecules.5,13−16 Moreover, Pickering emulsions can be used to template the formation of higher order composite structures including armored polymer particles, hollow capsules, and porous films.17−26 Graphene oxide (GO) nanosheets are of particular interest as surfactants in Pickering emulsions, as their polarity can be tailored by functionalization and they likely lay flat at the fluid−fluid interface based on their aspect ratio (i.e., an © XXXX American Chemical Society

Received: June 14, 2018 Revised: July 29, 2018 Published: July 30, 2018 A

DOI: 10.1021/acs.langmuir.8b02011 Langmuir XXXX, XXX, XXX−XXX

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Scheme 1. Idealized Structure of GO, Chemical Structure of Ionic Liquids (ILs), Schematic of IL-in-Water and IL-in-Oil Emulsions Stabilized by GO or C18-GO, and the Variables Studied in This Work

(C18-GO) adsorbs at the IL−oil interface of IL-in-oil emulsions. [Bmim][PF6] is used as the model IL, and the impact of nanosheet concentration, fluid−fluid ratio, and addition of acid and base are evaluated. We demonstrate that the emulsion interfacial area is dictated by the amount of nanosheet surfactant available. Addition of acid or base affects the surfactant polarity, changing the net charge of the nanosheets based on (de)protonation of different functional groups; unique trends are observed for the two systems, and reaction schemes are proposed. We further evaluate the preparation of Pickering emulsions using other ILs and illustrate the preparation of water-in-IL or oil-in-IL emulsions stabilized by GO functionalized with perfluorocarbons. The results reported herein provide complementary information to GO-stabilized oil-in-water emulsions and serve as a guideline for preparing IL-containing emulsions desirable for a variety of applications.

altered the polarity of GO nanosheets by functionalization with primary alkyl amines of varied chain length and used these modified nanosheets to stabilize oil-in-oil emulsions. The authors found that the length of the alkyl chain dictated whether DMF-in-octane or octane-in-DMF emulsions were formed.30 These water-free emulsion were used to fabricate hollow capsules, armored particles, and closed cell foams.21 Further, GO nanosheets were used by Luo et al. as particle surfactants to stabilize ionic liquid (IL)-in-water emulsions; carbon shells encapsulating IL were prepared and used as the active material in coin-cell supercapacitors. The tailored composites had superior performance at lower temperatures and faster scan rates than crushed shells mixed with IL, attributed to the better wetting of the carbon material with electrolyte.18 Expanding the applications of GO-stabilized Pickering emulsions requires the development of new particle surfactants and fluid−fluid interfaces, as well as understanding the factors that impact emulsion formation. Ionic liquids (ILs), organic salts that are liquid below 100 °C, have diverse applications in energy storage, gas handling, and carbon capture.40−43 ILs have garnered much attention for advanced applications due to their nearly negligible vapor pressure and high thermal stability (most decompose above 450 °C).44−46 Some ILs have surfactant properties themselves; for example, those containing bis-2-ethylhexyl sulfosuccinate (AOT) anions can form micelles in water.47,48 Further, IL-inwater and IL-in-oil (mini)emulsions stabilized by small molecule or polymeric surfactants have been reported and used in energy-related applications as well as in chemical sensing,49 drug delivery,50 lubrication,51 etc. Thus, ILcontaining emulsions are attractive for a number of different applications; given the unique properties of Pickering emulsions, accessing IL-containing systems and understanding their formation will help establish advanced applications. Herein we report the preparation of IL-containing emulsions stabilized by GO or functionalized GO and evaluate the impact of different factors on their formation and stability. As shown in Scheme 1, GO nanosheets adsorb to the IL−water interface in the formation of IL-in-water emulsions and alkylated GO

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EXPERIMENTAL SECTION

Materials and Instrumentation. Graphite, sulfuric acid (H2SO4), hydrogen peroxide (H2O2), toluene, sodium hydroxide (NaOH), 1H,1H-perfluorooctylamine, Sudan Blue II, trifluoroacetic acid (TFA), and N,N-dimethylformamide (DMF) were purchased from Sigma-Aldrich. 1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU) was purchased from Acros Organics. Potassium permanganate and 1octadecylamine were purchased from Alfa Aesar. Octane, 1-butyl-3methylimidazolium hexafluorophosphate, and 1-butyl-3-methylimidazolium tetrafluoroborate were purchased from Fisher. 1-Ethyl-3methylimidazolium bis(trifluoromethylsulfonyl)imide (99%) was purchased from Iolitec. Hydrochloric acid was purchased from BDH. All reagents were used as received, without further purification. Centrifugation was accomplished with an Eppendorf 5804 centrifuge. X-ray photoelectron spectroscopy (XPS) data were collected using a PHI Versaprobe 5000 scanning X-ray photoelectron spectrometer. Fourier Transform infrared (FTIR) spectroscopy was performed using an Aglient Cary 630 FTIR in ATR mode. Atomic force microscopy (AFM) was performed on a NX-10 Park System in tapping mode and imaged in topography mode; samples were prepared by drop casting from 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 B

DOI: 10.1021/acs.langmuir.8b02011 Langmuir XXXX, XXX, XXX−XXX

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Figure 1. Photographs and optical microscopy images of GO-stabilized [Bmim][PF6]-in-water Pickering emulsions illustrating the effect of the following: (A) Initial aqueous GO concentration of (i) 0.50, (ii) 1.0, (iii) 2.0, (iv) 4.0 mg/mL; (B) IL:water ratio of (i) 0.8:1, (ii) 0.4:1, (iii) 0.2:1, (iv) 0.1:1. Preparation of IL-in-Water Pickering Emulsions Stabilized by GO. The standard IL-in-water emulsion was prepared by dispersing GO in distilled water (2.0 mg/mL, 1.0 mL) in a 4 mL vial; [Bmim][PF6] (0.20 mL) was then added and the solution agitated by vortex (10 s), sonication (10 s), vortex (10 s), sonication (10 s), and vortex (10 s). Emulsions at different GO concentrations, IL-water ratio, and addition of HCl and NaOH were prepared in a similar manner (see Table S1). One molar HCl and 1 M NaOH aqueous solutions were prepared for use as the added acid and base to IL-in-water emulsions. Preparation of IL-in-Oil Pickering Emulsions Stabilized by C18-GO. For the standard IL-in-oil emulsion, C18-GO was dispersed in octane (2.0 mg/mL standardized by parental GO, 1.0 mL) in a 4 mL vial, and then [Bmim][PF6] (0.20 mL) was added. The system was agitated with a hand-held emulsifier for 30 s. Emulsions at different C18-GO concentrations, IL-water ratios, and additions of TFA and DBU were prepared in a similar manner (see Table S2). Preparation of [Emim][TFSI]-in-Water Emulsion and [Bmim][BF4]-in-Octane Emulsions. The sample preparation was similar to that discussed above. The [Emim][TFSI] emulsions were emulsified by vortex (10 s), sonication (10 s), vortex (10 s), sonication (10 s), and vortex (10 s). The [Bmim][BF4] system was emulsified by a hand-held emulsifier for 30 s. Preparation of Water-in-IL and Oil-in-IL Pickering Emulsions Stabilized by Perfluorocarbon-Functionalized GO (FGO). FGO was dispersed in a mixture of 5:1 by volume methanol: [Bmim][PF6] by sonication, and then methanol was then evaporated to obtain FGO in IL dispersion. For the water-in-IL and oil-in-IL emulsions, water or oil was added, and the mixture was emulsified by vortex (10 s), sonication (10 s), vortex (10 s), sonication (10 s), and vortex (10 s).

Products, Model 985370. Optical images were taken with an AmScope M150C microscope with AmScope MU500-CK 5.0 MP USB microscope camera. Emulsion samples were drop-cast on glass slides, spread out as a thin layer using a spatula, and then examined under the optical microscope. The optical images shown are representative and generally collected in the middle of the sample. Zeta potential measurements were performed on Mobius, Wyatt Technology. Particle analysis was performed using Image-J of optical microscopy images under the assumption that the droplets are spherical; droplets out of focus, fragmented, or overlapping were not included. This method was not suitable for characterization of emulsions with droplets >200 μm, due to the limit of image area. Stability of the emulsions was tested by letting them stand unagitated for 7 days and then performing optical microscopy. Preparation of Graphene Oxide (GO). Graphene Oxide (GO) was synthesized from natural graphite through a reported method.52 Briefly, natural graphite (1.0 g) was magnetically stirred in concentrated H2SO4 (134 mL) at room temperature. Then a batch of KMnO4 (1.0 g, 0.063 mol) was carefully added to the suspension, and the mixture was stirred for 24 h; the procedure was repeated three more times until a total of 4 g of KMnO4 was added (the reaction was stirred 24 h each time before addition of next batch of oxidant). The solution was then transferred to ice−water solution (0.70 L), followed by the slow addition of an aqueous H2O2 solution, until the pink color in the solution disappeared, which indicated quenching of excess KMnO4. Finally, centrifugation led to the isolation of a yellow brown solid as a pellet, the supernatant was discarded, and the pellet was washed repeated with 2-propanol until neutral and then dried under reduced pressure at room temperature. For GO aqueous dispersions, GO solid was vortexed and sonicated in water with concentrations of 0.50 mg/mL (0.050 wt %), 1.0 mg/mL (0.10 wt %), 2.0 mg/mL (0.20 wt %), and 4.0 mg/mL (0.40 wt %). GO dispersions flowed slower as the concentration increased (i.e., when tilting the vial). Preparation of Octadecylamine-Functionalized Graphene Oxide (C18-GO). C18-GO was prepared by suspending GO (100 mg) in DMF (50 mL) and octadecylamine (1.0 g) in DMF (50 mL)30 and then mixing the two solutions together and stirring for 5 min. The precipitate was isolated by centrifugation then dispersed in toluene (50 mL); octadecylamine (2.0 g) was added to this suspension, and the resulting system was loosely capped and stirred overnight at 50 °C. Finally, a dark brown solid was isolated by centrifugation, successively washed with toluene and octane, and dried under reduced pressure. Preparation of Perfluorooctylamine-Functionalized Graphene Oxide (FGO). GO (15.0 mg) was suspended in DMF (15 mL), and 1H,1H-perfluorooctylamine (0.058 mL) was added; the mixture was stirred under ambient atmosphere at 60 °C for 24 h. The reaction product was washed thoroughly with DMF and acetone.

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RESULTS AND DISCUSSION

Synthesis of GO and C18-GO. GO nanosheets were prepared and modified as previously reported. Graphite flakes were oxidized using potassium permanganate in concentrated acid and isolated by centrifugation, then characterized by X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and Fourier transform infrared (FTIR) spectroscopy. XPS revealed that the GO nanosheets are highly oxidized, with C:O ratio ∼2:1 (Figure S1), and AFM showed that they were microns in diameter with a thickness of ∼1.2 nm, in agreement with exfoliated nanosheets (i.e., single layer, Figure S2).53 The FTIR spectrum of GO is consistent with previous reports and contains peaks at ∼1720 cm−1 indicative of CO functionalities and ∼1620 cm−1 indicative of CC bonds (Figure C

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Figure 2. Photographs and optical microscopy images of GO-stabilized [Bmim][PF6]-in-water emulsions demonstrating the effect of the following: (A) Addition of acid (vol % 1 M HCl) of (i) 0.8%, (ii) 3.2%, (iii) 13%, or (B) addition of base (vol % 1 M NaOH) of (i) 0.4%, (ii) 0.8%, (iii) 1.6%.

S3).54 The epoxides and carboxylic acid groups of the GO nanosheets were modified with octadecylamine, yielding C18GO, which was characterized by XPS and FTIR. Octadecyl amine reacts with GO by two routes: (1) deprotonation of carboxylic acid groups on the nanosheet edges to functionalize the nanosheets by electrostatic interactions; (2) ring opening of epoxides to covalently modify both faces of GO (reaction scheme in Figure S4). XPS showed the C:O:N ratio of C18-GO is ∼25:2:1, consistent with the addition of 18 carbon atoms for each nitrogen atom within the limits of XPS (Figure S1). Compared to GO, the FTIR spectrum of C18-GO has significantly more intense peaks from 2700 to 3000 cm−1 as well as a peak at ∼1540 cm−1, assigned to C−H and N−H bonds, respectively. The reduction in intensity of the CO peak at 1720 cm−1 can be attributed to formation of carboxylates and/or a slight reduction of the nanosheets, as supported by a darkening of the color. GO nanosheets were dispersed in water and formed a transparent dispersion (Figure S5A), and C18-GO nanosheets were dispersed in octane, forming an opaque dispersion with some aggregates, as supported by optical microscopy (Figure S5B). [Bmim][PF6]-in-Water Emulsions. [Bmim][PF6] was chosen as the IL phase because it is commercially available and immiscible with both water and octane. This IL has been used for metal ion extraction, as well as a solvent for various reactions and electrolyte for supercapacitors.55−57 A standard GO-stabilized IL-in-water emulsion was prepared by dispersing GO nanosheets in water (2.0 mg/mL) and then adding 0.20 mL of [Bmim][PF6] to 1.0 mL of this GO solution; the resulting mixture was agitated by vortex and sonication to achieve emulsification. This standard emulsion had droplets 20−60 μm in diameter as determined by optical microscopy (Figure 1Aii, and Figure S9), with a clear layer of excess water on top of an opaque emulsion. The impact of GO concentration, IL:water ratio, and presence of acid/base on the ability to form an emulsion and droplet size are discussed below. The stability of GO-stabilized IL-in-water emulsions was also studied: higher GO concentration, lower IL:water ratio, and addition of acid led to little change in droplet shape or diameter after 7 days compared to lower GO concentration, higher IL:water ratio, and addition of base which led to droplet deformation or disappearance, with IL expelled from the emulsion (Figure S11).

Impact of GO Concentration and IL:Water Ratio on IL-inWater Emulsions. The formation of IL-in-water emulsions was studied as a function of GO concentration at constant IL:water ratio and as a function of IL:water ratio at constant GO concentration. Using an IL:water ratio of 0.2:1 (by volume), emulsions were prepared with aqueous GO concentrations of 0.50, 1.0, 2.0, and 4.0 mg/mL. Photographs and optical microscopy images of these emulsions are shown in Figure 1A. At all GO concentrations, emulsions formed with the dispersed IL droplets residing at the bottom of the vial (as expected based on density), with the color of the emulsion fraction darkening with increased GO concentration. For each emulsion, a distribution of droplet sizes is observed; as a general trend, from 0.50 to 2.0 mg/mL larger volume fractions of emulsion and smaller droplets were observed (emulsions fractions of ∼40%, 60%, 70%, and 70% for 0.50, 1.0, 2.0, and 4.0 mg/mL of GO, respectively, droplet size distribution shown in Figure S9A). This trend is in line with small molecule surfactants and indicates that more nanoparticle surfactant leads to a larger interfacial area.2,58 However, comparison of the emulsions formed using 2.0 and 4.0 mg/mL GO revealed a similar volume fraction of stable emulsion and droplet sizes (Figure 1Aiii and 1Aiv); this lack of change in droplet diameter may be attributed to aggregation of GO at higher concentration, as supported by the increased color contrast in optical images. The impact of IL:water ratio was studied using 2.0 mg/mL of GO and IL:water ratios of 0.8:1, 0.4:1, 0.2:1, and 0.1:1 (Figure 1B). Smaller droplet diameters were observed when less IL was used (Figure S9B), a trend that again reflects that the presence of more nanoparticle surfactants gives a larger interfacial area. If we assume all GO nanosheets adsorb to the IL−water interface, that they form a monolayer (or have similar extents of aggregation), and that there is enough water to form a continuous phase, then emulsion droplets with similar diameters will form at the same relative amounts of GO and IL. For example, compare Figure 1Ai and 1Bi: both samples contain a ratio of GO:IL 1:4 (mg:mL), but the sample in Figure 1Ai contains four times the amount of water (and thus lower GO concentration). Although the droplets should be the same size based on the argument above, the droplets in Figure 1Bi are larger than those in Figure 1Ai. This difference may be due to the nanosheets being more aggregated at higher D

DOI: 10.1021/acs.langmuir.8b02011 Langmuir XXXX, XXX, XXX−XXX

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Figure 3. Photographs and optical microscopy images of C18-GO stabilized [Bmim][PF6]-in-octane emulsion illustrating the effects of the following: (A) C18-GO concentration of (i) 0.5 mg/mL, (ii) 1.0 mg/mL, (iii) 2.0 mg/mL, (iv) 4.0 mg/mL, and (B) IL:oil ratio of (i) 0.8:1, (ii) 0.4:1, (iii) 0.2:1, (iv) 0.1:1. Concentration of particle (C18-GO) is based on GO itself.

decrease in the number of particles with an affinity for the water−IL interface due to more negatively charged nanosheets that are highly dispersible in water, indicated by the color of the aqueous phase. Of note, addition of acid or base to prepared emulsions led to deformation of droplets into irregular (i.e., nonspherical) shapes (Figure S6B and S6C). This subtle change may be due to the inability of the nanosheets to migrate or rearrange once assembled at the IL− water interface and may indicate a route to accessing stimuliresponsive IL-containing emulsions. [Bmim][PF6]-in-Octane Emulsions. Water-free emulsions attract attention for use of water-incompatible compounds, as well as coatings for water-sensitive substrates. IL-in-oil emulsions stabilized by polymer surfactants have previously been applied as drug carriers and lithium battery separators.61−63 We studied the preparation of IL-in-octane Pickering emulsions stabilized by alkylated GO nanosheets. Octane was chosen as the continuous phase because C18-GO nanosheets are dispersible in it, and [Bmim][PF6] was chosen as the IL because it is immiscible with octane and the system can be compared to the IL-in-water emulsions above. Similar to [Bmim][PF6]-in-water emulsions, the impact of C18-GO concentration, IL:octane ratio, and acid/base addition on emulsion formation was evaluated. For comparison to IL-inwater emulsions, the concentration of C18-GO was standardized to mass of GO present rather than C18-GO, so that approximately the same amount of nanoparticle surfactant is used. Of note, vortex and sonication were not effective for emulsion formation (Figure S6A) and thus a hand-held shearing emulsifier was used. The stability of these IL-in-oil emulsions was studied after 7 days unagitated: addition of base led to a stable emulsion with little-to-no change in droplets size or emulsion volume fraction, but other conditions led to increased droplet size distribution (Figure S13). The Impact of C18-GO Concentration and IL:Oil Ratio on IL-in-Octane Emulsions. C18-GO was dispersed in octane at different concentrations (0.50, 1.0, 2.0, and 4.0 mg/mL), and emulsions were prepared by adding [Bmim][PF6] and agitating, using an IL-oil ratio of 0.2:1. As seen in Figure 3A, at higher concentrations of C18-GO, a greater emulsion volume fraction was observed, as expected (∼40%, 50%, 60%, and 100% of the 0.50, 1.0, 2.0, and 4.0 mg/mL of C18-GO,

concentration (Figure 1Bi), or increased viscosity of solution at higher IL:water ratio. A similar comparison can be made between Figure 1Aii/1Bii and Figure 1Aiv/1Biv; however, these emulsions appear similar and even a qualitative statement cannot be made. These data illustrate that when preparing Pickering emulsions, and especially those making use of 2D particle surfactants, care must be taken to understand the impact of a number of variables on emulsion formation. Impact of Addition of Acid or Base on IL-in-Water Emulsions. Next, acid or base was added to the aqueous GO suspension before emulsion formation. Numerous studies indicate that in oil-in-water emulsions pH impacts the interfacial activity of GO nanosheets: under acidic conditions, GO nanosheets become less hydrophilic and under basic conditions GO becomes more hydrophilic due to protonation and deprotonation of carboxylates, respectively.27,39,59,60 Essentially, at low pH the increased hydrophobicity of the nanosheets renders them more likely to assemble at the oil− water interface. In IL-in-water emulsions, changes in the charge of GO nanosheets in response to acid/base are expected to be the same; however, the impact on interfacial activity is a priori unclear given the polarity difference between oil/water and IL/ water. To study the impact of added acid or base, the standard ILin-water emulsions were prepared (2.0 mg/mL, IL:water 0.2:1), but with addition of HCl or NaOH to the aqueous GO dispersion, prior to addition of the IL (see Experimental Section for details). Regardless of how much acid was added (0.8−13 vol % 1 M HCl), emulsions formed, and droplets of similar size were observed (Figures 2A and S9C). In contrast, emulsion formation was dramatically impacted by addition of base. With addition of a small amount of base (0.4 vol % 1 M NaOH), emulsion droplets were smaller and less aggregated, resulting in an increased volume fraction of the emulsion (Figures 2Bi and S9D). This observation can be attributed to deprotonation of carboxylic acid groups on the nanosheet edges which improves their dispersibility in water, thus giving a greater number of nanosheets (see Figure S10 for change in solution pH and zeta potential of nanosheets). Addition of more base led to a dramatic increase in droplet size, as well as the presence of GO nanosheets in the aqueous phase (Figure 2Bii and 2Biii). This observation can be attributed to a E

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Figure 4. Photographs and optical microscopy images of C18-GO stabilized [Bmim][PF6]-in-octane emulsion illustrating the impact of the following: (A) Addition of acid (TFA): (i) 2.0%, (ii) 4.0%, (iii) 6.0%; (B) addition of base (DBU) (i) 1.0%, (ii) 2.0%, (iii) 3.0%.

fluid interface is similar for all samples, the expected trend in droplet diameter is not observed, and other factors play critical roles in emulsion formation and droplet diameter. Impact of Addition of Acid or Base on IL-in-Oil Emulsions. To study the impact of added acid or base on IL-in-octane emulsions, the standard emulsions were prepared (2.0 mg/mL C18-GO, IL:oil 0.2:1) but with the addition of trifluoroacetic acid (TFA, an acid) or 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU, a base) to the IL prior to emulsification. In the presence of acid, the carboxylates on the nanosheet edges of C18-GO are protonated, and then secondary amines and epoxide groups on the faces of the nanosheets can be protonated. Alternatively, in the presence of base, the ammonium counterions of the carboxylates will be deprotonated, essentially leading to a cation exchange and formation of octadecylamine. An illustration of how protonation/deprotonation impacts the charge of C18-GO is shown in Figure S7; unfortunately, the zeta potential of alkylated nanosheets could not be determined due to lack of solubility in polar solvents. The impact of added acid or base on the interfacial activity of C18-GO is expected to be complementary to that observed for GO-stabilized IL-in-water emulsions. The difference in polarity of the IL and octane are distinct from IL/water: for C18-GO, a higher net charge of the particle leads to a greater affinity for the dispersed phase (IL), whereas for GO, the higher net charge of the nanosheets gives higher affinity for the continuous phase (water). The effect of acid addition on IL-in-oil emulsions is shown in Figure 4A. Addition of 2.0% TFA led to no emulsion formation and the observation of a clear IL layer at the bottom of the vial and a dark octane layer above (Figure 4Ai). Optical microscopy images show this upper layer is composed of aggregated C18-GO nanosheets and no droplets. This observation is attributed to protonation of carboxylates on the edges of the nanosheets, in effect decreasing the overall charge of C18-GO, increasing its affinity for octane, and decreasing its affinity for the IL−octane interface. In contrast, addition of 4.0% or 6.0% TFA led to emulsion formation, albeit without all nanosheets associated with the droplets (Figure 4Aii and 4Aiii). With 4.0% TFA, emulsion droplets and C18-GO nanosheet aggregates are observed along with excess IL at the bottom of the vial (Figure 4Aii), whereas

respectively, are the opaque emulsion layer, droplet diameter distribution shown in Figure S12). However, in contrast to ILin-water emulsions, the concentration of particle surfactant did not significantly impact droplet size but instead impacted the amount of particle surfactant dispersed in the oil. With 0.50 mg/mL of C18-GO, all of the [Bmim][PF6] was not incorporated into the emulsion, and excess IL was present at the bottom of the vial (Figure 3Ai). Increasing the concentration of C18-GO to 1.0 mg/mL led to efficient encapsulation of the IL, without the separate IL phase (Figure 3Aii); however, further increasing the C18-GO concentration led to emulsion formation in combination with nanosheets dispersed in the continuous octane phase, as observed by optical microscopy (Figure 3Aii−iv). Thus, attempts to incorporate all IL or all C18-GO into an emulsion under various conditions were not met with success, and similar emulsions formed (i.e., droplet sizes are similar, Figure S12A). Evaluation of the impact of different IL:oil ratios using 2.0 mg/ mL of C18-GO was also evaluated; decreased volume fraction of IL led to decreased droplet size, as expected, and at the highest volume fraction of IL used, excess IL was observed (Figure 3Bi−iv). As discussed for IL-in-water emulsions above, assuming that the same amount of C18-GO reside at the fluid−fluid interface, that a monolayer forms (or nanosheets have the same degree of aggregation), and that enough octane is present to form a continuous phase, then emulsion droplets of similar size will be formed with the same relative amounts of C18-GO and IL. Thus, compare Figure 3Ai and 3Bi: both samples contain a ratio of C18-GO:IL 1:4 (mg:mL), but the sample in Figure 3Ai contains four times the amount of octane. The droplets should be similar in size; however, droplets in Figure 3Bi are clearly larger. Further, because excess C18-GO and IL are both observed, the extent of incorporation of these two materials into the emulsion is not the same. Again, aggregation of nanosheets or the overall viscosity of solution may be responsible for this phenomenon. Comparison of the emulsions in Figure 3Aii/3Bii and 3Aiv/3Biv shows no significant differences, and both samples contain excess C18GO nanosheets suspended in the octane (i.e., not all nanosheets are at the fluid−fluid interface). If the distribution of nanosheets between the continuous phase and the fluid− F

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fluorophilic and dispersible in IL by modification with 1H,1H-perfluorooctylamine. The resulting FGO nanosheets were characterized by XPS and FTIR and were dispersed in [Bmim][PF6]. XPS revealed a C:F:O:N ratio of ∼24:11:7:1 (Figure S1), and the FTIR spectrum showed the characteristic N−H bending stretch at ∼1540 cm−1, and peaks at ∼1170 cm−1, 1210 cm−1, and 1350 cm−1 attributed to the C−F bonds (Figure S3). FGO was dispersed in [Bmim][PF6] (2.0 mg/mL, standardized with the mass of GO), and characterization of the dispersion by optical microscopy showed some aggregates (Figure S5C). Emulsions were prepared by adding either water or oil to the dispersion of FGO in [Bmim][PF6] followed by vortex/sonication agitation. The opaque emulsion layer resided atop the IL, as expected based on density difference. The clear color of the continuous IL phase of both water-in-[Bmim][PF6] (Figure 5C) and oil-in-[Bmim][PF6] (Figure 5D) emulsions illustrates that FGO nanosheets adsorb to the fluid−fluid interface. This facile ability to control Pickering emulsion formation with different liquids based on the tailored functionalization of GO will allow for these systems to be applied in diverse fields, including medicine, energy storage and harvesting, catalysis, coatings, etc.

addition of 6.0% TFA led to incorporation of all IL in the emulsion, along with C18-GO nanosheets dispersed in the oil (Figure 4Aiii). This observation can be explained by protonation of neutral functional groups (e.g., secondary amines) which gives an overall positive charge to the nanosheets and improves their affinity for IL (and the IL− octane interface). Evaluation of other amounts of TFA indicate that emulsion formation is only observed by addition of greater than 3.0% TFA (Figure S8). In contrast to the impact of acid addition, the effect of the addition of base on IL-in-oil emulsions is minimal and reflected only in slightly decreased droplet size, as shown in Figures 4B and S12D. Addition of DBU deprotonates the ammonium cations which electrostatically interact with the carboxylates on the nanosheet edges and thus undergo a net cation exchange. Emulsions with Other ILs and Emulsions with IL as Continuous Phase. We also examined the ability to form emulsions with ILs other than [Bmim][PF6], namely [Emim][TFSI]-in-water and [Bmim][BF4]-in-octane emulsions. The ability to form emulsions with a number of different ILs is critical for tailored applications, as the IL identity dictates operating the electrochemical window, thermal stability, etc. Figure 5A shows the photograph and optical microscopy image

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CONCLUSIONS Herein we have illustrated that GO nanosheets and their functionalized derivatives are particle surfactants for different IL-containing Pickering emulsions and have reported the impact of different factors on emulsion formation. Graphene oxide (GO) and octadecylamine-functionalized GO (C18-GO) stabilize IL-in-water or IL-in-oil emulsions, respectively, with [Bmim][PF6] serving as the IL and octane as the oil. For IL-inwater emulsions, increased concentration of GO resulted in larger emulsion volume fraction and smaller droplet size, whereas increased volume fraction of IL increased droplet size. In IL-in-water emulsions, addition of acid had limited effect on emulsion formation, whereas addition of base negatively impacted the emulsions, leading to larger droplet diameters and nanosheets remaining dispersed in the water phase. For IL-in-oil emulsions, all IL was incorporated into the emulsion only upon addition of a sufficient concentration of C18-GO, but this was accompanied by nanosheets in oil phase; increased volume fraction of IL led to increased droplet size, yet addition of too much IL led to not all being incorporated into droplets. IL-in-oil emulsions displayed the opposite trend in response to addition of acid and base compared to IL-in-water emulsions: addition of acid greatly impacted the ability to form emulsions with no emulsion formation upon addition of a small amount of acid, whereas addition of base did not significantly impact emulsion formation. The differences in response of the IL-inwater and IL-in-oil systems to acid and base are attributed to protonation/deprotonation of different functional groups of GO and C18-GO, as well as differences in relative polarity of the fluids used to make emulsions. The applicability of GO-based nanoparticle surfactants was expanded to other ILs, as well as other emulsion systems. Specifically, [Emim][TFSI]-in-water and [Bmim][BF4]-in-oil Pickering emulsions stabilized by GO and C18-GO, respectively, were prepared. Furthermore, the polarity of GO was tuned by functionalization with a perfluorinated amine and the resulting FGO nanosheets were dispersible in [Bmim][PF6] and used as particle surfactants for both octane-in-IL and water-in-IL emulsions. While many solid particle surfactants have been reported for stabilization of oil-in-water Pickering

Figure 5. Photographs and optical microscopy images of the following: (A) [Emim][TFSI]-in-water emulsion stabilized by GO; (B) [Bmim][BF4]-in-octane emulsion stabilized by C18-GO; (C) water-in-[Bmim][PF6] emulsion stabilized by FGO; (D) octane-in[Bmim][PF6] emulsion stabilized by FGO.

of GO-stabilized [Emim][TFSI]-in-water emulsions; these systems had a large emulsion volume fraction, and droplets that were slightly smaller (∼30−70 μm) than those of [Bmim][PF6]-in-water emulsions. This could be attributed to the difference in polarity of the ILs, or to the lower viscosity of [Emim][TFSI] which could facilitate emulsion formation. In addition, C18-GO-stabilized [Bmim][BF4]-in-octane emulsions were prepared. The photograph and optical microscopy image in Figure 5B show residual nanosheets in oil phase, similar to [Bmim][PF6]-in-octane emulsions, and a wider distribution of droplet diameters, likely due to differences in affinity of the nanosheets for the two ILs. Thus, GO and alkylated GO nanosheets can be used as particle surfactants to stabilize droplets of IL in water or oil, provided the IL is not miscible with the continuous phase. We further examined the accessibility of IL-containing emulsions stabilized by GO-based particle surfactants by preparing water-in-IL and oil-in-IL Pickering emulsions, inverted from those discussed above (i.e., IL is the continuous phase). For these systems, GO nanosheets were made G

DOI: 10.1021/acs.langmuir.8b02011 Langmuir XXXX, XXX, XXX−XXX

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Langmuir

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emulsions, particle surfactants for IL-containing emulsions have been less widely studied and require distinct characteristics (i.e., polarity and dispersibility). The work presented herein demonstrates that GO-based particle surfactants are attractive for IL-containing Pickering emulsions and also lays the foundation for fabrication of IL-containing hybrid structures for tailored applications. Ongoing work focuses on using these IL systems for the preparation of different architectures and illustrating their distinct properties and applications.18,64−67

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.8b02011. Additional experimental details (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Qinmo Luo: 0000-0003-4269-2642 Peiran Wei: 0000-0001-7820-1716 Emily Pentzer: 0000-0001-6187-6135 Author Contributions §

These authors equally contributed to this work

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank CWRU College of Arts and Sciences and NSF CAREER Award no. 1551943 for financial support. Q.L. thanks Mr. Kevin Pachuta for AFM.

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REFERENCES

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DOI: 10.1021/acs.langmuir.8b02011 Langmuir XXXX, XXX, XXX−XXX